STANDARD 90.1
USER’S MANUAL
THE COMPLETE GUIDE TO USING STANDARD 90.1-2016
ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings,
has been a benchmark for commercial building energy codes in the United States and a key basis for
codes and standards around the world for more than 40 years. The standard provides the minimum
requirements for energy-efficient design of most buildings, except low-rise residential buildings. It is an
indispensable reference for engineers and other professionals involved in design of buildings and building
systems.
With sample calculations, application examples, useful tools, forms to demonstrate compliance, and
references to helpful resources and websites, this Manual is intended for architects, engineers, contractors,
code officials, and other building professionals, and is also suitable for use in educational programs. In
addition, purchasers of this User’s Manual can download interactive compliance forms and tools from
ASHRAE’s website.
STANDARD 90.1 USER’S MANUAL
Because the standard is written in mandatory language and therefore not intended as a design specification
or an instruction manual, this User’s Manual was developed to minimize multiple interpretations of Standard
90.1 that may occur. This Manual helps users of Standard 90.1-2016 understand its principles and
requirements and how to comply with them. It is written in clear, direct language, making it understandable
to professionals and laymen alike. It also includes measurements and calculations in both I-P and SI units,
making it usable with either edition of Standard 90.1.
Based on
ANSI/ASHRAE/IES Standard 90.1-2016,
Energy Standard for Buildings Except Low-Rise
Residential Buildings
Includes Online Access to
Compliance Forms and Tools
ISBN: 978-1-939200-87-7 (PDF)
ISBN: 978-1-939200-86-0 (Softcover)
This manual provides
• Explanation of Standard 90.1’s requirements
• Detailed description of changes from the
previous edition
• Useful examples of compliance scenarios
• Access to online compliance forms and tools
• Useful references and resources
Product Code: 90319 (Print) D-90319 (PDF)
ASHRAE
90-1 UM Cover.indd 1
•
1791 Tullie Circle, NE, Atlanta, GA 30329
•
www.ashrae.org
12/13/2017 4:40:35 PM
Standard 90.1 User’s Manual
ANSI/ASHRAE/IES Standard 90.1-2016
Energy Standard for Buildings
Except Low-Rise Residential Buildings
I-P and SI
ATLANTA
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Printed in the United States of America.
ISBN 978-1-939200-86-0 (softcover)
ISBN 978-1-939200-87-7 (PDF)
Library of Congress Cataloging-in-Publication Data
Names: ASHRAE (Firm)
Title: 90.1 user's manual based on ANSI/ASHRAE/IES standard 90.1-2016, energy
standard for buildings except low-rise residential buildings.
Description: Atlanta, GA : ASHRAE, [2017] | Includes bibliographical
references and index.
Identifiers: LCCN 2017045504| ISBN 9781939200860 (softcover) | ISBN 9781939200877 (PDF)
Subjects: LCSH: Tall buildings--Thermal properties--Handbooks, manuals, etc. | Tall buildings--Energy conservation-Standards--United States--Handbooks, manuals, etc. | Buildings--Thermal properties--Handbooks, manuals, etc. | Buildings-Energy conservation--Standards--United States--Handbooks, manuals, etc. | Insulation (Heat)--Standards--United States-Handbooks, manuals, etc. |
ASHRAE (Firm). ANSI/ASHRAE/IESNA standard 90.1-2007 energy standard for
buildings except low-rise residential buildings.
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www.ashrae.org/publicationupdates.
Contents
Preface .............................................................................................................................................................. vii
Acknowledgments ......................................................................................................................................... xi
1. Purpose ...................................................................................................................................................... 1
Overview ...........................................................................................................................................................................................1
2. Scope ........................................................................................................................................................... 3
Authority of Standard 90.1 ........................................................................................................................................................3
Scope of the Standard ..................................................................................................................................................................3
3. Definitions, Abbreviations, and Acronyms ................................................................................... 5
Definitions ........................................................................................................................................................................................5
Abbreviations and Acronyms ...................................................................................................................................................5
4. Administration and Enforcement ..................................................................................................... 9
Scope and Compliance (4.1.1 and 4.2) .................................................................................................................................9
New Buildings (4.1.1.1 and 4.2.1.1).......................................................................................................................................9
Existing Buildings (4.1.1.2, 4.1.1.3, 4.1.1.4, 4.2.1.2, and 4.2.1.3) ........................................................................... 10
Administrative Requirements (4.1.2)................................................................................................................................ 12
Alternative Materials, Methods of Construction, or Design (4.1.3) ...................................................................... 12
Validity (4.1.4) .............................................................................................................................................................................12
Other Laws (4.1.5)...................................................................................................................................................................... 12
Referenced Standards (4.1.6)................................................................................................................................................ 12
Normative Appendices (4.1.7) .............................................................................................................................................. 12
Informative Appendices (4.1.8) ........................................................................................................................................... 13
Compliance Documentation (4.2.2).................................................................................................................................... 15
Labeling of Materials and Equipment (4.2.3)................................................................................................................. 14
Inspections (4.2.4) ..................................................................................................................................................................... 15
Verification and Commissioning Reporting (4.2.5) ..................................................................................................... 15
5. Building Envelope ................................................................................................................................ 17
General (5.1) .................................................................................................................................................................................17
Scope (5.1.1) .................................................................................................................................................................................17
Space-Conditioning Categories (5.1.2).............................................................................................................................. 17
Envelope Alterations (5.1.3) .................................................................................................................................................. 23
 Climate (5.1.4) .................................................................................................................................................................... 24
 Compliance Paths (5.2) ................................................................................................................................................... 27
Mandatory Provisions (5.4) ................................................................................................................................................... 30
Insulation (5.4.1) ........................................................................................................................................................................30
Fenestration and Doors (5.4.2) ............................................................................................................................................ 32
Air Leakage (5.4.3) ..................................................................................................................................................................... 32
Prescriptive Building Envelope Option (5.5) ................................................................................................................. 36
Using the Criteria Tables (5.5.1 and 5.5.2) ...................................................................................................................... 36
Opaque Areas (5.5.3) ................................................................................................................................................................ 37
Fenestration (5.5.4) ................................................................................................................................................................... 81
Building Envelope Trade-Off Option (5.6) ................................................................................................................... 104
Product Information and Installation Requirements (5.8) ................................................................................... 107
Insulation (5.8.1) ..................................................................................................................................................................... 107
Fenestration and Doors (5.8.2) ......................................................................................................................................... 110
 Inspection and Verification ........................................................................................................................................ 113
6. HVAC Systems ..................................................................................................................................... 127
General (6.1) .............................................................................................................................................................................. 127
Standard 90.1 User’s Manual
iii
Contents
Scope (6.1.1) .............................................................................................................................................................................. 127
 Compliance Paths (6.2)................................................................................................................................................ 128
Simplified Approach Option (6.3) .................................................................................................................................... 128
Prescriptive Path (6.5) .......................................................................................................................................................... 129
Alternative Compliance Path (6.6) .................................................................................................................................. 129
Energy Cost Budget Method (Section 11) .................................................................................................................... 129
 Performance Rating Method (Appendix G) ........................................................................................................ 130
Simplified Approach Option for HVAC Systems(6.3) .............................................................................................. 130
Scope (6.3.1) .............................................................................................................................................................................. 130
Criteria (6.3.2) .......................................................................................................................................................................... 130
Mandatory Provisions (6.4) ................................................................................................................................................ 139
Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1) .................................................... 139
Calculations (6.4.2) ................................................................................................................................................................. 147
Controls and Diagnostics (6.4.3) ...................................................................................................................................... 149
HVAC&R System Construction and Insulation (6.4.4) ............................................................................................ 172
Walk-In Coolers and Walk-In Freezers (6.4.5) ........................................................................................................... 180
Refrigerated Display Case (6.4.6)..................................................................................................................................... 182
Prescriptive Path (6.5) .......................................................................................................................................................... 182
Economizers (6.5.1) ............................................................................................................................................................... 182
Simultaneous Heating and Cooling Limitation (6.5.2) ............................................................................................ 202
Air System Design and Control (6.5.3) ........................................................................................................................... 214
Hydronic System Design and Control (6.5.4) .............................................................................................................. 234
Heat-Rejection Equipment (6.5.5) ................................................................................................................................... 241
Energy Recovery (6.5.6) ....................................................................................................................................................... 243
Exhaust Systems (6.5.7) ....................................................................................................................................................... 249
Radiant Heating Systems (6.5.8) ...................................................................................................................................... 252
Hot-Gas Bypass Limitation (6.5.9) ................................................................................................................................... 252
Door Switches (6.5.10) ......................................................................................................................................................... 252
Refrigeration Systems (6.5.11) ......................................................................................................................................... 253
Alternative Compliance Path (6.6) .................................................................................................................................. 254
Computer Room Systems (6.6.1) ...................................................................................................................................... 254
Submittals (6.7) ........................................................................................................................................................................ 257
Completion Requirements (6.7.2) ................................................................................................................................... 257
 Minimum Equipment Efficiency Tables (6.8) .................................................................................................... 262
7. Service Water Heating ..................................................................................................................... 269
General (7.1) .............................................................................................................................................................................. 269
Scope (7.1.1) .............................................................................................................................................................................. 269
 Compliance Paths (7.2)................................................................................................................................................ 270
Mandatory Provisions (7.4) ................................................................................................................................................ 272
Load Calculations (7.4.1) ..................................................................................................................................................... 272
Equipment Efficiency (7.4.2) .............................................................................................................................................. 273
 Service Hot-Water Piping Insulation (7.4.3) ...................................................................................................... 276
Service Water Heating System Controls (7.4.4)......................................................................................................... 278
Swimming Pools (7.4.5)........................................................................................................................................................ 279
Heat Traps (7.4.6) ................................................................................................................................................................... 280
Prescriptive Path (7.5) .......................................................................................................................................................... 282
Space Heating and Service Water Heating (7.5.1) .................................................................................................... 282
Service Water Heating Equipment (7.5.2) .................................................................................................................. 284
Buildings with High-Capacity Service Water Heating Systems (7.5.3) ........................................................... 284
8. Power ..................................................................................................................................................... 287
iv
General (8.1) .............................................................................................................................................................................. 287
Scope (8.1.1) .............................................................................................................................................................................. 287
Mandatory Provisions (8.4) ................................................................................................................................................ 287
Standard 90.1 User’s Manual
Contents
Voltage Drop (8.4.1) ............................................................................................................................................................... 287
Automatic Receptacle Control (8.4.2) ............................................................................................................................ 289
Electrical Energy Monitoring (8.4.3) .............................................................................................................................. 292
Low-Voltage Dry-Type Distribution Transformers (8.4.4) ................................................................................... 296
Submittals (8.7) ........................................................................................................................................................................ 296
Drawings (8.7.1) ...................................................................................................................................................................... 296
Manuals (8.7.2) ......................................................................................................................................................................... 296
9. Lighting ................................................................................................................................................. 299
General (9.1) .............................................................................................................................................................................. 299
 Scope (9.1.1) ..................................................................................................................................................................... 299
 Lighting Alterations (9.1.2)........................................................................................................................................ 300
Installed Lighting Power (9.1.3) ....................................................................................................................................... 300
Interior and Exterior Luminaire Wattage (9.1.4)...................................................................................................... 301
 Compliance (9.2) ............................................................................................................................................................ 303
Prescriptive Requirements (9.2.2) .................................................................................................................................. 304
Mandatory Provisions (9.4) ................................................................................................................................................ 307
Lighting Control (9.4.1) ........................................................................................................................................................ 307
Exterior Building Lighting Power ( 9.4.2) .................................................................................................................... 328
Functional Testing ( 9.4.3) .................................................................................................................................................. 333
 Dwelling Units (9.4.4)................................................................................................................................................... 334
 Building Area Method Compliance Path (9.5) ................................................................................................... 334
Alternative Compliance Path: Space-by-Space Method (9.6) .............................................................................. 338
Additional Interior Lighting Power (9.6.2) .................................................................................................................. 340
Additional Interior Lighting Power Using Nonmandatory Controls (9.6.3) ................................................. 345
Room Geometry Adjustment (9.6.4) ............................................................................................................................... 345
 Submittals (9.7)............................................................................................................................................................... 350
General (9.7.1) .......................................................................................................................................................................... 350
Completion Requirements (9.7.2).................................................................................................................................... 350
10. Other Equipment ............................................................................................................................... 353
General (10.1) ........................................................................................................................................................................... 353
Scope (10.1.1)............................................................................................................................................................................ 353
Mandatory Provisions (10.4) ............................................................................................................................................. 353
 Electric Motors (10.4.1)............................................................................................................................................... 353
Service Water Pressure Booster Systems (10.4.2) ................................................................................................... 355
Elevators (10.4.3) .................................................................................................................................................................... 355
Escalators and Moving Walks (10.4.4)........................................................................................................................... 356
11. Energy Cost Budget Method ........................................................................................................... 357
General (11.1) ........................................................................................................................................................................... 357
Scope and Limitations (11.1.1, 11.1.2 and 11.1.3) .................................................................................................... 358
Compliance (11.2) ................................................................................................................................................................... 361
Simulation General Requirements (11.4) ..................................................................................................................... 362
Simulation Program (11.4.1) .............................................................................................................................................. 362
Climatic Data (11.4.2) ............................................................................................................................................................ 364
Renewable, Recovered, and Purchased Energy (11.4.3) ....................................................................................... 364
Compliance Calculations (11.4.4) ..................................................................................................................................... 365
Exceptional Calculation Methods (11.4.5).................................................................................................................... 366
Calculating Design Energy Cost and Energy Cost Budget (11.5) ........................................................................ 366
Design Model (Table 11.5.1[1])......................................................................................................................................... 367
Alterations and Additions (Table 11.5.1[2]) ............................................................................................................... 367
Space Use Classifications (Table 11.5.1[3]) ................................................................................................................. 368
Schedules (Table 11.5.1[4]) ................................................................................................................................................ 368
Building Envelope (Table 11.5.1[5]) ............................................................................................................................... 369
Lighting Systems (Table 11.5.1[6]) ................................................................................................................................. 371
Standard 90.1 User’s Manual
v
Contents
Thermal Blocks—HVAC Zones Designed (Table 11.5.1[7]) ................................................................................. 374
Thermal Blocks—HVAC Zones Not Designed (Table 11.5.1[8]) ........................................................................ 374
Thermal Blocks—Multifamily Residential Buildings (Table 11.5.1[9]).......................................................... 376
HVAC Systems (Table 11.5.1[10] and Section 11.5.2) ............................................................................................ 376
Service Hot-Water Systems (Table 11.5.1[11]) ......................................................................................................... 386
Miscellaneous Loads (Table 11.5.1[12]) ....................................................................................................................... 387
Modeling Exceptions (Table 11.5.1[13]) ...................................................................................................................... 387
Modeling Limitations to the Simulation Program (Table 11.5.1[14]) ............................................................. 387
Documentation Requirements (11.7) ............................................................................................................................ 394
Considerations for the Adopting Authority ................................................................................................................. 395
G. Performance Rating Method.......................................................................................................... 399
General (G1)............................................................................................................................................................................... 399
Performance Rating Method Scope (G1.1) ................................................................................................................... 399
Performance Rating (G1.2) ................................................................................................................................................. 400
Documentation Requirements (G1.3) ............................................................................................................................ 404
Simulation General Requirements (G2) ........................................................................................................................ 404
Performance Calculations (G2.1) ..................................................................................................................................... 404
Simulation Program (G2.2) ................................................................................................................................................. 405
Minimum Modeling Capabilities (G2.2.1) ..................................................................................................................... 405
Proposed Building and Baseline Building Performance (G2.2.2) ...................................................................... 406
Design Load Calculations (G2.2.3) ................................................................................................................................... 406
Testing (G2.2.4) ........................................................................................................................................................................ 406
Climatic Data (G2.3) ............................................................................................................................................................... 406
Renewable, Recovered, and Purchased Energy (G2.4) ........................................................................................... 406
Exceptional Calculation Methods (G2.5) ....................................................................................................................... 407
Calculation of the Proposed Design and Baseline Building Performance (G3) ........................................... 407
Building Performance Calculations (G3.1) ................................................................................................................... 408
Design Model (Table G3.1[1]) ............................................................................................................................................ 408
Additions and Alterations (Table G3.1[2])................................................................................................................... 409
Space Use Classifications (Table G3.1[3]) .................................................................................................................... 409
Schedules (Table G3.1[4]) ................................................................................................................................................... 410
Building Envelope (Table G3.1[5]) .................................................................................................................................. 412
Lighting (Table G3.1[6]) ....................................................................................................................................................... 419
Thermal Blocks—General Information ......................................................................................................................... 422
Thermal Blocks—HVAC Zones Designed (Table G3.1[7])..................................................................................... 422
Thermal Blocks—HVAC Zones Not Designed (Table G3.1[8]) ............................................................................ 422
Thermal Blocks—Multifamily Residential Buildings (Table G3.1[9]) ............................................................. 423
HVAC Systems (Table G3.1[10]) ....................................................................................................................................... 424
Baseline HVAC System Type and Description (G3.1.1) .......................................................................................... 428
 General Baseline HVAC System Requirements (G3.1.2) ............................................................................... 433
System-Specific Baseline HVAC System Requirements (G3.1.3) ........................................................................ 441
Service Water Heating Systems (Table G3.1, No.11) ............................................................................................... 446
Receptacle and Other Loads (Table G3.1, No.12) ...................................................................................................... 448
Modeling Limitations to the Simulation Program (Table G3.1, No.13) ........................................................... 449
Exterior Conditions (Table G3.1, No.14) ....................................................................................................................... 450
Distribution Transformers (Table G3.1, No.15) ........................................................................................................ 451
Elevators (Table G3.1[16]) .................................................................................................................................................. 451
Refrigeration (Table G3.1[17]).......................................................................................................................................... 451
This User's Manual includes free online access to compliance forms for
chapters 5, 6, 7, 9, and 11 and Appendix G. These forms are provided to assist in
understanding and documenting compliance with the Standard’s requirements.
vi
Standard 90.1 User’s Manual
Preface
General Information
Standard 90.1 User’s Manual provides detailed instruction for the design of commercial and high-rise
residential buildings to ensure their compliance with ANSI/ASHRAE/IES Standard 90.1–2016 (referred
to in this manual as “Standard 90.1” or simply “the standard”).
In addition, this user’s manual
• offers information on the intent and application of Standard 90.1,
• illuminates the standard through the use of abundant sample calculations and examples,
• streamlines the process of showing compliance, and
• provides forms to demonstrate compliance.
This manual also instructs the user in the application of
• energy simulation programs used in conjunction with the Energy Cost Budget Method and
Performance Rating Method of compliance with the standard and
• the Performance Rating Method, used for calculating building energy performance ratings for certain
building rating systems and incentive programs.
This manual is intended to be useful to numerous types of building professionals, including
• architects and engineers who must apply the standard to the design of their buildings,
• plan examiners and field inspectors who must enforce the standard in areas where it is adopted as
code,
• general and specialty contractors who must construct buildings in compliance with the standard, and
• product manufacturers, state and local energy offices, policy groups, utilities, and others.
Addenda
Standard 90.1 is a dynamic document undergoing continuous maintenance. Addenda, errata, and
interpretations will be issued throughout its life. This edition of Standard 90.1 User’s Manual is consistent
with Standard 90.1–2016. Significant changes have been made to the standard since publication of the
2013 edition, including modifications by more than 110 addenda. In this manual, noteworthy changes are
summarized at the start of each technical chapter and are indicated by a  in the relevant sections. In
addition, Appendix H of the standard provides a detailed list of the addenda that define the differences
between the 2013 and 2016 editions of the standard.
The ASHRAE and IES boards will approve additional addenda in the future that could revise the intent of
the standard. Users should consult the ASHRAE website (www.ashrae.org) or other sources to obtain the
latest addenda.
When using this manual to comply with an energy code based on Standard 90.1, check whether any
addenda have been incorporated into that code, and read those addenda carefully. Also, if one or more of
the addenda or criteria of the standard are not incorporated into an energy code, be careful to apply the
recommendations of this manual appropriately.
Standard 90.1 User’s Manual
vii
Pr e fa c e
Official Interpretations of the Standard
Standing Standards Project Committee (SSPC) 90.1 provides official interpretations of the standard upon
written request. Address requests for interpretations to Senior Manager of Standards, ASHRAE, 1791
Tullie Circle, NE, Atlanta, GA, 30329-2305, or e-mail standards.section@ashrae.org.
Requests for interpretations are forwarded to SSPC 90.1. That committee usually assigns the request to a
subcommittee, which then reviews it and develops an interpretation. This interpretation is then voted on
by the full committee. A common timeframe for a response is six to twelve months.
Standard 90.1 Organization
Numbering System
Standard 90.1 is divided into 11 sections. Sections 1, 2, 3, 4, and 12 are administrative:
•
•
•
•
•
Section 1, Purpose, states the purpose of the standard.
Section 2, Scope, describes where the Standard applies and does not apply.
Section 3, Definitions, Abbreviations, and Acronyms, provides definitions of terms that are used
throughout the standard and a list of abbreviations, acronyms, and symbols.
Section 4, Administration and Enforcement, gives an overview of the standard’s compliance
requirements, compliance documentation, materials and equipment labeling, and other
administrative requirements.
Section 12, Normative References, lists references and citations used in the Standard.
Sections 5 through 11 are the technical sections of the standard. Sections 5 through 10 contain the
technical requirements for distinct components of the building’s design, while Section 11 offers an
alternative whole-building approach to complying with the standard:
•
•
•
•
•
•
•
Section 5, Building Envelope, discusses building envelope, including building envelope components
and installation and fenestration (glazing).
Section 6, Heating, Ventilating, and Air Conditioning, covers HVAC systems, equipment, and
controls.
Section 7, Service Water Heating, addresses service water heating equipment and systems.
Section 8, Power, applies to building power distribution systems.
Section 9, Lighting, sets requirements for interior and exterior lighting systems and controls.
Section 10, Other Equipment, covers permanently wired electric motors.
Section 11, Energy Cost Budget Method, lays out the requirements for developing a computer
model for the Energy Cost Budget (ECB) compliance method.
Sections 5 through 11 are further divided into thematic subsections, with each subsection number
identifying its use. The numbering system for Sections 5 through 10 is organized as follows:
•
•
•
•
viii
x.1 General. This provides a general description of a particular section, including the scope and, in
some instances, general requirements of the section.
x.2 Compliance Paths. This provides a description of the process of complying with the section of
the standard.
x.3 Simple Buildings or Systems. This only exists for Section 6, but a placeholder is held for all the
other sections in the event that a simple compliance approach is developed in the future.
x.4 Mandatory Provisions. These are the mandatory minimum requirements that all projects must
meet under all circumstances.
Standard 90.1 User’s Manual
•
•
•
•
Pr e fa c e
x.5 Prescriptive Requirements. Provides additional requirements that only apply when the
prescriptive method is used to show compliance. Only Sections 5, 6, 7, and 9 have prescriptive
requirements.
x.6 Alternative Compliance Path. This is an alternative approach to compliance. For the Building
Envelope section, a procedure is included that permits trade-offs between all elements of the
building envelope. For the Lighting section, a space-by-space method is provided for determining
lighting power allowances.
x.7 Submittals. Addresses information that needs to be provided by the designer to the building
official, or by the contractor to the designer, to verify that the building complies with the standard.
x.8 Product Information. Addresses product information, installation requirements, and equipment
efficiency tables.
Section 11 follows a somewhat different numbering system because this section describes an alternative
compliance method rather than requirements for specific components of the building’s design.
In addition to the eleven primary sections, the standard contains a Foreword, eight appendices, and an
annex. The Foreword provides a historical perspective on the development of the standard.
Appendices A, C, and G are normative appendices that are part of the standard. Appendices B, D, E, F and
H are informative and are not part of the standard. A brief description of each appendix follows:
•
•
•
•
•
•
•
•
•
Appendix A includes precalculated R-values, U-factors, C-factors, and F-factors for typical
construction assemblies and calculation methods for atypical construction assemblies.
Appendix B is retained for future use.
Appendix C describes the methodology for the Building Envelope Trade-Off Option in Section 5.6.
Appendix D is retained for future use.
Appendix E contains informative references for the convenience of users of the standard and to
acknowledge source documents.
Appendix F includes an informative listing of minimum efficiency requirements established by the
U.S. Department of Energy.
Appendix G provides a normative description of the procedure for showing compliance or
calculating building energy performance ratings using the Performance Rating Method.
Appendix H provides an informative listing of approved addenda. The approved addenda define the
differences between the 2013 and 2016 editions of Standard 90.1.
Annex 1 reproduces material from ASHRAE Standard 169, which defines the climate zones used
throughout Standard 90.1.
Organization and Use of Standard 90.1 User’s Manual
In general, the chapters of this user’s manual follow the major sectional organization of the standard. To
aid the user in correlating requirements of the standard with the explanations in the user’s manual, all
major headings in the manual contain section number references in parentheses. For example, a
discussion of lighting control requirements in this user’s manual begins with the heading “Lighting
Control (9.4.1).” This allows the user to quickly refer to Section 9.4.1 of the standard, which gives the
requirements for lighting control.
Each major section of the standard has a corresponding section in the user’s manual, unless that section
of the standard is so clear that no further explanation is required.
Standard 90.1 User’s Manual
ix
Pr e fa c e
Compliance forms for Chapters 5, 6, 7, 9, 11, and Appendix G of the standard are available for download
from ASHRAE’s website at http://www.ashrae.org/UM90.1-2016. These forms are provided to assist in
understanding and documenting compliance with the standard’s requirements.
Unless directly footnoted or contained within the text, full citations for documents referred to in this
manual are found in Standard 90.1, Section 12, “Normative References.”
I-P and SI units
Standard 90.1–2016 is available in two editions, one using inch-pound (I-P) units and the other using the
International System (SI). Throughout this manual, I-P numbers, equations, dimensions, and so forth are
followed by the corresponding SI values in parenthesis, for example: 500 ft² (46 m²). Wherever possible,
illustrations contain both I-P and SI units. Complex or lengthy tables and calculations are provided first in
I-P units and then in SI units.
Resources, Data, and Analysis Tools
The following is a list of publications and tools that are necessary to apply the standard. Some of these
items, as noted, are only applicable to specific sections of the standard:
•
•
•
•
•
•
•
A current copy of Standard 90.1–2016 with errata and interpretations.
Copies of any published addenda to Standard 90.1.
An energy simulation program for the analysis of energy consumption in buildings if the ECB Method
of Section 11 or the Performance Rating Method of Appendix G are to be used.
ASHRAE Handbook—Fundamentals, which is referenced throughout the standard.
ASHRAE Handbook—HVAC Systems and Equipment and ASHRAE Handbook—HVAC Applications,
which are referenced in Chapters 6 and 7 of the manual.
ANSI/ASHRAE Standard 62.1-2013, Ventilation for Acceptable Indoor Air Quality, which is referenced
in Section 6 of the standard.
In addition to the project plans and specifications, manufacturer data may be required for lighting,
motors, opaque envelope, fenestration, HVAC, control, and water heating systems and equipment.
NOTE: All compliance forms referenced in this user’s manual, as well as any related spreadsheets and
materials generated by the committee to assist in the use of the standard, are available for download at
http://www.ashrae.org/UM90.1-2016.
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Standard 90.1 User’s Manual
Acknowledgments
NORESCO, LLC, prepared this update to Standard 90.1 User’s Manual under contract to ASHRAE. The
user’s manual was prepared under ASHRAE Special Project 300.
Roger Hedrick, BEMP, LEED AP, was technical editor, and John Lisman of NORESCO provided updates to
graphics.
This new edition of Standard 90.1 User’s Manual, based on the 2016 edition of the standard, builds on the
work of those who have been acknowledged in previous editions. Existing and past members of the
Standard 90.1 Standing Standards Project Committee (SSPC) wrote the original technical content, much
of which is still largely intact.
The 2016 Project Monitoring Subcommittee, chaired by Dick Lord, guided the user’s manual project and
helped reach resolution on issues as they arose. The 2016 edition benefited from the careful review of
members of SSPC 90.1, particularly Sean Beilman, Jeff Boldt, Jay Crandell, Thomas Culp, Jason Glazer,
Krishnan Gowri, Ned Heminger, Amanda Hickman, John Hogan, Jonathan Humble, Harold Jepsen, Jay
Johnson, Benjamin Meyer, Frank Morrison, Eric Richman, Michael Rosenberg, Steve Rosenstock, Greg
Schluterman, Amy Schmidt, Kelly Seeger, Emily Smith, Frank Stanonik, Matt Swenka, Christian Taber, Bill
Talbert, Steve Taylor, and Martha Van Geem. Special acknowledgement is due to Lilas Pratt, ASHRAE
Manager of Special Projects; Stephanie Reiniche, Director of Technology; Steve Ferguson, Senior Manager
of Standards; and Drake Erbe, Chair of SSPC 90.1 for their dedication and the support they provided to
the Standard 90.1 User’s Manual project team.
Existing and past members of SSPC 90.1 also deserve thanks for their many years of labor. The user’s
manual springs from the firm foundation laid by the committee. Over the years, hundreds of SSPC 90.1
members have contributed to the standard, and thousands of persons provided useful comments during
the many public reviews. It is not possible to acknowledge everyone, but special recognition is due to all
of the past SSPC Chairs, who worked diligently to establish and maintain Standard 90.1 as an
international standard for the design of energy-efficient buildings and building systems.
Standard 90.1 User’s Manual
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1 Purpose
Overview
ANSI/ASHRAE/IES Standard 90.1 provides minimum requirements for the energy-efficient design of
buildings and building systems. It applies to all buildings except low-rise residential buildings (low-rise
means three habitable floors or less). The standard is written in building code language and is
intended for adoption by national, state/province, and local code jurisdictions. The standard specifies
reasonable design practices and technologies that minimize energy consumption while providing
comfortable and productive environments for the building occupants.
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2 Scope
Changes to the Scope
 The scope was modified to allow for the inclusion of new equipment or building systems that are
part of industrial or manufacturing processes.
These changes are marked with  in the margins of this chapter. For the specific addenda that define
the differences between the 2013 and 2016 editions of Standard 90.1, see Appendix H to the standard.
Authority of Standard 90.1
Standard 90.1 is an ANSI-approved national consensus standard cosponsored by ASHRAE and the
Illuminating Engineering Society (IES). As a product of consensus, and by virtue of the participants in
the consensus process, Standard 90.1 represents the collective views of the manufacturing, design, and
construction communities for an appropriate set of minimum requirements for energy-efficient design
and construction.
Participants in the development and review of the standard included, among others: professional,
technical, and trade organizations; environmental organizations; equipment manufacturers; utility
companies; code officials; and design professionals.
The standard is written in code-enforceable language. Although Standard 90.1 is not a code, it is
intended to be adopted as a code by governmental agencies that are empowered to enact codes
through legislative or regulatory processes. These agencies may (and often do) adopt consensus
standards published by organizations such as ASHRAE and IES. Until Standard 90.1 is adopted as code,
the sponsoring organizations—ASHRAE and IES—recommend its voluntary use.
Some agencies may use Standard 90.1 as the basis for their energy code but make modifications to suit
their local conditions. Some requirements may be identical to the standard, while others may be
modified. Unless the standard is adopted or referenced as a whole, care must be taken when using this
user’s manual; certain aspects of the standard may not apply or may apply differently, depending on
the modifications made by the adopting agency.
When Standard 90.1 is adopted and compliance is required, the authority having jurisdiction (AHJ) is
responsible for implementing and applying the standard. Interpretations of the standard may be
requested from ASHRAE at the address provided in the preface to this manual. However, the ultimate
authority for interpretation is the authority having jurisdiction over the building.
Scope of the Standard
The standard provides minimum energy-efficiency requirements for the design and construction of
new buildings and new construction in existing buildings. In particular, it applies to new buildings and
their systems, building additions and their systems, and new systems and equipment in existing
buildings. The standard has been expanded to include new equipment or building systems specifically
identified in the standard that are part of commercial, industrial, and manufacturing processes. This
empowers the Standard 90.1 committee to address specific commercial, manufacturing, and industrial
processes. Requirements have been added to address commercial coolers and freezers, escalators,
moving walkways, and elevator cab lighting. Future addenda will likely address other manufacturing
and/or industrial processes.
The scope of the requirements covers the design of the building envelope, lighting systems, HVAC&R
systems, and other energy-using equipment.
The standard applies to the building envelope when it encloses heated and/or cooled space where the
heating system has an output capacity greater than or equal to 3.4 Btu/h∙ft² (10 W/m²) of floor area or
where the cooling system has a sensible output capacity greater than or equal to 5 Btu/h∙ft² (15
W/m²) of floor area. The standard also applies to systems and equipment used in conjunction with
buildings, including systems for heating, ventilating and air conditioning, service water heating,
electric power distribution, electric motors, and lighting.
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The standard does not apply to
• single-family houses, multifamily structures of three stories or fewer above grade, and
manufactured houses (modular or mobile homes) or
• buildings that do not use either electricity or fossil fuel.
Certain other buildings or building components may be exempt by specific notations in the technical
sections of the standard.
The standard shall not be used to circumvent any safety, health, or environmental requirements. If
there is a conflict between the requirements of this standard and safety, health, or environmental
codes, interpretation should be requested from the local AHJ.
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3 Definitions, Abbreviations, and
Acronyms
Definitions
Standard 90.1 includes definitions for many of the technical terms used in the standard and in this
user’s manual. These terms and definitions are not repeated here, although the index provides a
reference to places in the user’s manual where many of the terms are discussed. Definitions, resources,
terms, and calculation methods are presented in the context where they are used in this manual.
Abbreviations and Acronyms
Abbreviations and acronyms used in the standard and this manual are listed below:
ac
alternating current
AHAM
Association of Home Appliance Manufacturers
ach, ACH
AFUE
ANSI
AHRI
ASTM
bhp
BSR
Btu
Btu/h
Btu/ft²∙°F
Btu/h∙ft²
Btu/h∙ft∙°F
Btu/h∙ft²∙°F
C
CDD
CDD10
CDD50
CFD
cfm
CHPS
c.i.
CLT
CMU
COP
CRAC
CRAH
CRRC
CTI
DASMA
DCV
DDC
air changes per hour
annual fuel utilization efficiency
American National Standards Institute
Air-Conditioning, Heating and Refrigeration Institute
American Society for Testing and Materials
brake horsepower
Board of Standards Review
British thermal unit
British thermal unit per hour
British thermal unit per square foot degree Fahrenheit
British thermal unit per hour square foot
British thermal unit per hour linear foot degree Fahrenheit
British thermal unit per hour square foot degree Fahrenheit
Celsius
cooling degree-day
cooling degree-days base 10°C
cooling degree-days base 50°F
computational fluid dynamics
cubic feet per minute
Collaborative for High Performance Schools
continuous insulation
cross laminated timber
concrete masonry unit
coefficient of performance
computer-room air conditioner
computer-room air handler
Cool Roof Rating Council
Cooling Technology Institute
Door and Access Systems Manufacturers Association
demand control ventilation
direct digital control
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DOE
U.S. Department of Energy
Ec
combustion efficiency
DX
direct expansion
EALP
exterior allowed lighting power
ECB
Energy Cost Budget
ECM
electronically commutated motor
EER
energy efficiency ratio
EF
energy factor
EIFS
exterior insulation and finishing system
EILP
exterior installed lighting power
EISA
EMS
ENVSTD
EPAct
Et
F
energy management system
Envelope System Performance Compliance Program
Energy Policy Act
thermal efficiency
Fahrenheit
FC
filled cavity
FEG
fan efficiency grade
ft
foot
gal
gallon
GFRC
glass fiber reinforced concrete
gpm
gallons per minute
h
hour
HC
HDD
HDD18
HDD65
h∙ft²∙°F/Btu
HID
hp
HSPF
HVAC
HVACR
Hz
IEC
IEER
heat capacity
heating degree-day
heating degree-days base 18°C
heating degree-days base 65°F
hour square foot degree Fahrenheit per British thermal unit
high-intensity discharge
horsepower
heating seasonal performance factor
heating, ventilating, and air conditioning
heating, ventilating, air conditioning, and refrigeration
hertz
International Electrotechnical Commission
integrated energy efficiency ratio
IES
Illuminating Engineering Society of North America
IILP
interior installed lighting power
ILPA
interior lighting power allowance
in.
inch
I-P
inch-pound
IPLV
integrated part-load value
J
joule
K
kelvin
kg
kilogram
kJ
kilojoule
kVA
kW
Energy Independence and Security Act of 2007
kilovolt-ampere
6
kilowatt
Standard 90.1 User’s Manual
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kWh
kilowatt-hour
LED
light emitting diode
L
lb
LEED
lin
lin ft
lin m
LPD
L/s
Ls
LSG
m
m²∙K/W
MICA
mm
MSH
NAECA
NEMA
NFPA
NFRC
NPLV
O&M
o.c.
o.d.
OSB
Pa
PF
pmd
PSC
psig
PSZ-AC
PSZ-HP
PTAC
PTHP
PUE
PVAV
PVC
R
Rc
RCR
RH
Ru
rpm
SC
SCR
SEER
SHGC
liter
pound
Leadership in Energy and Environmental Design
linear
linear foot
linear metre
lighting power density
liter per second
liner system
light-to-solar gain ratio
metre
square metre per kelvin per watt
Midwest Insulation Contractors Association
millimetre
monitor seal height
National Appliance Energy Conservation Act
National Electrical Manufacturers Association
National Fire Protection Association
National Fenestration Rating Council
nonstandard part-load value
operation and maintenance
on center
outside diameter
oriented strand board
pascal
projection factor
probable maximum demand
permanent split capacitor
pounds per square inch gauge
packaged rooftop air conditioner
packaged rooftop heat pump
packaged terminal air conditioner
packaged terminal heat pump
Power Usage Effectiveness
packaged variable air volume
polyvinyl chloride
R-value (thermal resistance)
thermal resistance of a material or construction from surface to surface
room cavity ratio
relative humidity
total thermal resistance of a material or construction including air film resistances
revolutions per minute
shading coefficient
silicon controlled rectifier
seasonal energy efficiency ratio
solar heat gain coefficient
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SI
Systeme International d’Unites (International System of Units)
SMACNA
Sheet Metal and Air Conditioning Contractors’ National Association
SIP
SL
SRI
SRR
SSPC
structurally insulated panel
standby loss
Solar Reflectance Index
skylight-roof ratio
ASHRAE Standing Standard Project Committee
T db
dry-bulb temperature
T wb
wet-bulb temperature
TMY
typical meteorological year
UL
Underwriters Laboratories Inc.
VAV
variable air volume
VFD
variable frequency drive
VRF
VT
VT/SHGC
W
WF
W/ft²
variable refrigerant flow
visible transmittance (also known as visible light transmittance [VLT])
ratio of VT divided by SHGC
watt
well factor
watts per square foot
Wh
W/m²
W/m²∙°C
W/m∙K
W/m²∙K
Wh/m²∙K
WWR
8
watt-hour
watts per square metre
watts per square metre degree Celsius
watts per metre per kelvin
watts per square metre per kelvin
Watt-hours per square metre per kelvin
window-wall ratio
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4 Administration and Enforcement
Changes to the Administration and Enforcement Section
 An additional compliance path has been added in Standard 90.1-2016, the Performance Rating
Method, described in Appendix G.
 Climatic data in Appendices B and D have been removed and an Annex has been added that
reprints relevant information from ASHRAE Standard 169-2013.
These changes are marked with  in the margins of this chapter. For the specific addenda that define
the differences between the 2013 and 2016 editions of Standard 90.1, see Appendix H to the standard.
Scope and Compliance (4.1.1 and 4.2)
This chapter addresses administration and enforcement issues as well as general methods and
requirements for demonstrating compliance with the standard. When the standard is adopted as a
code, the adopting jurisdiction may have some additional requirements. This chapter anticipates some
of these requirements, but designers using this manual should check with the adopting jurisdiction for
supplemental information on compliance.
Section 4 of the standard outlines the compliance options and specifies some requirements applicable
to all projects. The technical requirements of the standard are covered in Sections 5 through 10, which
deal, respectively, with the building envelope, HVAC, service water heating, electrical power, lighting,
and electrical motors (other equipment).
 Figure 4-A illustrates the general approach to compliance. The standard requires that the general
and mandatory provisions of each of the technical sections (Sections 5 through 10) of the standard
must always be met. Then there are three compliance paths, each of which has different additional
requirements. Each of the technical sections has prescriptive requirements, and compliance with all
applicable prescriptive requirements is required for the prescriptive path. The other two options are
different performance methods of compliance, both of which allow trade-offs between building
systems. For example, the efficiency of the lighting system might be improved in order to justify
fenestration that does not meet the prescriptive envelope requirements. Section 11 of the standard
describes the Energy Cost Budget (ECB) Method, and Appendix G describes the Performance Rating
Method. Both of these methods are similar in that they use building performance simulation to
compare the energy performance of a proposed building design to a baseline building design. But there
are significant differences because the baseline is defined differently when using the two methods. The
ECB Method is a pass/fail approach, whereas the Performance Rating Method provides a percentage of
baseline building energy cost. Because of this, the Performance Rating Method is widely used for
purposes other than just compliance with the standard.
With the ECB Method, compliance can be achieved by first meeting the general and mandatory
provisions of each of the technical sections. After that, the estimated annual energy cost of the
proposed building must be shown to be no more than the annual energy cost of a baseline building that
exactly complies with the prescriptive requirements (see Example 4-A). Similarly, compliance with the
general and mandatory provisions of each technical section is required when using the Performance
Rating Method, but compliance is shown when the annual energy cost of the proposed building is less
than or equal to a specified percentage of the baseline building annual energy cost.
New Buildings (4.1.1.1 and 4.2.1.1)
The standard’s main focus is new buildings. Every new building project is different: each building’s site
presents unique opportunities and challenges, each building owner or user has different requirements,
and climate and microclimate conditions can vary significantly among projects. Architects and
engineers need flexibility in order to design buildings that address these diverse requirements.
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 FIGURE 4-A COMPLIANCE PATHS
Example 4-A. Compliance Procedures, ECB Method
Corresponding sections: Scope and Compliance (4.1.1 and 4.2)
Q
A designer of a large shopping mall wishes to demonstrate compliance using the Energy Cost Budget
(ECB) Method of Section 11. The proposed design, which specifies HVAC equipment that does not meet
the efficiency equipment requirements of Section 6, can be shown to have a lower annual energy cost
than the budget building. Does this design comply with the standard?
A
No. Using the ECB Method does not release the designer from any of the mandatory provisions. The
HVAC equipment must meet the minimum efficiency requirements of Section 6. To demonstrate
compliance using the ECB Method, the designer must also show that the proposed project meets the
mandatory provisions of all the technical sections of the standard.
The standard provides this flexibility in a number of ways. Each of the technical sections has multiple
compliance paths. To use the building envelope section as an example, designers can choose a
prescriptive method that requires that insulation be installed with a minimum R-value. Alternatively, a
component performance method allows the designer to show compliance with the thermal
performance (U-factor) of construction assemblies for each component. Finally, a building envelope
trade-off option is provided that permits trade-offs between building envelope components. If more
flexibility is needed, the ECB Method is available.
The lighting and HVAC sections also offer flexibility and exceptions for special cases. The specifics of
the various compliance options are presented in each of the technical chapters in this manual.
Existing Buildings (4.1.1.2, 4.1.1.3, 4.1.1.4, 4.2.1.2, and 4.2.1.3)
The standard also applies to certain work in existing buildings. The requirements are triggered when
new construction is proposed, such as an addition, or when unconditioned space is converted to
conditioned space (that is, heating and/or cooling are added for the first time).
The standard applies to additions and alterations much as it does to new buildings: the mandatory
provisions must always be met; after that, multiple compliance options may apply. In existing
buildings, however, there is a general exception to the standard whenever compliance with the
requirements can be shown to cause an increase in the building’s annual energy use. Compliance
details are discussed below for additions, alterations, and changes in conditioned space.
Additions (4.1.1.2 and 4.2.1.2)
An “addition” is a new wing or new floor that extends or increases the building floor area or height of a
building outside the envelope of the existing building. The standard applies to the addition but does
not require any changes or upgrades to the existing building. As is the case with new buildings, the
mandatory provisions must be complied with; also, the addition must comply either with the
prescriptive or performance requirements of all the applicable technical sections or with the ECB
Method or the Performance Rating Method in Appendix G.
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The simplest compliance method for additions is to treat the addition as if it were its own separate
building. The mandatory provisions of the building envelope, lighting, and HVAC sections apply to the
addition; after that, the addition must meet either the prescriptive or performance requirements of
each of the technical sections or comply using the ECB Method or Performance Rating Method.
A second option is to make trade-offs between the addition and improvements to the existing building
so that the annual energy cost of the existing building plus the proposed addition is less than the
existing building plus an addition that exactly meets the prescriptive requirements. This approach,
known as the “addition + existing trade-off,” can only be applied using the ECB Method. For instance, it
may be desirable that the exterior envelope of the addition matches the existing building facades.
While the envelope might not meet the standard, other systems such as lighting might be improved to
make up for it. The Performance Rating Method also allows trade-offs between an addition and
existing building, but the rules are very different. While ECB allows a special baseline for existing
buildings, Appendix G does not. The Appendix G baseline is the same for an existing building and for
new construction.
When heating and/or cooling for the addition is provided by existing HVAC equipment or systems, the
existing equipment and systems do not have to be upgraded to comply with the standard. However, it
is necessary that any new HVAC equipment or systems comply. Likewise, if service hot water for the
addition is provided by an existing hot-water system, it is not necessary to upgrade the existing
system.
Table 4-A provides some examples of how the standard applies to existing HVAC equipment and
systems that are being extended to serve an addition.
Alterations (4.1.1.3 and 4.2.1.3)
The standard applies to certain aspects of new construction in existing buildings. In general, the
standard only applies to new building systems and equipment. The standard does not apply to building
systems or equipment that are not being altered or repaired unless there is a change in space
conditioning (see Section 4.1.1.5). Alterations may comply with the standard in two ways:
1. The first approach is to show that each system, piece of equipment, or component that is being
replaced complies individually with the applicable requirements of Sections 5, 6, 7, 8, 9, and 10.
With this approach, each component that is being replaced must separately comply with the
standard. There can be no trade-offs among components.
2. The second approach is to evaluate the alteration as a whole using the ECB Method or
Performance Rating Method. This approach permits trade-offs between components and
equipment. The proposed alteration must still comply with the mandatory provisions.
Historic buildings are exempt from the requirements of the standard for building alterations (see the
Exception to Section 4.2.1.3). In order to qualify for the exemption, the historic building must be
designated as historically significant by the authority having jurisdiction (AHJ) or listed (or eligible for
listing) in the National Register of Historic Places. The National Register is administered by the
National Park Service, which is part of the U.S. Department of the Interior.
TABLE 4-A. EXTENDING EXISTING HVAC EQUIPMENT AND SYSTEMS TO SERVE AN ADDITION
Corresponding sections: Additions (4.1.1.2 and 4.2.1.2)
Situation
An existing central plant will provide hot and cold
water to new fan coils in a building addition.
A variable-air-volume (VAV) air handler in the
existing building will provide cool air and outdoor air
ventilation to an addition.
An addition is served by its own single-zone HVAC
system.
Application of Standard
The standard applies to the fan coils and controls in
the addition but not to the existing central plant.
The standard applies to the VAV boxes and controls in
the addition but not to the existing air handler or the
central plant that serves it.
The standard applies to the HVAC system and controls
in the same way that it applies to new construction.
Building Alterations—Exceptions and Explanations
Several important exceptions and particulars apply specifically to the alteration of existing buildings.
These are organized by building system and are discussed in each respective chapter of this manual.
Standard 90.1 User’s Manual
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•
•
•
•
•
•
Building Envelope—Refer to Section 5.1.3 of the standard
Heating Ventilating, and Air Conditioning—Refer to Section 6.1.1.3
Service Water Heating—Refer to Section 7.1.1.3
Power—Refer to Section 8.1.4
Lighting—Refer to Section 9.1.2
Other Equipment—Refer to Section 10.1.1.3
Changes in Space Conditioning (4.1.1.5)
The standard applies in its entirety when previously unconditioned space or semiheated space is
converted to conditioned space (either heated or cooled); see Example 4-B. This includes building
envelope, heating, ventilating, air-conditioning, service water heating, power, lighting, and other
systems and equipment that serve the space that is being heated and/or cooled. Note that if a space is
already heated (i.e., conditioned), adding mechanical cooling does not trigger this requirement because
the space is already considered a conditioned space.
Administrative Requirements (4.1.2)
All administrative requirements related to building permits, enforcement procedures, interpretations,
claims of exemption, and rights of appeal are defined by the AHJ.
Alternative Materials, Methods of Construction, or Design (4.1.3)
There will be situations where equipment, materials, design, or products proposed for installation in a
building are not specifically addressed by the standard. This may be particularly true with new
materials or innovative products. It is not the intent of the standard to prevent the use of such new
products, designs, or construction technologies as long as their installation is consistent with the
requirements of other codes as they pertain to health and life safety.
Validity (4.1.4)
The standard’s language pertaining to validity is generally used within codes and provides that if one
particular part of the code is challenged and subsequently removed, that action does not invalidate the
remainder of the code’s requirements.
Other Laws (4.1.5)
The requirements of the standard do not nullify any provisions of local, state, or federal law. If there is
a conflict between a requirement of this standard and another building code requirement or law, the
AHJ determines precedence.
Referenced Standards (4.1.6)
The standards listed in Section 12 are considered to be normative references and, as such, are part of
the standard to the extent of the reference. Where differences occur between the provisions of the
standard and referenced standards, the provisions of the standard apply.
Normative Appendices (4.1.7)
The normative appendices to the standard are integral parts of the standard. They are included as a
matter of convenience. Appendix A contains precalculated building envelope performance factors that
can be used for compliance purposes, as well as descriptions of acceptable methods for calculating Ufactors. Appendix C contains the procedures for making building envelope trade-offs. Appendix G
describes the Building Performance Rating Method.
Example 4-B. Expansion of Office into Warehouse
Corresponding sections: Alterations (4.1.1.3 and 4.2.1.3) and Changes in Space Conditioning (4.1.1.5)
Q
An existing warehouse measures 400 × 200 ft (122 × 61 m). The warehouse is unconditioned, but
administrative offices are located in a 100 × 100 ft (30 × 30 m) corner. The offices are served by a
single-zone rooftop packaged HVAC system that provides both heating and cooling.
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The owner wants to expand the administrative offices into the warehouse. The new office space will
convert an area that measures 100 × 50 ft (30 × 15 m) from unconditioned to conditioned space. The
existing HVAC system has sufficient capacity to serve the additional space. However, new ductwork
and supply registers will need to be installed to serve the additional space.
Does the standard apply to this construction project?
A
The standard applies to the 100 × 50 ft (30 × 15 m) space that is being converted from unconditioned
to conditioned space. However, the standard does not apply to the existing office or the existing
warehouse space.
The new lighting system installed in the office addition must meet the requirements of Section 9. The
walls that separate the office addition from the unconditioned warehouse must be insulated to the
requirements for semiheated spaces. The exterior wall and roof are exterior building envelope
components and must meet the requirements for nonresidential spaces. The existing HVAC system
does not need to be modified, but the ductwork extensions must be insulated to the requirements of
Section 6.
Informative Appendices (4.1.8)
The standard also contains three informative appendices and an informative annex. Appendix E
provides references and acknowledges source documents. This informative appendix does not contain
requirements that are a part of the standard. Appendix F reproduces U. S. Department of Energy
efficiency standards that apply to equipment defined as “residential covered products” that may be
used in buildings covered by Standard 90.1. These efficiency requirements for single-phase air
conditioners and heat pumps, water heaters, and pool heaters are provided for convenience. Appendix
H describes the addenda that have been incorporated into Standard 90.1–2013 to create Standard
90.1-2016.
Annex 1 reproduces material from ASHRAE Standard 169-2013, Climatic Data for Building Design
Standards. Standard 169 provides climate zone data that are referenced by Section 5 and used
throughout the standard. Annex 1 is informative in that the climate zone information it contains is
provided for convenience, while the climate zone data that are required to be used are those contained
in Standard 169.
Compliance Documentation (4.2.2)
Documentation of compliance consists of all materials including plans, specifications, calculations,
diagrams, reports, and other data that have been submitted in support of a permit application and
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subsequently approved by a code enforcement official. All such documentation must be in sufficient
detail to permit a determination of compliance by the building official (Section 4.2.2.1). The building
official may request additional information if required to verify compliance (Section 4.2.2.2).
Compliance forms and worksheets are available for download from ASHRAE’s website at
http://www.ashrae.org/UM90.1-2016. These forms are intended to facilitate the process of complying
with the standard. These forms serve a number of functions:
• They help a permit applicant and designer know what information needs to be included on the
plans.
• They provide a structure and order for the necessary calculations. The forms allow information to
be presented in a consistent manner, which is a benefit to both the permit applicant and the
building official.
• They provide a checklist for the building official to help structure the plan check process.
• They promote communication between the plan’s examiner and the field inspector.
• They provide a checklist for the inspector.
Manuals (4.2.2.3)
Optimum energy efficiency requires that the building and the equipment installed in the building be
operated and maintained in accordance with the design intent. The standard requires that operating
and maintenance information be provided to the building owner. This information is specified in the
HVAC (Section 6.7.2.2), electric power (Section 8.7.2), and lighting (Section 9.7.2.2) technical sections
of the standard.
Labeling of Materials and Equipment (4.2.3)
The overall performance of fenestration products, insulation material, water heaters, and HVAC
equipment is determined through laboratory tests and calculations that cannot easily be performed in
the field. For this reason, labeling is frequently required so that construction managers, field
inspectors, design professionals, and general contractors can verify that the products, materials, and
equipment being installed comply with the standard. The intent of these labeling requirements is to
make it easier to do field verification and administration.
The standard requires labeling of the following products. In some cases, exceptions allow other
methods of performance certification:
• Fenestration. The U-factor, solar heat gain coefficient (SHGC), visible transmittance (VT) , and air
leakage rate for all manufactured fenestration products must labeled on the product by the
manufacturer. Alternatively, when a fenestration product does not have a label physically applied
to the product, the installer or supplier of the fenestration must provide a signed and dated
certification for the installed fenestration listing the U-factor, SHGC, VT, and air leakage rate.
• Doors. The U-factor and the air leakage rate for all manufactured doors used in the exterior or
semi-exterior envelope must be identified on a label installed on the product by the manufacturer.
As with fenestration products, this label is generally located on the side of the door or the door
frame and additionally includes information about the door’s fire rating. Alternatively, when a
door does not have a label, the installer or supplier must provide a signed and dated certification
for the installed door listing the U-factor and the air leakage rate.
• Insulation. The rated R-value must be clearly indicated by an identification mark applied by the
manufacturer to each piece of building envelope insulation. Alternatively, when insulation does
not have an identification mark, the supplier or installer must provide a signed and dated
certificate listing the type of insulation, the manufacturer, the rated R-value and, where
appropriate, the initial installed thickness, the settled thickness, and the coverage area. The
certificate is most common for blown-in and spray applied insulation products.
• Mechanical Equipment. Mechanical equipment that is not covered by the National Appliance
Energy Conservation Act (NAECA) must carry a permanent label installed by the manufacturer
stating that the equipment complies with the requirements of Standard 90.1. NAECA-regulated
equipment must also be labeled, but the labeling requirements are addressed by the federal act,
not by Standard 90.1.
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• Packaged Terminal Air Conditioners. The replacement of packaged terminal air conditioners in
some existing wall openings sometimes presents difficulties if the original wall opening is small.
Packaged terminal air conditioners that may be used in these situations are subject to specific
labeling requirements. Packaged terminal air conditioners and heat pumps with sleeve sizes less
than 16 × 42 in. (0.41 × 1.05 m) must be factory labeled as follows: “Manufactured for
replacement applications only: not to be installed in new construction projects.”
Inspections (4.2.4)
The standard requires that construction work be available for field inspections. For smaller buildings,
inspections are typically made during certain phases in the construction process, for example, during
foundation, rough-in, and final. Larger and more complex buildings will often have many more
inspections at additional times during the construction process. Table 4-B has examples of work that is
subject to field inspection.
Work that is critical to compliance with the standard must remain accessible and exposed for
inspection until approved in accordance with procedures specified by the building official.
TABLE 4-B. FIELD INSPECTIONS
Corresponding section: Inspections (4.2.4)
Discipline
Envelope—
Insulation
Envelope—
Other
Electrical
Inspection Phase
Foundation
Rough-in
Final
Foundation
Rough-in
When Inspected
Before backfill of foundation walls
Before interior finish materials are installed
but after fenestration and doors are in place
Before occupancy
Before cover-up
Before building insulation is installed
Foundation
Rough-in
Not applicable
Before interior finish materials are installed
Final
Final
Before occupancy
Before occupancy
Example of Things to Check
Slab edge insulation
Wall, roof, and floor insulation
Sealing and infiltration control
Window and skylight areas
That fenestration products match
plans
High-reflectance, high-emittance roof
surfaces
Transformer
That lighting controls are properly
located
Circuits
Verification and Commissioning Reporting (4.2.5)
Only a few provisions of the standard require verification and commissioning reporting. However, it is
important that buildings perform as intended. Requiring verification and commissioning reporting
provides confidence to the owner that the building features they desire to provide energy efficiency for
the life of the building are installed in a manner that is going to provide the expected performance. If a
lack of performance is discovered, the code official or other approved agency must report these
findings so they can be corrected.
Nonconformance (4.2.5.1)
This section further describes the process that is to be implemented when work is found to be
noncompliant. It further states that work that has not been corrected within the agreed upon time
frame shall be reported in writing to the building official and design professional. This emphasizes the
need for performance in the field and good communication among all stakeholders.
FYI
The Compliance and Enforcement Process
Although the compliance and enforcement process may vary somewhat with each adopting
jurisdiction, the enforcement authority is generally the building department or other agency that has
responsibility for approving and issuing building permits. When noncompliance or omissions are
discovered during the plan review process, the building official may issue a correction list and require
that the plans and applications to be revised to bring them into compliance prior to issuing a building
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permit. In addition, the building official has the authority to stop work during construction when a
code violation is discovered.
The local building department has jurisdiction for determining the administrative requirements
relating to permit applications. They are also the final word on interpretations, claims of exemption,
and rights of appeal. From time to time, ASHRAE will issue interpretations clarifying the intent of the
standard. The local building department may take these under consideration, but the local building
department still has the final word.
To achieve the greatest degree of compliance and to facilitate the enforcement process, the standard
should be considered at each phase of the design and construction process (see Figure 4-B).
FIGURE 4-B. THE BUILDING DESIGN AND CONSTRUCTION PROCESS
1.
At the design phase, designers must understand both the requirements and the underlying intent
of the standard. The technical sections of this Manual provide information that designers need to
understand how the standard applies both to individual building systems and to the integrated
building design.
2. At permit application, the design team must make sure that the construction documents submitted
with the permit application contain all the information that the building official will need to verify
that the building satisfies the requirements of the standard. (Compliance forms and worksheets to
help ensure that all the required information is submitted are available for download from
ASHRAE’s website at http://www.ashrae.org/UM90.1-2016.)
3. During plan review, the building official must verify that the proposed work satisfies the
requirements of the standard and that the plans (not just the forms) describe a building that
complies with the standard. The building official may also make a list of items to be verified later
by the field inspector.
4. During construction, the contractor must carefully follow the approved plans and specifications.
The design professional should carefully check the documentation and shop drawings that
demonstrate compliance and should observe the construction in progress to see that compliance is
achieved. The building official must verify that the building is constructed according to the plans
and specifications.
5. After completion of construction, the contractor and/or designer should provide information to
the building operators on maintenance and operation of the building and its equipment. Although
only minimal completion and commissioning is required by the standard, most energy-efficiency
experts agree that full commissioning is important for proper building operation and
management.
6. After occupancy, the building and its systems must be correctly operated and properly maintained.
In addition, building users should be advised of their opportunities and responsibilities for saving
energy (for example, by turning off lights when possible).
Effective compliance and enforcement requires coordination and communication among all parties
involved in the building project.
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5 Building Envelope
Changes to the Building Envelope Section
 Reference Standard Reproduction Annex 1 (located at the end of the standard) identifies the
appropriate climate zones in the United States, Canada, and other international locations.
(Appendix B has been deleted.) Approximately 10% of the counties in the U.S. have been
reassigned to a different climate zone due to updates in climatic data. Further, the previous
Climate Zone 1 has been subdivided into Climate Zones 0 and 1. This material from ASHRAE
Standard 169, Climatic Data for Building Design Standards, is now cited in Section 5.1.4 and applies
to all sections of the standard.
 The thresholds for conditioned space (which are based on the output capacity of space heating and
space cooling systems, and are located in Table 3.2 in Section 3 Definitions) have been lowered to
catch up with the reduction in loads due to the greater building envelope efficiency in Section 5
that has been achieved over the last dozen years.
 Whole-building air leakage testing was added to Section 5.4.3.1.3. Testing is now an option for
compliance in addition to the existing continuous air barrier options.
 The opaque door and fenestration prescriptive requirements in Tables 5.5-0 through 5.5-8 and
associated text in section 5.5.4.5 have been modified. While Tables 5.5-0 through 5.5-8 are not
reproduced in this user’s manual, significant changes have been made to the content of these
tables.
 Changes have been made to the SHGC credit for shading by permanent projections in Section
5.5.4.4.1. The SHGC multipliers in Table 5.5.4.4.1 now only apply to south, east, and west-facing
fenestration shaded by permanent projections. North-facing fenestration is addressed through a
newly revised Exception 5 to Section 5.5.4.4.1. (5.5.4.4.1).
 Vertical fenestration orientation requirements were modified by adding requirements based on
the solar heat gain coefficients (SHGCs) of the glazing and the climate zone (5.5.4.5).
 A new Section 5.9 was added to the standard to increase delivered performance by inspecting and
verifying that the requirements of Section 5 are adequately met. New Section 5.9 was added to the
list of mandatory requirements in Section 5.2.1 Compliance.
 The relevant sections of Appendix A have been adjusted to reflect the new metal building
calculation procedure (Appendix A).
 R-values assigned to air spaces have been revised in Appendix A (A9.4.2).
These changes are marked with  in the margins of this chapter. For the specific addenda that define
the differences between the 2013 and 2016 editions of Standard 90.1, see Appendix H to the standard.
General (5.1)
Scope (5.1.1)
Section 5 contains the standard’s requirements for the building envelope.
Space Conditioning Categories (5.1.2)
The building envelope requirements vary depending on the type of space: nonresidential, residential,
and semiheated. Both nonresidential and residential are conditioned spaces; for these the standard
calls for more insulation and more control of heat gain through fenestration compared to semiheated
spaces. Most spaces within buildings that are covered by the standard will fall into one of these three
categories.
The standard distinguishes between conditioned, semiheated, unconditioned, and indirectly
conditioned space classifications. The building envelope requirements do not apply to unconditioned
spaces, except for skylight requirements in Section 5.1.2.2. These space-conditioning categories are
discussed below.
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Residential and Nonresidential Spaces
Residential conditioned space includes spaces used primarily for living and sleeping. The occupants
can be either permanent or transient in nature. Consequently, the residential space category includes
dwelling units (apartments and condominiums), dormitories, and fraternities and sororities, as well as
hotel/motel guest rooms, boarding houses, and hostels.
However, note that the residential space category also includes the living and sleeping portions of
institutional and other facilities. Examples include, but are not limited to, congregate care, assisted
living facilities, group homes, nursing homes, convalescent facilities, hospital patient rooms, prisons,
detention centers, and fire stations. Thus, the Standard 90.1 definition of residential spaces is more allinclusive than the International Building Code definition of residential spaces. This is because the
building envelope criteria in Standard 90.1 are driven by thermal concerns, while the International
Building Code definition of residential spaces is geared toward life-safety issues and focuses on the
capability of occupants to self-evacuate in the case of an emergency.
Nonresidential conditioned space includes all other conditioned spaces covered by the standard,
including, but not limited to, offices, retail shops, shopping malls, theaters, restaurants, and meeting
rooms. The defining characteristic of nonresidential spaces is that they are not continuously
conditioned. Offices, for instance, are typically conditioned only during the day on weekdays and part
of the day on Saturday; they are generally not conditioned on Sundays and holidays or at night.
Residential spaces, on the other hand, are conditioned on a more-or-less continuous basis. A greater
investment in energy efficiency can be justified for spaces that are continuously conditioned, and this
is the basis of the distinction between these two space categories.
Note that the building envelope criteria apply on a space-by-space basis, not on a floor-by-floor basis
and not on an overall building basis. Consequently, it is necessary to identify the space uses on the
perimeter of the building to determine the building envelope criteria that apply to the specific portion
of the building. Many buildings are mixed-use with some portion of the building envelope required to
comply with the residential building envelope criteria, while other portions are subject to the
nonresidential building envelope criteria. For example, take a ten-story hotel having a ground floor
containing the registration area, restaurant, bar, and indoor swimming pool; the second floor has
administrative offices and meeting rooms; and the upper floors consist solely of guest rooms. In this
case, the building envelope of the first two floors is subject to the criteria for nonresidential spaces,
and the building envelope of the upper floors must comply with the criteria for residential spaces. It
may be necessary to divide a single floor into both residential and nonresidential spaces. For example,
a five-story apartment building might have retail spaces just on the street-side of the ground floor,
while the remainder of the ground floor has apartments. In this case, the building envelope of the
street side of the ground floor must comply with the criteria for nonresidential spaces, while the
building envelope of the rear half of the ground floor and all the upper floors must comply with the
criteria for residential spaces. For other examples addressing hospitals, prisons, and fire stations, see
Example 5-A.
Example 5-A Application of Residential and Nonresidential Building Envelope Criteria in Buildings with
Mixed Use
Corresponding section: Space-Conditioning Categories (5.1.2)
Q
A hospital consists of a central tower and two wings of patient rooms. What are the building envelope
criteria for various portions of the facility?
A
Some portions of the building envelope for this facility must comply with the residential requirements
and other portions with the nonresidential requirements. The building envelope of the two wings of
patient rooms must comply with the building envelope requirements for residential spaces, because
patient rooms fall within the scope of residential spaces (per the definition of “residential” in
Section 3). The other spaces are considered nonresidential.
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An urban detention facility consists of eight stories of prison above two floors of administrative offices.
What are the building envelope criteria for various portions of the facility?
A2
Some portions of the building envelope for this facility must comply with the residential requirements
and other portions with the nonresidential requirements. The building envelope of the eight floors of
prison must comply with the building envelope requirements for residential spaces, because prisons
fall within the scope of residential spaces (per the definition of “residential” in Section 3). The other
spaces are considered nonresidential.
Q3
A fire station facility consists of an engine room, a training facility, and sleeping quarters for the
firefighters. What are the building envelope criteria for various portions of the facility?
A3
Some portions of the building envelope for this facility must comply with the residential requirements
and other portions with the nonresidential requirements. The building envelope of the sleeping
quarters for the firefighters must comply with the building envelope requirements for residential
spaces, because sleeping quarters fall within the scope of residential spaces (per the definition of
“residential” in Section 3). The other spaces are considered nonresidential.
Space Classifications
The terms conditioned space (which includes indirectly conditioned space), semiheated space, and
unconditioned space are defined in the Section 3 of the standard. Designating a space as conditioned,
semiheated, or unconditioned affects whether the building envelope requirements apply and how
much insulation must be installed. For shell or speculative buildings that do not have a heating system
shown on the plans, all spaces must be considered conditioned unless approval is granted by the
building official to designate the space as semiheated or unconditioned. For an example application of
these space-conditioning categories, see Example 5-C.
•  Conditioned space is space that has a space heating and/or space cooling system of sufficient
size to maintain temperatures suitable for human comfort. For simplicity of compliance, the
definition of conditioned space is expressed in terms of installed space heating and/or space
cooling equipment capacity per square foot (square meter) of floor area. For cooling, the
threshold is 3.4 Btu/h∙ft² (10 W/m²) and for heating the threshold depends on the climate zone of
the building location (as indicated in Table 3.2 in the standard).
Some spaces are considered conditioned even though they may not have a heating system or
cooling system that directly serves the space. This type of space is called indirectly conditioned.
The nonresidential and residential building envelope requirements apply to indirectly
conditioned space in the same way that they apply to directly conditioned space. Examples of
indirectly conditioned spaces are storage rooms that are adjacent to conditioned spaces, toilets
that exhaust air from conditioned spaces, or electrical closets that are adjacent to conditioned
spaces.
Most of the time it will be easy to identify indirectly conditioned spaces. When there is
uncertainty, the standard has two criteria to determine what constitutes indirectly conditioned
space:
1. The heat transfer rate to conditioned space is larger than the heat transfer rate to the exterior
(ambient conditions), assuming the temperature differences are the same. Technically, this is
determined by calculating the UA value of each envelope component separating the space
from conditioned space and from the exterior. UA value is the product of multiplying the
average U-factor of the surface construction and the area of that surface. See FYI, Steady-State
Heat Flow: Understanding U-Factor and R-Value (Section 5.4.1), for more information on U-
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factor. The total UA value for all components separating the space from the exterior must be
smaller than the total for all components separating the space from conditioned space. This
will cause the temperature of the space to more closely track interior temperature.
2. There is an air transfer rate between the space and conditioned space that exceeds three air
changes per hour (ACH).
It is really up to the designer to make a space either unconditioned or indirectly conditioned. This
can be achieved by the placement of insulation or by providing (or not providing) ventilation to
the space. A space on the exterior of a building can be made indirectly conditioned by placing the
insulation on the exterior wall, such as with an enclosed exit stairway. This is the common
approach, since usually less insulation is required. Likewise, by providing ventilation vents or
fans, a space can be made indirectly conditioned.
Figure 5-A illustrates the two criteria for indirectly conditioned space.
Example 5-B shows how to make the necessary calculations when applying the heat transfer
criterion.
• Semiheated space. Semiheated space has a heating system with a capacity greater than 3.4
Btu/h∙ft² (10 W/m²) of floor area but smaller than that needed to qualify for conditioned space
(as shown in Table 3.2 of the standard). Declaring a space as semiheated is only allowed through
an exception that must be approved by the building official. The designer must also label
semiheated spaces on the construction plans that are submitted with the building permit
application. This will enable the building official to verify that the spaces are truly semiheated and
to provide documentation to the field inspector. Examples spaces that may sometimes qualify as
semiheated are warehouses or light manufacturing facilities that have only a very limited space
heating system and no space cooling.
• Unconditioned space. This is a space that does not have a space cooling system and either does
not have a space heating system or the space heating system has a capacity that is less than 3.4
Btu/h∙ft² (10 W/m²). The default assumption is that all spaces are conditioned or semiheated. As
noted below, in many cases design choices such as insulation placement will determine whether a
space is unconditioned or indirectly conditioned. The designation of a space as semiheated or
unconditioned (rather than conditioned) must be approved by the building official and labeled on
the construction plans. This determination is based on the intended use of the space, regardless of
whether mechanical equipment is included with the building permit application.
Note that unconditioned spaces are not automatically exempt from all building envelope requirements.
For example, the minimum skylight area and automatic daylighting control requirements still apply to
certain large open spaces with tall ceilings (e.g., unconditioned warehouses). Also, certain boundaries
of the unconditioned space may be considered semiexterior building envelope components (see
discussion below) and must meet the requirements for semiheated spaces.
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FIGURE 5-A. EXAMPLES OF INDIRECTLY CONDITIONED SPACES
Corresponding section: Space-Conditioning Categories (5.1.2)
Example 5-B. Indirectly Conditioned Space, Application of Heat Transfer Criteria
Corresponding section: Space-Conditioning Categories (5.1.2)
Q
The following figure shows an example of a 100 × 100 × 10 ft (30 × 30 × 3 m) space that is adjacent to
conditioned space but does not have a heating or cooling system. The walls that separate this space
from the U-shaped conditioned space are uninsulated steel-framed walls. The exterior wall of the space
is 6 in. (152 mm) actual depth steel-framed, 16 in. (400 mm) on center (o.c.), with R-19 (R-3.3)
insulation. The floor is an uninsulated concrete slab. The roof has metal framing at 48 in. (1.2 m) o.c.,
an attic, and R-38 (R-6.7) insulation.
According to the heat transfer criteria, does the space qualify as indirectly conditioned?
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A
Yes. The heat transfer criteria state that the space is considered indirectly conditioned if the total UA
value of the surfaces separating the space from conditioned space is larger than the total UA value of
the surfaces separating the space from the outdoors, unconditioned space and semiheated spaces. The
total UA value between the space and the outdoors includes the sum for the product of the areas and
U-factors of the roof, exterior wall, and slab. The UA value to the exterior is 532 Btu/h∙°F (282 W/°C)
while the UA value to adjacent conditioned space is 1056 Btu/h∙°F (557 W/°C). The space is therefore
considered indirectly conditioned because the UA value to conditioned space is greater than it is to the
exterior.
UA value to the exterior:
Component
Area/
Length
U-factor/
F-factor
UA Value
(Btu/h∙°F)
UA Value (W/°C)
Roof
10,000 ft² (929 m²)
0.035 (0.20)
350
186
Table A2.5.2
Slab length
100 ft (30 m)
0.73 (4.14)
73
38
Table A6.3.1
Exterior wall
Overall UA value
1,000 ft² (93 m²)
0.109 (0.619)
109
532
UA value between the space and adjacent conditioned space:
58
282
Component
Area/
Length
U-factor/
F-factor
UA Value
(Btu/h∙°F)
UA Value (W/°C)
Interior wall
3,000 ft²
(279 m²)
0.352 (2.0)
1056
557
Data Source
Table A3.3.3.1
Data Source
Table A3.3.3.1
FYI
Understanding Exterior and Semiexterior Building Envelope Components
Building envelopes consist of opaque components and fenestration components. Opaque envelope
components include walls, roofs, floors, slab-on-grade floors, below-grade walls, and opaque doors.
Fenestration envelope components include windows, skylights, and doors that are more than one-half
glazed.
A building envelope component can be either exterior or semiexterior:
•
Exterior building envelope components separate conditioned space from outdoor conditions,
including portions of the envelope separating conditioned space from ventilated crawlspaces and
attics.
• Semiexterior building envelope components separate conditioned space from unconditioned
space (which excludes ventilated crawlspaces and attics) or from semiheated space. Semiexterior
envelope components also separate semiheated space from exterior (outdoor) conditions or from
unconditioned space.
Being able to identify exterior and semiexterior building envelope components is essential for the
proper use of the standard. The requirements for semiheated spaces apply to semiexterior building
envelope components, while the requirements for nonresidential or residential spaces apply to
exterior building envelope components. The requirements for exterior building envelope components
are more stringent than those for semiexterior building envelope components.
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FIGURE 5-B. BUILDING ENVELOPE COMPONENTS AND SPACE TYPES
Figure 5-B shows a section through a building. This figure shows many types of spaces in order to
illustrate the distinctions between exterior and semiexterior envelope components. The middle floor
and part of the basement are conditioned. The upstairs is semiheated, and a portion of the basement is
unconditioned. In addition, the building has a ventilated crawlspace and a ventilated attic. In this
figure, exterior envelope components are shaded dark and semiexterior envelope components are
lightly shaded.
The standard does not apply to the envelope components that are shown without shading because
these are neither exterior nor semiexterior. Notice that all the envelope components surrounding the
semiheated space are semiexterior. The exterior envelope components separate the conditioned space
from the outdoors or from the ventilated attic or crawlspace. Envelope components that separate
conditioned space from unconditioned space are also semiexterior.
Shell Buildings (5.1.2.3)
Shell buildings are a special case. The building shell is constructed before it is known how the building
will be used. The HVAC and lighting systems are installed later at the time of tenant improvements.
Shell buildings have consistently created code enforcement problems, as tenants assume that the
building envelope already complies with the code. The mechanical contractor’s responsibility,
however, is limited to the HVAC system. The electrical contractor’s responsibility is also limited. The
mechanical and electrical permit applications are reviewed and inspected by different staff at the
building department than those involved in the building shell.
To address this issue, the standard assumes that all buildings will be conditioned, although the
building official can make an exception to this rule for special cases. If the building official approves a
space as semiheated or unconditioned, it must be clearly designated as such on the construction
documents (Section 5.7.2).
Envelope Alterations (5.1.3)
The standard applies in its entirety when previously unconditioned space or semiheated space in
existing buildings is converted to conditioned space (either heated or cooled). For further information,
see Section 4.1.1.5.
For other alterations to existing buildings, the standard’s requirements for insulation, air leakage, and
fenestration, apply when a new envelope component is added or an existing envelope component is
modified. However, the following types of building envelope alterations are exempt from compliance
with the standard, provided they do not increase the energy usage of the building:
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1.
2.
3.
4.
5.
6.
7.
8.
Installing storm windows or glazing panels over existing glazing, provided the storm window or
glazing panel contains a low-emissivity coating. This improves the performance of the building
envelope by significantly reducing the U-factor as well as the solar heat gain coefficient (SHGC) to a
larger or smaller degree, depending on the glazing type. Technology has advanced such that lowemissivity or “low-e” coatings can be provided on both exterior and interior storm windows and
glazing panels to provide additional energy savings. A low-e coating is not required if the existing
window already has a low-e coating.
Replacing broken or damaged glazing in an existing sash and frame, provided that the U-factor and
SHGC of the replacement glass are equal to or lower than those of the original glass. In-kind
replacement glazing will always satisfy this exception. This exception applies to replacements of
the glass alone, not to alterations that involve replacement of the sash or frame. See number 8
below if glass and sash are being replaced in an existing frame or if glass, sash, and frame are being
replaced.
Altering roofs, ceilings, walls, or floors that have cavities, as long as the cavity is filled with
insulation having an insulating value of at least R-3.0 per inch (R-0.02 per millimetre).
Altering walls and floors that have no framing cavities and such that no new cavities are created.
Roof recovering (defined in Section 3), which is where an additional roof covering or membrane is
added over the existing roof covering without removing the existing roof covering.
Removal and replacement of an existing roof covering where there is existing roof insulation,
integral to or below the roof deck. Again, the key nuance here is the fact that the existing roof
insulation is not touched or modified in any substantial way. This would apply to roofs constructed
of structurally insulated panels (SIPs) or roof decks with spray-on insulation on the underside of
the roof deck, for example.
Replacing exterior doors such that it does not trigger the requirement for a vestibule or revolving
door. However, if a vestibule or revolving door exists, it must not be removed.
Replacing existing fenestration (windows, plastic panels, glass blocks, glass doors, or skylights), as
long as the area of fenestration that is being replaced is less than 25% of the total fenestration area
of the existing building. Also, the U-factor and SHGC of the replacement fenestration must be equal
to or less than the original fenestration. If the replacement fenestration area exceeds 25%, then
the replacement fenestration that is installed must meet the requirements of the standard.
 Climate (5.1.4)
Reference Standard Reproduction Annex 1 (located at the end of the standard) identifies climate zones
in the United States, Canada, and other international locations. (Annex 1 supersedes the former
Appendix B, which has been deleted.) In the 2016 version of the standard, approximately 10% of the
counties in the United States have been assigned to a different climate zone due to updates in climatic
data and changes in methodology. Further, the previous Climate Zone 1 has been subdivided into
Climate Zones 0 and 1. This material from ASHRAE Standard 169, Climatic Data for Building Design
Standards, is now cited in Section 5.1.4 and is used throughout the standard.
Figure 5-C shows climate zone boundaries for the United States. Each county in the United States
belongs to one and only one climate zone. The climate zone is defined by a primary thermal zone
indicated by a number and then a secondary moisture subzone indicated by a letter (except for Climate
Zones 7 and 8, which do not have any moisture subzones).
Climate Zones 0 through 8 generally move from south to north but also from lower to higher elevation,
becoming gradually colder as the number gets higher. Climate Zone 0 is the hottest, but there are no
locations in the 50 United States in this climate zone (although Climate Zone 0 does include the United
States territories of Guam and American Samoa). Within the 50 United States, Climate Zone 1 is the
warmest and includes Hawaii and the southern tips of Florida and Texas (as well as the United States
territories of the Virgin Islands and most of Puerto Rico). Climate Zone 8 is the coldest and includes the
north slope of Alaska, Nome, and Fairbanks. Anchorage, Juneau, and the Kenai peninsula in Alaska are
in Climate Zone 7. Climate Zone 7 is the coldest in the continental United States. It includes northern
portions of Maine, Minnesota, North Dakota, and Michigan.
In addition to thermal characteristics, a location is also assigned to a climate zone based on its wetness
or humidity. The moisture subzone is identified by a letter: A (humid), B (dry), or C (marine). Zone A
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includes the eastern part of the United States, where summers are usually humid and air conditioners
are typically required to remove moisture from outdoor air in order to maintain comfortable
conditions. Zone B includes the generally dry western states, where HVAC humidity control is
generally not an issue in the summer. Zone C includes the cool southern Alaska panhandle and the
Washington, Oregon, and California coasts, which are strongly influenced by cold Pacific Ocean waters.
The standard’s prescriptive option (Section 5.5) has nine sets of building envelope criteria, one for
each of the nine thermal climate zones. Each building envelope criteria set is presented as a separate
table in Section 5.5. These sets of building envelope criteria depend only on the thermal zones and not
the moisture subzones. (Moisture subzones are used extensively in Section 6 for specifying criteria for
HVAC&R systems.)
The easiest way to determine the climate zone for a particular location is to look at Reference Standard
Reproduction Annex 1 located at the end of the standard. Within Annex 1 (extracted from ASHRAE
Standard 169), Table Annex1-1 includes all of the counties in the United States, and a climate zone is
identified for each. Table Annex1-2 lists climate zone for over 500 locations in Canada, and Table
Annex1-3 lists climate zone for over 4000 locations in many other countries.
For Canadian or international cities that are not listed in Annex 1, you can select a city that has similar
climate conditions. Alternatively, if you have climate data for the city, you can use the climate zone
definitions in Section A3 of Annex 1 to determine the climate zone.
For most United States cities, the climate zone map in Figure 5-C and the listing in Table Annex1-1 will
be enough to determine the appropriate climate zone. Some U.S. counties, however, have significant
elevation changes within the county that affect climate. In these instances, if there are recorded
historical climatic data available for a building site, the climate zone definitions in Section A3 of Annex
1 may be used to determine the climate zone. Such a determination requires the approval of the AHJ.
FIGURE 5-C. CLIMATE ZONES FOR UNITED STATES LOCATIONS
Corresponding section: Climate (5.1.4)
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FYI
Integrated Design and Thermal Balance
The building envelope is one of the most important factors in designing energy-efficient buildings.
While the envelope does not directly use energy, its design strongly affects space heating, space cooling
loads, and space lighting. For example, insulation affects heat transfer with the outdoors, which the
space heating and cooling system must offset. Also, glazing can introduce daylight into the space,
reducing the need for electric lighting.
Integrated design considers multiple elements—including the building envelope, the HVAC system,
and the lighting system—as a whole. It recognizes that changing one can affect the others. For instance,
investments in insulation or energy-efficient windows can result in smaller HVAC systems, which will
help pay for the better envelope.
The envelope design must take into consideration both external loads and internal loads as well as
daylighting benefits. External loads (Figure 5-D) include solar gains, conduction losses across envelope
surfaces, and infiltration, while internal loads (Figure 5-E) include heat gain from lights, equipment,
and people.
The temperature at which losses through the building envelope balance internal heat gains is the
building’s balance-point temperature. The balance-point temperature depends on the magnitude of
internal gains, the rate of heat loss through the building envelope, and the quantity of outdoor air
brought into the building through the HVAC system.
The balance point varies by building use and is different for occupied and unoccupied hours. For
example, a laundry or a commercial kitchen will likely have a lower balance-point temperature
because of high internal loads. By contrast, a high-rise residential building will have relatively low
internal loads and a higher balance point. A typical office building has a low balance-point temperature
during daytime occupied periods and a higher balance-point temperature during unoccupied evening
hours. As a result, the office may require cooling during the day and heating at night and for early
morning warm-up.
FIGURE 5-D. EXTERNAL LOADS
FIGURE 5-E. INTERNAL LOADS
The ideal building envelope would control exterior loads in response to coincident internal loads to
achieve a thermal balance for each set of conditions. When the building is in a cooling mode, solar
gains should be reduced while still admitting daylighting, and outdoor air should be introduced if
outdoor conditions are suitable. Outdoor air could also be introduced during evening hours to cool
thermal mass in preparation for the next day’s loads. If the building is in a heating mode during the
day, solar gains should be increased and heat losses due to both conduction and infiltration should be
reduced.
Solar gains, heat loss, thermal bridging, infiltration, and other aspects of design have a considerable
impact on the thermal comfort and performance of the building. All of these elements should be
carefully considered by the designer. The integrated design and selection of building envelope and air
sealing components is critical to a well performing and durable building.
The standard sets minimum levels of thermal performance for all components of the building envelope
and limits solar gain through fenestration, based on climate zone, space-conditioning category, and
class of construction.
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Example 5-C. Applying the Building Envelope Standards, Warehouse in Oakland, California
Corresponding section: General (5.1)
Q
A 40,000 ft² (3700 m²) warehouse in Oakland, CA, will be used to store household appliances until they
are distributed to retail outlets. A 2000 ft² (185 m²) office is attached to the warehouse (see the figure
below). The warehouse is 20 ft (6 m) high and designed with two unit heaters, each with an output
capacity of 100,000 Btu/h (29 kW) and is not air conditioned. A packaged single-zone heating and
cooling system will serve the office area.
How do the building envelope standards apply to this facility?
A
The envelope standards clearly apply to the office portion of the building—the portion that is both
heated and cooled. The space heating system in the warehouse area has an output capacity of 200,000
Btu/h divided by 40,000 ft² = 5.0 Btu/h∙ft² (58,000 W divided by 3700 m² = 15.7 W/m²). Oakland is in
Climate Zone 3C. For this climate zone, the heating system would have to have an output capacity
larger than 7 Btu/h∙ft² (21 W/m²) in order for the space to be considered conditioned space. However,
the space is considered semiheated because the space heating system output capacity is greater than
3.4 Btu/h∙ft² (10 W/m²).
The walls and roofs that separate the office from the outdoors are exterior building envelope
components and the nonresidential criteria apply. The walls and roofs that separate the warehouse
either from the exterior or from the office are semiexterior envelope components, and the criteria for
semiheated spaces apply. Because Oakland is in Climate Zone 3, the building official must approve
designation of the warehouse as semiheated space. Because the warehouse is greater than 2500 ft²
(232 m²), the warehouse must also have a minimum skylight area and automatic daylighting controls,
in accordance with Section 5.5.4.2.3, because the ceiling height is greater than 15 ft (4.6 m).
 Compliance Paths (5.2)
Compliance with the standard requires that the mandatory provisions be satisfied in all cases. Section
5.4 contains the mandatory requirements for the building envelope. The mandatory requirements
include insulation installation requirements (applicable to the Prescriptive Building Envelope Option
and Building Envelope Trade-Off Options only) and requirements for reducing air leakage.
As shown in Figure 5-F, the designer must also comply with one of the following sections:
• Prescriptive Building Envelope Option (Section 5.5)
• Building Envelope Trade-Off Option (Section 5.6)
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• Energy Cost Budget Method (Section 11)
• Performance Rating Method (Appendix G)
If the Prescriptive Building Envelope Option or the Building Envelope Trade-Off Option are used, the
project must also comply with Section 5.1, General; Section 5.7, Submittals; Section 5.8, Product
Information and Installation Requirements; and Section 5.9, Inspection and Verification. These
sections include requirements for installing insulation, rating doors and windows, and limiting air
leakage.
These compliance paths are presented briefly here and discussed in detail later in this chapter.
 FIGURE 5-F. ENVELOPE COMPLIANCE OPTIONS
Corresponding section: Compliance Methods (5.2)
Prescriptive Building Envelope Option
The Prescriptive Building Envelope Option in Section 5.5 consists of nine climate-zone-specific sets of
criteria (see Section 5.1.4 above). Each set of criteria is on a single page that summarizes all of the
prescriptive requirements for that climate zone, including insulation levels for opaque components
such as roofs, walls, and floors. For opaque portions of the building envelope, two methods are
allowed, with the design criteria expressed in terms of (a) a minimum R-value for the insulation
included in the construction or (b) a maximum U-factor/C-factor/F-factor for the entire assembly .
For method (a), if insulation is installed that has the prescribed R-value, it is not necessary to assess
the performance of the overall assembly. There is no need to demonstrate compliance with assembly
U-factor.
For method (b), when using the maximum assembly U-factor/C-factor/F-factor criteria, it is necessary
to refer to Appendix A of the standard, which addresses all classes of construction. If the proposed
assembly is listed in one of the tables in Appendix A, then Section A1.1 requires that the U-factor/Cfactor/F-factor for that assembly be taken from the appropriate table in Appendix A. Appendix A of the
standard provides precalculated U-factors/C-factors/F-factors for most opaque assemblies so that one
rarely has to calculate a U-factor/C-factor/F-factor to show compliance. For most opaque assemblies,
the criteria are expressed in terms of a U-factor. For below-grade walls, C-factors are used instead of U-
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factors. For slab-on-grade floors, F-factors are used instead of U-factors. The differences between these
metrics are explained in later sections.
Prescriptive design criteria are also provided for fenestration (windows, glass doors, glass block,
plastic panels, and skylights). The Prescriptive Building Envelope Option limits the window-wall ratio
(WWR) to 40% of the gross exterior wall area and limits the skylight-roof ratio (SRR) to 3% of the
gross roof area (or 6% if certain daylighting requirements are met as specified in Section 5.5.4.4.2). If
the fenestration area exceeds these percentages, then the Prescriptive Building Envelope Option
cannot be used, and another compliance option must be selected.
The fenestration criteria are expressed in terms of maximum U-factor, maximum solar heat gain
coefficient (SHGC), and a minimum ratio of visible light transmission (VT) to SHGC. While all
fenestration criteria vary by climate zone, the U-factor criterion for vertical fenestration also varies by
class of construction.
With the prescriptive option, each envelope component must separately satisfy the requirements of
the standard. Exceptions allow area-weighted averaging of U-factors, C-factors, F-factors, and SHGCs,
which makes it possible for portions of a building’s envelope to fall short of the standard as long as
other portions of the envelope exceed the standard. Area-weighted averaging is allowed when all
portions of the envelope being averaged are from the same construction class and space-conditioning
category. R-values cannot be averaged, only U-factors, C-factors, F-factors, and SHGCs. For more
information about area-weighted averaging within a construction class, see FYI, Area-Weighted
Averages (5.5) later in this chapter.
Building Envelope Trade-Off Option
The Building Envelope Trade-Off Option (Section 5.6) offers the designer more flexibility. The thermal
performance of one envelope component, such as the roof, is allowed to fall below the prescriptive
requirements as long as the performance of one or more envelope components is raised to provide an
overall envelope performance equal to that of the prescriptive path. Trade-offs are permitted only
between building envelope components. It is not possible, for instance, to make trade-offs against
improvements in the lighting or HVAC systems.
Using the Building Envelope Trade-Off Option is more work than the Prescriptive Building Envelope
Option. It is necessary to calculate the surface area of each component of the exterior envelope and
semiexterior envelope. Wall areas must also be calculated separately for each orientation. The
methods used to make envelope trade-offs are documented in Appendix C of the standard.
The major differences between the Prescriptive Building Envelope Option and the Building Envelope
Trade-Off Option are shown in Table 5-A.
TABLE 5-A. COMPARISON OF BUILDING ENVELOPE PRESCRIPTIVE AND TRADE-OFF OPTIONS
Fenestration area
Area take-offs
U-factor compliance
Prescriptive Option
Vertical fenestration area is limited to
40% of the gross exterior wall area,
and skylights are limited to 3% of the
roof area (6% as permitted by Section
5.5.4.4.2).
It is only necessary to verify that the
vertical fenestration area is less than
40% of the gross exterior wall area and
that the total skylight area meets the
prescriptive requirements.
Not necessary if the R-value option is
used.
Trade-Off Option
Fenestration area greater than 40% is
permitted if the performance of
envelope components is improved over
that required by the prescriptive
requirements.
Surface areas must be calculated for
each type and class of construction.
Vertical fenestration and wall areas
must be separately calculated for
surfaces facing the major compass
points (N, S, E, W) plus NE, SE, SW, and
NW.
Required.
 Energy Cost Budget Method and Building Performance Rating Method
If neither the Prescriptive Building Envelope Option nor the Building Envelope Trade-Off Option are
suitable, the Energy Cost Budget (ECB) Method (Section 11) or the Performance Rating Method (PRM)
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(Appendix G) can be used. With these options, trade-offs can be made between the building envelope
and the lighting and/or mechanical systems. In all cases, however, the design must comply with the
mandatory provisions in Section 5.4.
The Performance Rating Method in Appendix G is useful when you want to know how much more
energy efficient a building is than a standard building design intended to meet but not exceed the
minimum prescriptive requirements of the standard. Several labeling and recognition programs exist
that require buildings to perform a certain percentage better than Standard 90.1. Two examples
include the U.S. Environmental Protection Agency’s ENERGY STAR program and the U.S. Green
Building Council’s LEED rating system.
Compliance Forms
Compliance forms and worksheets intended to facilitate the process of complying with the standard
are available for download from ASHRAE’s website at http://www.ashrae.org/UM90.1-2016.
Mandatory Provisions (5.4)
This section discusses the standard’s mandatory provisions. Before reading this section, you should
review the General (5.1) section at the beginning of this chapter so that you understand concepts such
as conditioned, semiheated, and unconditioned spaces, as well as concepts such as exterior and
semiexterior envelope components. The General section also explains how to find the criteria set that
applies to your building location. It is important to have a good grasp of these concepts before
reviewing the envelope requirements.
Insulation (5.4.1)
The mandatory provisions state that where insulation is required by Section 5.5 or 5.6, it must also
meet the requirements of Sections 5.8.1.
FYI
Steady-State Heat Flow: Understanding U-Factor and R-Value
When it is colder on one side of an envelope element, such as a wall, roof, floor, or window, heat will
conduct from the warmer side to the cooler side. Heat conduction is driven by temperature differences
and represents a major component of heating and cooling loads in buildings. The standard’s building
envelope requirements address heat conduction by specifying maximum U-factors for building
envelope construction assemblies and/or minimum R-values for insulation.
U-Factor
The U-factor is the rate of steady-state heat flow. In I-P units, it is the amount of heat in British thermal
units (Btus) that flows each hour through one square foot when there is a one-degree Fahrenheit
temperature difference between the indoor air and outdoor air (see Figure 5-G). (In SI units, it is the
amount of heat in watts that flows through a one square metre area with a one-degree Celsius
temperature difference.) The heat flow can be in either direction, as heat will flow from the warmer
side to the cooler side. With some constructions, the rate of heat flow may vary with direction of flow.
Steady-state heat flow assumes that temperatures on both sides of the building envelope element
(while different) are held constant for a sufficient period so that heat flow on both sides of the
assembly is steady. The steady-state heat flow method is a simplification, because in the real world
temperatures change constantly. However, it can predict average heat flow rates over time for
nonmass construction and is used by the standard to limit conductive heat losses and gains.
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FIGURE 5-G. THE U-FACTOR CONCEPT
R-Value
R-values are also used to describe steady-state heat flow but in a slightly different way. The R-value is
the thermal resistance to heat flow. A larger R-value has greater thermal resistance, or more insulating
ability, than a smaller R-value.
R-value is widely recognized in the building industry and is used to describe insulation effectiveness.
Consequently, the prescriptive criteria tables contain a compliance option based on the R-value of the
insulation alone. The insulation R-value does not describe the overall performance of the complete
assembly, however. It only describes the thermal resistance of the insulation material.
Heat Transfer in Construction Assemblies
Each layer of a building assembly, such as the sheathing and the insulation, has its own conductance, or
rate of heat transfer. The conductance for an individual layer is similar to the U-factor and has the
same units. When there are multiple elements in a layer, such as wood studs and cavity insulation, the
calculations must adjust for the different heat flow rates. When metal framing is used, the highly
conductive metal provides a pathway for significant heat transfer around, rather than through, the
insulation. These penetrations through an assembly of more highly conductive materials form what are
referred to as thermal bridges, and they can have a significant impact on the performance of the overall
assembly, sometimes reducing the resistance to heat transfer to less than half.
The U-factor accounts for the conductance of every element of the construction assembly, including the
air films on the interior and exterior surfaces. The air film conductances quantify the rate at which heat
is transferred between the surface of the construction assembly and the adjacent air. This conductance
depends on the orientation and roughness of the surface, the direction of heat flow, and the wind
speed across the surface. In Standard 90.1, air film conductance used in all U-factor and R-value
calculations is standardized as described in Section A9.4.1 Air Films. Similarly, standardized R-values
are used for compliant air spaces, as described in Section A9.4.2.
Appendix A contains tables of U-factors for a range of insulation options for many construction
assemblies. These have been carefully calculated using ASHRAE procedures and are to be used for
compliance with the U-factor options. This simplifies compliance for the designer and the building
official by eliminating the need to perform and review U-factor calculations. However, there may be
some cases where an assembly is not adequately represented in Appendix A. Where allowed by Section
A1.2, the standard requires that the U-factor of each envelope assembly be calculated taking into
account framing and other thermal bridges within the construction assembly. The method to be used
depends on the class of construction and other factors.
For light-frame walls, U-factors provide an adequate description of heat transfer. For heavy concrete
and masonry walls, however, this is only true under steady-state conditions. The dynamic heat storage
properties of the concrete and masonry alter the thermal behavior of the wall, and the U-factor
becomes less accurate as a predictor of heat flow.
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Fenestration and Doors (5.4.2)
The mandatory provisions require that fenestration and doors be rated using procedures and methods
specified in Section 5.8.2 of the standard.
Air Leakage (5.4.3)
The standard requires that the building envelope be carefully designed to limit the uncontrolled air
leakage into and out of the building. Controlling air leakage is important to achieving energy-efficient
buildings. Air leakage introduces sensible heat into conditioned and semiheated spaces. In climates
with moist outdoor conditions, it is also a major source of latent heat. Latent heat that enters the
building must be removed by the air-conditioning system at considerable expense. The standard has
requirements for a continuous air barrier and sealing of building envelope elements in order to reduce
air leakage through doors and windows, air seals at loading dock doors, and vestibules to limit air
leakage at building entrances. As with all of the mandatory requirements, the air leakage requirements
must be met regardless of the compliance method chosen, even the Energy Cost Budget (ECB) Method
(Section 11) and Performance Rating Method (Appendix G).
Continuous Air Barrier (5.4.3.1)
This section requires that the entire building envelope be designed and constructed with a continuous
air barrier that complies with the requirements for design (Section 5.4.3.1.1), installation (Section
5.4.3.1.2), and testing, materials, and assemblies (Section 5.4.3.1.3).
Continuous Air Barrier Design (5.4.3.1.1)
The first air leakage requirement is for a continuous air barrier enveloping the entire boundary of the
building. The purpose of the air barrier system is to resist air movement into and out of the building
through the building envelope caused by pressures from wind, stack effect, and fans. The air barrier
system consists of interconnected materials, assemblies, and flexible sealing of joints and penetrations
that mitigate air leakage between conditioned space and the exterior, as well as between semiheated
space and the exterior. All components of the air barrier must be specifically identified on the
construction plans and specifications, including details of sealing joints, interconnections, and sealing
of penetrations.
Continuous Air Barrier Installation (5.4.3.1.2)
A continuous air barrier requires special attention to joints and penetrations during installation. To
address this, the standard requires approved sealing, caulking, gasketing, or taping in the following
locations:
a. Joints around fenestration and door frames
b. Junctions between walls and foundations, between walls at building corners, between walls and
structural floors or roofs, and between walls and roof or wall panels
c. Openings at penetrations of utility services through roofs, walls, and floors
d. Building assemblies used as ducts or plenums
e. Joints, seams, connections between planes, and other changes in air barrier materials
Special attention is needed in the construction phase to ensure proper workmanship. A quality air
barrier system is largely achieved through careful construction practices and attention to detail. Poorly
sealed buildings frequently cannot maintain thermal comfort when the actual infiltration load exceeds
the HVAC design assumptions. This can be a significant problem in high-rise buildings due to stack
effect and exposure to stronger winds.
The standard also has requirements for limiting air leakage through mechanical air intakes and
exhausts. These requirements are addressed in the mechanical section (Section 6) of the standard, not
in the building envelope section.
Testing, Acceptable Materials, and Assemblies (5.4.3.1.3)
This section requires that one of three options be used to demonstrate compliance:
a. Whole-building air leakage testing with a maximum air leakage rate of 0.40 cfm/ft² under a
pressure differential of 0.3 in. of water in accordance with ASTM E779 or ASTM E1827.
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b.
Install the continuous air barrier using materials with a maximum air permeance of 0.004
cfm/ft² under a pressure differential of 0.3 in. of water in accordance with ASTM E2178. The
standard includes a list of materials that meet this requirement.
c. Install the continuous air barrier using assemblies with a maximum air permeance of 0.04
cfm/ft² under a pressure differential of 0.3 in. of water in accordance with ASTM E2357, ASTM
E1677, ASTM E1680, or ASTM E283. The standard lists two masonry wall constructions that
meet this requirement.
The whole-building air leakage option (Option a) has two exceptions. The first exception allows
buildings over 50,000 ft² to test prescribed representative portions of the building instead of the whole
building to ease the cost and complexity of the testing. The second allows flexibility of the air leakage
rate of 0.4 cfm/ft² to a maximum 0.6 cfm/ft² when a whole-building test has been performed and it has
been demonstrated that prescribed air leakage mitigation techniques have been employed to improve
the building air leakage rate. Every reasonable effort should be made to reach the 0.4 cfm/ft² as a
maximum rate. With this exception, a report to the building owner and code official is required that
describes the air leakage test results and the mitigation techniques employed to reduce the leakage
rate.
Fenestration and Doors (5.4.3.2)
Fenestration products, including doors, can significantly contribute to air leakage (infiltration and
exfiltration). The standard sets maximum air leakage rates as follows (when tested in accordance with
the specified test procedures):
a. Glazed swinging entrance doors, glazed
power-operated sliding or folding entrance
doors, and revolving doors
b. Curtain wall and storefront glazing
c. Skylights
d. High-speed nonswinging doors for
vehicles or materials
e. Nonswinging opaque doors and glazed
sectional garage doors
f. All other products (including windows)
1.0 cfm/ft² (5.1 L/s∙m²)
0.06 cfm/ft² (0.3 L/s∙m²)
0.3 cfm/ft² (1.5 L/s∙m²)
1.3 cfm.ft² (6.6 L/s∙m²)
0.4 cfm/ft² (2.0 L/s∙m²)
0.2 cfm/ft² (1.0 L/s∙m²) or
0.3 cfm/ft² (1.5 L/s∙m²) when tested at 6.24 lb/ft² (300 Pa)
These requirements are all based on a minimum pressure difference of 1.57 lb/ft² (75 Pa), although it
is common to test commercial fenestration products at 6.24 lb/ft² (300 Pa). Skylights with
condensation weepage openings and windows tested at 6.24 lb/ft² (300 Pa) are allowed somewhat
higher maximum air leakage.
Overhead doors are categorized in two ways:
• High-speed doors, typically fabric or coiling, where the opening speed is at least 32 in./s (0.81 m/s).
Typically used for vehicle access or material transportation.
• Nonswinging opaque doors, including glazed sectional garage doors and coiling doors.
Either type of overhead door could be considered glazed or opaque, depending on the construction.
The amount of glazing in these doors does not affect the maximum air leakage requirement because
the principal air leakage pathway is at the perimeter.
The final air leakage category is a catchall for all other products that are not specifically called out
above. See Section 5.4.3.2 for details and a reference to the test procedures.
Exemptions to Door and Fenestration Maximum Allowable Leakage
Metal coiling doors in semiheated spaces when installed in Climate Zones 0 through 6 shall have a
leakage requirement of no more than 1.0 cfm/ft² (5.1 L/s∙m²) at a pressure of at least 1.57 lb/ft² (75
Pa) when tested in compliance with any one of three specified test standards. (As per item [d] of the
table above, the requirement for the same application in Climate Zones 7 and 8 is 1.3 cfm/ft² [6.6
L/s∙m²].)
Field-fabricated fenestration is exempted from the air leakage requirements, but this must not be
confused with site-built fenestration. Field-fabricated fenestration does not include stick-built curtain
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wall and storefront systems that are designed to be assembled and glazed on site. These site-built
products must still meet the air leakage requirements. The exemption for field-fabricated fenestration
only applies when the frame is made at the construction site of materials that were not previously cut
or designed to be used as part of a fenestration system, such as using dimensional lumber to manually
create a window or door frame.
Individual products need not satisfy the air leakage requirements of the standard if the whole-building
leakage is 0.4 cfm/ft² (2 L/s∙m²) or less at a pressure differential of 1.57 lb/ft² (75 Pa) when tested in
accordance with ASTM E779. Although whole-building leakage testing is allowed in lieu of individual
product testing, it is unlikely a building using products with elevated leakage rates will be able to
perform within the allowable whole-building leakage. Note that regardless of product testing, wholebuilding leakage testing may be beneficial for gaining credit when using the Performance Rating
Method.
Loading Dock Weatherseals (5.4.3.3)
In Climate Zones 0 and 4 through 8, cargo doors and loading dock doors must be equipped with
weatherseals to restrict infiltration when vehicles are parked in the doorway. Manufacturers of loading
dock doors offer these devices as an option. They usually consist of a vinyl-wrapped compressible
foam block that is mounted around the perimeter of the door. The device forms a seal between the
truck and the dock when the truck is parked at the dock (see Figure 5-H).
FIGURE 5-H. LOADING DOCK WEATHERSEAL
Corresponding section: Loading Dock Weatherseals (5.4.3.3)
Vestibules (5.4.3.4)
Vestibules or revolving doors are required for building entrances (unless the entrance qualifies for one
of the exceptions described below). Building entrances are defined in Section 3.2 as the means
ordinarily used to gain access to the building, so this does not include exits from fire stairwells or the
handicapped access doors that might be adjacent to a revolving door. However, it does include all the
doors that are ordinarily used to gain access to the building. An urban building that occupies a full
block might conceivably have building entrances on all four sides of the block. A suburban office
building might have a front entrance for the public and a rear entrance that staff enters through. Both
of these are considered building entrances and would need to have vestibules. For buildings with
below-grade parking, the elevator doors in the parking garage are also considered building entrances
and must have vestibules (see the exception below).
All the doors entering and leaving required vestibules must be equipped with self-closing devices, and
the vestibule must comply with both a minimum dimension and a maximum size. For most buildings,
the minimum distance between the doors must be at least 7 ft (2.1 m) so as to allow operation where
both doors would not be open at the same time. The maximum vestibule area is the greater of 50 ft² (5
m²), which would apply to smaller buildings, or 2% of the gross conditioned floor area for that level of
the building, which would be allowed for larger buildings such as office towers. However, for buildings
with a very large footprint, i.e., those having a gross conditioned floor area for that level of the building
of 40,000 ft² (4,000 m²) and greater, such as big-box retail stores, Section 5.4.3.4.1 specifies larger
dimensions.
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If the vestibule contains any heating or cooling equipment, the building envelope requirements for
conditioned space must apply to exterior surfaces separating the vestibule from the outside, and there
are no requirements for the interior surface of the vestibule. If the vestibule does not contain any
heating or cooling equipment, then the building envelope requirements for semiheated space must
apply to the exterior and interior surfaces separating the vestibule from the outside and inside,
respectively (see Figure 5-K).
There are a number of exceptions to the vestibule requirement:
1. Revolving doors in building entrances are exempt.
2. Doors other than building entrances are exempt, such as those leading to service areas, mechanical
rooms, electrical equipment rooms, or exits from fire stairways. There is less traffic through these
doors, and the vestibule may limit access for large equipment.
3. Doors opening directly from dwelling units are exempt in all climate zones and for any number of
stories or amount of building area. Therefore, for example, sliding and swinging doors in high-rise
residential buildings opening out to decks or balconies are exempt.
4. Climate Zones 1 and 2 are exempt because the use of vestibules does not generate significant
energy savings in these warmer climate zones. However, vestibules may still be a good idea in tall
buildings, particularly air-conditioned buildings, as stack effect will increase the electrical load
during peak hours.
5. For Climate Zone 3, vestibules are not required in buildings that are both (a) less than four stories
above grade and (b) less than 10,000 ft² (1000 m²) in gross conditioned floor area. This is because,
with less height and less extreme temperatures, the stack effect is smaller. The stack effect (along
with wind effects) is one of the main drivers of infiltration. In addition, low-rise buildings are
generally smaller and there is less traffic through the door. However, large low-rise buildings
(such as big-box retail stores and supermarkets) have more foot traffic and so are not exempt.
6. For Climate Zones 0 and 4 through 8, vestibules are not required when the building is smaller than
1000 ft² (100 m²) in gross conditioned floor area.
7. Doors that are not building entrances and that open from a space with an area less than 3,000 ft²
(300 m²) in gross conditioned floor area are exempt. This is intended to apply to small retail
tenants on the ground floor of a multistory building that have entrances directly from the outside
into their small retail space. Doors that are dedicated to mechanical, electrical, and other service
equipment rooms are not considered entrance doors and are exempt from the vestibule
requirement.
8. Spaces that qualify as semiheated per the definition in Section 3 are not required to have
vestibules. Note that the definition of semiheated space is based on the output capacity of the
heating system and/or the cooling system. It does not matter at what temperature a space is
maintained. Spaces with installed heating or cooling equipment capacity that exceed these
thresholds, or which are indirectly conditioned, do not qualify for this exception.
9. Building entrances that are elevators from parking garages are not required to have vestibules,
provided that they have enclosed lobbies around the elevator. These lobbies serve to reduce stack
effect and have the additional benefit of reducing the amount of carbon monoxide that is drawn
into the building from a parking garage.
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FIGURE 5-I. VESTIBULE REQUIREMENTS
Corresponding section: Vestibules (5.4.3.4)
Prescriptive Building Envelope Option (5.5)
This section describes the Prescriptive Building Envelope Option. This is the easiest way to comply
with the building envelope requirements. All of the design criteria (also called “criteria set,” see Table
5-B) for a particular climate zone are contained on a single page, including the criteria for
nonresidential, residential, and semiheated space categories.
To determine the climate zone and criteria set for a location in the United States, look up the county in
Table Annex1-1 of Reference Standard Reproduction Annex 1 (located at the end of the standard). For
Canadian and other international locations, refer to Tables Annex1-2 or Annex1-3, respectively. For
international locations that are not listed, use the procedures described in Section 5.1.4, Climate. When
the standard is adopted as a code, the process is often simplified by the adopting jurisdiction
identifying the criteria set or sets that are to be used. For example, a state may choose to specify that a
particular criteria table be used for multiple adjacent counties to simplify implementation.
While the Prescriptive Building Envelope Option is simpler to apply, one cannot make trade-offs when
using this option. Each envelope component must comply with the requirements for that component. If
one needs more design flexibility, they can instead use the Building Envelope Trade-Off Option
(Section 5.6), the Energy Cost Budget (ECB) Method (Section 11), or the Performance Rating Method
(PRM) (Appendix G). All of these permit trade-offs between envelope components and, in the case of
the ECB Method and PRM, trade-offs between building systems. None of the compliance methods, the
prescriptive tables, the Building Envelope Trade-Off Option, the ECB Method or the PRM, can be used
to bypass any of the mandatory requirements.
Using the Criteria Tables (5.5.1 and 5.5.2)
A separate criteria table is provided for each of the nine climate zones. The criteria tables contain three
columns: nonresidential, residential, and semiheated. The nonresidential and residential columns are
for conditioned spaces. The criteria in the semiheated column applies to elements of the building
envelope that are considered semiexterior (see Figure 5-A) or spaces that are either semiheated or
unconditioned (see Section 5.1.2). The semiheated column applies to a building, or spaces within a
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building, that is heated by a mechanical system with an output capacity greater than or equal to 3.4
Btu/h·ft² (10 W/m²) of floor area and that is not mechanically conditioned (heated or cooled) as
defined by the requirements for a conditioned space (see Section 5.1.2).
Notice that the requirements for residential space categories are slightly more stringent than the
nonresidential requirements. The reason is that space heating and space cooling of residential space
categories is assumed to occur throughout the day and weekends, while nonresidential spaces are
assumed to be conditioned only during the day and only partly on weekends. The nonresidential and
residential criteria apply to the exterior building envelope. For the semiexterior envelope, use the
criteria for the semiheated space category, which assumes that these spaces will be heated constantly
by a space heating system with an output capacity that does not exceed the limits specified in Section
5.1.2.
Opaque Areas (5.5.3)
Opaque areas of the building envelope include the following construction elements: roofs; walls, above
grade; wall, below grade; floors; slab-on-grade floors, and opaque doors. The standard identifies
classes of construction and gives separate design criteria for each class. Table 5-B shows example
prescriptive criteria (in I-P and SI units) for Climate Zone 4.
Most of the time, the appropriate class of construction will be obvious. Table 5-C summarizes the
defining characteristics of the various classes of constructions for opaque elements of the envelope and
gives a thumbnail sketch of each.
There are two methods that can be used to meet the prescriptive requirements for opaque elements of
the envelope. The easiest way is to install insulation with an R-value that meets or exceeds the criteria
shown in the column “Insulation Min. R-value.” R-value criteria are given for all construction classes
except opaque doors. The R-value criteria apply only to the insulation materials and do not include any
other building materials (such as structural sheathing), air gaps, interior finishes, or air films. For
compliance using this method, each assembly is required to have the specified minimum R-value of
insulation throughout the assembly (including roofs with tapered insulation). Averaging of R-value is
not allowed.
When a single R-value is given without a modifier, the standard usually assumes that the insulation is
located within a cavity in the construction. For instance, for wood-framed walls, a requirement of R-20
(R-3.5) means that the insulation installed between the framing members has a thermal resistance that
is at least R-20 (R-3.5). In some cases, however, the R-value criteria have “c.i.” next to them. This stands
for continuous insulation. This “c.i.” notation means that the insulation must be installed in a manner
that is continuous and is uninterrupted by thermal bridges other than fasteners and service openings
that would reduce the thermal resistance of the assembly. Notice that for the “Insulation Entirely
above Deck” class of roof construction, all the R-value criteria have the notation “c.i.,” as do most of the
above grade mass walls, mass floors, and below-grade walls.
When two R-values are given, the first is the minimum R-value required for insulation in the cavity,
and the second R-value is almost always the minimum R-value required for continuous insulation,
denoted by “c.i.” For example, for steel-framed walls, a requirement of R-13 + R-7.5 c.i. (R-2.3 + R-1.3
c.i.) means that the insulation installed between the framing members has an insulation R-value of R13 (R-2.3) or more and that there is additional continuous insulation that has an insulation R-value of
R-7.5 (R-1.3) or more. However, for metal building roofs, the notation “Ls” often appears, which stands
for “liner system.”
The other method that can be used to meet the prescriptive requirements for opaque construction is
by using the values in the column titled “Assembly Maximum,” which contains the criteria for the
overall thermal performance of the construction assembly. These criteria are specified in terms of Ufactor, F-factor or C-factor, depending on the type of opaque element. When using the maximum
assembly U-factor/F-factor/C-factor method, it is necessary to refer to Appendix A of the standard.
Appendix A of the standard has tables of precalculated assembly U-factor/F-factor/C-factors for all
classes of construction. If the proposed assembly is listed in one of the tables in Appendix A, then
Section A1.1 requires that the U-factor/F-factor/C-factor for that assembly be taken from the
appropriate table in Appendix A.
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If an assembly is significantly different from those listed in Appendix A for either the Insulation
Minimum R-value or Assembly Maximum approaches, then the assembly U-factor must be determined
in accordance with Section A9, Determination of Alternate Assembly U-Factors, C-Factors, F-Factors, or
Heat Capacities.
For roofs, above grade walls, and floors, the overall thermal performance is expressed as a U-factor.
The U-factor takes into account all elements and layers in the construction assembly, including the
framing, sheathing, interior finishes, and air gaps, as well as exterior and interior air films. For opaque
doors, the U-factor is the only compliance option.
For below-grade walls, the overall thermal performance criteria are expressed as a C-factor. The Cfactor includes all layers in the construction assembly but excludes the exterior air film and the soil’s
effect on the outside of the wall.
For slab-on-grade floors, the overall thermal performance criteria are expressed as an F-factor. The Ffactor describes heat loss relative to the length (linear foot or metre) of slab perimeter.
When a building has more than one assembly within a single class of construction that falls within the
same space-conditioning category, the standard allows area-weighted averaging using the U-factor, Cfactor, or F-factor compliance option. Area-weighted averaging is not allowed for R-value compliance.
Area-weighted averaging enables one construction assembly within the class of construction to fail to
meet the criteria as long as other constructions within the same class exceed the requirement. The
area-weighted average U-factor/F-factor/C-factor of all constructions within the class must not exceed
the corresponding criterion. When performing area-weighted averaging, up to 1% of openings due to
recessed equipment can be ignored. If the openings are greater than 1%, they need to be accounted for
in the area-weighted average. For more information, see FYI, Area-Weighted Averages.
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TABLE 5-B. EXAMPLE PRESCRIPTIVE CRITERIA SET, CLIMATE ZONE 4 — I-P AND SI
Corresponding section: Opaque Areas (5.5.3)
(This is Table 5.5-4 in the Standard.)
Building Envelope Requirements for Climate Zone 4 (A,B,C) — I-P
NONRESIDENTIAL
OPAQUE ELEMENTS
Roofs
Insulation Entirely
above Deck
Metal Buildinga
Attic and Other
Walls, Above-Grade
Mass
Metal Building
Steel-Framed
Wood-Framed and Other
Wall, Below-Grade
Below-Grade Wall
Floors
Mass
Steel-Joist
Wood-Framed and Other
Slab-On-Grade Floors
Unheated
Heated
Opaque Doors
Swinging
Nonswinging
FENESTRATION
Vertical Glazing,
0-40% of Wall
Nonmetal framing, all
Metal framing, fixed
Metal framing, operable
Metal framing,
entrance door
Skylights, 0%-3% of Roof
All types
RESIDENTIAL
SEMIHEATED
Assembly
Insulation
Assembly
Insulation
Assembly
Insulation
Maximum
Min. R-Value
Maximum
Min. R-value
Maximum
Min. R-Value
U-0.032
R-30.0 ci
U-0.032
R-30.0 ci
U-0.093
R-10.0 ci
U-0.034
R-30
U-0.037
U-0.021
U-0.104
U-0.060
U-0.064
U-0.064
C-0.119
U-0.057
U-0.038
U-0.033
F-0.520
F-0.843
U-0.370
U-0.310
R-19.0+R-11 Ls or
R-25 + R-8 Ls
R-49
R-9.5 ci
R-0 + R15.8 ci
R-13.0 + R-7.5 ci
R-13.0 + R-3.8 ci
or R-20
R-7.5 c.i.
R-14.6 ci
R-30
R-30
R-15 for 24 in.
R-20 for 24 in.
U-0.037
U-0.021
U-0.090
U-0.050
U-0.064
U-0.064
C-0.092
U-0.051
U-0.038
U-0.033
F-0.520
F-0.688
U-0.370
U-0.310
R-19.0+R-11 Ls or
R-25 + R-8 Ls
R-49
R-11.4 ci
R-0 + R-19
R-13.0 + R-7.5 ci
R-13.0 + R-3.8 ci
or R-20
R-10 ci
R-16.7 ci
R-30.0
R-30.0
R-15 for 24 in.
R-20 for 48 in.
U-0.082
R-19
U-0.580
U-0.162
U-0.124
NR
R-13.0
R-13.0
C-1.140
NR
U-0.089
U-0.107
U-0.052
U-0.051
F-0.730
F-0.900
U-0.370
U-0.360
R-13.0
R-6.3 ci
R-19.0
R-19.0
NR
R-10 for 24 in.
Assembly Assembly Assembly Assembly Assembly Assembly Assembly Assembly Assembly
Max.
Min.
Min.
Min.
Max.
Max.
Max.
Max.
Max.
VT/SHGC
VT/SHGC
VT/SHGC
U
SHGC
U
SHGC
U
SHGC
U-0.31
U-0.38
U-0.46
U-0.68
U-0.50
(for all frame types)
SGHC-0.36
all
1.10
U-0.31
U-0.38
U-0.46
SHGC-0.40
NR
U-0.50
U-0.68
(for all frame types)
SGHC-0.36
all
1.10
U-0.51
U-0.73
U-0.81
SHGC-0.40
NR
U-1.15
U-0.77
(for all frame types)
SGHC-NR
all
NR
NR
NR
* The following definitions apply: c.i. = continuous insulation (see Section 3.2), FC = filled cavity (see Section A2.3.2.5), Ls = liner system (see Section A2.3.2.4),
NR = no (insulation) requirement.
a. When using the R-value compliance method for metal building roofs, a thermal spacer block is required (see Section A2.3.2).
Standard 90.1 User’s Manual
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Building Envelope Requirements for Climate Zone 4 (A,B,C) — SI
NONRESIDENTIAL
OPAQUE ELEMENTS
Roofs
Insulation Entirely
above Deck
Metal Buildinga
Attic and Other
Walls, Above-Grade
Mass
Metal Building
Steel-Framed
Wood-Framed and Other
Wall, Below-Grade
Below-Grade Wall
Floors
Mass
Steel-Joist
Wood-Framed and Other
Slab-On-Grade Floors
Unheated
Heated
Opaque Doors
Swinging
Nonswinging
FENESTRATION
Vertical Glazing,
0-40% of Wall
Nonmetal framing, all
Metal framing, fixed
Metal framing, operable
Metal framing,
entrance door
Skylights, 0%-3% of Roof
All types
RESIDENTIAL
SEMIHEATED
Assembly
Insulation
Assembly
Insulation
Assembly
Insulation
Maximum
Min. R-Value
Maximum
Min. R-Value
Maximum
Min. R-Value
R-5.3 ci
U-0.182
R-5.3
U-0.527
R-1.8 ci
U-0.192
R-5.3
U-0.182
U-0.210
U-0.119
U-0.592
U-0.341
U-0.365
U-0.365
C-0.676
U-0.321
U-0.214
U-0.188
F-0.900
F-1.460
U-2.101
U-1.760
R-3.3 + R-1.9 Ls
or R-4.4 + R-1.4 Ls
R-8.6
R-1.7 ci
R-0 + R-2.8 ci
R-2.3 + R-1.3 ci
R-2.3 + R-0.7 ci
or R-3.5
R-1.3 ci
R-2.6 ci
R-5.3
R-5.3
R-2.6 for 600mm
R-3.5 for 600mm
U-0.210
U-0.119
U-0.513
U-0.286
U-0.365
U-0.365
C-0.522
U-0.287
U-0.214
U-0.188
F-0.900
F-1.191
U-2.101
U-1.760
R-3.3 + R-1.9 Ls
or R-4.4 + R-1.4 Ls
R-8.6
R-2.0 ci
R-0 + R-3.3 ci
R-2.3 + R-1.3 ci
R-2.3 + R-0.7 ci
or R-3.5
R-1.8 ci
R-2.9 ci
R-5.3
R-5.3
R-2.6 for 600mm
R-3.5 for 1200mm
U-0.466
R-3.3
U-3.293
U-0.920
U-0.705
NR
R-2.3
R-2.3
U-0.504
R-2.3
C-6.473
U-0.606
U-0.296
U-0.288
F-1.264
F-1.558
U-2.101
U-2.044
NR
R-1.1 ci
R-3.3
R-3.3
NR
R-1.8 for 600mm
Assembly Assembly Assembly Assembly Assembly Assembly Assembly Assembly Assembly
Max.
Min.
Min.
Min.
Max.
Max.
Max.
Max.
Max.
VT/SHGC
VT/SHGC
VT/SHGC
U
SHGC
U
SHGC
U
SHGC
U-1.76
U-2.16
U-2.61
U-3.86
U-2.84
(for all frame types)
SGHC-0.36
all
1.10
U-1.76
U-2.16
U-2.61
SHGC-0.40
NR
U-2.84
U-3.86
(for all frame types)
SGHC-0.36
all
1.10
U-2.90
U-4.14
U-4.60
SHGC-0.40
NR
U-6.53
U-4.37
(for all frame types)
SGHC-NR
all
NR
NR
NR
* The following definitions apply: c.i. = continuous insulation (see Section 3.2), FC = filled cavity (see Section A2.3.2.5), Ls = liner system (see Section A2.3.2.4),
NR = no (insulation) requirement.
a. When using the R-value compliance method for metal building roofs, a thermal spacer block is required (see Section A2.3.2).
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TABLE 5-C. SUMMARY OF OPAQUE CONSTRUCTION CLASSES
Cha p t e r 5 | Bu il d i ng E n vel o p e
Corresponding section: Opaque Areas (5.5.3)
Sketch
Roofs
Single Layer
System
Double Layer
System
Liner System
Filled Cavity
Class of Construction
Description
Insulation Entirely above
Deck
The insulation is installed above a
concrete, wood, or metal deck in a
continuous manner.
Metal Building
The construction typically has an
exterior metal panel attached using
screws (for through-fastened roof) or
clips (for standing seam roof) attached
to metal purlins. A layer of blanket
insulation is typically draped over the
purlins or joists and compressed at the
purlins by the roof panels. For filled
cavity and liner systems, a layer of
insulation is first installed in the cavity
between the purlins, then a second layer
of insulation is installed running
perpendicular to the purlins. Thermal
spacer blocks are used with standing
seam roof systems. Continuous
insulation may also be used
independently or in combination with
blanket insulation.
Includes all roof constructions that do
not qualify for one of the other classes of
roof construction.
Attic and Other
Roofs
Insulation Entirely above
Deck
Metal Building
Single Layer
System
Liner System
Double Layer
System
Filled Cavity
Attic and Other
Standard 90.1 User’s Manual
The insulation is installed above a
concrete, wood, or metal deck in a
continuous manner.
The construction typically has an
exterior metal panel attached using
screws (for through-fastened roof) or
clips (for standing seam roof) attached
to metal purlins. A layer of blanket
insulation is typically draped over the
purlins or joists and compressed at the
purlins by the roof panels. For filled
cavity and liner systems, a layer of
insulation is first installed in the cavity
between the purlins, then a second layer
of insulation is installed running
perpendicular to the purlins. Thermal
spacer blocks are used with standing
seam roof systems. Continuous
insulation may also be used
independently or in combination with
blanket insulation.
Includes all roof constructions that do
not qualify for one of the other classes of
roof construction.
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Sketch
Walls, Above-Grade
Class of Construction
Description
Mass
Any concrete or masonry wall with a heat
capacity exceeding 7 Btu/ft²·°F (143
kJ/m²·K). If the mass elements are
constructed with lightweight materials
with a unit weight not greater than 120
lbs/ft3 (1920 kg/m3) then the heat
capacity must be greater than 5 Btu/ft²·°F
(102 J/m²·K) in order to qualify as a mass
wall.
The construction typically has an exterior
metal panel attached directly to
horizontal metal girts that span between
the vertical building columns. Fiberglass
insulation can be installed as a single
layer compressed at the girts by the wall
panels, as single layer in the cavity, or as a
double layer with one layer in the cavity
and one layer compressed at the girts by
the wall panels. Continuous insulation
may also be used independently or in
combination with the blanket insulation.
Walls with metal framing members. This
is a very common construction type in
nonresidential and some residential
buildings, as noncombustible
construction is required for many classes
of construction.
Metal Building
Single Layer
Compressed
Single Layer
in Cavity
Double Layer
Steel-Framed
Wood-Framed and
Other
Walls, Below-Grade
Floors
Below-Grade Wall
Any type of wall that is below grade. The
outer surface of the wall is in contact with
the earth, and the inside surface is
adjacent to conditioned or semiheated
space.
Mass
Any floor with a heat capacity exceeding
7 Btu/ft²·°F (143 kJ/m²·K). If the mass
elements are constructed with
lightweight materials with a unit weight
not greater than 120 lbs/ft3 (1920
kg/m3), then the heat capacity must be
greater than 5 Btu/ft²·°F (102°J/m²·K) in
order to qualify as a mass floor.
Any floor that is constructed with metal
joists or purlins in such a manner that the
metal framing members interrupt the
insulation continuity.
Steel-Joist
Wood-Framed and
Other
42
Walls with wood framing or any type of
wall construction that does not qualify as
mass, metal building, or steel-framed.
Floors that are framed with wood
members and any other type of floor
construction that is not of mass or steeljoist construction.
Standard 90.1 User’s Manual
Slab-On-Grade Floors
Opaque Doors
Cha p t e r 5 | Bu il d i ng E n vel o p e
Unheated
No heating elements either within or
below the slab.
Heated
Heating elements located within or below
the slab.
Swinging
Nonswinging
Opaque doors, access hatches, and smoke
vents with hinges on one side (operable
glazed doors are included with vertical
fenestration and operable glazed smoke
vents are included with skylights).
Rollup, sliding, and other doors, access
hatches, and smoke vents that are not
swinging (fixed glazed door panels and
sidelights are included with vertical
fenestration and glazed smoke vents are
included with skylights).
FYI
Area-Weighted Averages
When using the standard, there are several exceptions that permit one to perform area-weighted
averaging in specific cases (but not for insulation R-values). Building designs can be complex and
include many different types of roof, wall, and floor assemblies. Also, more than one type of window or
overhang will often exist in a building. In these cases, the added flexibility provided by allowing the use
of area-weighted averages can be beneficial. Area-weighted averages may only be performed, however,
within a single class of construction (e.g., roof with insulation entirely above deck) for a single space
conditioning category (i.e., nonresidential conditioned, residential conditioned, or semiheated). An
exception allows area-weighted averaging across multiple classes of construction for vertical
fenestration only (e.g., operable metal framing and fixed metal framing) but this is still limited to a
single space-conditioning category.
For instance, if a building has a number of different roof assemblies, but all of the same class of
construction (e.g., roof with insulation entirely above deck) in a single space-conditioning category
(e.g., nonresidential conditioned), you may opt to calculate the area-weighted average in order to
determine compliance. If all of the assemblies within this group independently meet the requirement,
then the area-weighted average would also meet the requirement and there would be no need to
perform the calculation. However, if one or more of this group of assemblies fails to meet the
requirement, the building may still comply with the standard if the area-weighted average U-factor of
all the assemblies for that same class of construction meets the criterion. Thus the standard allows
better-performing assemblies to compensate for an otherwise noncompliant assembly within a single
class of construction for a single space-conditioning category per Section 5.5.3(b). For example, for a
roof with insulation entirely above-deck, the insulation can be tapered to accommodate drainage
needs as long as the area-weighted average of all the roof assemblies meets the U-factor requirement.
Area-weighted averaging can be done with U-factors, C-factors, F-factors, solar heat gain coefficients
(SHGCs), and overhang projection factors (PFs). However, you must not average R-values.
To demonstrate compliance for tapered insulation, the U-factor must be calculated in a way that
accounts for the variable heat flow through various thickness of insulation. Calculus can provide a
completely accurate calculation. However, this is not necessary, as numerical approaches can be used
to provide an acceptable calculation. An acceptable procedure would be to divide the overall area into
subareas, each of which has insulation thickness in a specific range. Then the thinnest insulation in the
Standard 90.1 User’s Manual
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range is used to calculate the U-factor for the subarea. Then the U-factors for all the subareas are area
weighted over the entire area.
The area-weighted average is like a simple average, except that larger surfaces are weighted more
heavily than smaller surfaces. To illustrate the difference between simple averaging and area-weighted
averaging, suppose that a building has two roof constructions, both of the same class. The first
construction represents an area of 9000 ft² (836 m²) and has a U-factor of 0.030 (U-0.170). The second
construction represents an area of 1000 ft² (92.9 m²) and a U-factor of 0.100 (U-0.568). A simple
average of 0.065 (0.369) is calculated as shown here:
0.030+0.100
𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 =
2
0.170+0.568
𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 =
2
= 0.065
(I-P)
= 0.369
(SI)
Because the higher U-factor represents only 10% of the roof area, the simple average is inaccurate. The
true area-weighted average is 0.037 (0.209), almost half the simple average. The area-weighted
average is calculated by multiplying each U-factor by its area, adding these products, and dividing the
sum by the total area. The area-weighted average calculation is shown here:
𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊ℎ𝑡𝑡𝑡𝑡𝑡𝑡 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 =
9,000×0.030+1,000×0.100
𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊ℎ𝑡𝑡𝑡𝑡𝑡𝑡 𝐴𝐴𝐴𝐴𝐴𝐴𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 =
10,000
836×0.170+93×0.568
929
= 0.037
= 0.209
(IP)
(SI)
Roof Insulation (5.5.3.1 and A2)
The standard establishes three classes of constructions for roofs: roofs with insulation located entirely
above the deck, metal building roofs, and all other roofs. For each of these classes, Tables 5.5-0 through
5.5-8 provide insulation requirements for nonresidential, residential, and semiheated applications by
climate zone. Section 5.5.3.1 also specifies that skylight curbs must be insulated to the same level as a
roof with insulation entirely above deck, or R-5, whichever is less.
This section describes the differences between these classes of construction and reviews methods that
can be used to determine the U-factor of different types of constructions. Information in this section is
applicable to both the Prescriptive Building Envelope Option and the Building Envelope Trade-Off
Option.
FIGURE 5-J. ROOF, INSULATION ENTIRELY ABOVE DECK
Corresponding section: Roof Insulation (5.5.3.1)
Insulation Entirely above Deck
The defining characteristic of this class of construction (Figure 5-J) is that all insulation is located
above the structural deck. Roof constructions that have no insulation cannot belong to this class;
neither can constructions that have insulation both above and below the structural deck. The
insulation is usually a rigid foam or high-density mineral fiber.
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R-Value Criteria
When using the R-value criteria for this class of construction, the insulation must be installed in a
continuous manner and must have only limited interruptions (the R-value criteria have the “c.i.”
notation). Some interruptions in the insulation are permitted as long as they do not exceed 1% of the
total roof area, such as structural supports for mechanical or other roof-mounted equipment. For
situations where greater than 1% interruption, tapering, or other deviation from a continuous,
consistent insulation depth is required, U-factor compliance must be used.
U-Factor Criteria
When using the U-factor criteria, the thermal performance of the entire construction assembly,
including any thermal bridges, is taken into account. With this option, the U-factor of the proposed
assembly must be less than or equal to the criteria. When buildings have more than one construction
belonging to this class, an area-weighted average can be calculated, and it is only necessary that the
area-weighted average U-factor be less than or equal to the criteria (for information about areaweighted averaging, see the FYI, Area-Weighted Averages).
For demonstrating U-factor compliance, use the U-factors from Table A2.2.3. Alternatively, if allowed
by Section A1.2, U-factors can be determined using the methods and procedures described in Appendix
A. This class of construction is simple and does not introduce thermal bridges. If your construction
assembly has materials other than the insulation that contribute to the thermal resistance, you can use
the series calculation method to calculate your own U-factor (see the Determination of Alternate
Assembly U-factors, C-factors, F-factors, or Heat Capacities [Section A9] section later in this chapter).
Metal Building Roofs
Metal building roofs are a component of metal buildings. Structural metal roof panels are supported
over metal structural supports and serve as both the roof deck and the waterproof roof surface. There
are two primary types: standing seam metal roofs and through-fastened metal roofs. A standing seam
metal roof is attached to the structural framing using metal panel clips located along the side-seams of
the roof panels. A standing seam metal roof typically includes a thermal spacer block. A throughfastened metal roof is attached using screws with rubber washers that penetrate the roof panels.
Typically, a through-fastened metal roof does not include a thermal spacer block unless approved by
the metal roof product manufacturer.
In single-layer systems, a layer of faced blanket insulation is draped over the purlins and compressed
by the roof panel attachment. In double-layer systems, the first layer (towards the interior) of faced
blanket insulation is draped over the purlins, then a second layer of unfaced blanket insulation is
installed on top of the first layer, between the purlins. The first layer of insulation is then compressed
at the purlins, and both layers are slightly compressed when the roof panels are attached. For singlelayer and double-layer systems, the amount and effect of insulation compression is considered in the
U-factor analysis per Section A9.4.6 as part of the geometric inputs for the insulation configuration that
should represent as installed field conditions.
Liner system (Ls) insulation is installed by fastening a continuous membrane to the underside of the
purlins with a series of bands. The membrane must be continuous, meaning framing members must
not interrupt or penetrate the membrane. Unfaced blanket insulation is placed on the membrane in the
cavity space between the purlins. For multilayer installations, a second layer of unfaced blanket
insulation is draped over the purlins. It is compressed when the metal roof panels are attached. To
properly install the insulation, the lower layer must be retained in a manner that maintains its contact
with the insulation above but does not compress the insulation and compromise the insulation value.
Thermal spacer blocks may be required when specified in Table A2.3.3.
Standard 90.1 User’s Manual
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FIGURE 5-K. METAL BUILDING ROOF INSULATION
Corresponding section: Roof Insulation (5.5.3.1)
Filled cavity (FC) systems are similar to liner systems in that both use two layers of blanket insulation.
One layer is installed in the cavity space between purlins and one layer is installed above and
perpendicular to the purlins. The difference between the two systems relates to the orientation of the
facing at the purlins. With the filled cavity system, the facing tabs run alongside of and over the purlins.
Both systems require banding to support the insulation and allow it to fill the cavity space and
maintain contact with the purlins. Thermal spacer blocks may be required when specified in Table
A2.3.3.
Detailed descriptions are provided in Sections A2.3.2.4 and A2.3.2.5. Higher-performance assemblies
are available that use continuous insulation and insulated metal panels. Metal roof panels commonly
compress some insulation at the supports (see Figure 5-M). Although not required by the standard, in
most cases, the inner face of the insulation is sealed to prevent the movement of air and moisture
across the assembly. However, in warm/moist climate zones it is desirable for the inner face to be
moisture-vapor permeable and not sealing facer joints may not be consequential. Additionally, a
thermal spacer block between the metal roof deck and the top of the purlin may be required with
standing seam roofs to reduce thermal bridging.
R-Value Criteria
When using the R-value criteria for metal building roofs, it is important to understand the insulation
must be installed in accordance with the details provided in the Appendix A, Section A2.3 for metal
building roofs.
When a single R-Value, such as R-19 (R-3.3) is specified, it is typically installed by draping the faced
blanket insulation over the structural supports. For higher-performing insulations systems, multiple Rvalues will be shown, such as R-13 + R-13 (R-2.3 + R-2.3). This indicates that two layers of R-13 (R-2.3)
blanket insulation should be used. When the two R-Values are listed without a suffix such as “Ls” or
“FC”, the first layer of faced insulation is installed perpendicular to, and draped over, the purlins. The
second layer is unfaced insulation installed on top of the first layer and between the purlins. When a
suffix such as “FC” or “Ls” follows the last R-value, the system being specified is either a filled cavity or
liner system. For these insulations systems, the supplier’s installation procedures must be followed.
Figure 5-K shows the methods of complying with the R-value criteria for metal building roofs. Note the
tables in Appendix A have assemblies “with thermal spacer block” and “without thermal spacer block.”
Where indicated by Tables 5.5-0 through 5.5-8, thermal spacer blocks must be used (if using R-value
compliance method). If thermal spacer blocks are not used, the U-factor equivalent must be used in
lieu of the R-value method.
U-Factor Criteria
Heat transfer in metal buildings is complex. The construction consists of a combination of highly
conductive metal components and compressed blanket insulation at the supports, unless continuous
insulation is used above the supports and compressible insulation is not draped over supports for the
roofing. U-factors for common metal building roof assemblies are shown in Table A2.3.3. For
assemblies not shown, the U-factor must be determined by other methods described in Section A9.
These methods include two- or three-dimensional heat transfer computer models, laboratory testing
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or calculation procedures as provided in Section A9.4.6. Table A2.3.3 gives data for single-layer,
double-layer, Ls, and FC. Continuous insulation can be used alone or in combination with the blanket
insulation options. For single- and double-layer systems, a calculation procedure is provided in Section
A9.4.6 to determine the roof assembly U-factor. Note, the U-factor for insulated metal panel systems
would be determined by modeling or testing.
Attics and Other Roofs
This class of construction includes all roof constructions that are a not metal building roofs or that do
not have the insulation installed entirely above the deck. Roofs that have no insulation fall into this
category (see Example 5-C). This class covers many different kinds of construction because it is a
catchall for all roofs not in one of the other classes. Examples of roof constructions in this class include:
•
•
•
•
Attics with either wood or metal trusses
Roofs above plenum spaces where the insulation is installed on the underside of the deck
Single-rafter roofs
Any other type of roof that is not a metal building roof and does not have insulation entirely
above the deck
Figure 5-L shows examples of roof constructions that belong to this class. They include attic roofs with
either metal or wood framing members and single-rafter roofs where the interior finish is installed on
the bottom of the rafter and the structural deck above. Concrete roofs or metal deck roofs can also
belong to this class, depending on the position of the insulation.
Attics are a common roof construction in this class. Attics are usually ventilated to the exterior, and the
insulation is installed above the ceiling. Section A2.4.2.3 of the standard permits the insulation depth to
be reduced near the eaves, as this was accounted for in developing the R-value requirements and in
developing the precalculated assembly U-factor tables in Appendix A. When the depth of the insulation
required by the Prescriptive Building Envelope Option is greater than the depth of the bottom chord of
the truss, the insulation must extend over the top of the bottom chord of the truss.
Single-rafter roofs are another common roof construction that belong to this class. For this
construction, framing members (usually wood framing members) have exterior sheathing attached to
one side and the interior finish attached to the other side. The depth of the framing member limits the
depth of the cavity. Per Section A2.4.2.4, when insulation required by the standard has a thickness too
large to fit in the cavity, an exception applies, and it is only necessary to install insulation at a depth
that will fill the cavity and still leave an inch or so for ventilation. Table 5-D shows the minimum Rvalue of insulation that must be installed for three different depths of wood framing members. For
single-rafter roofs, the minimum insulation that must be installed is the lesser of the values in Table
5-D or the requirement in the criteria set. Note that the categories in Table 5-D are based on the actual
depth of the rafters and not the nominal depth. The first category is rafters having a maximum actual
depth of less than or equal to 8 in. (200 mm); thus, this category includes nominal 8 in. (200 mm) deep
wood rafters, as their actual depth is only 7.25 in. (184 mm). The second category is rafters having a
maximum actual depth of less than or equal to 10 in. (250 mm); correspondingly, this category
includes nominal 10 in. (250 mm) deep wood rafters, as their actual depth is only 9.25 in. (235 mm).
The third category is rafters having a maximum actual depth of less than or equal to 12 in. (300 mm);
correspondingly, this category includes nominal 12 in. (300 mm) deep wood rafters, as their actual
depth is only 11.25 in. (286 mm).
Appendix A includes a number of data tables that can be used for this class of roofs. Table A2.4.2 has
data for attic roofs with wood joists. These are common for low-rise residential construction but are
used for light commercial buildings as well. Data are provided in the table for both standard trusses
and advanced framing. The difference is that advanced framing has a raised heel or other framing
technique that permits the full depth of insulation to extend to the building walls. With a standard
truss, the insulation must be tapered or compressed near the eaves because the clearance is reduced.
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Wood Joists, Standard Truss
Wood Joists, Raised Truss
Wood Joists, Single Rafter
Steel Joists, Rigid Insulation
Steel Joists, Batt Insulation
Steel Joists, Batt Insulation
FIGURE 5-L. ATTICS AND OTHER ROOFS
Corresponding section: Roof Insulation (5.5.3.1)
Table A2.4.2 also provides data for single-rafter wood roofs. When using the single-rafter data, the
specified insulation must not be compressed. U-factors in Table A2.4.2 account for a layer of 5/8 in. (16
mm) gypsum board (R-0.56 [R-0.10]), an interior air film (R-0.61 [R-0.11]), and a semiexterior air film
(R-0.46 [R-0.08]). The semiexterior air film resistance is higher than the exterior air film resistance
because the air is assumed to be inside an attic, thus reducing the effect of outside wind.
If allowed by Section A1.2, you can also calculate the U-factor for wood-framed attics and single-rafter
roofs using the parallel path calculation method or based on hot-box apparatus testing in accordance
with ASTM C1363 as specified in Section A9.3.
Use the U-factor data in Table A2.5.2 for any attic roof with steel joists. These U-factors are based on
steel joists spaced at 48 in. (1.2 m) o.c. Data in the table include the thermal resistance of an interior air
film (R-0.61 [R-0.11]) and an exterior air film (R-0.17 [R-0.03]). Batt insulation is assumed to be
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installed on the underside of a metal deck. The metal deck is assumed to have no significant thermal
resistance. The steel joists interrupt the continuity of the insulation. Steel joists are more conductive
than wood, and acceptable procedures for calculating U-factors are more complex. Acceptable
calculation methods include laboratory testing, the modified zone method, and the isothermal planes
method in combination with the effective R-values from Table A9.2-1 of Appendix A.
Example 5-C. Concrete Roof with No Insulation
Corresponding section: Roof Insulation (5.5.3.1)
Q
A building in a hot climate has a roof assembly that consists of lightweight concrete over a metal deck.
The assembly is not insulated, but a roof coating is used that has both a high reflectance and a high
emittance. What class of roof construction does this assembly fall into?
A
This construction is in the “attic and other” class of construction, as it is not insulated. If it had
insulation above the deck, then it would belong to the insulation-entirely-above-deck class of
construction. Because of the concrete deck, this construction cannot be a member of the metal building
class of construction.
TABLE 5-D. SINGLE-RAFTER ROOFS
Corresponding section: Roof Insulation (5.5.3.1)
(This is Table A2.4.2 in the Standard)
Minimum Insulation R-Value or Maximum Assembly U-Factor
Wood Rafter Depth, d (actual)
Climate Zone
0 to 7
8
d ≤ 8 in.
(d ≤ 200 mm)
R-19 (3.3)
U-0.055 (0.31)
R-21 (3.7)
U-0.052 (0.29)
8 < d ≤ 10 in.
(200 < d ≤ 250 mm)
R-30 (5.3)
U-0.036 (0.20)
R-30 (5.3)
U-0.036 (0.20)
10 < d ≤ 12 in.
(250 < d ≤ 300 mm)
R-38 (6.7)
U-0.028 (0.16)
R-38 (6.7)
U-0.028 (0.16)
Roof Solar Reflectance and Thermal Emittance (5.5.3.1.1)
“Cool roof” is a term that applies to roof surfaces that have both a high solar reflectance and a high
thermal emittance. In hot climates, cool roofs are an effective way to reduce solar gains through the
roof. The properties of a cool roof can be achieved with roofing products that are manufactured as cool
roofs or by field-applying a coating to the roof’s outside surface. The high solar reflectance reflects
sunlight and heat away from the building, and the high thermal emittance allows heat to escape when
the surface becomes heated. Some surfaces, such as galvanized steel, have a high solar reflectance but
low thermal emittance. These surfaces reflect heat, but heat that is absorbed cannot easily escape.
Other surfaces, such as traditional dark paint, have a high thermal emittance but a low solar
reflectance. These surfaces allow heat to escape, but do a poor job of reflecting heat that strikes the
surface. However, roof products, coatings, and paints are now available in dark visible colors that test
successfully for a high solar reflectance.
In Climate Zones 0 through 3, the standard recognizes the cooling benefits of a cool roof surface and
requires that a qualifying cool roof be installed. See Example 5-D. In order to comply with the
prescriptive requirements, the roof must comply with one of the following requirements:
a. The surface has a three-year-aged solar reflectance equal to or greater than 0.55 and a three-yearaged thermal emittance equal to or greater than 0.75.
b. The surface has a three-year-aged Solar Reflectance Index (SRI) of at least 64. This procedure
considers both thermal emittance and solar reflectance and rates a surface based on these
properties.
c. The roof construction has additional roof insulation as specified in Table 5.5.3.1.1.
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Qualifying roof products must be tested by the Cool Roof Rating Council’s CRRC-1 standard. Some
sources for determining the solar reflectance and thermal emittance of roofing products are the
manufacturer of the roofing through data sheets, the Cool Roof Rating Council directory
(www.coolroofs.org), or the U.S. Environmental Protection Agency’s ENERGY STAR Roofing Program
roof product list (www.energystar.gov).
There are numerous exceptions to the cool roof requirement. The requirement does not apply for the
following conditions:
a. Roofs having a stone ballast with a weight of at least 17 lb/ft² (83 kg/m²) or stone or concrete
pavers with a weight of at least 23 lb/ft² (112 kg/m²).
b. Roofs with a vegetated roof system over at least 75% of the surface. To qualify for this exception,
there must be at least 2.5 in. (64 mm) of soil that supports durable plantings.
c. Roofs that are shaded over at least 75% of their surface or covered with solar panels (photovoltaic
or thermal).
d. Steep-sloped roofs (usually >2:12 per code).
e. Low-sloped (≤2:12) metal building roofs in Climate Zones 2 and 3.
f. Roofs over ventilated attics or over conditioned space that is not air conditioned.
g. Roofs that have an asphaltic membrane in Climate Zones 2 and 3.
Example 5-D and Example 5-E address cool roofs.
FYI
Cool Roof Terms
Solar Reflectance
Solar reflectance is the portion of the sun’s radiation that is reflected by a surface. A perfect reflector
has a reflectance of 1.0, and a perfect absorber has a reflectance of 0. These are both physical
impossibilities. No surface (not even mirrors) reflects all radiation and no surface (not even flat black
paint) absorbs all the heat from the sun. Radiation that is not reflected from an opaque surface is
absorbed. The sum of the fractions of radiation that is reflected, transmitted, and absorbed must equal
1. In hot climates, it is desirable that surfaces—especially roof surfaces—have a high solar reflectance.
Note that the solar spectrum includes a much larger spectrum of light than the visible spectrum. Thus,
visible color is not a reliable indicator of a roof product’s solar reflectance.
Thermal Emittance
Thermal emittance is the ability of a surface to radiate heat. This is in contrast to reflectance and
absorptance, which describe a surface’s ability to receive radiation. Like reflectance and absorptance,
the emittance is a property of the surface and not the material. For instance, polished aluminum and
brushed aluminum have very different values for reflectance, absorptance, and thermal emittance.
When the building needs cooling, it is desirable for exterior surfaces, especially roofs, to have a high
thermal emittance. This allows heat absorbed by the roof to escape through radiation. At night, this is
especially important because the temperature of the night sky is low and a great deal of heat can
escape by radiation.
Solar Reflectance Index (SRI)
SRI is a measure of a constructed surface’s ability to reflect solar heat, as shown by a small
temperature rise. A standard black surface (solar reflectance 0.05, thermal emittance 0.90) is 0 and a
standard white surface (solar reflectance 0.80, thermal emittance 0.90) is 100. SRI is generally
between 0 and 100 but can be greater than 100 or less than 0. (As opposed to solar reflectance, which
is always between 0 and 1.0).
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Example 5-D. Cool Roofs in Georgia
Cha p t e r 5 | Bu il d i ng E n vel o p e
Corresponding section: Roof Solar Reflectance and Thermal Emittance (5.5.3.1.1)
Q
The building plans for a proposed residential building in Savanna, Georgia, (Climate Zone 2A) call for a
reflective roof coating with an aged thermal emittance of 0.40 and an aged solar reflectance of 0.60.
The roof has insulation entirely above deck and has a U-factor of 0.028 (0.159). Does this building
comply with the prescriptive roof criteria?
A
Yes. Table 5.5-2 allows a maximum roof U-factor of 0.039 (0.220), so the proposed roof construction
satisfies this requirement. Because the site is in Climate Zone 2, a cool roof is required. The proposed
roof complies with the minimum aged solar reflectance requirement of 0.55 but fails to comply with
the minimum aged thermal emittance requirement of 0.75. However, Sections 5.5.3.1.1(b) and
5.5.3.1.1(c) offer alternative paths for demonstrating cool roof compliance. Section 5.5.3.1.1(b) allows
compliance with an SRI of 64 or greater. For this example, the ASTM E1980 calculated SRI is 55, which
does not comply. Alternatively, Section 5.5.3.1.1(c) allows cool roof compliance by reducing the roof Ufactor to 0.029 (0.165) or below (for residential roofs with insulation entirely above the deck). This
roof meets the prescriptive requirements of Section 5.5.3.1.1(c).
Example 5-E. High Solar Reflectance/High Thermal Emittance Roof Surface
Corresponding section: Roof Solar Reflectance and Thermal Emittance (5.5.3.1.1)
Q
A nonresidential building located in Kuala Lumpur, Malaysia, has a concrete roof with insulation
entirely above deck. The roof is covered with a white elastomeric roof product that qualifies as a high
solar reflectance/high thermal emittance surface: its aged solar reflectance is greater than 0.55, and its
aged thermal emittance is greater than 0.75 when tested according to the CRRC-1 Standard.
What is the minimum R-value needed for compliance?
A
Kuala Lumpur is in Climate Zone 0—it is very near the equator and is one of the hottest places in the
world. Because the proposed building qualifies with the cool roof requirements of Section 5.5.3.1.1, the
standard U-factor and R-value criteria apply. The maximum U-factor criterion for a roof with insulation
entirely above deck is 0.039 (0.220), and the minimum R-value criterion for the insulation alone is R25 c.i. (R-4.4 c.i.).
Above-Grade Wall Insulation (5.5.3.2 and A3)
There are four classes of construction for above-grade walls: mass walls, metal building walls, steelframed walls, and wood-framed and other walls. This section describes the differences between these
classes of construction and reviews methods that can be used to determine compliance.
Like roofs, the criteria for walls are expressed in two ways. First, minimum R-value criteria are given
for the insulation alone. This is the easiest way to comply with the requirement. Another way is to
comply with the U-factor requirement for the overall assembly, including thermal bridges. The U-factor
method must be used when one or more of the wall constructions in a class do not comply with the Rvalue requirement, and area-weighted averaging is necessary (see FYI, Area-Weighted Averages earlier
in this chapter). The U-factor method may also be appropriate when a wall construction is significantly
different from those used to generate the precalculated assembly U-factor tables in Appendix A.
Usually it is very clear if a wall is above grade or not. However, in some cases, a portion of a wall may
be above grade and a portion below grade. When a wall is both above and below grade and insulated
on the interior, the above-grade insulation requirement applies to the entire wall. When the insulation
is installed on the exterior of the wall or is integral to the wall (for instance, the cells of a concrete
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masonry wall are filled), then the wall is divided between the above-grade and below-grade portions,
and the separate requirements apply to each.
Mass Walls
A mass wall is a heavyweight wall, generally weighing more than 25 lb/ft² (122 kg/m²) per wall area.
The definition used in the standard is that the wall has a heat capacity (HC) greater than 7.0 Btu/ft²∙°F
(143 kJ/m²∙°C) for normal density mass materials and 5.0 Btu/ft²∙°F (102 kJ/m²∙°C) for lighter density
mass materials weighing less than 120 lb/ft³ (586 kg/m³). Mass wall heat capacity is determined from
Table A3.1-2 for concrete or Table A3.1-3 for concrete masonry, as appropriate. For more information
on heat capacity, see FYI, Understanding Heat Capacity, and Example 5-F.
Figure 5-M shows examples of walls in this class.
R-Value Criteria
When the R-value method is used for compliance, the mass wall insulation must be continuous, i.e., the
“c.i.” notation is used with the R-value specification. However, the R-value method can still be used
when the insulation is installed with small metal clips no more than 1 in. (25 mm) in length that are
spaced no more frequently than 16 in. (400 mm) on center (o.c.) vertically and 24 in. (600 mm) o.c.
horizontally. If other framing (or furring) materials are used, such as wood framing, metal studs, or
continuous metal channels, the U-factor compliance method must be used. Furthermore, if insulation
were installed so that it is completely continuous (for instance, on the exterior), it would be
advantageous to use the U-factor method as the insulation would be uninterrupted.
For some criteria sets, the mass wall criteria have footnote “b,” which indicates that the exception to
Section 5.5.3.2 applies. This exception permits compliance by insulating the cells of concrete masonry
units with any material (such as perlite) that has a thermal conductivity of 0.44 Btu∙in./h∙ft²∙°F (0.063
W/m∙K) or less. This exception applies only when the concrete masonry units are ungrouted or
partially grouted. Partially grouted means that the cells are grouted no more frequently than 32 in.
(800 mm) o.c. vertically and 48 in. (1200 mm) o.c. horizontally. This exception does not apply to solid
grouted walls or concrete masonry walls that do not meet the criteria for ungrouted or partially
grouted.
Note that the R-values in Table A3.1-1 can be used for R-value compliance with the standard only for
assemblies with continuous insulation. These include the ones under the subheadings “1 in. (25 mm)
Metal Clips at 24 in. (600 mm) on Center Horizontally and 16 in. (400 mm ) Vertically,” “Continuous
Insulation Uninterrupted by Framing,” and “Brick Cavity Wall with Continuous Insulation.” Any other
wall assembly that does not have continuous insulation, or has insulation that is interrupted by metal
framing, must use the U-factor for compliance.
The table has data for 8 in. (200 mm) thick solid normal-weight concrete and medium-weight concrete
masonry unit (CMU) walls. The CMU data are given for solid grouted and partially grouted walls.
Ungrouted CMU walls must use data from the partially grouted column. Concrete walls should use the
8 in. (200 mm) concrete column regardless of thickness. Similarly, solid grouted CMU walls of any
thickness must use the solid grouted column.
A “Brick Cavity Wall with Continuous Insulation” has the base wall described in the columns of Table
A3.1-1 on the inside, brick on the outside, and continuous insulation uninterrupted by framing
between the two.
U-Factor Criteria
Appendix A has several ways to determine the U-factor of mass walls. The easiest method is to use data
from Table A3.1-1. The table has data for 8 in. (200 mm) thick solid normal-weight concrete and
medium-weight CMU walls. The CMU data are given for solid grouted and partially grouted walls.
While the table is based on the mass constructions described above, it can be used for any mass wall
described in this table, provided the U-factor is used for compliance. Ungrouted CMU walls should use
data from the partially grouted column; however, lower U-factors may be obtained by performing
calculations described in Section A3.1.3.2(b). Concrete walls must use the 8 in. (200 mm) concrete
column regardless of thickness. Similarly, solid grouted CMU walls of any thickness may use the solid
grouted column.
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The “Continuous Insulation Uninterrupted by Framing” section can be used for concrete walls, solid
grouted masonry walls, or partially grouted masonry walls as long as the total amount of continuous
insulation has the R-value indicated by the appropriate row. This can be for walls with the continuous
insulation on the inside or outside (e.g., an exterior insulation and finishing system [EIFS]) or when the
continuous insulation is integral, such as between concrete layers (e.g., precast concrete sandwich
panel wall) or when continuous insulation is on either side of the concrete layer (e.g., an insulating
concrete form [ICF] wall).
The "Brick Cavity Wall with Continuous Insulation" section can be used for concrete walls, solid
grouted masonry walls, or partially grouted masonry walls with an added layer of brick, as long as the
amount of continuous insulation has the R-value indicated by the appropriate row.
For the section for continuous insulation with stucco and metal framing at 24 in. (600 mm), the first
designated R-value is between metal framing at 24 in. (600 mm) o.c., and the second R-value of R-19
c.i. (R-3.35 c.i.) in all cases is continuous insulation. These values can also be used for concrete walls,
solid grouted masonry walls, or partially grouted masonry walls with this type and amount of
insulation on the inside, outside, integral to the wall, or on either side as long as the insulation is as
described.
For uninsulated mass walls or mass walls where the insulation is interrupted by framing members or
clips with greater size or more frequency than described in Table A3.1-1, Tables A3.1-2, A3.1-3, and
A3.1-4 may be used. These tables can also be used where the insulation is interrupted by wood
furring/framing or other unique assemblies where the total U-factor is calculated. These tables are a
little more complicated to use than Table A3.1-1, but they provide considerable flexibility for a wide
variety of walls. For an example U-factor calculation for mass walls, see Example 5-G.
• Table A3.1-2 has data for concrete walls with a thickness ranging from 3 to 12 in. (75 to 300
mm) and densities ranging from 20 to 144 lb/ft³ (320 to 2304 kg/m³). For each case, the table
provides an overall U-factor and total R-value (Ru). The overall U-factor may be used directly for
compliance if the wall does not have exterior insulation, interior insulation, or interior furring.
The table also contains the heat capacity (HC). This value can be used to verify that the wall
qualifies as a mass wall. In order to qualify, the HC must be equal to or greater than 7.0 Btu/ft²⋅°F
(143 kJ/m²·°C) for mass materials that have a density equal to or greater than 120 lb/ft³ (1920
kg/m³). HC must be greater than 5.0 Btu/ft²⋅°F (102 kJ/m²·°C) for mass materials that have a
density less than 120 lb/ft³ (1920 kg/m³). Note that not all the constructions in Table A3.1-2
actually qualify as mass walls. Table A3.1-2 is used with both above-grade mass walls and belowgrade walls. For this reason, it has U-factors and Ru for above-grade walls and C-factors and Rc for
below-grade walls. Be careful which you use in your calculations.
• Table A3.1-3 has data for CMU walls with 6 in., 8 in., 10 in., and 12 in. (150 mm, 200 mm, 250
mm, and 300 mm) thicknesses and densities ranging from 85 to 135 lb/ft³ (1,360 to 2,160
kg/m³). Data are also provided for five different treatments of the cells of the concrete blocks:
solid grouted, partially grouted with the cells empty, partially grouted with the cells insulated,
unreinforced with the cells empty, and unreinforced with the cells insulated. “Partially grouted”
means that cells are grouted no more than 32 in. (800 mm) o.c. vertically and 48 in. (1200 mm)
o.c. horizontally. As with Table A3.1-2, the table provides the HC and an overall U-factor that may
be used directly for compliance if the wall does not have exterior insulation, interior insulation, or
an interior furring space.
• Table A3.1-4 has the effective R-value of insulation/framing layers that may be added to the
thermal resistance of the concrete or CMU mass wall selected from Table A3.1-2 or Table A3.1-3.
The table has data for R-values ranging from zero to R-25 (R-4.40). The table also has data for
metal framing, wood framing, and no framing (continuous insulation). The metal and wood
framing can have depths ranging from 0.5 to 5.5 in. (13 to 140 mm). Data from this table is added
to the Ru taken from either Table A3.1-2 or Table A3.1-3. The sum is the thermal total resistance.
The overall U-factor is the reciprocal of the total resistance.
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FYI
Understanding Heat Capacity
Heat capacity (HC) is the amount of heat that must be added to one square unit of surface area in order
to elevate the temperature of the construction uniformly by one degree. The inch-pound (I-P) units are
British thermal units per square foot per degree Fahrenheit (Btu/ft²∙°F). The metric or SI units are
kilojoules per square metre per degree Celsius (kJ/m²∙°C).
HC is used in the standard to quantify the amount of thermal mass in exterior walls and floors. With
the prescriptive option, HC must be known to determine if a wall is a mass wall or if a floor is a mass
floor. It is used the same way in the Building Envelope Trade-Off Option, but in addition, HC is a
significant factor in determining the envelope performance factor. HC may also be used with the ECB
Method, although in this case the various construction layers are usually modeled separately.
Heat capacity for mass walls is to be taken from Table A3.1-2 or Table A3.1-3. The HCs in Table A3.1-2,
but not the U-factors, are also appropriate for solid concrete mass floors. Where these are not
adequate, HC is calculated as follows:
n
HC = ∑ Density i × Specific Heat i × Thickness i
i =1
Essentially, HC is the sum of the HC of each individual layer in the wall. The HC of each layer is the
density of the material multiplied by the thickness times the specific heat (all in consistent units). In
the equation above, i is an index of each layer in the construction and n is the total number of layers in
the construction. Layers that have insignificant thermal mass (such as air films) can be ignored. When
layers have more than one material, for instance a framed wall with insulation in the cavity, each
separate material is weighted in proportion to its projected area.
Example 5-F illustrates how to calculate heat capacity.
Concrete
Solid Grouted Concrete Block
Unreinforced Concrete Block with
Empty Cells
Metal Framing
Metal Clips
Wood Framing
Rigid Insulation
FIGURE 5-M. MASS WALLS
Corresponding sections: Above-Grade Wall Insulation (5.5.3.2 and A3)
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Example 5-F. Heat Capacity Calculation
Corresponding sections: Opaque Areas (5.5.3)
Q
What is the heat capacity (HC) for the wall construction depicted below? The exterior wall consists of 4
in. (100 mm) of face brick, a 1.5 in. (38 mm) air gap, 8 in. (200 mm) partially grouted concrete masonry
unit (CMU) with a density of 105 lb/ft³ (1680 kg/m³) (cells uninsulated). The interior has R-11 (R-1.9)
batt insulation between nominal 2 × 4 in. (50 × 100 mm) wood studs spaced at 16 in. (400 mm) o.c. The
interior finish is 5/8 in. (16 mm) gypsum board.
A
The HC is the sum of the weight (density times thickness) times the specific heat for each layer of the
wall. The calculation can be structured in tabular form as shown below. The table is shown first in I-P
units and then repeated in SI units.
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Item
Density
(lb/ft³)
120
Weight
(lb/ft²)
Fraction
of Wall
Specific Heat
(Btu/lb∙°F)
HC
(Btu/ft²∙°F)
Data Source
40
1.00
0.19
7.60
ASHRAE
Handbook—
Fundamentals
Air Gap
8 in. Concrete block, partially
grouted, cells uninsulated
2 in. × 4 in. nominal oven dried
pine studs (1.5 in. x 3.5 in.
actual) @ 16 in. o.c.
R-11 Fiberglass Batt
0
105
0
51
1.00
1.00
0
0.20
0
10.2
0.7
0.20
0.75
0.2
0.0
0.625 in. Gypsum Board
40
2.1
1.00
0.21
0.4
Density
(kg/m³)
1920
Weight
(kg/m²)
Fraction
of Wall
Specific Heat
(kJ/kg∙°C)
18.9
HC
(kJ/m²∙°C)
Data Source
192
1.00
0.8
154
ASHRAE
Handbook—
Fundamentals
Air Gap
200 mm Concrete block, partially
grouted, cells insulated
50 × 100 mm nominal oven
dried pine studs (38 mm x 89
mm actual) @ 400 mm o.c.
R-1.9 Fiberglass Batt
0
1,680
0
0
370
14
1.00
1.00
0.25
1.88
0
216
12
0.5
0.75
0.8
0
16 mm Gypsum Board
640
10.2
1.00
1.15
12
4 in. Brick (fired clay)
Total
Item
100 mm Brick (fired clay)
23
6.7
0.25
0.45
0.7
7
Total
Standard 90.1
(Table A3.1-3)
ASHRAE
Handbook—
Fundamentals
ASHRAE
Handbook—
Fundamentals
ASHRAE
Handbook—
Fundamentals
Standard 90.1
(Table A3.1-3)
ASHRAE
Handbook—
Fundamentals
ASHRAE
Handbook—
Fundamentals
ASHRAE
Handbook—
Fundamentals
389
Example 5-G. U-Factor Calculation, Mass Wall
Corresponding section: Above-Grade Wall Insulation (5.5.3.2)
Q
What is the U-factor of a 10 in. (250 mm) solid grouted CMU wall with a block density of 95 lb/ft³
(1522 kg/m³)? The wall has a furred interior wall with wood framing members that are 3.5 in. (89
mm) deep and R-11 (R-1.94) in the cavity.
A
The first step is to find the total thermal resistance of the CMU wall and air films from Table A3.1-3.
The total thermal resistance (Ru) is R-2.15 (R-0.38) and the HC is 19.7 Btu/ft²∙°F (402 kJ/m²∙K). The
second step is to find the additional thermal resistance from Table A3.1-4. For 3.5 in. (89 mm) deep
wood studs and R-11 (R-1.94), the effective R-value (REff) of the framing cavity layer including drywall
is R-9.3 (R-1.64). The overall thermal resistance is R-11.45 (R-2.02) and the U-factor is 0.087 (0.50).
The details of the U-factor calculation are:
1
1
1
=
=
= 0.087
𝑅𝑅𝑢𝑢 + 𝑅𝑅𝐸𝐸𝐸𝐸𝐸𝐸 2.15 + 9.3 11.45
1
1
1
𝑈𝑈 =
=
=
= 0.50
𝑅𝑅𝑢𝑢 + 𝑅𝑅𝐸𝐸𝐸𝐸𝐸𝐸 0.38 + 1.64 2.02
𝑈𝑈 =
Metal Building Walls
Metal building walls are a component of metal buildings. The exterior surface and the weather barrier
is a metal panel that usually runs vertically and spans between horizontal girts that are supported at
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the building columns. Various insulation options are available for metal building walls that include
single-layer compressed, single-layer in cavity, double layer, as well as continuous insulation (c.i.) and
insulated metal panels, used in combination with insulation between girts or independently.
In single-layer compressed blanket insulation systems, the blanket insulation is draped down on the
outside of the girts. It is compressed between the metal wall panels and the girt, as illustrated in Figure
5-N.
Single-layer in cavity has blanket insulation that fills the girt cavity space, with no insulation
compressed between the metal wall panel and the girts, as illustrated in Figure 5-O. The insulation
placed between the girts will include a facing that is installed separately or adhered to the insulation. A
thermal spacer block or thermal break strip may be required between the metal wall panel and the
girts when specified in Table A3.2.3. Alternatively, continuous insulation may be used in lieu of the
thermal block and to supplement insulation located between the girts.
Double-layer insulation, as illustrated in Figure 5-P, is similar to single-layer in cavity insulation in that
the entire girt cavity space is filled with blanket insulation. However, two layers of insulation are
installed. One layer of insulation is draped down on the outside of the girts. It is compressed between
the metal wall panel and the girts, and the other layer of insulation is installed in the cavity between
the girts (not compressed by framing). The insulation placed between the girts will include a facing
that is installed separately or adhered to the insulation. A thermal spacer block or thermal break strip
may be required between the metal wall panel and the girts when specified in Table A3.2.3.
Continuous insulation (c.i.) on a metal building wall may be installed on the outside or inside of the
girts, uncompressed and uninterrupted by the framing members, per Section A3.2.2.2. Additionally,
continuous insulation may be installed independently or in combination with blanket insulation
systems, as shown in Table A3.2.3.
Typically, the inner face of the insulation is sealed to prevent the movement of air across the assembly
and, in some climate zones, vapor migration into the assembly from the conditioned space. However,
the standard no longer requires the inner face to be sealed and now allows joints to be lapped or
sealed. Where not sealed, the vapor retarding capability of the facer may be compromised by air
leakage into the assembly. Particularly for colder climates, it is advisable to seal the seams of the facer
and any penetrations or discontinuities. With a sufficient amount of exterior continuous insulation,
sealing of seams of the facer becomes less important due to warmer interior conditions within the
assembly, but sealing is still advisable in colder climates. In warm/moist climates, a more vapor
permeable interior facer is generally more desirable, and the choice of sealing or lapping seams is
generally understood to be less consequential.
Detailed metal building wall descriptions are provided in Section A3.2.2.
When using the R-value criteria for metal building walls, the criteria is expressed either as a singlelayer compressed system or a continuous insulation (c.i.) system. For example, R-13 (R-2.3) refers to a
single-layer compressed system and R-19 c.i. (R-3.3 c.i.) refers to a continuous insulation system.
However, other metal building wall insulation assemblies are available if the U-factor criterion is
chosen.
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FIGURE 5-N. SINGLE-LAYER COMPRESSED METAL BUILDING WALL
Corresponding sections: Above-Grade Wall Insulation (5.5.3.2 and A3)
FIGURE 5-O. SINGLE-LAYER IN CAVITY METAL BUILDING WALL
Corresponding sections: Above-Grade Wall Insulation (5.5.3.2 and A3)
FIGURE 5-P. DOUBLE LAYER METAL BUILDING WALL
Corresponding sections: Above-Grade Wall Insulation (5.5.3.2 and A3)
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U-Factor Criteria
As with metal building roofs, the heat transfer in metal building walls is complex. The U-factors for
metal building wall assemblies not accounted for in Table A3.2.3 must be determined by methods
described in Section A9. These methods include two-dimensional heat transfer computer models,
laboratory testing, or calculation procedure as provided in Section A9.4.6. Table A3.2.3 provides singlelayer compressed, single-layer in cavity, and double-layer systems, as well as continuous insulation
installed by itself or in combination with the blanket insulation. The Section A9.4.6 calculation
procedure is applicable for single-layer compressed, single-layer in cavity, double-layer and c.i.
systems. Note, the U-factor for insulated metal panel systems would be determined by modeling or
testing.
When using the U-factor approach for metal building walls, including calculation methods in Section
A9.4, values are based on a minimum girt spacing of 60 in. The standard allows an average girt spacing
to be no less than 52 in. o.c. when using the U-factor approach with tabulated values from Table A3.2.3.
If the girt spacing is less than 52 in. o.c., then U-factors must be determined as per Section A9.2 using
testing or calculations provided for specific construction types: single-layer compressed, single-layer
in cavity, double-layer systems, and continuous insulation. When using the R-value method (per tables
in the standard with “c.i.” R-values), the girt spacing is of much less significance and there is no limit on
girt spacing.
Steel-Framed Walls
Steel-framed walls are quite common in nonresidential building construction. Life-safety codes require
that many building types be constructed of noncombustible materials; this means that steel studs are
commonly substituted for wood studs. The construction techniques are similar for metal and wood
studs. In both cases, an interior finish material (usually gypsum board) is attached to the inside
surface. Any number of materials can be used for the exterior finish, including glass fiber reinforced
concrete (GFRC), precast concrete, stucco, or glass curtain walls. Steel studs are much more conductive
than wood studs, and the economics of providing insulation are quite different. This is the defining
characteristic of this class of construction. Figure 5-Q shows an example of a wall in this class. It is
important to note that the opaque portions of curtain wall construction are considered steel-framed
walls and need to be insulated as such.
R-Value Criteria
If the R-value criteria are given as a single specification, for instance R-13 (R-2.3), this represents the
thermal resistance of uncompressed insulation that must be installed in the steel stud cavity.
Obviously, it would also be acceptable to use continuous insulation with the specified R-value, as the
overall thermal performance of the wall would be improved. If there are two values in the R-value
specification, for instance R-13 + R-7.5 c.i. (R-2.3 + R-1.3 c.i.), the second rated R-value of insulation
must be installed in addition to the first and must be continuous (uninterrupted by framing).
When using the R-value method for compliance, the opaque portions of curtain walls must comply
with the cavity insulation and c.i. criteria contained in the steel-framed wall class of construction..
U-Factor Criteria
If the U-factor criteria are used, take the data from Table A3.3.3.1 or, if allowed by Section A1.2, Ufactors can be calculated using one of the methods specified in Appendix A.
Table A3.3.3.1 has U-factor data for both 3.5 in. (89 mm) deep and 6.0 in. (152 mm) deep metal studs
spaced at both 16 in. (400 mm) o.c. and 24 in. (600 mm) o.c. Data are also provided for different levels
of both cavity insulation and continuous insulation. The cavity insulation is interrupted by the metal
framing, while the continuous insulation is not. U-factors in the table include an R-0.17 (R-0.03)
exterior air film, R-0.08 (R-0.01) stucco, R-0.56 (R-0.10) exterior gypsum board, R-0.56 (R-0.10)
interior gypsum board, and an R-0.68 (R-0.12) interior air film. The effective R-value of the
insulation/framing layer is taken from Table A9.2-2 of Appendix A. When using U-factors from Table
A3.3.3.1, the continuous insulation (if applicable) must be uninterrupted and the cavity insulation (if
applicable) must be uncompressed.
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FIGURE 5-Q. STEEL-FRAMED WALL
Corresponding sections: Above-Grade Wall Insulation (5.5.3.2 and A3)
If your steel-framed wall construction is significantly different from that assumed to develop the
values in Table A3.3.3.1 (see Section A1.2), you can calculate your own U-factor. Appendix A9.2
specifies methods for each class of construction including steel-framed walls. U-factors must be
calculated in one of three ways: laboratory tests, the parallel path calculation method using the
insulation/framing layer adjustment factors in Table A9.2-2, or the modified zone method. The
modified zone method is documented in ASHRAE Handbook—Fundamentals and is also described in
the Determination of Alternate Assembly U-factors, C-factors, F-factors, or Heat Capacities (Section A9)
section of this user’s manual. In addition, for opaque assemblies, it is always acceptable to use two- or
three-dimensional finite difference and finite volume computer models.
The values in Table A9.2-2 represent effective R-values for the insulation/framing layer and were
derived from laboratory tests. An effective R-value is the thermal resistance that must be added to the
thermal resistance of the other layers in the wall that results in the correct heat transfer. When the
heat transfer is determined through laboratory tests and the thermal resistance of all the other layers
is known, the effective R-value of the insulation/framing layer can be calculated with simple algebra.
This is the basis of the values in Table A3.3.3.1. See Example 5-H.
Curtain walls are commonly used in buildings within the scope of Standard 90.1 and merit moredetailed discussion. Curtain walls typically comprise fenestration portions and opaque spandrel
portions. The fenestration portions (including glass, sash, and frame) must comply with the fenestration
criteria using the NFRC rating procedures. As mentioned above, the opaque portions (including
spandrel panels and framing) of curtain walls must comply with the opaque criteria in the steel-framed
construction class.
As the opaque portions generally use the same framing as the fenestration portions, this provides an
opportunity for product manufacturers, designers, and contractors. When particular software is
already being used for the fenestration portions of the project, it makes sense to consider also using
that same software to perform the U-factor calculations for the opaque portions of the curtain wall.
Remember that, for opaque assemblies, it is always acceptable to use two- or three-dimensional finite
difference and finite volume computer models. Given that even the opaque portions of the curtain wall
can be a significant component of the building envelope in large buildings, it is certainly worthwhile to
consider using sophisticated software to obtain a better understanding of potential thermal bridging
and how best to detail the insulation in the opaque portion of the curtain wall.
The following is a summary of material from the ASHRAE Handbook—Fundamentals regarding an
ASHRAE research project for the opaque spandrel portion of curtain walls. As noted in Chapter 15,
“The spandrel portion of curtain walls usually consists of a metal pan filled with insulation and covered
with a sheet of glass or other weatherproof covering. Although the U-factor in the center of the spandrel
panel can be quite low, the metal pan is a thermal bridge, significantly increasing the U-factor of the
assembly. Two-dimensional simulation, validated by testing of a curtain wall having an aluminum frame
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with a thermal break, found that the U-factor for the edge of the spandrel panel (the 2 1/2 in. [65 mm]
band around the perimeter adjacent to the frame) was 40% of the way toward the U-factor of the frame.
The U-factor was 0.06 Btu/h· ft²·°F [0.34 W/(m²·K)] for the center of the spandrel, 0.45[2.56] for the
edge of the spandrel, and 1.06 [6.02] for the frame (Carpenter and Elmahdy 1994). Two-dimensional
heat transfer analysis or physical testing is recommended to determine the U-factor of spandrel panels.”
See FYI, Estimating the U-factor for the Opaque Spandrel Portion of a Curtain Wall, for an example of
applying this information reprinted from ASHRAE Handbook—Fundamentals.
Example 5-H. U-Factor Calculation, Steel-Framed Wall, Effective R-Value Method
Corresponding section: Above-Grade Wall Insulation (5.5.3.2)
Q
What is the U-factor of the steel-framed wall represented in the following sketch? The wall has stucco,
R-7 (R-1.23) rigid insulation, an insulation/framing layer, and interior gypsum board. The metal
framing is 8 in. (200 mm) deep and is spaced at 24 in. (600 mm) o.c. Insulation with an R-value of R-25
(R-4.40) is installed in the cavity. (Hint: use the effective R-values from Table A9.2-2).
A
Section A9 of the standard discusses requirements for determination of alternate assembly U-factors,
C-factors, F-factors, or heat capacities. Per Section A9.2(b)(3), the parallel path calculation method is
used as shown below. (This is the easiest option, but it can only be used for assemblies with metal
framing if the effective R-value of the insulation/framing layer is from Table A9.2-2. Otherwise, the
modified zone method must be used.) The thermal resistance of each layer of the construction
assembly is listed, including the insulation/framing layer. The effective R-value of the
insulation/framing layer is R-9.6 (R-1.7) from Table A9.2-2. This is added to the thermal resistance of
the other layers.
Layer
Exterior air film
Stucco
Continuous rigid insulation
0.625 in. (16 mm) gypsum board
Insulation/framing layer
0.625 in. (16 mm) gypsum board
Interior air film
Total
U-factor
R-value
0.17 (0.03)
0.08 (0.01)
7.00 (1.23)
0.56 (0.01)
9.6 (1.7)
0.56 (0.01)
0.68 (0.12)
18.65 (3.29)
0.054 (0.304)
Standard 90.1 User’s Manual
Source of Data
Standard 90.1 (Section A9.4.1)
Standard 90.1 (Table A9.4.4-1)
Manufacturer’s data
Standard 90.1 (Table A9.4.4-1)
Standard 90.1 (Table A9.2-2)
Standard 90.1 (Table A9.4.4-1)
Standard 90.1 (Section A9.4.1)
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FIGURE 5-R. WALL, WOOD-FRAMED AND OTHER
Corresponding sections: Above-Grade Wall Insulation (5.5.3.2 and A3)
FYI
Estimating the U-factor for the Opaque Spandrel Portion of a Curtain Wall
The following excerpted from ASHRAE Handbook—Fundamentals, Chapter 15.
Example 3. Estimate the overall average U-factor for the opaque portion of a multifloor curtain wall
assembly that is part vision glass and part opaque spandrel. The typical floor-to-floor height is 12 ft
(3.6 m), and the building module is 4 ft (1.2 m) as reflected in the spacing of the mullions both
horizontally and vertically. For a representative section 4 ft (1.2 m) wide and 12 ft (3.6 m) tall, one of
the modules is glazed and the other two are opaque. The mullions are aluminum frame with a thermal
break 3 in. (80 mm) wide and centered on the module. Assume that the frame U-factor is determined
in accordance with NFRC procedures and is 1.75 Btu/h·ft²·°F (9.94 W/[m²·K]). The glazing unit is
double glazing with a pyrolytic low-e coating (e = 0.40) and has a 1/2 in. (13 mm) gap filled with air
and a metal spacer. The spandrel panel has a metal pan backed by R-20 (R = 3.5 [m²·K]/W) insulation
and no intermediate reinforcing members. Per the ASHRAE research project cited ASHRAE
Handbook—Fundamentals, assume that the edge-of-spandrel U-factor is 40% of the way from the
center-of-spandrel U-factor to the frame U-factor.
Solution: … Calculate center-of-glass, edge-of-glass, and frame areas. The total glazed area is 48 by 48
in. (1200 by 1200 mm). The frame is 3 in. (80 mm) wide, 1.5 in. (40 mm) on each edge, so the glazed
area is 45 by 45 in. (1120 by 1120 mm). The edge of glass is assumed to be 2.5 in. (130 mm) wide
along each edge, so the center of glass area is 40 by 40 in. (990 by 990 mm).
Acg = (48 – 3 – 5)(48 – 3 – 5) = 1,600 in.²
Aeg = [(48 – 3) × (48 – 3)] – 1,600 = 425 in.²
Af = (48 x 48) – – 1,600 – 425 = 279 in.²
Acg = (1,200 – 80 – 130)(1,200 – 80 – 130)/106 = 0.9801 m²
Aeg = [(1,200 – 80) × (1,200 – 80)]/106 – 0.9801 = 0.2743 m²
Af = [(1,200 × 1,200)/106 –– 0.9801 – 0.2743 = 0.1856 m²
…
Calculate the overall U-factor for the two opaque spandrel modules. The center-of-spandrel, edge-of
spandrel, and frame areas are the same as for the glazed module, shown above. In calculating the
center-of-spandrel U-factor, the R-value of the insulation does not need to be derated, because there
are no intermediate framing members penetrating it, avoiding thermal short circuits. When the
resistance of the insulation (20 ft²·°F·h/Btu [3.5 (m²·K)/W]) is added to the exterior air film resistance
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of 0.17 (0.03 [m²·K]/W) and the interior air film resistance of 0.68 ft²·°F·h/Btu (0.12 [m²·K]/W), the
total resistance is 20.85 ft²·°F·h/Btu (3.65 [m²·K]/W), and the U-factor is 1/20.85 = 0.05 Btu/h·ft²·°F
(1/3.65 = 0.274 W/[m²·K]). The edge-of-spandrel U-factor is 40% of the way to the frame U-factor, which
is 0.05 + [0.40 × (1.75 - 0.05)] = 0.73 Btu/h·ft²·°F (0.274 + [0.40(9.94 – 0.274)] = 4.14 W/[m²·K]). The
overall spandrel module U-factor is then determined using area weighting.
(0.05 ∙ 1600) + (0.73 ∙ 425) + (1.75 ∙ 279)
𝑈𝑈𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 =
(48 ∙ 48)
Btu
= 0.38
h∙ft 2 ∙°F
(0.274 ∙ 0.980) + (4.14 ∙ 0.2743) + (9.94 ∙ 0.1856)
(1.2 ∙ 1.2)
W
= 2.26 2
m ∙°C
Consequently, while most of the area of the opaque spandrel portion of the curtain wall is cavity filled
with R-20 (R-3.5) insulation, the overall U-factor for the opaque spandrel portion of the curtain wall is
U-0.38 (U-2.26), which is equivalent to continuous insulation of only R-1.78 (R-0.29) plus air films.
Thus, it is very important to carefully assess the potential thermal bridging in the opaque spandrel
portion of the curtain wall and then thoughtfully detail the assembly.
𝑈𝑈𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 =
Wood-Framed and Other Walls
This class of construction includes all wall constructions that do not qualify for one of the other wall
classifications. Examples of wall assemblies that fall under “Other” would be structural insulated
panels (SIPs) and cross-laminated timber (CLT) assemblies. Mainly, however, this class includes walls
constructed of wood framing members. Wood studs are generally spaced at either 16 or 24 in. (400 or
600 mm) o.c. An exterior sheathing is applied directly to the outer surface of the studs and an interior
finish is applied to the inner surface. Figure 5-R shows an example of a wall in this class.
R-Value Criteria
If a single R-value is specified, for instance R-13 (R-2.3), then insulation with at least this thermal
resistance must be installed in an uncompressed manner within the cavity formed by the wood studs.
You can also use continuous insulation, as this would perform better. When two R-values are specified,
for instance R-13 + R-7.5 c.i. (R-2.3 + R-1.3 c.i.), the second R-value must be installed as continuous
insulation. Usually this means that the insulation is a rigid board and is applied on the exterior of the
wall.
U-Factor Criteria
When using the U-factor criteria, you can take into account factors in the wall construction that are
significantly different from the assumptions underlying the R-value criteria. Table A3.4.3.1 in Appendix
A has precalculated U-factor data for wood-framed and other walls. This table is organized by the
wood stud spacing—either 16 or 24 in. o.c. (400 or 600 mm o.c.), and by the depth of the stud—either
3.5 or 5.5 in. (89 or 140 mm).
For the 5.5 in. (140 mm) stud depth case, there is also an option for insulated headers (+ R-10 [R-1.8]
headers). Headers are the horizontal supports over doors and windows. Normally these are
constructed of solid wood, which is more conductive than the insulated cavities. With the R-10 (R-1.8)
header option, the header is also insulated by sandwiching 2.5 in. (64 mm) of rigid insulation between
1.5 in. (38 mm) framing members.
Table A3.4.3.1 has data for insulation installed in the cavity and insulation installed in a continuous
manner and uninterrupted by the framing members. The continuous insulation can be installed on
either the interior or the exterior of the wall. You can select any combination of cavity and continuous
insulation, and the table provides the U-factor for the construction. Constructions in the table include
an exterior air film, stucco, exterior gypsum board, the insulation/framing layer, interior gypsum
board, and an interior air film. The calculations are performed using the parallel path calculation
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method. The percentage of the wall that is assumed to be insulated cavity, studs, and headers is shown
in Table 5-E.
For walls that are constructed with significant differences from the assumptions used to generate
Table A3.4.3.1 (as defined in Section A1.2), you can determine your own U-factor. The acceptable
procedures for wood-framed walls are specified in Section A9.2(b)(4) and include including laboratory
tests and the parallel path calculation method. These calculation options are described later in this
chapter.
Table 5-E. Framing Percentages for Wood-Framed Walls
Corresponding sections: Above-Grade Wall Insulation (5.5.3.2 and A3)
Insulated
Cavity
Studs
Headers
Standard framing—16
in. (400 mm) o.c.
75
21
4
Advanced framing with
insulated headers
78
18
4
78
18
4
Advanced framing—24
in. (600 mm) o.c.
With the parallel path calculation method, the wall is divided into subareas. For wood-framed walls,
the subareas are typically the insulated cavity, the portion that is solid wood studs, and the portion
that is a header (the horizontal members that span over doors and windows). Heat is assumed to flow
straight across the wall. The heat that passes through each subarea is directly proportional to the area
of that wall and its U-factor. The overall U-factor of the wall is the area-weighted average of the Ufactors through the subareas. Example 5-I shows how this calculation is performed.
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Example 5-I. U-Factor Calculation, Wood-Framed Wall, Parallel Path Calculation Method
Corresponding section: Above-Grade Wall Insulation (5.5.3.2)
Q
Determine the U-factor of the wood-framed wall represented in the following sketch.
A
First, check Appendix A to see if the U-factor for the proposed assembly has already been calculated. If
the assembly is listed in one of the tables in Appendix A, Section A1.1 requires that the U-factor be
taken from the Appendix A table. The U-factors for wood-framed wall assemblies are listed in Table
A3.4.3.1. However, the proposed wall has deeper wood framing than the assemblies listed in Table
A3.4.3.1. Consequently, it is necessary to use another methodology to determine the U-factor for the
assembly.
Section A9 of the standard provides requirements for determination of alternate assembly U-factors, Cfactors, F-factors, or heat capacities, discussed below. Section A9.2 specifies the required procedures
for determining U-factors for opaque assemblies not listed in the tables in Appendix A. Two- or threedimensional finite difference and finite volume computer models are always acceptable for opaque
assemblies. Section A9.2(b)(4) for wood-framed walls indicates that testing of the assembly is also
allowed but so is the parallel path calculation method. Of the options allowed for wood-framed walls,
the parallel path calculation method is typically the easiest.
With the parallel path calculation method, a wood-framed wall is divided into three parts: the portion
that is insulated cavity, the portion that is solid wood framing (the studs, plates and sills), and the
portion that is an insulated header (composed of wood framing with insulation between the headers).
Per Section 9.4, the framing factors in the standard must be used. The proposed assembly is the same
configuration as the advanced framing with insulated header assembly described in Section A3.4.1; the
only difference is the depth of the wood framing. Consequently, the framing factors for the advanced
framing with insulated header specified in Section A3.4.1 are to be used. The cavity is assumed to
represent 78% of the wall area, the stud/plat sill area 18%, and the headers 4%. These are the
assumptions that were used to generate the values in Table A3.4.3.1 and should be used when you
make your own calculations.
The next step is to make a list of all the different materials or layers through the wall, starting with the
exterior air film and continuing through to the interior air film. Some layers, such as the gypsum board,
are common to all subareas. Others, such as the cavity insulation, are unique to a particular subarea.
Per Section 9.4, the R-values for air films, air spaces, insulation, and building materials must be taken
from Sections A9.4.1 through A9.4.4, and assumptions must correspond with Sections A2 through A8.
Per the definition of rated R-value of insulation in Section 3.2, the thermal resistance for insulation
alone is to be as specified by the manufacturer at a mean temperature of 75°F (24°C). Note that some
manufacturers rate their insulation products at several different mean temperatures—e.g., rigid
insulation may also be rated at 40°F (4°C); however, the R-value rated at 75°F (24°C) is the one to be
used for demonstrating compliance with the standard. If the insulation is compressed, or is installed
between metal framing, then further adjustments are necessary, as specified in Section A9.4.3. The
thermal resistance of building materials must be taken from Table A9.4.3-1 of the standard for all
materials listed in the table. For other building materials, ASHRAE Handbook—Fundamentals or test
data are appropriate sources. Build a table with three columns, one for each of the three parallel heat
flow paths, as shown in Table 5-E. If a material does not apply, enter “n.a.”
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Air film, exterior surface
Cavity
0.17 (0.03)
Studs
0.17 (0.03)
Headers
0.17 (0.03)
Continuous rigid insulation
7.00 (1.23)
7.00 (1.23)
7.00 (1.23)
Stucco
Gypsum board, 0.625 in. (16 mm)
Cavity insulation
0.08 (0.01)
0.56 (0.10)
25.00 (4.40)
0.08 (0.01)
0.56 (0.10)
n.a.
0.08 (0.01)
0.56 (0.10)
n.a.
Wood studs/plates/sill,
8 in. (200 mm) nominal,
7.25 in. (184 mm) actual
Wood header, 2 in. (50 mm)
nominal, 1.5 in. (39 mm) actual
n.a.
9.06 (1.60)
n.a.
n.a.
n.a.
1.88 (0.34)
Rigid insulation installed between
headers, 4.25 in. (106 mm) actual
Wood header, 2 in. (50 mm)
nominal, 1.5 in. (39 mm) actual
n.a.
n.a.
17.50 (3.08)
Gypsum board, 0.625 in. (16 mm)
0.56 (0.10)
0.56 (0.10)
0.56 (0.10)
Total thermal resistance
U-factor
Weight
34.05 (5.99)
0.0294 (0.167)
78%
18.11 (3.19)
0.0552 (0.313)
18%
30.31(5.35)
0.0330 (0.187)
4%
Air film, interior vertical surface
n.a.
0.68 (0.12)
n.a.
0.68 (0.12)
1.88 (0.34)
0.68 (0.12)
Data Source
Standard 90.1
(Section A9.4.1)
Standard 90.1
(Table A9.4.4-1)
Manufacturer’s data,
rated at 75°F (24°C)
Standard 90.1
(Table A9.4.4-1)
Manufacturer’s data,
insulation not
compressed
Standard 90.1
(Table A9.4.4-1)
Derived from Standard
90.1 (Table A9.4.4-1),
wood at R-1.25/in.
(R-0.0087/mm)
Manufacturer’s data
rated at 75°F (24°C)
Derived from Standard
90.1 (Table A9.4.4-1),
wood at R-1.25/in.
(R-0.0087/mm)
Standard 90.1
(Table A9.4.4-1)
Standard 90.1
(Section A9.4.1)
The next step is to calculate the thermal resistance through each subarea of the wall. This is the sum of
each thermal resistance in each parallel heat flow path. The total thermal resistance is 34.05 (5.99)
through the cavity, 18.11 (3.19) through the studs/plates/sills, and 30.31 (5.35) through the header.
The U-factor through each subarea is the reciprocal of the total thermal resistance or 1 divided by the
total thermal resistance. Carrying an additional decimal place for this intermediate step in the
calculation only, the U-factor is 0.0294 (0.167) through the cavity, 0.0552 (0.314) through the studs,
and 0.0323 (0.187) through the header.
The final step is to perform an area-weighted average of the U-factors to determine the overall Ufactor. The overall U-factor is 0.034 (0.19) as calculated below.
UOverall = WCavity × UCavity + WStuds × UStuds + WHeader × UHeader
I-P:
UOverall = (0.78 × 0.0294) + (0.18 × 0.0552) + (0.04 × 0.0330)
UOverall = 0.034
SI:
UOverall = (0.78 × 0.167) + (0.18 × 0.313) + (0.04 × 0.187)
UOverall = 0.19
Below-Grade Wall Insulation (5.5.3.3 and A4)
Below-grade walls have conditioned or semiheated space on the inside and earth on the outside. Walls
below grade on a sloping site or basement walls are good examples. The criteria for below-grade walls
are given either as a minimum R-value for the insulation alone or as a maximum C-factor for the
overall assembly. A C-factor is like a U-factor except that it does not include the interior air film, the
exterior air film, or the effect of the earth. While the effects of air films and earth were included in
establishing the criteria, they have been removed to simplify compliance.
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R-Value Criteria
If the R-value method is used for below-grade walls, then insulation with the specified thermal
resistance must be installed in a continuous manner with no interruptions by framing members. If
framing members interrupt the insulation, then only the C-factor method can be used. Insulation for
below-grade walls is required in Climate Zones 4 through 8.
Often, the same wall may be partly below grade and partly above grade. When this is the case, and
when insulation is installed on the interior, the R-value requirement for the above-grade portion
applies to the entire wall per Section 5.5.3.2.
C-Factor Criteria
Table A4.2.1 of Appendix A contains C-factors for below-grade walls. The table has data for three
conditions:
1. Insulation that is continuous and uninterrupted by framing members of any kind. This can
be achieved by installing the continuous insulation either inside or outside the below-grade wall.
2. Insulation installed between steel framing members or studs that are spaced at 24 in. (600
mm) o.c. This will typically be achieved by furring the inside wall and installing insulation in the
cavity created by the steel studs.
3. 1-in. (25 mm) metal clips that are spaced at 24 in. (600 mm) o.c. horizontally and 16 in.
(400 mm) o.c. vertically. These are small clips no more than 1 in. (25 mm) in length used to
support the insulation and to attach the interior finish material (usually gypsum board). This
system performs better than standard steel studs because there is much less metal to provide a
thermal bridge past the insulation.
For each condition, Table A4.2.1 gives the C-factor for varying levels of insulation R-value. The C-factor
does not include the air films or the effect of the earth. The values in Table A4.2.1 are based on an 8 in.
(200 mm) solid grouted CMU wall; however, the C-factors in the table can be used for any below-grade
wall. For insulated walls, the thermal resistance of 0.5 in. (13 mm) thick gypsum board is also assumed
to be R-0.45 (R-0.08).
As an alternative to using Table A4.2.1, and if allowed by Section A1.2, C-factors can be calculated using
data from Tables A3.1-2, A3.1-3, and A3.1-4. The procedure is similar to that described for above-grade
mass walls. This procedure is a little more complicated than just finding values from Table A4.2.1, but
it provides considerable flexibility for a wide variety of walls.
• Table A3.1-2 has data for concrete walls with a thickness ranging from 3 to 12 in. (75 to 300
mm) and densities ranging from 20 to 144 lb/ft³ (320 to 2304 kg/m³). For each case, the table
provides a C-factor and total R-value (Rc) that excludes the air films and earth. Table A3.1-2 is
used with both above-grade mass walls and below-grade walls. For this reason, it has U-factors
and Ru for above-grade walls and C-factors and Rc for below-grade walls. Be careful which you use
in your calculations. The C-factor must be used directly for compliance if the below-grade wall
does not have exterior insulation, interior insulation, or interior furring.
• Table A3.1-3 has data for CMU walls with 6 in., 8 in., 10 in., and 12 in. (150 mm, 200 mm, 250
mm, and 300 mm) thicknesses and densities ranging from 85 to 135 lb/ft³ (1360 to 2160 kg/m³).
Data are also provided for five different treatments of the cells of the concrete blocks: solid
grouted, partially grouted with the cells empty, partially grouted with the cells insulated,
unreinforced with the cells empty, and unreinforced with the cells insulated. “Partially grouted”
means that cells are grouted no more than 32 in. (800 mm) o.c. vertically and 48 in. (1200 mm)
o.c. horizontally. As with Table A3.1-2, the C-factor must be used directly for compliance if the
wall does not have exterior insulation, interior insulation, or an interior furring space. The total Rvalue (Rc) is also provided, which excludes the air films and the soil.
• Table A3.1-4 has the effective R-value of insulation/framing layers that may be added to the
thermal resistance (Rc) of the concrete or CMU mass wall selected from Table A3.1-2 or A3.1-3.
Table A3.1-4 has data for R-values ranging from zero to R-25 (R-4.4). The table also has data for
metal framing, wood framing, and no framing (continuous insulation). The metal and wood
framing can have depths ranging from 0.5 to 5.5 in. (13 to 140 mm). Data from this table are
added to the Rc taken from either Table A3.1-2 or Table A3.1-3. The sum is the total thermal
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resistance (excluding air films and soil). The overall C-factor is the reciprocal of this total
resistance. A C-factor calculation is shown in Example 5-J.
Example 5-J. C-Factor Calculation, Below-Grade Wall
Corresponding section: Below-Grade Wall Insulation (5.5.3.3)
Q
What is the C-factor of a 12 in. (300 mm) solid grouted concrete masonry unit (CMU) wall with a block
density of 85 lb/ft³ (1360 kg/m³)? The wall has continuous exterior insulation with a thermal
resistance of R-10 (R-1.76) and interior furring with no insulation. The furring space is 1.5 in. (39 mm)
deep and the furring members are constructed of wood.
A
Starting from the outside, going through the assembly layer by layer. The thermal resistance of the
exterior continuous rigid insulation is R-10 (R-1.76) from the figure above.
The next step is to find the thermal resistance (Rc) of the CMU component of the wall in Table A3.1-3.
Identify the appropriate product size, density, and concrete block grouting and cell treatment, then
select the corresponding value shown for Rc. The total thermal resistance (Rc) for the CMU component
of the wall is 1.68 (0.30).
The last step is to find the additional thermal resistances for the interior framed component wall from
Table A3.1-4. Using the “0” column in Table A3.1-4 under “Rated R-Value of Insulation,” from the row
where “Depth” = 1.5 in. and “Framing Type” = wood, the effective R-value of the furring space and
gypsum board is found to be 1.3 (0.23). The overall thermal resistance is 12.98 (2.29) and the U-factor
is 0.077 (0.44). The details of the calculation are as follows:
𝑈𝑈 =
𝑈𝑈 =
1
𝑅𝑅𝐸𝐸𝐸𝐸𝐸𝐸 +𝑅𝑅𝐶𝐶 +𝑅𝑅𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹
1
𝑅𝑅𝐸𝐸𝐸𝐸𝐸𝐸 +𝑅𝑅𝐶𝐶 +𝑅𝑅𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹
=
=
1
10+1.68+1.3
1
=
1.76+0.30+0.22
1
12.98
=
= 0.077 (I-P)
1
2.29
= 0.44 (SI)
Floor Insulation (5.5.3.4 and A5)
The standard includes three classes of construction for floors: mass floors, floors supported by metal
joists, and wood-framed and other floors. The floor insulation requirements are expressed as either a
minimum R-value for the insulation alone or a maximum U-factor for the overall assembly, including
thermal bridges. Compliance can be achieved using either method.
Mass Floors
Mass floors are heavyweight floors, generally greater than 25 lb/ft² (122 kg/m²) of floor area. The
technical definition of a mass floor is that the heat capacity is greater than 7.0 Btu/ft²∙°F (143
kJ/m²∙°C) or greater than 5.0 Btu/ft²∙°F (102 kJ/m²∙°C) for lighter density mass materials weighing less
than 120 lb/ ft3 (1920 kg/ m3). Use Tables A3.1-2 and A3.1-3 to determine heat capacity (HC). You can
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also calculate HC yourself if the assembly is not adequately represented in those tables (see FYI,
Understanding Heat Capacity, earlier in this chapter).
R-Value Criteria
When using the R-value method, the insulation must be continuous and uninterrupted by framing
members. Insulation sprayed to the underside of a concrete slab qualifies as continuous as long as it
also covers structural supports such as steel beams and concrete girders that extend 24 in. (600 mm)
or less below the exposed floor. For deeper beams and girders, the surfaces that extend 24 in. (600
mm) or less below the exposed floor must be insulated (see Section A5.2.2.4). For waffle slabs, sprayapplied insulation must cover all surfaces of the waffle in order to be considered continuous. Another
method for providing continuous insulation is to place rigid insulation above the concrete slab. This
system will have better thermal performance, if the insulation is continuous and not interrupted by
columns. Also, this minimizes thermal bridging to interior courtyards or adjacent unconditioned space.
In this case, a thin concrete topping slab or a plywood layer is also usually provided for attachment of
the interior finish floor.
U-Factor Criteria
When the insulation is not continuous, then the U-factor method must be used. Table A5.2.3.1 has Ufactors for mass floors. The table takes account of continuous insulation, spray-applied insulation, and
pinned batt insulation. In all cases, the insulation is assumed continuous; this is a restriction on the use
of this table. Development of the data in Table A5.2.3.1 assumes an R-0.92 (R-0.16) inside film
resistance, R-1.23 (R-0.22) carpet and rubber pad, 8 in. (200 mm) of concrete with an R-value of R-0.50
(R-0.09), and an R-0.46 (R-0.08) semiexterior air film. Insulation specified in the table is added to these
base thermal resistances.
Table A5.2.3.1 must not be used if framing members of any kind interrupt the continuity of the mass
floor insulation. For these types of floor systems, you can calculate your own U-factor but must use
advanced calculation techniques. Per Section A9.2(c)(1), the U-factor must be determined using
laboratory tests, two-dimensional heat transfer analysis, or the parallel path calculation method if the
concrete is solid and uniform, or by using the isothermal planes calculation method if concrete has
hollow sections. These calculation methods are described later in this chapter, in the Acceptable
Calculation Methods section. Example 5-K shows how the U-factor is determined for a concrete floor
on steel supports.
Example 5-K. U-Factor Calculation, Concrete Floor on Steel Supports
Corresponding section: Floor Insulation (5.5.3.4)
Q
What is the U-factor of the mass floor over a parking garage represented in the following sketch? The
top of the floor is covered with a carpet and pad. The structural portion of the floor consists of an 8 in.
(200 mm) reinforced concrete slab with a density of 144 lb/ft³ (2304 kg/m³), supported by steel joists
located at 48 in. (1200 mm) o.c. The underside of the floor is insulated with R-11 (R-1.94) sprayapplied insulation.
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A
First, check in Appendix A to see if the U-factor for the proposed assembly has already been calculated.
If the assembly is listed in one of the tables in Appendix A, then Section A1.1 requires that the U-factor
be taken from the appropriate table in Appendix A. The U-factors for mass floor assemblies are listed
in Table A5.2.3.1. However, the proposed floor has spray-applied insulation penetrated by metal joists.
Section A5.2.3.2 states that Table A5.2.3.1 cannot be used for any assemblies where the insulation is
not continuous. Consequently, it is necessary to use another methodology to determine the U-factor for
the assembly.
Section A9 of the standard provides requirements for determination of alternate assembly U-factors, Cfactors, F-factors, or heat capacities, discussed below. Section A9.2 specifies that the parallel path
calculation method can be used if the concrete is solid. As Table 9.2-1 provides insulation/framing
layer R-values for the insulation layer penetrated by metal joists, there is really only one path, not
multiple parallel paths. Consequently, this becomes a series calculation.
The R-values for the air films are from Section A9.4.1. Per Section A9.4.1.2, the semiexterior air film is
used for the bottom of the assembly because it is located over a parking garage. The R-values for the
carpet and pad, and for the concrete floor are from Table A9.4.4-1. The R-value for the
insulation/framing layer is from Table A9.2-1. The U-factor determined from this method is 0.076
(0.43) as shown below.
Layer
Air film, interior horizontal surfaces, heat flow down
Carpet and pad
8.0 in. (200 mm) concrete, density 144 lb/ft³ (2304 kg/m³)
Insulation/framing layer with rated R-11 (R-1.94) insulation
between metal joists at 4 ft (1,200 mm) o.c.
Air film, semiexterior surface
Total R-value
U-factor
R-Value
0.92 (0.17)
1.23 (0.22)
0.50 (0.09)
10.01 (1.76)
0.46 (0.08)
13.12 (2.32)
0.076 (0.43)
Data Source
Standard 90.1 (Section A9.4.1)
Standard 90.1 (Table A9.4.4-1)
Standard 90.1 (Table A9.4.4-1)
Standard 90.1 (Table A9.2-1)
Standard 90.1 (Section A9.4.1)
Steel-Joist Floors
Steel-joist floors include any floor that is supported by steel bar joists or purlins except those that
qualify as a mass floor. If the floor has a heat capacity (HC) large enough to qualify as a mass floor, then
the mass floor class of construction must be used, even if metal joists support the mass floor. By
definition then, a steel-joist floor has a HC less than 7.0 Btu/ft²∙°F (143 kJ/m²∙°C) if constructed of
normal-weight concrete, or less than 5 Btu/ft²·°F (102 kJ/m²∙°C), provided that the floor has a material
unit mass not greater than 120 lb/ft3 (1920 kg/m3). This limits the thickness of normal-weight
concrete in a steel-joist floor to approximately 2.5 in. (64 mm), although the thickness can be greater
for very low density concrete.
R-Value Criteria
The steel joists that support the floor can be either open web joists or steel purlins. The key
characteristic is that metal framing members interrupt the insulation. When a single R-value is given in
the criteria tables in Section 5.5, this means insulation with no less than this thermal resistance may be
installed between the joists and is therefore interrupted by the steel joists. Insulation installed in a
continuous manner is also acceptable, as is spray-applied insulation, but these do not allow for a
reduction in required R-value.
U-Factor Criteria
When using the U-factor method, select data from Table A5.3.3.1 or, if allowed by Section A1.2,
calculate your own U-factor using methods defined in Appendix A. Table A5.3.3.1 may be used with any
type of steel-joist floor; however, the values are based on an R 0.92 (R-0.16) interior air film, an R-1.23
(R-0.22) carpet and pad, and an R-0.46 (R-0.08) semiexterior air film. The thermal resistance of the
assumed metal deck and concrete topping is ignored.
The table has assembly U-factors for steel-joist floors with insulation sprayed on to the bottom surface
of the deck and for insulating batts pinned or otherwise fastened to the underside of the deck.
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Continuous insulation can be added in addition to one of these options. When calculating the U-factor
for assemblies substantially different from the base assembly (see Section A1.2), you are allowed to
use the parallel path method, isothermal planes, or laboratory testing. (See Determination of Alternate
Assembly U-Factors, C-Factors, F-Factors, or Heat Capacities [A9] later in this chapter.) Example 5-L
shows how the U-factor is determined for a steel-joist floor.
Example 5-L. U-Factor Calculation, Steel-Joist Floor
Corresponding section: Floor Insulation (5.5.3.4)
Q
What is the U-factor of the steel-joist floor construction represented in the following sketch and
located over a parking garage? The construction has a carpet and pad, 2 in. (50 mm) of lightweight
concrete with a density of 85 lb/ft³ (1360 kg/m3), a metal deck, and spray-applied insulation having a
rated R-value of R-11 (R-1.94).
A
First, while the floor has steel joists, the proposed assembly contains some concrete, so it is necessary
to assess whether or not the floor is considered a mass floor. The definition of a mass floor has two
different categories. The floor in this example, with a density of 85 lb/ft³ (1360 kg/m3), falls into the
second category, which limits the heat capacity (HC) to less than 5 Btu/ft²·°F (102 kJ/m²·°C) where the
floor has a material unit mass not greater than 120 lb/ft3 (1920 kg/m3).
Table A3.1-2 contains the HC for a range of thicknesses for different densities of concrete, but it does
not contain an entry for 2 in. (50 mm) of concrete. However, there is an entry for 4 in. (100 mm) of
concrete, which has an HC of 5.7 (116) for concrete with a density of 85 lb/ft³ (1360 kg/m3).
Consequently, a 2 in. (50 mm) layer of this concrete has a heat capacity half that of a 4 in. (100 mm)
layer. Thus, the heat capacity for the proposed floor in this example is 2.9 (58) for 2 in. (50 mm) of
concrete with a density of 85 lb/ft³ (1360 kg/m3). As the proposed floor has a heat capacity less than 5
(102), the floor in this example is not a mass floor. Therefore, the proposed floor is in the steel-joist
floor class of construction.
Having determined that this is a steel-joist floor, check Appendix A to see if the U-factor for the
proposed assembly has already been calculated. If the assembly is listed in one of the tables in
Appendix A, Section A1.1 requires that the U-factor be taken from the appropriate table in Appendix A.
The U-factors for steel-joist floor assemblies are listed in Table A5.3.3.1. Assemblies with spray-applied
insulation are listed in the table, but this specific insulation R-value is not listed. As Table A5.3.3.1 does
not contain R-11 (R-1.94) spray-applied insulation, it is necessary to interpolate, which is allowed by
Section A1.1. In Table A5.3.3.1, the U-factor for R-8 (R-1.41) spray-applied insulation is 0.096 (0.54)
and the U-factor for R-12 (R-2.11) spray-applied insulation is 0.073 (0.41). Both U-factors are from the
column for the base floor assembly, as there is no continuous insulation. Interpolation for sprayapplied insulation rated R-11 (R-1.94) results in a U-factor of 0.079 (0.44). Consequently, this U-factor
must be used for the assembly because interpolation is allowed.
Note, however, that Section A1.1 does not allow extrapolation beyond the assemblies listed in the
tables in Appendix A. Thus, if the spray-applied insulation for this steel-joist floor assembly had a rated
R-value greater than R-24 (R-4.23), Table 5.3.3.1 could not be used. The calculation below provides an
example of the methodology to be used in such a case.
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This example is very similar to the previous Example 5-K. As with Example 5-K, the series calculation
method can also be used with the effective R-values from Table A9.2-1. The basis for all of the R-values
is the same as for Example 5-K except that the concrete R-value from Table A9.4.4-1 is for 2 in. (50
mm) of concrete rather than 8 in. (200 mm) of concrete. The U-factor determined from this method is
0.078 (0.44) as shown below.
Layer
Air film, interior horizontal surface, heat flow down
Carpet and pad
2 in. (50 mm) lightweight concrete, density 85 lb/ft³ (1,360 kg/m³)
Insulation/framing layer, rated R-11 (R-1.94) insulation
between metal joists at 4 ft (1,200 mm) o.c.
Air film, semiexterior surface
Total R-value
U-factor
R-value
0.92 (0.17)
1.23 (0.23)
0.13 (0.02)
10.01 (1.76)
0.46 (0.08)
12.75 (2.25)
0.078 (0.44)
Data Source
Section A9.4.1
Table A9.4.4-1
Table A9.4.4-1
Table A9.2-1
Section A9.4.1
Wood-Framed and Other Floors
Wood-framed and other floors include any floor that does not qualify as a mass floor or a steel-joist
floor. If the floor has a heat capacity (HC) large enough to qualify it as a mass floor, then the mass floor
class of construction must be used, even if wood joists support the mass floor. By definition then, a
wood-framed floor has an HC less than 7.0 Btu/ft²∙°F (143 kJ/m²∙°C) if constructed of a material with a
unit mass greater than or equal to 120 lb/ft3 (1920 kg/m3), or less than 5 Btu/ft²·°F (102 kJ/m²∙°C),
provided that the floor has a material unit mass not greater than 120 lb/ft3 (1920 kg/m3).
R-Value Criteria
The wood joists that support the floor can be either solid or open-web joists. The key characteristic is
that wood framing members interrupt the insulation. When a single R-value is given in the criteria
tables, this means that insulation with this minimum thermal resistance is allowed to be installed in
the cavity between the joists and is therefore interrupted by the wood joists. Insulation installed in a
continuous manner is also acceptable.
U-Factor Criteria
When using the U-factor method, select data from Table A5.4.3.1, or, if allowed by Section A1.2,
calculate your own U-factor using methods defined in Appendix A. Table A5.4.3.1 may be used with any
type of wood-framed floor; however, the values are based on an R-0.92 (R-0.16) interior air film, an R1.23 (R-0.22) carpet and pad, R-0.94 (R-0.17) for 0.75 in. (19 mm) wood subfloor, and an R-0.46 (R0.08) semiexterior air film. The framing fractions are 91% for the insulated cavity and 9% for the wood
joists.
The table has assembly U-factors for wood-framed floors with various depths of floor joists.
Continuous insulation can be added in addition to one of these options. When calculating the U-factor
for assemblies substantially different from the base assembly (see Section A1.2), you are allowed to
use the parallel path method, isothermal planes, or laboratory testing. (See Determination of Alternate
Assembly U-Factors, C-Factors, F-Factors, or Heat Capacities [A9] later in this chapter.) Example 5-M
shows how the U-factor is determined for a wood-framed floor.
Example 5-M. U-Factor Calculation, Wood-Framed Floor
Corresponding section: Floor Insulation (5.5.3.4)
Q
What is the U-factor of the wood-framed floor represented in the following sketch and located over a
crawlspace? The floor has a carpet and pad, 0.75 in. (19 mm) wood subfloor, 2 × 14 in. (50 × 350 mm)
nominal wood joists (1.5 × 13.25 in. [39 × 337 mm] actual) at 12 in. (300 mm) o.c., with R-49 (R-8.6)
high-density insulation in the cavity between the joists, extending from the top to the bottom of the
wood floor joists.
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A
First, check Appendix A to see if the U-factor for the proposed assembly has already been calculated. If
the assembly is listed in one of the tables in Appendix A, Section A1.1 requires that the U-factor be
taken from the appropriate table in Appendix A. The U-factors for wood-framed floor assemblies are
listed in Table A5.4.3.1. However, the proposed floor has more insulation than the assemblies listed in
Table A5.4.3.1. Consequently, it is necessary to use another methodology to determine the U-factor for
the assembly.
Section A9 of the standard provides requirements for determination of alternate assembly U-factors, Cfactors, F-factors, or heat capacities, discussed below. Section A9.2 specifies the required procedures
for determining U-factors for opaque assemblies not listed in the tables in Appendix A. Two- or threedimensional finite difference and finite volume computer models are always acceptable for opaque
assemblies. Section A9.2(d)(3) for wood-framed floors indicates that testing of the assembly is also
allowed but so is the parallel path calculation or the isothermal planes calculation method. Of the
options allowed for wood-framed floors, the parallel path calculation method is the easiest.
With the parallel path calculation method, a wood-framed floor is divided into two parts: the portion
that is insulated cavity and the portion that is wood framing. Per Section 9.4, the framing factors in the
standard must be used. Per Section A5.4.1, the cavity is assumed to represent 91% of the floor area,
and the framing represents the remaining 9%. These are the assumptions that were used to generate
the values in Table A5.4.3.1 and are to be used when you make your own calculations.
The next step is to make a list of all the different materials or layers through the floor starting with the
interior air film and extending all the way through the exterior air film. Some layers, such as the
gypsum board, are common to all subareas. Others, such as the cavity insulation, are unique to a
particular subarea. Note that in this particular example, the insulation in the wood joist floor assembly
is exposed to the crawlspace. The question states that there is R-49 (R-8.6) high-density insulation in
the cavity between the joists, which extends all the way from the top of the wood floor joists to the
bottom of the wood floor joists. Consequently, the heat flow path for the framing extends to the full
depth of the wood joist, i.e., 13.25 in. (337 mm). However, if the insulation only partly fills the cavity,
then credit for the heat flow path through the framing can only be claimed for the same depth as the
insulation—e.g., if the insulation only extended 9.25 in. (235 mm), then credit for the wood joist in the
heat flow path through the framing must be calculated for only 9.25 in. (235 mm) of the wood joist.
Per Section 9.4, the R-values for air films, airspaces, insulation, and building materials must be taken
from Sections A9.4.1 through A9.4.4, and assumptions must correspond with Sections A2 through A8.
The thermal resistance of air films must be taken from Section A9.4.1. The thermal resistance for
insulation is to be determined as specified in Section A9.4.3. Per the definition of rated R-value of
insulation in Section 3.2, the thermal resistance for insulation alone is to be as specified by the
manufacturer at a mean temperature of 75°F (24°C). Note that some manufacturers rate their
insulation products at several different mean temperatures—e.g., rigid insulation may also be rated at
40°F (4°C); however, the R-value rated at 75°F (24°C) is the one to be used for demonstrating
compliance with the standard. If the insulation is compressed, or is installed between metal framing,
then further adjustments are necessary, as specified in Section A9.4.3. The thermal resistance of
building materials must be taken from Table A9.4.4-1 of the standard for all materials listed in Table
A9.4.4-1. For other building materials, ASHRAE Handbook—Fundamentals or test data are appropriate
Standard 90.1 User’s Manual
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sources. Build a table with two columns, one for each of the two parallel heat flow paths, as shown in
Table 5-E. If a material does not apply, enter “n.a.”
Calculate the thermal resistance through each subarea of the floor. This is the sum of each thermal
resistance in each parallel path to heat flow. The total thermal resistance is 52.55 (9.24) through the
cavity and 20.11 (3.56) through framing. The U-factor through each subarea is the reciprocal of the
total thermal resistance, or one divided by the total thermal resistance. Carrying an additional decimal
place for this intermediate step in the calculation only, the U-factor is 0.0190 (0.108) through the
cavity and 0.0497 (0.281) through the framing.
The final step is to perform an area-weighted average of the U-factors to determine the overall Ufactor. The overall U-factor is 0.022 (0.12) as calculated below.
UOverall = (WCavity × UCavity) + (WFraming × UFraming)
I-P:
UOverall = (0.91 × 0.0190) + (0.09 × 0.0497)
UOverall = 0.022
SI:
UOverall = (0.91 × 0.108) + (0.09 × 0.281)
UOverall = 0.12
Layer
Air film, interior horizontal surface,
heat flow down
Carpet and pad
Wood subfloor,
0.75 in. (19 mm)
Cavity insulation, high density, to full
depth of cavity
Wood joists. 14 in. (350 mm)
nominal,
13.25 in. (337 mm) actual
Air film, semiexterior surface
Total R-value
U-factor
Weighting
Weighted average
Cavity
0.92 (0.17)
Framing
0.92 (0.17)
Data Source
Standard 90.1 Section A9.4.1
49 (8.6)
n.a.
Manufacturer’s data,
insulation not compressed
Standard 90.1 Table A9.4.4-1
1.23 (0.22)
0.94 (0.17)
n.a.
0.46 (0.08)
52.55 (9.24)
0.0190 (0.108)
91%
0.022 (0.12)
1.23 (0.22)
0.94 (0.17)
Standard 90.1 Table A9.4.4-1
Standard 90.1 Table A9.4.4-1
16.56 (2.92)
0.46 (0.08)
20.11 (3.56)
0.0497 (0.281)
9%
Standard 90.1 Section A9.4.1
Slab-on-Grade Floor Insulation (5.5.3.5 and A.6)
Slab-on-grade floors are defined as being in direct contact with the earth and either above grade or less
than or equal to 24 in. (600 mm) below the final elevation of the nearest exterior grade. They are
generally made of concrete and can have several edge conditions. The standard includes two classes of
construction for slab-on-grade floors: heated and unheated. Heated slab-on-grade floors have hotwater pipes or heating coils embedded within the slab or located beneath the slab to provide space
heating. Heat losses from heated slab-on-grade floors are greater because of the higher temperature of
the slab. As a result, perimeter insulation is required for heated slab-on-grade floors in all climate
zones, and insulation under the entire slab is required for residential spaces in Climate Zone 8. For
unheated slab-on-grade floors, perimeter insulation is required in Climate Zones 3 through 8 for
residential spaces and in Climate Zones 4 through 8 for nonresidential spaces.
The R-value specification gives both the R-value of the insulation and the depth or width of the
insulation. An example is R-20 for 48 in. (R-3.5 for 1200 mm). This means that insulation with a
thermal resistance of 10 (3.5) must be installed and that the insulation must extend a distance of 48 in.
(1200 mm) starting from the top surface of the slab.
Per Section A6.2.2, if the insulation is installed on the inside surface of the concrete foundation wall,
the insulation must extend downward from the top of the slab the distance specified or to the top of
the foundation, whichever is less. Per Section A6.2.3, if the insulation is installed outside the
foundation wall, it must extend from the top of the slab directly downward for the full distance, or at
least down to the bottom of the slab and then horizontally until the specified distance is achieved. Any
horizontal insulation extending outside of the foundation shall be covered by pavement or by soil a
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minimum of 10 in. (250 mm) thick. For monolithic slab-on-grade floor and footing, the insulation must
extend downward from the top of the slab only to the bottom of the footing or the distance specified,
whichever is less.
Figure 5-S gives examples of acceptable and unacceptable installations of slab-on-grade insulation. The
insulation locations shown in the first, second, and third graphics are allowed in the Prescriptive
Building Envelope Option. Installing the insulation vertically on either the interior or exterior of the
foundation wall or footing has the greatest thermal benefit because this results in a longer heat flow
path from the slab-on-grade floor to the surface of the ground on the exterior.
The insulation locations shown in the fourth, fifth, and sixth graphics in Figure 5-S are not allowed in
the Prescriptive Building Envelope Option. Installing the insulation horizontally below the perimeter of
the slab, as shown in the fourth graphic, does not thermally isolate the edge of the floor slab from the
outdoor temperatures. In this case, the heat flows directly through the edge of the slab to the outdoors.
The horizontal insulation provides very little benefit as it does not block the primary heat flow path.
Installing a little additional vertical insulation on the exterior of the slab-on-grade, as shown in the fifth
graphic, does decrease the heat transfer at the edge of the floor slab somewhat, but there is still a
major thermal bridge due to the discontinuity between the interior and exterior insulation. The sixth
graphic in Figure 5-S provides proper direction in terms of detailing the slab edge to prevent thermal
bridging through the foundation perimeter. However, because this detail is sometimes overlooked in
the field, it is not allowed in the Prescriptive Building Envelope Option.
Insulation Inside—Permitted
Insulation Outside—Permitted
Monolithic Slab—Permitted
Insulation Beneath Slab—
Not permitted
Insulation Beneath Slab—
Not permitted
Insulation Beneath Slab—
Not permitted
FIGURE 5-S. SLAB-ON-GRADE INSTALLATIONS
Corresponding section: Slab-on-Grade Floor Insulation (5.5.3.5 and A.6)
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Table A6.3.1 of Appendix A has F-factors for various combinations of insulation R-value and insulation
depths and configurations. Using this table in conjunction with the F-factor criteria is a flexible way of
meeting the requirements. Heat loss through concrete slabs is complex, and the only method to
determine F-factors is to use the data in Table A6.3.1.
As can be seen by comparing the F-factors in Table 6.3.1, the heat loss for the best horizontal insulation
is higher than the heat loss for the worst vertical insulation. Consequently, if the slab-on-grade floor
insulation does not have the vertical position required to comply with the Prescriptive Building
Envelope Option, it will be necessary to use another compliance option (such as the Building Envelope
Trade-Off Option in Section 5.6) to make up for the additional heat loss when slab-on-grade floor
insulation is installed in the horizontal position.
Opaque Doors (5.5.3.6 and A7)
Opaque doors include operable opening components of the building envelope: both those installed
vertically, such as swinging doors, roll-up doors, and fire doors, as well as those installed horizontally,
such as opaque smoke vents and access hatches.
The criteria for opaque doors are expressed only as maximum U-factors. Section 5.8.2.3 of the standard
specifies NFRC ratings for doors in the same way that it does for fenestration. NFRC Standard 100
applies to doors as well as windows and other fenestration products. An exception to Section 5.8.2.3
allows the use of DASMA Standard 105 for sectional garage doors and metal coiling doors. When doors
have NFRC or DASMA ratings, those U-factors must be used for compliance. For unlabeled doors,
Section A7 in Appendix A prescribes the U-factors to use. These are summarized in Table 5-F of this
user’s manual. Opaque smoke vents and access hatches must use the appropriate U-factors in Table 5F, which vary depending on whether the smoke vents and access hatches are single-layer uninsulated,
double-layer uninsulated, or double-layer insulated.
TABLE 5-F. U-FACTORS FOR UNLABELED DOORS
Corresponding section: Opaque Doors (5.5.3.6 and A7)
U-factor
Construction Description
Btu/h∙ft²∙°F
W/m²∙°C
(a) Uninsulated single-layer metal swinging doors or nonswinging doors, including singlelayer uninsulated access hatches and uninsulated smoke vents
1.45
8.2
(c) Uninsulated double-layer metal swinging doors or nonswinging doors, including doublelayer uninsulated access hatches and uninsulated smoke vents
0.70
(b) Insulated double-layer metal coiling doors
(d) Insulated metal swinging doors, including fire-rated doors, insulated access hatches,
insulated smoke vents, and other insulated metal non-swinging doors
(e) Wood doors, minimum nominal thickness of 1 3/4 in. (44 mm), including panel doors with
minimum panel thickness of 1 1/8 in. (28 mm), and solid core flush doors, and hollow core
flush doors
(f) Any other wood door
1.00
5.68
0.50
2.8
0.50
0.60
4.0
2.8
3.4
Determination of Alternate Assembly U-factors, C-factors, F-factors, or
Heat Capacities (Section A9)
Per Section A1.1, in most cases, the default tables in Appendix A are to be used to determine U-factors,
F-factors, C-factors, and other figures of merit. Section A1.2 contains criteria for a building official to
determine if a proposed construction assembly is adequately represented. This determination is
related to whether the base assembly type is the same as that in Appendix A. If the building materials
differ significantly from those described in Sections A2 through A8, it is necessary to calculate the Ufactor using Section A9 of the standard, which specifies acceptable calculation methods. In particular,
Section A9 calculations are required if building materials added to the base assembly increase or
decrease the R-value by more than 2 from that indicated in the descriptions in Sections A2 through A8.
Changes such as adding a layer of plywood or siding, or even doubling-up the layers of gypsum board
on both sides of a wall, have a minor impact on R-value and do not meet this threshold.
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These are related to the classes of construction for opaque assemblies that are identified in the
standard, although in some cases a class is expanded. Table 5-G of this user’s manual shows the
required procedures for each class of construction.
Note, however, that per Section A9.1 these alternate procedures in Section A9 are only applicable to
opaque assemblies, and not all of the opaque assemblies at that. Thus, U-factors for opaque doors must
be determined in accordance with Section 5.5.3.6 or Section A7 only. U-factors for fenestration must be
determined in accordance with Section 5.8.2.3 or Section A8 only.
Testing
Laboratory tests are the most accurate way to determine the U-factor of a construction assembly and
are acceptable for all types of construction except slab-on-grade floors. In these tests, an 8 × 8 ft (2.4 ×
2.4 m) sample of the construction assembly is placed in a test unit. For steady-state measurements, the
temperatures on either side of the wall are held constant until temperatures within the construction
have stabilized; then the rate of heat flow is measured. Heat flow is typically measured by metering the
heat energy required to maintain the temperature on the warm side of the assembly.
The biggest advantage of laboratory testing is that it produces equally good results for just about any
type of construction assembly. The major disadvantage is that it is costly and time consuming. There
are a large variety of possible construction assemblies, and it is impractical to test them all. For this
reason, it is usually more cost-effective to use calculation methods if allowed. Laboratory
measurements must use one of the following test procedures for determining building material Rvalues or thermal conductivities:
• Guarded hot plate (ASTM C-177)
• Heat flowmeter (ASTM C-518)
• Hot-box apparatus (ASTM C-1363)
For determining assembly U-factors, the only the following test procedure is allowed:
• Hot-box apparatus (ASTM C-1363)
Series Calculation Method
The series calculation method is the easiest way of calculating the U-factor. However, its use is limited
to constructions that have no framing and are made of homogenous materials. In reality, few
construction assemblies meet these strict requirements. With the series calculation method, the
thermal resistance of each layer in the construction assembly is determined. Section A9.4.1 and Tables
A9.2-1, A9.2-2, A9.4.2-1, A9.4.3, and A9.4.4-1 of Appendix A have data on the thermal resistance of
materials that can be used in the calculations. Test data may be used for materials not listed in
Appendix A. The total thermal resistance is the sum of individual resistances, and the U-factor is the
reciprocal of the total resistance. In Equation 5-A, R1 and R4 are the air film resistances, while R2 and
R3 are the resistances of the two materials in the construction.
U=
1
Equation 5-A,
Series Calculation
R1 + R 2 + R 3 + R 4
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TABLE 5-G. REQUIRED PROCEDURES FOR DETERMINING ALTERNATIVE U-, C-, AND F-FACTORS FOR
OPAQUE ASSEMBLIES
Corresponding section: Determination of Alternate Assembly U-factors, C-factors, F-factors, or Heat Capacities (A9)
Acceptable Calculation Methods
Class of Construction
Testing
per
Section
A9.3
Series
Calculation
Method







Parallel Path
Calculation
Method
Isothermal
Planes
Calculation
Method
Modified
Zone
Calculation
Method
Two- or Three
Dimensional Finite
Difference and
Finite Volume
Computer Models
Roofs
Insulation entirely above deck
Metal building
Attic (wood joists)
Attic (steel joists)
Attic (concrete joists)
Other
Walls, Above-Grade
Mass
Metal building
Steel-framed
Wood-framed
Other
Wall, Below-Grade
Mass
Other
Floors
Mass
Steel-joist
Wood-framed
Other
Slab-On-Grade Floors





(4)
 (2)

(1)
 (2)

(5)

(1)








 (3)

(2)

 (3)















Unheated
Heated
Notes:
1. Must use the insulation/framing layer adjustment factors from Tables A9.2-1 or A9.2-2 of Appendix A.
2. Use only if concrete is solid and uniform.
3. Use if the concrete has hollow sections.
4. Use only for single-layer and double-layer systems.
5. Use only for single-layer compressed, single-layer in cavity, double-layer systems, and continuous insulation.






Parallel Path Calculation Method
The parallel path calculation method is a simple extension of the series calculation method that can be
used for wood-framed assemblies. Essentially, a series calculation method is performed twice, once for
the cavity portion of the surface (roof, wall, or floor) and once for the framing portion of the wall
(Equation 5-B). In some cases, it may be necessary to divide a surface into more than two parts (for
instance, see Example 5-I). The U-factor is calculated for each subarea (U1 and U2 in the equations)
and weighted according to surface area. The W1 and W2 terms in the equations are weightings for
each subarea. The sum of all weightings must equal 1.
With the parallel path method, the temperature of the outdoor air (TOut) and the inside air (TIn) are the
same for each path; however, the surface temperatures may be different through each path. To put it
another way, the outside wall temperature will be warmer near framing members on a cold day. These
temperature differences can be detected by infrared photography, which is a useful tool for finding
thermal bridges in construction facilities.
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U1 =
U2 =
1
R1 + R 2 + R 3 + R 4 + R 5
Equation 5-B,
Parallel Path
Calculation
1
R1 + R 2 + R 6 + R 4 + R 5
U = U1 ⋅ W1 + U 2 ⋅ W2
Isothermal Planes Calculation Method
The isothermal planes calculation method uses principles similar to the series and parallel path
calculation methods, except the temperature through one or more planes in the construction assembly
is assumed constant (iso is the Greek word for equal). The isothermal planes method is appropriate for
walls made of concrete or concrete masonry units (CMUs) where high material conductance causes
equal (or near equal) temperatures across one or more planes in the construction assembly. In the
network diagram accompanying Equation 5-C, the temperature across the R3 and R6 thermal
resistances is assumed equal. A parallel path calculation method can be performed to determine the
effective R-value through the R3 and R6.
In Equation 5-D, the effective R-value across resistances R3 and R6 is calculated using the parallel path
method. However, for many construction types, such as steel-framed walls, the parallel path method is
inappropriate and must not be used. For steel-framed constructions, the overall U-factor can be
determined through laboratory tests, then the effective R-value can be calculated as shown below. This
procedure is the basis of the effective R-values published in Tables A9.2-1 and A9.2-2 of Appendix A.
Using these effective R-values is really a variation on the isothermal planes method covered in
Equation 5-C.
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1




1
+R +R
R1 + R 2 + 
4
5
  +   
 W1  1  W1  1  
 R6  
  R3 
1
U=
R1 + R 2 + R Effective + R 4 + R 5
U=
U=
Equation 5-C,
Isothermal
Planes
Calculation
Equation 5-D
1
R1 + R 2 + R Effective + R 4 + R 5
R1 + R 2 + R Effective + R 4 + R 5 =
R Effective =
1
U
1
− (R1 + R 2 + R 4 + R 5 )
U
Modified Zone Calculation Method
The modified zone method can be used with roof, floor, and wall constructions that have metal
framing. The method may be used when roofs, walls, or floors are not adequately addressed in Tables
A9.2-1 or A9.2-2. The method is documented in ASHRAE Handbook—Fundamentals. It involves dividing
the construction assembly into zones. Heat flow in the zone near the metal framing is directed toward
the framing, and the thermal resistance is smaller.
Section A9.4.6 Calculation Method
The Section A9.4.6 calculation method is only applicable to metal buildings. For metal building roofs, it
is only to be used for single-layer and double-layer systems. For metal building walls, it is only to be
used for single-layer compressed, single-layer in cavity, double-layer systems, and continuous
insulation. Sections A9.4.6.1 through A9.4.6.3 provide calculations of overall assembly U-factors for
single-layer roof and single-layer compressed wall assemblies, double-layer roof assemblies and
single-layer in cavity and double-layer walls, respectively. The calculations are quite detailed and are
based on construction geometry and the properties of insulation and thermal spacers used in the
construction. The user is also advised that the calculation methods in A9.4.6 are calibrated to give
appropriate results for girt spacing of 60 in. only. Calculations for this girt or purlin spacing can then be
used conservatively for larger spacings. For smaller spacings, it is advisable to consider use of test data
or an acceptable 2-D or 3-D computational method (see discussion below).
Two- or Three-Dimensional Finite Difference and Finite Volume Computer Models
Two- or three-dimensional heat flow analysis may be used to accurately predict the U-factor of a
complex construction assembly. While the series and parallel path calculation methods assume that
heat flows in a straight line from the warm side of the construction to the cooler side, with two-
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dimensional models, heat can also flow laterally in the construction, following the path of least
resistance. Calculating two-dimensional heat flow involves advanced mathematics and is best
performed with a computer.
To use the method, you divide the construction into a large number of small pieces and define the
thermal resistance between each piece. The result is analyzed with electric circuit theory. The network
consists of a rectangular array of nodes connected by resistances. As in the real material, the energy
flow will take the path of least resistance. The computer can perform the complicated calculations
necessary to solve the network, yielding the U-factor for the unit at steady state. It can also solve the
network for dynamic energy conditions.
Short of performing laboratory tests, this is the most accurate method available for determining the Ufactors of concrete and masonry walls. Three-dimensional heat flow analysis follows the same process,
except that the thermal grid extends in three dimensions rather than just two.
Fenestration (5.5.4)
The fenestration design criteria apply to fenestration, including windows, glass doors, glass block,
plastic panels, and skylights. The prescriptive criteria limit the fenestration area to a maximum of 40%
of the gross wall area and the skylight area to a maximum of 3% (or 6% if certain daylighting
requirements are met as specified in Section 5.5.4.4.2) of the gross roof area. However, for some
spaces, a minimum skylight area is also required. For both vertical fenestration (windows, glass doors,
glass block, and plastic panels) and skylights (glass and plastic panels) there are two performance
requirements, a maximum U-factor, and a maximum solar heat gain coefficient (SHGC). In spaces
required to have automatic daylighting controls, there is also a requirement for a minimum ratio of
visible transmittance (VT) divided by SHGC.
Vertical Fenestration Classes of Construction
There are four classes of construction for vertical fenestration, which are based on frame material and
operator type: all nonmetal-framed products; metal-framed products, which are fixed in place and do
not move (including both fixed-unit windows as well as curtain walls and storefronts); metal-framed
products that are operable (including sliding windows, awning and casement windows, as well as
sliding glass doors that are not entrance doors); and metal-framed products that are used as entrance
doors. The standard has separate U-factor criteria for these classes of construction for vertical
fenestration, but the SHGC and VT/SHGC criteria are the same for all classes of construction for vertical
fenestration.
For determining compliance, there are some additional classifications, however. These include the
following:
• Labeled Fenestration. This subclass includes all fenestration products that have an NFRC rating.
Such products are required to be labeled or to be listed on a signed label certificate. Information
on the label certificate includes the U-factor, SHGC, VT, and other data. For this subclass,
fenestration performance data used for compliance with the standard must be taken from the
label, certificate, or NFRC rating.
• Unlabeled Vertical Fenestration (Section A8.2). This subclass includes all fenestration
products that do not have NFRC ratings. Compliance data for this subclass must be taken from
Table A8.2 of Appendix A.
Skylights
The standard has U-factor and SHGC criteria for skylights, but all skylights are included in one class of
construction, regardless of frame material or operator type. For demonstrating compliance, there are
additional classifications similar to vertical fenestration:
• Labeled Fenestration. All skylights with NFRC ratings are required to be labeled with those
values or to be listed on a signed certificate.
Unlabeled Skylights (Section A8.1). For unlabeled skylights, U-factors must be taken from Table A8.1-1
of Appendix A; overall product SHGC values must be taken from Table A8.1-2 of Appendix A or from
manufacturers’ shading coefficient (SC) or SHGC data for the center of the glass, provided that the data
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are established using a spectral data file determined in accordance with NFRC 300. If manufacturers’
SC data are used, convert to SHGC by multiplying the SC by 0.86.
Fenestration Area (5.5.4.2)
Vertical Fenestration Area (5.5.4.2.1)
The prescriptive requirements allow vertical fenestration areas up to 40% of the gross wall area. This
limit applies separately to each space conditioning category in the building (i.e., nonresidential
conditioned space, residential conditioned space, and semiheated space). However, the standard
provides the opportunity for additional glazing for nonresidential spaces on the street side of the
street-level story only (see Vertical Fenestration on the Street Side of the Street-Level Story in
Nonresidential Spaces below). In addition, there is an orientation requirement for vertical fenestration
in Section 5.5.4.5. Buildings that have vertical fenestration areas greater than 40% of the gross wall
area must use either the Building Envelope Trade-Off Option or the Energy Cost Budget (ECB) Method.
Example 5-N illustrates how to determine the gross wall area.
FYI
Fenestration and Energy Use
“Fenestration” refers to the light-transmitting areas of a wall or roof, mainly windows and skylights
but also glass doors, glass block walls, and translucent plastic panels. Depending on the area, heat
losses and gains through fenestration can be very significant and are carefully addressed by the
standard.
Controlling solar gains through fenestration and maximizing daylighting can significantly affect energy
use in buildings. Solar gains through windows add to cooling loads in the summer and during other
times when the building is air conditioned. On cold days, solar gains can also offset heating loads,
although this may or may not be a significant benefit in nonresidential buildings, depending on
building type and whether high internal heat gains reduce the hours heating is needed when the
building is occupied.
The more significant benefit of sunlight is daylighting. Light is solar radiation in the visible spectrum,
with a wavelength between about 380 and 770 nanometers. With the right type of electric lighting
system and controls, daylight can be a significant benefit. The ideal fenestration would allow light to
enter the building but block solar radiation outside of the visible spectrum (in the ultraviolet and near
infrared part of the solar spectrum). Residential buildings and nonresidential buildings with lower
internal heat gains, on the other hand, can benefit from passive solar gains, depending on the climate
and building design.
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Example 5-N. Determining Gross Wall Area
Cha p t e r 5 | Bu il d i ng E n vel o p e
Corresponding section: Fenestration Area (5.5.4.2)
Q
A two-story building in Nashville, Tennessee, is sited on a sloping site such that the first floor of the
north wall is below grade. The first floor of the east and west walls are partially below grade, as the
ground slopes. The building is rectangular in shape with a 200 ft (61 m) dimension in the east-west
direction and a 100 ft (30.5 m) dimension in the north-south direction. The floor-to-floor height is 12 ft
(3.7 m).
What is the gross wall area for this building? This is significant because the maximum allowable
window area requirement is based on the gross wall area.
A1
The gross wall area includes both above-grade walls and below-grade walls. The gross wall area is
simply the perimeter of the building times the building height. For the two stories, the building height
totals 24 ft (7.3 m). In I-P units the gross wall area is (200 + 100 + 200 + 100 ft) × 24 ft, or 14,400 ft². In
SI units, it is (61 + 30.5 + 61 + 30.5 m) × 7.3 m, or 1336 m².
FYI
Fenestration Area Terminology
Vertical Fenestration Area
The maximum allowed vertical fenestration area when using the Prescriptive Building Envelope
Option for compliance is based on a percentage of the gross wall area. The fenestration area is the area
of the rough opening, including the frame, sash, and other nonglazed window components.
Fenestration area does not include opaque spandrel area, which is considered opaque wall even if the
surfacing material is glass. The gross wall area is measured horizontally from the exterior surface; it is
measured vertically from the top of the floor to the bottom of the roof. The gross wall area includes
below-grade as well as above-grade walls.
It is necessary to calculate the vertical fenestration area as a percentage of the gross exterior wall area
with most compliance options, because this information is needed with the Prescriptive Building
Envelope Option (Section 5.5), the Building Envelope Trade-off Option (Section 5.6), and the Energy
Cost Budget (ECB) Method (Section 11). For the Performance Rating Method (Appendix G),
fenestration areas are entered directly and must be consistent with the design documents.
The gross wall area includes all surfaces that are vertical or tilted at an angle of 60 degrees from
horizontal or greater. Sloping glazing falls in the vertical category if it has a slope equal to or more than
60 degrees from the horizontal. If it slopes less than 60 degrees from the horizontal, the fenestration
falls in the skylight category (see Figure 5-T). This means that clerestories, roof monitors, and other
such fenestration fall in the vertical category.
Skylight Area
Skylights are fenestration with a slope less than 60 degrees from the horizontal (see figure below). The
maximum allowed skylight area in the Prescriptive Building Envelope Option is based on a percentage
of the gross roof area. The skylight area is the rough opening and includes the frame, sash, and other
components of the skylight. The gross roof area is measured to the outside surface of the roof. The roof
area is measured along the surface that encloses the conditioned space.
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For a flat roof and flat ceiling, the roof area is the same as shown in plan view. For an attic with a
pitched roof over a flat ceiling enclosing conditioned space, the roof area is again the same as shown in
plan view. However, for sloped ceilings or vaulted ceilings, roofs are measured along the slope, as
opposed to the projection onto a horizontal plane that would show on a floor plan.
Maximum Skylight Fenestration Area (5.5.4.2.2)
When using the Prescriptive Building Envelope Option for compliance, the skylight area is generally
limited to not exceed 3% of the gross roof area. This limit applies separately to each space conditioning
category in the building (i.e., nonresidential conditioned space, residential conditioned space, and
semiheated space). However, the skylight area is allowed to be increased to 6% of the gross roof area,
provided certain daylighting requirements are met. These requirements include skylights satisfying
the minimum haze (diffusion) value and visible light transmittance, inclusion of automatic daylight
controls, and a daylight area under the skylight that covers at least half the floor area of the space.
Buildings that have a skylight area greater than these limits must use the Building Envelope Trade-Off
Option, the ECB Method, or the Performance Rating Method.
Minimum Skylight Fenestration Area (5.5.4.2.3)
Some enclosed spaces may require skylights when the following conditions are met:
• The floor area of the enclosed space is greater than 2500 ft² (232 m²).
• The space is directly under a roof and has a ceiling height greater than 15 ft (4.6 m).
• The space is an office, lobby, atrium, concourse, corridor, nonrefrigerated warehouse or storage,
gymnasium/exercise center, gymnasium seating, playing area, convention center, automotive
space, manufacturing, retail, library reading and stack area, distribution/sorting area,
transportation facility, or workshop.
• The building is located in Climate Zones 1 through 5 (exception).
• The skylights would not be significantly shaded by neighboring buildings or other obstructions
(exception).
The standard provides multiple exceptions to the above requirements. Exceptions exist for cold
climates, shaded roof areas, and the following:
• When an enclosed space uses clerestories or roof monitors for toplighting in lieu of skylights, the
daylighted area from these sources must exceed 50% of the total floor area.
• Enclosed spaces that are almost entirely sidelighted may be exempt from the minimum skylight
area. This exemption applies when 2500 ft² (232 m²) or less of an enclosed space remains after
subtracting the total sidelighted area from the enclosed space floor area.
This requirement applies to unconditioned spaces as well as conditioned and semiheated spaces.
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When skylights are required, they must be appropriately sized and spaced such that they create a
daylight area that is at least half the area of the enclosed space. The daylight area created under a
skylight is equal to the horizontal projection of the bottom of the skylight well, plus a distance equal to
70% of the floor-to-ceiling height. See Example 5-O. Note that vertical opaque obstacles that fall within
the daylight area may reduce the daylight area. The definition in Section 3 of the standard for daylight
area under skylights limits the height of obstacles within the daylight area. The closer the object is to
the edge of the skylight, the taller it may be without cutting off the daylight area. The further away the
obstacle is from the edge of the skylight, the shorter it must be until the obstacle is outside of the
daylight area. Refer to Figure 3.2-2 in the standard.
The minimum skylight area can be determined in one of two ways:
• The skylight area is greater than 3% of the gross roof area and has a visible transmittance (VT)
greater than or equal to 0.40.
• The skylight effective aperture is at least 1%. The skylight effective aperture is defined in the
standard as 85% of the product of the skylight area, the VT, and the well factor (WF), divided by
the daylight area.
Additionally, automatic daylighting controls must be used in the daylight area, in accordance with
Section 9.4.1.1 of the standard.
Examples 5-O through 5-R address minimum skylight requirements.
Example 5-O. Daylighted Area under One Skylight
Corresponding section: Minimum Skylight Fenestration Area (5.5.4.2.3)
Q
A 4 × 8 ft (1.2 × 2.4 m) skylight with a straight light well is located in a space with a 20 ft (6.1 m)
ceiling. Without considering the impact of adjacent skylights or sidelighted areas, what is the
daylighted area that is created?
A
The band around the horizontal projection of the skylight is 70% of the 20 ft (6.1 m) ceiling height, or
14 ft (4.3 m). The dimension of the daylighted area in the long direction of the skylight (the skylight
well dimension) is 8 ft (2.4 m) plus two times 14 ft (4.3 m), or 36 ft (11 m). The dimension in the short
direction of the skylight is 4 ft (1.2 m) plus two times 14 ft (4.3 m), or 32 ft (9.8 m). The overall
dimensions of the daylighted area created are therefore 36 × 32 ft (11 × 9.8 m), or 1152 ft² (108 m²).
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Example 5-P. Minimum Skylight Requirements, Simple Warehouse I
Corresponding section: Minimum Skylight Fenestration Area (5.5.4.2.3)
Q
A simple warehouse measuring 120 × 120 ft (37 × 37 m) with a 20 ft (6 m) ceiling height is shown in
the following illustration. Sixteen skylights with a rough framed opening of 4 × 8 ft (1.2 × 2.4 m) are
spaced at 30 ft (9 m) centers, as shown. The skylights are made of double dome acrylic with a prismatic
pattern and are mounted on curbs (see detail below). The visible light transmission of the skylight
glazing is 0.45. The building’s walls are 6 in. (150 mm) thick, and the building has no vertical
fenestration.
Ignoring possible obstructions on the interior of the building, what is the daylighted area under these
skylights, and does the design comply with Section 5.5.4.2.3?
A
Yes, the design complies.
Before consideration of the building’s walls, each skylight creates a 36 × 32 ft (11 × 9.8 m) daylighted
area. The daylighted areas created by each skylight overlap because the 30 ft (9 m) skylight spacing is
smaller than either the 36 ft (11 m) or 32 ft (9.8 m) dimension. The figure below shows the
overlapping daylighted areas. Because the daylighted areas also extend beyond the walls, the entire
120 × 120 ft (36.6 × 36.6 m) space is daylighted. The building’s walls are 6 in. (150 mm) thick, and the
daylighted area extends only to the surface of these walls, so the daylighted area is 119 × 119 ft (36.3 ×
36.3 m), or 14,161 ft² (1318 m²).
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The minimum skylight area must be either 3% of the daylighted area when the visible light
transmission of the skylight is greater than 40%, which is the case in this example, or the skylight
effective aperture must be at least 1% (see Section 5.5.4.2.3). The total skylight area is 16 × 4 × 8 ft =
512 ft² (16 × 1.2 × 2.4 m = 46.1 m²). This represents 3.6% of the 14,161 ft² (1318 m²) daylighted area,
which is greater than 3%, so the first requirement is met.
Testing for the 1% skylight effective aperture criteria is a bit more complicated. The skylight effective
aperture accounts for the depth of the skylight well, the area of the skylights, the daylighted area, and
the visible light transmission of the skylights. It is given by the following equation, where the well
factor (WF) is 0.70:
𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑡𝑡 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 =
=
0.85 × 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑡𝑡 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 × 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝐿𝐿𝐿𝐿𝐿𝐿ℎ𝑡𝑡 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 × 𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹
𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷ℎ𝑡𝑡𝑡𝑡𝑡𝑡 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑈𝑈𝑈𝑈𝑈𝑈𝑈𝑈𝑈𝑈 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑡𝑡𝑡𝑡
0.85 × 512 × 0.45 × 0.7 0.85 × 46.1 × 0.45 × 0.7
=
= 0.0097 = 0.97%
14,161
1318
The skylights fail to meet the second test (0.97% is less than 1%), but because they pass the first, the
building complies. The WF is typically calculated as a function of the length, width, and depth of the
skylight and the reflectance of the skylight. However, the standard takes a simple approach and
specifies that the WF must be 0.7 for skylight wells with a depth of 2 ft (0.6 m) or greater and 0.9 for
skylight wells that have a depth less than 2 ft (0.6 m). In this example, the depth is exactly 2 ft (0.6 m),
so the lower WF applies.
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Example 5-Q. Minimum Skylight Requirements, Simple Warehouse II
Corresponding section: Minimum Skylight Fenestration Area (5.5.4.2.3)
Q
A design modification in Example 5-P requires that the center four skylights be removed so that
equipment can be installed on the roof. The interior of the space is still unobstructed. Does the building
still meet the minimum skylight requirements of Section 5.5.4.2.3?
A
Yes.
First it is necessary to calculate the area that is not daylight area under skylights. As the skylights are
spaced at 30 ft (9.1 m) centers from Example 5-P, the center of the skylights around the perimeter is
15 ft (4.6 m) from the outside edge of the building’s walls. As a result, the distance between the center
of the skylights on opposite walls is 90 ft (27.4 m). As the dimension of the daylight area under
skylights in the long direction of the skylight is 36 ft (11 m) from Example 5-P, and half of that
dimension extends toward the center of the building from either side, the dimension of the area in the
center without daylight is 90 ft (27.4 m) minus 18 ft (5.5 m) minus 18 ft (5.5 m) equals 54 ft (16.4 m).
Correspondingly, the dimension in the short direction of the skylight is 32 ft (9.8 m) from Example 5-P,
and half of that dimension extends toward the center of the building from either side, the dimension of
the area in the center without daylight is 90 ft (27.4 m) minus 16 ft (4.9 m) minus 16 ft (4.9 m) equals
58 ft (17.6 m). Thus, the portion of the building that is not daylighted measures 54 × 58 ft (16.4 ×
17.6 m), as shown below, for a total floor area of 3132 ft² (289 m²).
The area that is still daylighted is 14,161 ft² from Example 5-P, less 3132 ft², or 11,029 ft² (1318 m²
from Example 5-P, less 289 m², or 1029 m²). This is more than half of the enclosed space, so the
building still meets the requirement of Section 5.5.4.2.3 that the daylighted area must be a minimum of
half the area of the enclosed space.
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Example 5-R. Minimum Skylight Requirements, Simple Warehouse III
Corresponding section: Minimum Skylight Fenestration Area (5.5.4.2.3)
Q
Assume that the building described in Example 5-Q is used as a warehouse for heavy automobile parts.
Each storage rack is 10 ft (3 m) wide and 16 ft (4.9 m) high. They are spaced 10 ft (3 m) apart, as
shown below, to enable forklift access to both sides of the storage racks. As a result, there are four
aisles plus a perimeter area.
What is the daylighted area for this situation, and does the design comply with the requirements of
Section 5.5.4.2.3?
A
The daylight area under skylights includes the perimeter area around the outside of the storage racks
plus the two center aisles. The two outer aisles (of the four total) are not considered daylighted space
because light (assumed to be at an angle of 35 degrees from plumb) strikes the top of the storage racks
and does not make its way to these two aisles.
The daylight area under skylights for the perimeter area is the gross interior area minus the square
area in the center defined by the edges of the storage racks. The storage area in the center is 90 × 90 ft
(27.4 × 27.4 m) equaling an area of 8100 ft² (751 m²). Thus, the perimeter area is 14,161 ft² from
Example 5-Q, less 8100 ft², for a net area of 6061 ft² (1318 m² from Example 5-Q, less 751 m², or
567 m²).
The daylight area under skylights for the two center aisles is 10 × 90 ft (3 × 27.4 m) per aisle times 2
aisles, equaling an area of 1800 ft² (164 m²). Therefore, the daylight area under skylights for this
design is 6061 ft² (567 m²) for the perimeter area, plus 1800 ft² (164 m²) for the two center aisles,
yielding a total of 7861 ft² (731 m²).
Consequently, the daylight area under skylights is 7861 ft² (731 m²), divided by 14,161 ft² (1318 m²)
from Example 5-Q, giving a daylight fraction of 55% of the enclosed space; therefore, the space meets
the requirements. This includes the perimeter around the storage racks and the two central aisles.
Again, the two outermost of the four aisles are not daylighted because the 35 degree path of light
intersects the racks before it reaches the aisles.
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Fenestration U-Factor (5.5.4.3)
Vertical fenestration has four classes of construction that are based on frame material and operator
type: nonmetal framed products; metal framed products, which are fixed in place and do not move
(including fixed-unit windows as well as curtain walls and storefronts); metal-framed products that
are operable (including sliding windows, awning and casement windows, as well as sliding glass doors
that are not entrance doors); and metal-framed products that are used as entrance doors. Each of these
four vertical fenestration classes of construction have separate U-factor criteria to account for product
differences in each class and how each class is rated. The SHGC and VT/SHGC criteria, however, are the
same for all vertical fenestration classes of construction.
Skylights have only one class of construction.
Vertical Fenestration U-Factor
The U-factor of the fenestration depends on the class of construction (framing and operator type). For
the proposed design, the U-factor must be determined in accordance with NFRC rating procedures (see
Mandatory Provisions [5.4 and 5.8] earlier in this chapter). For products with NFRC ratings, those Ufactors that are provided on the NFRC label or label certificate must be used. For unlabeled windows,
the default values in Table A8.2 of Appendix A must be used.
When a building has more than one type of vertical fenestration, it is not necessary for every one to
meet the U-factor criteria. An area-weighted average calculation can be performed. To show
compliance with the standard, the area-weighted average U-factor must be less than or equal to the
criteria. (See FYI, Area-Weighted Averages earlier in this chapter.) This area-weighted average U-factor
calculation must be based on the NFRC ratings for the standard reference size at the standard rating
conditions.
Note that while the NFRC ratings are provided at a standard reference size as part of the standard
rating conditions for comparative purposes and compliance with Standard 90.1, the actual
performance in the building will vary based on the ratio of glass to frame area and the climate of the
building site. For other purposes, such as HVAC sizing, some designers may find it to be worth the
effort to determine project-specific values based on actual product sizes and local conditions.
Skylight U-Factor
Like vertical fenestration, maximum U-factor requirements are provided based on space-conditioning
category. Skylights, however, have only one class of construction, regardless of frame material or
operator type. If NFRC ratings are available for the skylight, then the NFRC U-factor must be used. For
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unlabeled skylights, take the U-factor of the proposed design from Table A8.1-1 of Appendix A of the
standard.
Fenestration Solar Heat Gain Coefficient (SHGC) (5.5.4.4)
SHGC of Vertical Fenestration (5.5.4.4.1)
For the proposed design, the Solar Heat Gain Coefficient (SHGC) is to be determined in accordance with
NFRC rating procedures by a laboratory accredited by NFRC or a similar organization. For products
with NFRC ratings, the NFRC-rated SHGC must be used. For unlabeled products, the values in Table
A8.2 of Appendix A must be used.
Exception 1 to Section 5.8.2.4 also allows the shading coefficient of the center of the glass multiplied by
0.86 to be an acceptable alternative to SHGC if the shading coefficient has been determined using a
spectral data file determined in accordance with NRFC 300. See the Mandatory Provisions (5.4 and 5.8)
section earlier in this chapter for details. In addition, Exception 2 permits the SHGC for the center-ofglass to be used for compliance calculations.
Example 5-S illustrates how to calculate an area-weighted average SHGC. Example 5-T addresses the
application of U-factor, SHGC, and VT criteria for fenestration.
Example 5-S. SHGC, Office Tower with Lower-Level Retail
Corresponding section: SHGC of Vertical Fenestration (5.5.4.4.1)
Q
What is the area-weighted average vertical fenestration SHGC for a 15-story rectangular building that
has two floors of retail at the ground level and 16 stories of office above?
Each retail story has 500 ft² (50 m²) of fenestration on the south side, 850 ft² (85 m²) on the east side,
and none on the other two sides. Each office floor has 400 ft² (40 m²) on both the north and south
sides and 480 ft² (48 m²) on the east and west sides. All the fenestration is double-glazed with a low-e
coating. The clear low-e on the retail stories has an SHGC of 0.60, while the SHGC is 0.30 for the tinted
low-e on the office floors.
A
For the prescriptive option, calculate an area-weighted average SHGC for all fenestration.
I-P:
SHGCoverall = {[(500 + 850) × 0.60 × 2 stories]
+ [(400 + 480 + 400 + 480) × 0.30 × 16 stories]}/{[(500 + 850) × 2 stories]
+ [(400 + 480 + 400 + 480) × 16 stories]} = (1620 + 8448)/(2700 + 28,160) = 0.33
SI:
SHGCoverall = {[(50 + 85) × 0.60 × 2 stories]
+ [(40 + 48 + 40 + 48) × 0.30 × 16 stories]}/{[(50 + 85) × 2 stories]
+ [(40 + 48 + 40 + 48) × 16 stories]} = (162 + 844)/(270 + 2816) = 0.33
Note: When using the Building Envelope Trade-Off Option, there is no need to calculate the area-weighted
average SHGC. Just enter each window separately into your chosen program.
Example 5-T. Prescriptive Building Envelope Option, Seattle Waterfront Restaurant
Corresponding sections: Fenestration U-factor (5.5.4.3) and SHGC of Vertical Fenestration (5.5.4.4.1)
Q
A restaurant is being designed for a location in Seattle, Washington, that has good views across Puget
Sound to the Olympic Mountains. The wood-framed building will be insulated to comply with the
standard. The schematic design has the west facade almost entirely glazed, but there aren’t many
windows in the other walls, so the overall fenestration area is 37% of the gross exterior wall area. The
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picture windows are wood framed and double glazed with a low-e coating on the third surface. The
windows are manufactured locally and are NFRC rated with a U-factor of 0.52, an SHGC of 0.55, and a
VT of 0.56. The glass is clear and there are no overhangs.
Will this comply with the standard, or are modifications necessary?
A
The windows do not comply as designed. Seattle is in King County, and Table B-1 in Reference
Standard Reproduction Annex 1 specifies that King County is in Climate Zone 4. Therefore, the building
envelope criteria set for Seattle is Table 5.5-4. For the nonresidential space category (a restaurant
belongs to this category), the vertical fenestration criteria call for a U-factor of 0.31 (1.76) maximum
for a nonmetal frame. The maximum SHGC is 0.36 for all orientations. A VT/SHGC ratio of 1.10
minimum is required because the space must have automatic daylighting controls in accordance with
Section 9.4.1.4. In addition, Section 5.5.4.5 requires that the area of the west-oriented fenestration be
less than one-quarter of the total vertical fenestration area.
The building fails to comply with the U-factor criteria, the SHGC criteria, and the VT/SHGC criteria in
Table 5.5-4. Additionally, the distribution of the fenestration fails to comply with Section 5.5.4.5. The
designer has a couple of choices for compliance. Another fenestration product can be selected that has
a U-factor less than 0.31 (1.76) and an SHGC less than 0.36 while maintaining a high VT. Some westfacing fenestration will also need to be relocated to the north or south facade. In lieu of selecting an
alternate fenestration product with a lower SHGC, sun shades consisting of fixed louvers with a
projection factor (PF) greater than 0.50 could be added, as the SHGC-0.55 multiplied by the 0.61 factor
for this projection factor in Table 5.5.4.4.1 would achieve an effective SHGC of 0.34. Alternatively, the
Energy Cost Budget Method in Section 11 or the Performance Rating Method in Appendix G may be
used if there are other elements of the building, for instance the lighting system, that improve on the
standard’s requirements.
Example 5-U. U-Factor Criteria for Vertical Fenestration in Extremely Hot Climates
Corresponding sections: Fenestration U-factor (5.5.4.3)
Q
A high-tech development center is being constructed in Singapore. What are the U-factor criteria for
the vertical fenestration?
A
Per Table A-6 in Reference Standard Reproduction Annex 1, Singapore is located in Climate Zone 0.
Table 5.5-0 addresses the building envelope criteria for Climate Zone 0.
Consequently, the maximum allowed U-factors for vertical fenestration are: U-0.32 (U-1.82) for
nonmetal framing, U-0.50 (U-2.84) for metal framing fixed, U-0.65 (U-3.69) for metal framing operable,
and U-0.83 (U-4.71) for metal framing entrance doors.
 TABLE 5-H. SHGC MULTIPLIERS FOR PERMANENT PROJECTIONS
Corresponding section: SHGC of Vertical Fenestration (5.5.4.4.1)
(This is Table 5.5.4.4.1 in the Standard)
92
Projection Factor
0 – 0.10
<0.10 – 0.20
<0.20 – 0.30
<0.30 – 0.40
<0.40 – 0.50
<0.50 – 0.60
<0.60 – 0.70
<0.70 – 0.80
<0.80 – 0.90
<0.90 – 1.00
SHGC Multiplier (South, East and West Orientations)
1.00
0.91
0.82
0.74
0.67
0.61
0.56
0.51
0.47
0.44
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 Opaque Permanent Projections (Exception 1 to 5.5.4.4.1)
Overhangs and architectural shading features such as sun shades can reduce solar gains through
vertical fenestration such as windows and sliding glass doors opening on to balconies. The standard
allows credit for overhangs and other architectural shading features that provide significant and
permanent shading. In order for credits to be applied, overhangs must be a permanent part of the
building. The standard credits overhangs by allowing an adjustment to the SHGC when overhangs or
other architectural shading features exist. See Table 5-H (Table 5.5.4.4.1 in the standard) for these
SHGC multipliers. Examples 5-V through 5-Y address opaque and louvered overhangs. Because solar
exposure on the north side of a building is inherently smaller than on other orientations, horizontal
permanent projections over north-facing fenestration have limited impact on total building energy use.
(This is for the northern hemisphere. For the southern hemisphere, reverse references to north and
south.)
When trying to achieve a certain SHGC multiplier, the needed size of the overhang is determined by the
projection factor (PF), which is the ratio of the overhang projection to the distance from the windowsill
to the bottom of the overhang. The overhang projection is measured from the surface of the glass to
the outer edge of the overhang. See Figure 5-T and Example 5-V.
Neither the Prescriptive Building Envelope Option nor the Building Envelope Trade-Off Option gives
additional credit to overhangs with PFs greater than 1.00. An overhang with a PF of 1.00 has a
projection equal to the distance from the windowsill to the bottom of the overhang.
FIGURE 5-T. OVERHANG PROJECTION FACTOR
Corresponding section: SHGC of Vertical Fenestration (5.5.4.4.1)
When fenestration products have different SHGC values, or different overhang conditions exist, it is
necessary to calculate an adjusted effective SHGC separately for each fenestration product. If each
fenestration product does not comply separately, then it is acceptable to determine an area-weighted
average if using the Prescriptive Building Envelope Option. The weighting is based on the fenestration
area that is shaded.
The designer should be aware that the projection factor reduction in radiation applies to both solar
radiation and to visible light transmittance. This means that although overhangs provide beneficial
reductions in solar gain and glare, they may also reduce useful daylight if not sized appropriately.
Full credit is offered through Exception 1 for louvered sunshades and overhangs only when the louvers
are angled and spaced such that no direct sunlight passes through the louvers at solar noon on the
summer solstice (June 21 in the northern hemisphere). Partial credit is offered through Exception 2 for
partially opaque (louvered and translucent) sunshades and overhangs that do not satisfy the criteria to
use Exception 1. Sun angle calculator tools are available on the Internet to determine the position of
the sun at solar noon and at other times.
A concept useful in making this determination is the profile angle. The profile angle is the angle
between the window or surface normal and the altitude angle of the sun. It is necessary to determine
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the profile angle in order to determine if a louvered overhang qualifies for credit under either the
prescriptive requirements or the Building Envelope Trade-Off Option.
The profile angle depends on the orientation of the window as well as the altitude and azimuth of the
sun. For south-facing windows, the altitude of the sun at noon is equal to the profile angle. For other
window orientations, the profile angle can be determined using the Sun Angle Calculator, reference
tables, or other appropriate tools.
Partially Opaque Permanent Projections (Exception 2 to 5.5.4.4.1)
Conventional overhangs are solid. However, the standard also offers credit for louvered overhangs and
overhangs made of transparent or translucent materials such as tinted or fretted glass.
Credit for translucent overhangs is determined based on the average opacity of the overhang (a solid
overhang is 100% opaque). The average opacity of translucent overhangs is calculated using Equation
5-E.
Os = (Ai × Oi) + (Af × Of)
Equation 5-E
where
Os = opacity of the shading device
Oi = opacity of the translucent infill; this is calculated as 1 minus the solar transmittance of the glass
Os = opacity of the framing (generally unity)
Ai = percent area of the infill
As = percent area of the framing
Once the opacity is calculated, it is multiplied by the PF to determine the adjusted PF. This adjusted PF
is then used to obtain the SHGC multiplier from Table 5-H. See Example 5-Z.
Example 5-V. Projection Factor, Supermarket with Awning
Corresponding section: Opaque and Partially Opaque Permanent Projections (5.5.4.4.1, Exception 1)
Q
What is the PF for a single-story supermarket with a sloped metal awning that extends 12 ft (3.66 m)
out from the surface of the glass and at its lowest point is 10 ft (3.05 m) above the sidewalk?
The storefront window starts at 2 ft (0.61 m) above the floor and has a height of 9 ft (2.74 m). Assume
that the sidewalk and the floor are at the same level. The fenestration is only on the west side of the
building facing the parking lot; all other facades are opaque.
A
The PF is the ratio of the horizontal projection of the overhang to the distance from the windowsill to
the bottom of the overhang. The horizontal projection is 12 ft (3.66 m), and the vertical distance from
the windowsill to the bottom of the overhang is 8 ft (2.44 m). The PF is, therefore, 12 ft (3.66 m)
divided by 8 ft (2.44 m), or 1.5.
The overhang multiplier is 0.44 (see Table 5.5.4.4.1). The SHGC of the supermarket fenestration is
multiplied times 0.44, and this product is compared to the criteria SHGC. Note that Table 5.5.4.4.1 does
not offer additional shading credit for PFs greater than 1.
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Example 5-W. Opaque Overhang Credit
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Corresponding section: SHGC of Vertical Fenestration (5.5.4.4.1, Exceptions 1 and 5)
Q
A travel agency under design in Fayetteville, Arkansas, has typical windows on all sides of the building
as shown in the figure below. The fenestration area is 32% of the gross wall area. Each window is 5 ft
(1.5 m) high and 10 ft (3 m) wide, has an opaque overhang that projects 3 ft (0.9 m) from the surface of
the glass, and is positioned 6 in. (150 mm) above the window head. The window has an NFRC rating
and label. The NFRC-rated SHGC is 0.45. Does the building comply with the nonresidential SHGC
fenestration criteria of the standard?
A
Yes.
Fayetteville is in Washington County, and Table B-1 in Reference Standard Reproduction Annex 1
specifies that Washington County is in Climate Zone 4. Therefore, the appropriate criteria table for
Fayetteville is Table 5.5-4, which provides the SHGC criterion of 0.36.
The windows in this building qualify for the overhang credit in Exception 1 to Section 5.5.4.4.1. The
first step in determining the credit is to calculate the overhang PF, which is the ratio of the 3 ft (0.9 m)
projection to the distance from the windowsill to the bottom of the overhang (5.5 ft [1.65 m]). PF is
then 3 ft/5.5 ft = 0.9 m/1.65 m = 0.55.
Reading from Table 5-H, the overhang multiplier is 0.61 for fenestration with south, east, and west
orientations. There is no adjustment for north-oriented fenestration. The effective SHGC for south,
east, and west orientations is 0.61 × 0.45 = 0.27. The effective SHGC for north-oriented fenestration
remains 0.45. In this case, the south-, east-, and west-facing windows comply, but the north-facing
windows do not comply through Exception 1 to Section 5.5.4.4.1 alone.
However, Exception 5 to Section 5.5.4.4.1 allows north-oriented vertical fenestration to have SHGC
equal to the area-weighted average SHGC of the south-, east-, and west-oriented vertical fenestration
before any reductions are made for permanent projections in Exception 1, regardless of overhang
length. In this case, SHGC of 0.45 of the north-oriented windows matches the SHGC of the windows
with other orientations. Consequently, the windows comply for all orientations.
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Example 5-X. Louvered Overhang Credit
Corresponding section: SHGC of Vertical Fenestration (5.5.4.4.1, Exception 3)
Q
A Fresno, California, office building proposes to use louvered overhangs on the south side of the
building to provide shade for the windows. The louvers are positioned vertically, and the space
between them is equal to the height of the louvers. Is such an overhang fully credited by the standard?
A
No.
Louvered overhangs are fully credited under Exception 1 to Section 5.5.4.4.1 as long as they block the
sun at solar noon on the summer solstice. Fresno is located at 36.77 degrees north latitude. At solar
noon on the summer solstice, the sun has an altitude that rounds to 77 degrees. The cut-off angle of the
louvers is only 45 degrees, so there would be sun penetration through the louvers, and the overhang
could not be fully credited. The cut-off angle is the angle above which sun would pass through the
louvers. No sun will pass as long as the profile angle is less than the cut-off angle, so the best option is
to use a sun shade/overhang with appropriately angled louvers. If the louver design is not modified,
another option is to take partial credit using Exception 2 to Section 5.5.4.4.1. In this case, the percent
opacity must be calculated at solar noon on the summer solstice. This percent then is used in the
equation in Exception 2 to Section 5.5.4.4.1.
Example 5-Y. Louvered Overhang Credit, Determining the Profile Angle
Corresponding section: SHGC of Vertical Fenestration (5.5.4.4.1)
Q
A designer of a building at north latitude 40 degrees wants to use louvered overhangs to shade a
window that faces southeast. The louvers are oriented parallel to the window. What is the cut-off angle
needed in order for the louvers to be credited by the standard?
A
The profile angle for a southeast-facing window at 40 degrees north latitude at solar noon on the
summer solstice is 78 degrees. The louvers would have to be spaced such that the sun would not
penetrate at this angle.
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Example 5-Z. Translucent Overhang Credit
Cha p t e r 5 | Bu il d i ng E n vel o p e
Corresponding section: SHGC of Vertical Fenestration (5.5.4.4.1, Exception 2)
Q
An Atlanta office building proposes to use continuous glass overhangs over windows on the south
facade supported by welded steel tubes. The overhangs project 6 ft (1.8 m) from the building and are
positioned 1 ft (0.3 m) above the window head. The glass in the overhang structure has a solar
transmittance of 0.30. The window is 6 ft (1.8 m) high and is rated by NFRC to have an SHGC of 0.40.
The steel tubes are 4 in. (100 mm) wide and 6 in. (150 mm) deep and are spaced 4 ft (1.2 m) apart,
center-to-center. Atlanta is in Climate Zone 3 and the SHGC criterion is 0.25.
Does the proposed window with its overhang meet the prescriptive requirement?
A
The overhang has a PF of 0.86 (the overhang projection of 6 ft [1.8 m] divided by the 7 ft [2.1 m]
distance from the windowsill to the bottom of the overhang). The percentage of opaque overhang
framing is 8% (4 in. [0.1 m] of framing divided by 48 in. (1.2 m) of center-to-center width), leaving the
percent of glass at 92%. The opacity of the overhang is 0.725 as calculated below. This adjusted PF is
0.62, also calculated below. The SHGC multiplier is 0.56, reading from Table 5.5.4.4.1. This results in an
adjusted SHGC for the shaded window of 0.22, which complies with the criteria of 0.25.
6
1.8
PF =
=
= 0.86
1 + 6 0.3 + 1.8
𝑂𝑂𝑠𝑠 = (𝐴𝐴𝑖𝑖 × 𝑂𝑂𝑖𝑖 ) + �𝐴𝐴𝑓𝑓 × 𝑂𝑂𝑓𝑓 � = �0.92 × (1 − 0.30)� + (0.08 × 1.0) = 0.64 + 0.08 = 0.72
PF𝐴𝐴𝐴𝐴𝐴𝐴 = PF × 𝑂𝑂𝑠𝑠 = 0.86 × 0.72 = 0.62
𝑀𝑀 = 0.56 (From Table 5.5.4.4.1)
SHGC𝐴𝐴𝐴𝐴𝐴𝐴 = SHGC × 𝑀𝑀 = 0.40 × 0.56 = 0.22
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Vertical Fenestration on the Street Side of the Street-Level Story in Nonresidential Spaces
(Exception 3 to 5.5.4.4.1)
Tenants in retail and other ground-level spaces often want to have good visibility through their
storefront windows facing the street. The standard has an exception that waives the SHGC
requirements when all the following requirements are satisfied:
• The street side of the street-level story does not exceed 20 ft (6 m) in height. This requirement
does not apply to tall spaces such as multistory atriums.
• The fenestration has a continuous overhang with a weighted average projection factor (PF)
greater than 0.5. This overhang provides shading to partially compensate for the SHGC exception.
• The fenestration area for the street side of the street-level story is less than 75% of the gross wall
area for the street side of the street-level story. This exception does not apply to sides of the
building that do not face the street.
Figure 5-U illustrates these requirements.
Note that when this exception is used, these areas must be calculated separately and not averaged with
any others. No fenestration area can be credited or used elsewhere in the building, even if the full 75%
allowance is not used. Also, this exception only applies when using the Prescriptive Building Envelope
Option for compliance. This exception does not apply to the Building Envelope Trade-Off Option in
Section 5.6, the Energy Cost Budget Method in Section 11, or the Performance Rating Method in
Appendix G.
Also, be aware that this exception is limited to the SHGC criteria. The fenestration must still comply
with the U-factor criteria. Clear low-e coatings can be used to comply with the U-factor while still
providing good visibility for store windows.
FIGURE 5-U. VERTICAL FENESTRATION ON THE STREET SIDE OF THE STREET LEVEL STORY
Corresponding section: SHGC of Vertical Fenestration (5.5.4.4.1, Exception 3)
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Example 5-AA. Prescriptive Building Envelope Option, Tucson Supermarket
Corresponding sections: Opaque Areas (5.5.3) and Street Side Street-Level Fenestration (5.5.4.4.1,
Exception 3)
Q
A proposed 20,000 ft² (1858 m²) supermarket in Tucson, Arizona, has all of its fenestration oriented
south in one wall facing the street. The initial design is for clear, single glazing so that shoppers can
easily see all the products inside. Clear glass has an SHGC of 0.82. The building has no overhangs. The
fenestration area is 17% of the gross wall area. The walls are 8 in. (200 mm) normal weight concrete
block and the ungrouted cores are filled with insulation.
Will this comply with the standard or are modifications necessary?
A
This design does not comply without modification.
Tucson is in Pima County, and Table B-1 in Reference Standard Reproduction Annex 1 specifies that
Pima County is in Climate Zone 2. Therefore, the envelope criteria table for Tucson is Table 5.5-2. For
the nonresidential space category, the mass wall U-factor criteria is 0.151 (0.857) or R-5.7 (R-1.0) c.i.
or insulation in the cores (the exception to Section A3.1.3.1 applies), so the CMU walls comply.
However, the criteria for vertical fenestration limit the U-factor to a maximum of 0.54 (3.07) for fixed
metal-framed storefront windows. The SHGC criterion is 0.25. The proposed single glass does not meet
the U-factor criterion, nor does the SHGC of 0.82 meet the 0.25 criterion.
The designer must modify the design in one of two ways to become compliant:
Option 1 is to select a glazing material that meets the 0.25 SHGC criterion and the 0.54 (3.07) U-factor
criterion.
Option 2 is to take advantage of the exception for fenestration on the street side of the street-level
story. Exception 3 to Section 5.5.4.4.1 exempts such fenestration from the SHGC criterion provided that
the fenestration has an overhang with a PF of at least 0.5, the floor-to-floor height at the street level
does not exceed 20 ft (6 m), and the exempt fenestration does not exceed 75% of the street-level
facade. Using this exception, the clear glass can be used in the intended application to satisfy the SHGC
requirement. However, a clear glass would have to be selected that meets the U-factor criterion, which
would generally require double glazing, unless Appendix C, Section 11, or Appendix G are used with
other trade-offs to show equivalent energy performance.
Dynamic Glazing (5.5.4.4.1, Exception 4)
Dynamic glazing has the ability to modify its performance parameters, such as SHGC and VT, to
optimize cooling loads, daylighting, and energy performance throughout the day and over different
seasons. Because there is not a single SHGC value as with traditional fenestration products, NFRC
ratings for dynamic glazing list both higher and lower performance values on the label or certificate. If
dynamic glazing is installed, the lower SHGC from the NFRC rating must be used to demonstrate
compliance with this section. For the purpose of area-weighted averaging of SHGC, dynamic glazing
and nondynamic glazing must be treated separately and not combined.
 SHGC of North-Oriented Vertical Fenestration (5.5.4.4.1, exception 5)
Vertical fenestration oriented within 45 degrees of north (in the northern hemisphere; trade south for
north in the southern hemisphere) is permitted to be treated differently for compliance with SHGC,
because solar gains are less on the north side of a building, and also to encourage the use of daylighting
on the north side of buildings where higher VT glazing may be used with less concern of direct solar
gains or glare. Additionally, a technical flaw was noted in the calculation from the previous edition of
the standard where the SHGC multipliers could sometimes illogically require fenestration to have a
lower SHGC on the north side of a building than on the west side of the building when projections were
used. To correct this issue, exception 5 for north-oriented fenestration has been changed to allow the
same fenestration product SHGC to be used on the north side of the building as the area weighted
average of the SHGC of fenestration products on the other orientations before any adjustments are
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made for projections. In other words, compliance with maximum SHGC requirements for south, east,
and west-facing fenestration may be demonstrated by using low SHGC glazing, or somewhat higher
SHGC glazing in combination with shading by permanent projections (using exceptions 1 and 2). In the
latter case, exception 5 allows the same glazing to be used on the north-facing fenestration either with
or without a shading projection as long as the SHGC for the north-facing fenestration is the same or
lower than the area-weighted average SHGC for the fenestration on the other sides (alone, without
including the impact of overhangs or permanent projections.) See Example 5-Z for use of this
exception.
SHGC of Skylights (5.5.4.4.2)
As with vertical fenestration, there is a single maximum SHGC for all skylights. If the skylight has an
NFRC rating, then that value must be used for compliance. For unlabeled skylights, Table A8.1-2 of
Appendix A has values that are to be used. Alternatively, you can obtain manufacturer’s shading
coefficient data for the glazing used in the skylight and use 86% of this value as the SHGC, provided
that the shading coefficient is established using a spectral data file determined in accordance with
NFRC 300 (see Exception 1 to Section 5.8.2.4).
Exception 1 to Section 5.5.4.4.2 provides an exemption from the SHGC requirements for skylights that
are used to comply with the minimum skylight area requirement when the skylight has a VT greater
than or equal to 0.40 and a haze value greater than 90%. The exception also requires that daylighting
controls be installed in compliance with Section 9.4.1.1(f).
If dynamic glazing is installed, the SHGC for the bottom of the SHGC range must be used to demonstrate
compliance with this section. For the purpose of area-weighted averaging of SHGC, dynamic glazing
and nondynamic glazing must be treated separately.
 Fenestration Orientation (5.5.4.5)
The orientation of fenestration is an important building feature. Fenestration that faces east and west
experiences low solar angles in the morning and afternoon, respectively. As a result, east- and westoriented fenestration has a higher solar gain than north- or south-oriented fenestration of the same
area. In addition to the maximum fenestration area, the standard requires that solar gain attributed to
fenestration be minimized by one of the following strategies:
• Reduced fenestration area on the east and west facade
• Improved SHGC of east- and west-facing fenestration by improving the glazing properties,
including external shading, or a combination of both
• A combination of the previous two strategies
The reduced east- and west-oriented fenestration requirement encourages building owners and
designers to orient buildings with the long dimension along the east-west axis, placing more
fenestration on the north and south facades.
Compliance must be shown using one of two paths. The first path (Section 5.5.4.5[a]) requires the
west-oriented fenestration area be less than or equal to 1/4 of the total building fenestration area. This
requirement also applies to east-oriented fenestration. The second path (Section 5.5.4.5[b]) uses the
concept of solar aperture (effective solar heat gain times area) and varies by climate zone. For Climate
Zones 0 through 3, this path requires that the product of the fenestration area and SHGC of the westoriented fenestration be less than or equal to one-quarter of the product of the fenestration area of the
entire building and the SHGC criteria established in Tables 5.5-0 through 5.5-3. For Climate Zones 4
through 8, this path requires that the product of the fenestration area and SHGC of the west-oriented
fenestration be less than or equal to one-fifth of the product of the fenestration area of the entire
building and the SHGC criteria established in Tables 5.5-4 through 5.5-8. This requirement also applies
to the east-oriented fenestration. The SHGC of the east- and west-oriented fenestration is allowed to
include the effect of external shading from permanent projections established through Section
5.5.4.4.1.
The standard defines both east- and west-oriented fenestration areas in Section 5.5.4.5. West-oriented
fenestration is considered to be oriented within 45 degrees of true west to the south but only within
22.5 degrees of true west to the north (in the northern hemisphere). Similarly, east-oriented
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fenestration is considered to be oriented within 45 degrees of true east to the south but only within
22.5 degrees of true east to the north (in the northern hemisphere). South and north directions must
be reversed for buildings in the southern hemisphere. See Example 5-BB.
The following exceptions to the fenestration orientation requirement allow buildings with special
constraints to comply with the requirement:
1. Street-level fenestration that complies with Exception 3 to Section 5.5.4.4.1.
2. The east-facing requirement is excepted when existing structures or topography shade at least
75% of the east fenestration at 9:00 a.m. on the summer solstice. The west-facing requirement is
excepted when existing structures or topography shade at least 75% of the west fenestration at
3:00 p.m. on the summer solstice. This determination must be made for solar time, which may vary
significantly from clock time.
3. Alternations and additions are excepted when there is no increase in vertical fenestration area.
4. When the east-oriented or west-oriented fenestration area is less than 20% of the gross wall area
for that facade, the requirement will be considered to have been met as long as the SHGC of
fenestration on that facade is not greater than 90% of the SHGC criteria given in Tables 5.5-0
through 5.5-8.
5. Buildings in Climate Zone 8.
Visible Transmittance/SHGC Ratio (5.5.4.6)
Section 9.4.1.4 requires automatic daylighting controls in certain spaces to reduce energy use by
turning general lighting down or off when there is sufficient natural daylight. In those spaces where
automatic daylighting controls are required, Section 5.5.4.6 also requires the vertical fenestration in
that space to have a minimum ratio of VT divided by SHGC as specified in Tables 5.5-0 through 5.5-8.
The minimum VT/SHGC ratio is 1:1 in residential and nonresidential conditioned spaces to ensure that
solar selective glazing is used, admitting more daylight than solar heat gain. A minimum VT/SHGC ratio
is not required in semiheated spaces or where automatic daylighting controls are not required.
The VT/SHGC ratio must be determined from the NFRC ratings for the whole assembly, including both
the glazing and framing. While it is also permitted to use the default values for unlabeled fenestration
from Table A8.2, the ratio of the default VT and SHGC values will not satisfy the 1:1 ratio. Exception 1
to Section 5.5.4.6 also permits the use of light-to-solar gain ratio (LSG) for glazing not less than 1.25.
The LSG is the ratio of VT divided by SHGC for just the glazing, based on center-of-glass properties
instead of the whole assembly including frame. LSG values are commonly supplied by the glazing
manufacturer but must be determined in accordance with NFRC procedures by an independent
laboratory or listed in a governmental database, such as the International Glazing Database (IGDB)
managed by Lawrence Berkeley National Laboratory.
Exceptions 3, 4, and 5 to Section 5.5.4.6 all provide exceptions to the VT/SHGC requirement if sufficient
daylighting is provided by alternate means. Exception 3 exempts spaces where at least half the floor
area is covered by daylight provided by clerestories or rooftop monitors, and Exception 4 exempts
spaces with toplighting that meets the minimum skylight area requirements of Section 5.5.4.2.3.
Exception 5 exempts spaces where sufficient sidelighting is provided as determined by a minimum
sidelighting effective aperture of at least 0.15. Sidelighting effective aperture accounts for the
combination of fenestration VT and area. For instance, a larger window with lower VT can bring in the
same amount of daylight as a smaller window with higher VT. The sidelighting effective aperture is
calculated according to the following formula:
𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 =
∑(𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 × 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑉𝑉𝑉𝑉)
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑡𝑡𝑡𝑡𝑡𝑡 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴
Finally, Exception 6 to Section 5.5.4.6 provides information on how to determine compliance when
dynamic glazing is used. Dynamic glazing has the ability to modify its performance parameters, such as
SHGC and VT, to optimize cooling loads, daylighting, and energy performance throughout the day and
over different seasons. Because there is not a single SHGC or VT value as with traditional fenestration
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products, NFRC ratings for dynamic glazing list both higher and lower performance values on the label
or certificate.
If dynamic glazing is installed in a space that requires automatic daylighting controls, the VT/SHGC or
LSG used to demonstrate compliance with Section 5.5.4.6 must be determined using the ratio of the
higher VT rating divided by the higher SHGC value. For the purpose of area-weighted averaging of
VT/SHGC, dynamic and nondynamic glazing must be treated separately and not combined.
Example 5-BB. Fenestration Orientation, Site Constraints
Corresponding section: Fenestration Orientation (5.5.4.5)
Q
A building is constrained by its site and has a 2:1 aspect ratio with the long sides of the building facing
east and west. How can this building comply with the requirements of Section 5.5.4.5?
A
The building can comply by reducing the fenestration area or SHGC or a combination of both area and
SHGC of the east-oriented and west-oriented fenestration.
Option (a) can be satisfied if the west-oriented fenestration area is less than 1/4 of the total
fenestration area and the east-oriented fenestration area is less than 1/4 of the total fenestration area.
Given the 2:1 aspect ratio, the fenestration area as a percentage of the west and east facades would
need to be only about 1/2 of that on the south and north facades in order to achieve this.
By using permanent projections for shading, such as fins and overhangs on the east- and west-oriented
fenestration, it could be possible to reduce the effective SHGC so that option (b) in the main
requirement would be satisfied. Permanent projections could also be designed such that the
fenestration is shaded during the periods stated in Exception 2. Exception 4 could also be used to show
compliance. Other exceptions should also be investigated to show compliance. Finally, compliance
could be demonstrated using the Building Envelope Trade-Off Option, the Energy Cost Budget Method
(Section 11), or the Performance Rating Method (Appendix G).
Example 5-CC. East- and West-Oriented Fenestration Area Calculation
Corresponding Section: Fenestration Orientation (5.5.4.5)
Q
The building in the figure below is located in Denver, Colorado, in Climate Zone 5. It has a 75 ft
(22.9 m) long, 12 ft (3.7 m) tall west-oriented wall with 8 equally sized windows. Each window is a
4.5 × 5.5 ft (1.4 × 1.7 m) fixed metal frame type. The east-oriented wall also has 8 of these windows.
The north- and south-oriented walls each have 10 of these windows, and the south-oriented wall has a
54 ft² (5.0 m²) glazed entrance door. The windows and front entrance door all comply with the
maximum U-factors allowed by Tables 5.5-5. All of the windows have an NFRC-rated SHGC-0.32 and
the entrance door has an NFRC-rated SHGC-0.44. Therefore, the area-weighted average SHGC complies
with the maximum allowed in Table 5.5-5.
Does the west-oriented fenestration satisfy either option (a) or (b) of Section 5.5.4.5? If the building is
rotated 90 degrees counterclockwise, and the south-oriented wall becomes east-oriented, does the
answer change?
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A
Denver is in Climate Zone 5 with criteria specified in Table 5.5-5, which provides a maximum allowed
SHGC (SHGCC) of 0.38. The area of each window is 25 ft² (2.3 m²), and windows are of fixed metal
frame type. The west-oriented wall has 8 of these windows for a total fenestration area of 200 ft²
(18.4 m²). The building has 36 of these windows totaling 900 ft² (82.8 m²) as well as a glazed metal
frame entrance door of 54 ft² (5.0 m²) for a total fenestration area of 954 ft² (87.8 m²).
Option (a) (original orientation)
Equations for Option (a) apply to all climate zones:
AW = 200 ft² (18.4 m2)
AT = 900 ft2 + 54 ft2 = 945 ft2 (82.8 m2 + 5.0 m2 = 87.8 m2)
AT/4 = 239 ft2 (22.0 m2)
Because AW is less than AT/4, the example complies with Option (a).
Option (b) (original orientation)
It is necessary to select the correct equation based on climate zone. The following equation applies to
Climate Zones 4 through 8:
AW × SHGCW = 200 ft² (18.4 m²) × 0.32 = 64 ft² (5.9 m²)
(AT × SHGCC)/5 = (954 ft² (87.8 m²) × 0.38)/5 = 73 ft² (6.7 m²)
Because AW × SHGCW is less than (AT × SHGCC)/5, the example complies with Option (b) as well.
The east- oriented fenestration area is the same as the west, so the above calculation would be
identical for the east- oriented wall. This design orientation complies with both options due to the east-
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and west-oriented fenestration areas being only about one-fifth of total fenestration area, coupled with
the actual SHGC being lower than the maximum SHGC criteria in Table 5.5-5.
However, if the building above is rotated 90 degrees counterclockwise (such that the entry door is
east-oriented), the situation changes.
Option (a) (90 degrees counterclockwise rotation)
AW = 250 ft² (23.0 m2)
AT = 900 ft2 + 54 ft2 = 945 ft2 (82.8 m2 + 5.0 m2 = 87.8 m2)
AT/4 = 239 ft2 (22.0 m2)
Because AW exceeds AT/4, the example does not comply using Option (a).
Option (b) (original orientation)
AW × SHGCW = 250 ft² (23.0 m²) × 0.32 + 54 ft² (5.0 m²) × 0.44 = 104 ft² (9.6 m²)
(AT × SHGCC)/5 = (954 ft² (87.8 m²) × 0.38)/5 = 73 ft² (6.7 m²)
In this situation, AW × SHGCW is larger than (AT × SHGCC)/5, so the example design does not comply
with Option (b). Therefore, it is necessary to modify the design to reduce solar gains.
The Option (b) equation can be used to solve for the required SHGCE for this fenestration orientation:
Required SHGCE = (AT × SHGCC)/(5 × AE)
Required SHGCE = [954 ft² (87.8 m²) × 0.38]/[5 × 304 ft² (28.0 m²)]
Required SHGCE = 0.24
The area-weighted average SHGC for the original design for the now east-oriented fenestration is:
SHGCE = [(250 ft2 (23.0 m2) × 0.32] + [54 ft2 (5.0 m2) × 0.44)]/[304 ft2 (28.0 m2)] = 0.34
To achieve compliance with the new fenestration orientation, two design changes can be used. One is
to reduce the area-weighted average SHGC of the east-oriented fenestration to 0.24 or less through
changes to the fenestration product. The other is to reduce the effective SHGC of the fenestration by
adding fixed louvers or an overhang to provide external shading with a projection factor of 0.40 or
larger. (Reducing the effective SHGC from 0.34 to 0.24 requires a multiplier of 0.7 or less which can be
provided by a 0.40 projection factor from Table 5.5.4.4.1.) A combination of these two approaches
could also be used.
The fixed louvers or overhang would not need to be as great for the now west-oriented fenestration, as
this side of the building does not have a glazed entrance door so the fenestration area is not as large.
Building Envelope Trade-Off Option (5.6)
Section 5.6 and Appendix C of the standard cover the Building Envelope Trade-Off Option. With the
Building Envelope Trade-Off Option, the performance of one building envelope component can be
improved to make up for another building envelope component that may not meet the standard’s
requirements. While area-weighted averaging in the Prescriptive Building Envelope Option allows
these types of trade-offs to be made within a single class of construction based on steady-state Ufactors/F-factors/C-factors, the Building Envelope Trade-Off Option permits trade-offs between all
building envelope components. The Building Envelope Trade-Off Option uses annual energy flows
through those components based on an energy simulation to determine acceptable trade-offs.
The Building Envelope Trade-Off Option involves a little more work because it is necessary to measure
all the surface areas and to tabulate wall areas by orientation. However, it does provide considerable
design flexibility beyond what is offered by the Prescriptive Building Envelope Option. This flexibility
is often extremely important; it helps designers respond to a building’s unique characteristics,
including different user needs, a different site, and often a different climate.
There are times when the Building Envelope Trade-Off Option in Section 5.6, the Energy Cost Budget
(ECB) Method in Section 11, or the Performance Rating Method in Appendix G must be used, for
instance, when the vertical fenestration area exceeds 40% of the gross wall area or when the total
skylight area exceeds 3% of the gross roof area. The Building Envelope Trade-Off Option cannot be
used to make trade-offs between the building envelope and any other systems, including the heating,
ventilating, and air-conditioning system (Section 6), the service water heating system (Section 7), the
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power system (Section 8), the lighting system (Section 9), or other equipment system (Section 10). The
ECB Method in Section 11 must be used if trade-offs are desired with these other systems.
Simulation Method
The method used to make the trade-offs is documented in Appendix C of the standard. The method
consists of rules for building simulation modeling that results in a figure of merit called the “envelope
performance factor” that is compared to a baseline envelope performance factor. Appendix C allows for
a compliant energy modeling program to create the envelope performance factor and baseline
envelope performance factor. Appendix C, much like Appendix G, sets a series of rules to create the
baseline envelope performance factor based on characteristics of the proposed design, such as lighting
schedules, HVAC system, and operating schedules. The Building Envelope Trade-Off Option is meant to
be used with simulation software that automatically implements the rules of Appendix C based on
limited user input of the proposed building design alone. The schedules and internal loads, by building
area type, to be used in the Building Envelope Trade-Off Option simulations are included in Table 5-I
through Table 5-U below. The tables can also be found on the AHSRAE website at
http://sspc901.ashraepcs.org/content.html.
Daylighting Potential
When using the Building Envelope Trade-Off Option, you must specify the U-factor, SHGC, and VT for
all fenestration products, including windows, glazed doors, and skylights. For nonresidential zones, the
calculation method includes the effects of VT, daylighting, and photocontrols upon energy use. This is
to encourage fenestration design that optimizes the use of daylight.
Limits of the Building Envelope Trade-off Option
The Building Envelope Trade-Off Option in Section 5.6 cannot be used to bypass any of the standard’s
requirements in Section 5.1 General, Section 5.2 Compliance Paths, Section 5.4 Mandatory Provisions,
Section 5.7 Submittals, Section 5.8 Product Information and Installation Requirements, and Section 5.9
Inspection and Verification. See the discussions of those requirements in the corresponding sections.
Furthermore, the trade-off option cannot be used to make trade-offs between the building envelope
and any other systems. The ECB Method (Section 11) or the performance Rating Method (Appendix G)
must be used to make envelope trade-offs against any other systems in the building. When using the
Building Envelope Trade-Off Option, other systems are modeled, but the parameters of those systems
are prescribed by Appendix C and are held constant between the proposed and baseline building
models.
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TABLE 5-I BUILDING ENVELOPE TRADE-OFF OPTION SCHEDULES AND LOADS
Building Area Type
Automotive facility
Schedule
Index
J
Misc.
Loads,
W/ft²
625
0.25
0.41
14
250
200
B
6
Dormitory
B
B
E
1.5
Gymnasium
I
Health-care clinic
E
Hotel
Hospital
6
0.6
H
Fire station
6
D
Exercise center
0.5
0.5
0.93
0.71
10
10
14
0.11
100
0.21
33
0.26
0.3
100
33
200
F
1.11
0.08
250
J
1
2
C
Motel
F
1.11
D
0.62
Motion picture theater
H
Museum
C
Multifamily
0.93
20
0.47
Library
Manufacturing facility
0.31
2
E
1.5
0.54
1.5
0.47
0.11
0.25
0.08
1.19
200
100
143
250
7
0.06
380
0.09
200
0.36
25
Office
A
0.75
Penitentiary
F
0.5
0.25
40
E
1.5
0.21
33
Parking garage
Performing arts theater
Police station
Post office
K
H
A
0
0.5
1
0
0.76
0.21
Religious building
H
0.96
School/university
G
1.39
A
1
0.09
0.24
0.06
Retail
C
Sports arena
H
Transportation
E
Town hall
Warehouse
Workshop
106
L
J
Latent
375
Dining: bar lounge/leisure
Dining: family
Sensible
143
0.25
Dining: cafeteria/fast food
Occupant Heat Gain
(Btu/h⋅person)
0.25
H
A
Occupant
Density,
ft²/person
1
Convention center
Courthouse
Ventilation
Rate,
cfm/ft²
0.3
1
0.5
1
0.66
0
14
33
8
0.23
67
0.3
0
0.47
0.81
0.25
40
200
10
0
143
250
275
275
275
250
200
275
275
275
200
710
1090
710
1090
250
200
250
250
250
250
580
250
225
250
250
250
250
250
225
250
250
245
250
250
245
250
225
275
635
200
200
200
200
870
200
105
200
200
200
200
200
105
200
200
155
200
200
105
200
105
475
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Product Information and Installation Requirements (5.8)
Insulation (5.8.1)
Labeling of Building Envelope Insulation (5.8.1.1)
Premanufactured insulation must be labeled such that field inspectors can verify the R-value of the
product. Faced batt insulation and board insulation typically have the labeling printed directly on the
product. An exception is provided for insulation that cannot be easily labeled, such as loose-fill
insulation, unfaced batt insulation, or spray-applied foam insulation. In this case, the insulation
installer must provide a certificate that states the thickness and R-value of the installed product.
Compliance with Manufacturers’ Requirements (5.8.1.2)
Section 5.8.1.2 requires that insulation materials be installed according to the manufacturer’s
recommendations and in a manner that will achieve the rated insulation R-value. For example, you
can’t take credit for R-21 (R-3.7) insulation if you squeeze it into a 2 × 4 ft (50 × 100 mm) wall space;
doing so would compress its normal 5.5 in. (150 mm) thickness to 3.5 in. (100 mm) and reduce the
effective insulation to R-14 (R-2.5). Compressing the insulation reduces the effective R-value and the
thermal performance of the construction assembly (see Table A9.4.3 in Appendix A of the standard).
Insulation can be compressed if you perform U-factor calculations and account for the effect of
compression; in other words, you can’t use the precalculated U-factor tables published in Appendix A
of the standard if the insulation is compressed. However, there is an exception to the insulation
compression rule for metal buildings where compression at the purlins is already accounted for in the
tables. For metal buildings, insulation is typically draped over the metal purlins and compressed at the
supports.
Loose-Fill Insulation Limitation (5.8.1.3)
Section 5.8.1.3 limits the use of loose-fill or blown insulation to ceilings that have a slope not exceeding
three in twelve. The obvious reason for this is to prevent the insulation from tumbling to one side,
leaving the top portion of the ceiling uninsulated. See Figure 5-V.
Baffles (5.8.1.4)
Baffles must be installed in conjunction with loose-fill insulation at the eaves if the attic is ventilated
from that location. The purpose of the baffles is to prevent loose insulation from blocking the vent area
or being lost through the ventilation opening. See Figure 5-V.
FIGURE 5-V. BLOWN INSULATION ABOVE SLOPING CEILING
Corresponding section: Loose-Fill Insulation Limitation and Baffles (5.8.1.3 and 5.8.1.4)
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Substantial Contact (5.8.1.5)
Section 5.8.1.5 requires that insulation be installed in a permanent manner and in substantial contact
with the inside surface of the construction assembly. If the insulation does not entirely fill the cavity,
the air gap must be on the outside surface.
Maintaining substantial contact is particularly important for air-permeable insulation such as
fiberglass batt. Where substantial contact is not maintained, air currents find their way to air space
around insulation, substantially reducing the effectiveness of the insulation. See Figure 5-W for correct
substantial contact.
There is an exception for construction assemblies that use reflective materials and rely on an air space
next to the interior surface. This exception is meant to allow these types of products to be used when
appropriate. Where reflective air-spaces are used, they must comply with the R-values, requirements,
and use conditions provided in Section A9.4.2, and the appropriate R-value may then be used when
calculating the U-factor for the wall, floor, or ceiling/attic assembly. R-values for cavity air spaces shall
be obtained from Table A9.4.2-1. The R-value depends on climate zone, building component, average
air-space thickness (T) in the direction of heat flow, and effective emittance (E) of the surface
perpendicular to the direction of heat flow. Effective emittance is calculated using Equation A9.4-1.
When the values for T and E are between values listed in Table A9.4.2-1, linear interpolation can be
used to obtain the R-value. Additional guidance on the use of reflective air spaces can be found in
ASHRAE Handbook—Fundamentals.
Recessed Equipment (5.8.1.6)
Section 5.8.1.6 requires that recessed equipment not reduce the insulation thickness. Examples of
recessed equipment are lighting fixtures, wall heaters, HVAC ducts, diffusers, VAV boxes, and other
types of electrical or mechanical equipment. There are some exceptions to this requirement:
FIGURE 5-W. FLOOR INSULATION IN SUBSTANTIAL CONTACT
Corresponding section: Substantial Contact (5.8.1.5)
a.
b.
Equipment can be recessed if the area affected is less than 1% of the total roof/ceiling area. For
instance, lighting fixtures may penetrate an insulated ceiling as long as the area of the openings in
the insulation is less than 1% of the total ceiling area. It is acceptable for all the 1% to be located in
one roof/ceiling area; there is no need for the recessed equipment to be uniformly distributed
across all roof/ceiling surfaces.
A second exception applies to cases where the entire construction assembly is covered to the full
depth required. This might be achieved if Type IC (insulation contact) lighting fixtures were used
and additional insulation were placed over the top of the fixtures. Most building codes require that
a minimum clearance of 3 in. (76 mm) be maintained between the lighting fixture and the
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insulation for non-Type IC equipment. This is a problem with all insulation systems, because large
holes or discontinuities result. Type IC lighting fixtures are rated by Underwriters Laboratory (UL)
and permit insulation to be in direct contact. The additional cost of Type-IC fixtures is offset by the
savings from not having to construct dams around fixtures to maintain minimum clearance.
c. A third exception applies when the effect of the holes in the insulation or the reduced insulation
thickness is taken into account in the calculations. In this case, the designer might divide the
ceiling into areas that have penetrations and those that don’t and then show that the areaweighted average U-factor is less than the standard requires. For more information about areaweighted averaging.
The infiltration barrier must be maintained according to Section 5.4.3.1. This will generally prohibit
drop-in ceilings from being used as the exterior envelope element (see additional restrictions below).
Insulation Protection (5.8.1.7)
The standard requires that insulation be protected from sunlight, moisture, landscaping equipment,
wind, and other physical damage. Rigid insulation used at the slab perimeter of the building must be
covered to prevent damage from gardening or landscaping equipment. Rigid insulation used on the
exterior of walls and roofs must be protected by a permanent waterproof membrane or exterior finish
or be approved for use as a water-resistive barrier. If mechanical or other equipment is installed in
attics, access to this equipment must be provided in a way that won’t cause compression or damage to
the insulation. This may mean using walking boards, access panels, and other techniques to prevent
damage to the insulation.
In situations such as vinyl-faced insulation installed inside warehouse roofs, where there is no
ventilated airspace above the insulation and no solid surface such as gypsum board immediately below
the insulation, the standard requires that all seams be sealed with tape in order to provide an adequate
vapor retarder. In this application, simply stapling the insulation is not adequate.
Location of Roof Insulation (5.8.1.8)
The standard specifically prohibits installing insulation directly over suspended ceilings with
removable ceiling panels. This is because the insulation’s continuity is likely to be disturbed by
maintenance workers. Also, suspended ceilings do not meet the standard’s infiltration requirements
(Section 5.4.3) unless they are properly sealed (making the panels nonremovable).
In the past, it was not unusual to install insulation over suspended ceilings. However, this practice
must be avoided. If the thermal envelope is located at the ceiling, many building codes will consider the
space above the ceiling to be an attic and require that it be ventilated to the exterior. If vented to the
exterior, air in the attic could be quite cold (or hot) and the impact of the leaky suspended ceiling
would be made worse.
Extent of Insulation (5.8.1.9)
Insulation must extend over the full area of the insulated surface. When insulation is fitted in the
cavities of wall or roof constructions, ensure that the insulation is carefully cut to fit snugly in the
cavity (without being compressed), leaving no gaps or voids. If batt insulation is compressed in the
cavity, the labeled R-value is no longer valid and a compressed R-value must be used. Refer to Section
A9 for the effective R-values of compressed insulation.
FYI
Moisture Migration and Vapor Barriers
Except for the situation described in the Insulation Protection (5.8.1.7) section, where insulation is
exposed, Standard 90.1 does not address moisture migration. However, the designer should pay
attention to moisture migration and should consult the appropriate building standards and codes.
Joints in Rigid Insulation (5.8.1.10)
When two or more layers of rigid insulation are used to meet the requirements of the standard, the
joints for each layer should not align with each other. They must be staggered such that each new layer
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covers the joints of the previous layer, leaving the new joints over continuous section of the underlayer.
Fenestration and Doors (5.8.2)
Four fenestration performance characteristics are significant in the Standard: U-factor, solar heat gain
coefficient (SHGC), visible transmittance (VT), and air leakage. The first three of these are reviewed
briefly below; air leakage is addressed in Section 5.4.3.2 of the standard and is reviewed later. Shading
coefficient (SC) is also described, although it has largely been replaced by SHGC. Example 5-DD
addresses fenestration performance characteristics.
Rating and Labeling (5.8.2.1 and 5.8.2.2)
Section 5.8.2.1 requires the performance of windows and other fenestration products be determined
by an NFRC accredited laboratory or other nationally recognized rating authority. This requirement
applies to the U-factor, SHGC, VT, and air leakage rate. Section 5.8.2.2 requires that all manufactured
and site-built fenestration and door products must state the rated performance factors: U-factor, SHGC,
VT, and air leakage by either a label or a signed and dated manufacturer’s certificate provided with the
product. Doors with less than 25% glazing are not required to list SHGC and VT.
U-Factor (5.8.2.3)
The U-factor of fenestration is very important to the energy efficiency of buildings, especially in cold
climates. Fenestration U-factor is the rate of heat flow through a unit area of fenestration when there is
a one-degree temperature difference between the air on one side and the air on the other side. The I-P
units are Btu per hour per degree Fahrenheit, or Btu/h∙°F (the metric or SI units are W/m²∙°C). The Ufactor includes consideration of the whole fenestration product. Heat loss is accounted for through the
glass and edge of glass as well as the sash and frame elements. For skylights that sit on a curb, heat loss
also includes the skylight curb. The heat loss is then normalized for the area of the rough frame
opening provided for the fenestration. Expressing U-factor in this manner simplifies calculations for
sizing HVAC equipment and systems and for estimating annual energy use; however, performance at
actual fenestration size may vary from performance at the standard reference sizes that are used in
NFRC rating for code-compliance purposes.
U-factor does not consider solar gains through the fenestration; this is addressed by the SHGC or the
shading coefficient (SC).
Fenestration products are complex systems that use a wide variety of materials, systems, and
techniques. The standard requires that fenestration U-factors be determined in accordance with NFRC
Standard 100, which accounts for the complexity through a combination of testing and simulations.
NFRC is a membership organization of window manufacturers, researchers, and others that develops,
supports, and maintains fenestration rating and labeling procedures. Most fenestration manufacturers
have their products rated and labeled through the NFRC program. Default U-factor values are provided
in the standard’s Appendix A for fenestration products that do not have NFRC ratings. These default
values assume the worst in terms of thermal performance, so design professionals are encouraged to
use the NFRC values.
While the NFRC certification program has included skylights since its inception, the basis for skylight
ratings was shifted to a 20-degree slope in 2001. NFRC ratings are not commonly available for dome
skylights because of the plastic material and the varying air gap. Consequently, a more extensive
default table is provided.
Table A8.1-1 of Appendix A includes U-factors that can be used for skylights. The table offers credit for
low-e coatings, frame types, and other factors that affect thermal performance. When using the default
table to take credit for low-e coatings, the emissivity of the low-e coating must be determined using
NFRC Standard 301 and must be verified and certified by the glass manufacturer. NFRC ratings for
specific products are always preferable to the generic values in Table A8.1-1.
Glazed wall systems, including glass curtain walls used on large buildings, storefront glazing systems,
and other similar products that are assembled at the construction site instead of at the factory, must be
rated using either NFRC procedures or the default U-factor, SHGC, and VT from Table A8.2. Because the
performance values in Table A8.2 are based on uncoated clear glass in poorly performing metal frame,
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they do not offer any credit for advanced design features. In general, values from Table A8.2 will not
achieve compliance with the fenestration requirements.
The NFRC procedure for site-built fenestration is described in NFRC 100. The NFRC ratings are based
on computer simulations of various product options at standard sizes. (For curtain walls, the standard
size specified is 2000 × 2000 mm, or approximately 79 × 79 in.) Multiple glass options can be included
in one simulation matrix. The entire simulation matrix is then validated by a single physical test at the
standard size. If the matrix for a product has previously been validated, then a new glass option can be
added to the matrix by simulation alone. Simulations and tests must be performed by NFRC-accredited
simulation and test laboratories.
The manufacturer must provide a signed and dated certificate showing the fenestration U-factor
determined by an accredited laboratory in accordance with NFRC 100. One option is the NFRC label
certificate that lists the U-factor, SHGC, and VT as well as the project address, how many of these
fenestration products will be installed in the building project, the frame material supplier, the glazing
material supplier, the glazing contractor, and the certification authorization. For additional
information, visit the NFRC website at http://www.nfrc.org.
For sectional garage doors and metal coiling doors that do not have NFRC ratings, U-factors may be
determined in accordance with Door and Access Systems Manufacturers Association (DASMA)
Standard 105. Additionally, curtain wall spandrel panels are considered opaque wall elements and
need to be insulated to those requirements.
Solar Heat Gain Coefficient (5.8.2.4)
The solar heat gain coefficient (SHGC) is the ratio of solar radiation that passes through fenestration to
the amount of solar radiation that falls on the fenestration. Perfectly transmitting fenestration would
have an SHGC of 1.0, but this is a physical impossibility because even the most clear glass absorbs or
reflects some solar radiation. As with U-factor, SHGC is also a whole-product rating and accounts for
the glazing material as well as the frame and sash. The SHGC is a property of the fenestration product
and does not account for interior shading from Venetian blinds, vertical blinds, or draperies.
In hot climates, SHGC is the most important performance characteristic of fenestration, more
important than U-factor.
The standard requires that SHGC be determined in accordance with NFRC Standard 200 and by a
laboratory that has accreditation by NFRC or a similar organization. The fenestration product must
also be labeled and certified by the manufacturer. Fenestration products that have an NFRC rating
report the SHGC as well as the U-factor.
SHGC has replaced shading coefficient (SC) as the measurement for solar heat gain through
fenestration products. The SC is a number between 0 and 1 that indicates the amount of solar heat gain
that will pass through the center of the glazing, relative to 1/8 in. (3 mm) clear glass, not including the
effects of the sash and frame. By definition, the shading coefficient of 1/8 in. (3 mm) thick, clear,
double-strength window glass is 1.0. All other fenestration is rated relative to this.
For skylights, the standard provides default SHGC values in Table A8.1-2. For other unlabeled
fenestration products, see Table A8.2. However, the values in Table A8.2 do not account for low-e
coatings, reflective coatings, and other technologies commonly used to reduce solar heat gains. In most
instances, designers should obtain either SHGC or SC data from the manufacturer and use these data in
compliance calculations.
Visible Transmittance (5.8.2.5)
Visible transmittance (VT) is the third important performance characteristic of fenestration products.
VT is the ratio of light passing through the glazing to light passing through perfectly transmissive
glazing. VT is concerned only with the visible portion of the solar spectrum, as opposed to SHGC, which
is the ratio of all solar radiation. VT is important for buildings that incorporate daylighting. It is also
important in order to enjoy views from windows.
There is a strong relationship between the VT and the SHGC. The lower the SHGC, generally the lower
the VT. Some glazing products, however, have a VT higher than other products with the same SHGC.
For instance, bronze, gray, and green tinted glass all have about the same SHGC for a given glass
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thickness, but green glass has a significantly higher visible transmittance. Likewise, some coatings
applied to the surface of glazing reduce the SHGC more than they do the VT. For these reasons,
manufacturers’ literature should be carefully consulted in the selection of glazing products. For good
daylighting without excessive solar gain, look for a product whose VT/SHGC ratio is higher.
The standard requires that VT be determined in accordance with NFRC Standard 200 and that the VT
be verified and certified by the glazing manufacturer. For unlabeled glazed wall systems and skylights,
default values are provided in Table A8.1-2 of Appendix A. For other unlabeled products, use Table
A8.2. While VT may be listed in some glass manufacturers’ catalogs, remember that this value applies
just to the glass. The VT to be used in daylighting calculations or in trade-off options must be an overall
fenestration product value that considers glass, sash, and frame.
FYI
Interior and Exterior Shading Devices
Interior Fenestration Shading
Interior shading devices are not considered for compliance calculations in Standard 90.1. However,
interior shading was taken into account when determining the fenestration criteria. Had interior
shading not been considered, the criteria would be more stringent.
The main reason that interior devices are not credited in the compliance process is that that they are
usually not known at the time a building permit is issued for the building envelope. Interior shading
devices are more often included in the construction for tenant improvements, which comes later in the
building process. However, even when installed, their use is unpredictable and they can be readily
changed or replaced by users unaware of the energy implications. Consequently, their long-term
effectiveness cannot be counted on.
The benefit of interior shading devices depends on the glazing material. A white roller shade, for
instance, is more effective with clear glass than with low-transmission reflective glass. This is because
its effectiveness depends on the ability of the shade to reflect solar radiation back out the window, and
this ability is increased with high-transmission glass.
Exterior Fenestration Shading
The most effective way to control solar heat gains through windows is to intercept the sun before it
strikes the window. Exterior shading devices can be an effective means of achieving this. Exterior
shading devices include horizontal or vertical fixed-position louvers, moveable louvers, and
sunscreens. Sunscreens are often decorative in nature and range in style from large pattern aluminum
or metal screens to miniature louvers that enable less-obstructed views.
Exterior shading devices can be considered in complying with the standard if they are permanent
projections that will last as long as the building itself.
Example 5-DD. Determining Fenestration Performance Characteristics for Curtain Wall in High-Rise Office
Corresponding section: Fenestration and Doors (5.4.2 and 5.8.2)
Q
The designers of a glass curtain wall for a Boston office high-rise are proposing to use a standard low-e
double-glazed material and a standard thermally improved curtain wall framing system. The glazing
manufacturer’s literature shows a center-of-glass U-factor of 0.29, SHGC of 0.23, and VT of 0.32.
What are the options for determining the performance characteristics (U-factor, SHGC, and VT) for this
fenestration system?
A
There are two choices for determining the U-factor: either obtain NFRC data or use the defaults from
Table A8.2. The U-factor default is 0.90. Table A8.2 also gives the SHGC default as 0.50 and the VT
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default as 0.40. While these defaults could be used, they do not even come close to achieving
compliance with the standard.
The window-wall ratio (WWR) for the proposed high-rise is 38%, so from Table 5.5-5, the U-factor
criterion is 0.38 (2.16) and the SHGC criterion is 0.38. In addition, for spaces that require automatic
daylighting controls, the ratio of VT to SHGC must be 1.10 or greater. Note that the WWR calculation
only includes glass, sash, and framing in the vision area and not the opaque spandrel area. Spandrel
area is considered opaque wall, and must separately meet those requirements.
However, Exception 2 to Section 5.8.2.4 allows the glass manufacturer’s center-of-glass SHGC to be
used for compliance, because the total SHGC, including the effect of the opaque frame, will almost
always be lower, so it is conservative. The real problem in terms of compliance is the U-factor.
The NFRC option takes more time but produces performance data that are fair and reasonable. NFRC
has recently introduced the Component Modeling Approach, which simplifies the process of
determining the U-factor, SHGC, and VT for commercial fenestration. With this approach, the designer
separately selects a frame, glazing type, and spacer. NFRC online software then takes this information
and produces a Label Certificate that may be used for code compliance.
 Inspection and Verification (5.9)
This section helps to ensure that the performance claimed to meet the requirements of Section 5 is
realized in the field.
Section 5.9.1 requires a number of inspections that are crucial to the performance of the building
envelope, including but not limited to the use of compliant materials, components, and assemblies; the
proper installation of those elements; the condition of those elements; and the proper operation of
components.
Section 5.9.2 requires that the building envelope air leakage that is claimed is verified by one of two
options:
a. The first option is an air barrier design and installation program that includes a design review,
several field inspections, and compliance reporting.
b. The second option can be met by demonstrating compliance with the whole-building air
leakage test option in Section 5.4.3.1.3(a) and reporting the results.
Standard 90.1 User’s Manual
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114
0.1
0.2
0.3
0.3
0.3
0.3
0.95 0.95 0.95 0.95
0.1
0.5
0.1
0.1
0.1
0.1
0.95 0.95 0.95 0.95
60
60
60
60
60
60
60
60
60
60
60
64
64
60
67
67
80
77
77
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
60
70
80
80
75
1
60
60
70
80
80
75
1
1
0.3
60
60
70
80
80
75
1
1
0.3
60
60
70
80
80
75
1
1
0.3
60
60
70
80
80
75
1
1
0.3
0.3
0.4
60
60
60
80
80
80
1
1
1
0.3
0.3
0.4
60
60
60
80
80
80
1
1
1
0.3
0.3
0.4
60
60
60
80
78
0.3
0.3
0.4
Sun
Sat
60
80
80
0.3
0.3
0.4
60
80
80
78
0.3
0.3
0.4
Mon – Fri
80
80
80
0.3
0.3
0.5
0.05
Heating Set Point
80
80
80
0.3
0.9
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.3
0.9
0.35 0.35 0.35 0.35 0.35
0.9
0.1
80
80
80
1
1
0.3
0.5
0.9
0.2
Sun
Sat
80
1
1
0.3
0.5
0.8
0.2
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.3
0.5
0.9
0.3
80
1
1
1
0.3
0.5
0.9
0.3
Mon – Fri
1
1
1
0.3
0.4
0.9
0.5
Cooling Set Point
1
1
1
0.3
0.4
0.9
0.9
1
1
1
0.3
0.3
0.4
0.9
Sun
Sat
1
0.3
0.3
0.4
0.9
1
0.3
0.3
0.4
0.9
0.15 0.15 0.15 0.15 0.15 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.9
Mon – Fri
0.3
0.3
0.4
0.3
0.9
0
0
Infiltration
0.3
0.3
0.4
0.3
0.9
0
0
0.05 0.05
0.3
0.3
0.4
0.3
0.9
0
0
0.1
Sun
Sat
0.4
0.3
0.9
0
0
0.1
0.4
0.1
0.3
0
0
0.1
Mon – Fri
0.1
0.1
0
0.1
0.05 0.05
0.3
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.1
0.1
Plug Loads
Sat
0.1
0
0
0
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0
0
0
Sun
0
0
0
0.05 0.05 0.05 0.05 0.05
0
0
0
Mon – Fri
Lighting
0
0
0
0.05 0.05 0.05 0.05 0.05 0.05
0
0
Sun
Sat
0
Mon – Fri
Occupants
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-J SCHEDULE A
Cha p t e r 5 | Bu il d i ng E n vel o p e
Standard 90.1 User’s Manual
Standard 90.1 User’s Manual
1
86
86
60
60
0.25
86
86
60
60
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
0.6
0.9
0.9
0.6
0.9
0.9
0.6
0.9
0.9
60
86
0.25
60
86
1
60
60
60
86
86
86
1
1
1
60
60
60
86
86
86
1
1
1
60
60
60
86
86
86
1
1
1
0.3
0.3
0.3
65
65
65
80
80
80
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.18 0.09 0.03
0.18 0.09 0.03
0.5
0.5
0.5
0.2
0.18 0.09 0.03
0.6
0.9
0.9
0.2
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.2
0.6
0.9
0.9
0.35
0.65 0.55 0.35
Sun
Sat
0.6
0.8
0.9
0.7
0.7
1
0.9
0.2
0.25
0.7
0.8
0.7
0.35
Mon – Fri
0.7
0.8
0.3
0.25 0.35 0.55 0.65
0.3
0.5
Infiltration
0.7
0.8
0.9
0.2
0.3
0.8
0.2
0.7
0.8
0.9
0.3
0.35
0.8
0.03 0.02 0.03 0.02 0.05 0.12 0.13 0.15 0.18 0.21 0.26 0.29 0.27 0.25 0.23 0.23 0.26 0.26 0.24 0.22
0.7
0.8
0.9
0.5
0.5
0.8
0.03 0.02 0.03 0.02 0.05 0.12 0.13 0.15 0.18 0.21 0.26 0.29 0.27 0.25 0.23 0.23 0.26 0.26 0.24 0.22
0.7
0.8
0.9
0.5
0.5
0.5
Sun
Sat
0.5
0.6
0.9
0.3
0.45
0.25
0.2
0.5
0.6
0.9
0.2
0.2
0.2
0.03 0.02 0.03 0.02 0.05 0.12 0.13 0.15 0.18 0.21 0.26 0.29 0.27 0.25 0.23 0.23 0.26 0.26 0.24 0.22
0.3
0.3
0.6
0.2
0.4
0.4
Mon – Fri
0.3
0.3
0.6
0.5
0.5
0.7
Plug Loads
0.15 0.15 0.15 0.15 0.15
0.15 0.15 0.15 0.15 0.15
0.4
0.5
0.5
0.8
0.2
0.2
0.4
0.05
0.05
0.5
Sun
Sat
0.2
0
0
0.2
0.15 0.15 0.15 0.15 0.15
0
0
0.4
Mon – Fri
0
0
0.4
Lighting
0
0
0.4
0
0
0.1
0.05
0.05
0.05
Sun
Sat
0
Mon – Fri
0
0
0.05
Occupants
0
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-K SCHEDULE B
Cha p t e r 5 | Bu il d i ng E n vel o p e
115
116
0
0.3
85
85
60
60
85
85
60
60
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
60
85
60
85
1
1
1
1
Sun
Sat
1
1
Mon – Fri
Infiltration
0.2
0.2
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
0.6
0.9
0.9
0.4
0.9
0.9
0.2
0.8
0.7
0.8
0.9
0.9
0.6
0.9
0.9
0.4
0.8
0.7
0.8
0.9
0.9
0.6
0.9
0.9
0.4
0.8
0.7
0.8
0.9
0.9
0.6
0.9
0.9
0.4
0.8
0.7
0.8
0.9
0.9
0.6
0.9
0.9
0.4
0.8
0.8
0.8
0.9
0.9
0.6
0.9
0.9
0.4
0.8
0.7
0.6
0.9
0.9
0.4
0.9
0.9
0.2
0.6
0.5
0.4
0.7
0.9
0.2
0.5
0.5
0.1
0.2
0.5
0.3
0.5
0
0.2
0.3
0.1
0.2
0
0.1
0
60
65
65
85
80
80
1
60
70
70
85
75
75
1
65
70
70
80
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.05
0.05
0
0
0
0.5
0.7
0.3
0.2
0.15
0.2
60
70
70
85
75
75
1
60
70
70
85
75
75
1
60
70
60
85
75
85
1
1
60
60
60
85
85
85
1
1
1
0.15 0.15 0.15 0.15
0.5
0.7
0.05 0.05 0.05 0.05
0.3
0.5
0
0.2
0.3
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.6
0.9
0.9
0.4
0.9
0.9
0.2
0.6
0.5
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.8
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.2
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.2
0.5
Sun
Sat
0.2
0.3
0.9
0.2
0.4
0.6
0.2
0.6
0.2
0.3
0.1
0.5
0.5
Mon – Fri
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.1
0
0.2
0.2
Plug Loads
Sat
0.2
0
0.1
0.1
0.1
0
0
0
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0
0
0
Sun
0
0
0
0.9
0
0
0
0.4
0
0
0
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0
0
0
Mon – Fri
Lighting
0
0
0
Sun
Sat
0
0
Mon – Fri
Occupants
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-L SCHEDULE C
Cha p t e r 5 | Bu il d i ng E n vel o p e
Standard 90.1 User’s Manual
0.1
0.1
0.4
0.4
1
1
75
75
70
70
0.1
0.1
0.5
0.5
1
1
75
75
70
70
Lighting
Mon – Fri
Sun
Plug Loads
Mon – Fri
Standard 90.1 User’s Manual
Sun
Infiltration
Mon – Fri
Sun
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
Sat
Sat
Sat
70
75
1
0.5
0.1
70
75
1
0.4
0.1
1
1
1
1
Sun
Sat
1
1
Mon – Fri
Occupants
70
70
70
75
75
75
1
1
1
0.4
0.4
0.4
0.1
0.1
0.1
1
1
1
70
70
70
75
75
75
1
1
1
0.4
0.4
0.4
0.1
0.1
0.1
1
1
1
70
70
70
75
75
75
1
1
1
0.4
0.4
0.4
0.2
0.2
0.2
1
1
1
70
70
70
75
75
75
1
1
1
0.4
0.4
0.4
0.4
0.4
0.4
1
1
1
70
70
70
75
75
75
1
1
1
0.5
0.5
0.5
0.4
0.4
0.4
1
1
1
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.4
0.4
0.4
0.9
0.9
0.9
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.2
0.2
0.2
0.4
0.4
0.4
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.1
0.1
0.1
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.1
0.1
0.1
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.1
0.1
0.1
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.1
0.1
0.1
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.1
0.1
0.1
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.1
0.1
0.1
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.2
0.2
0.2
0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.25 0.25 0.25 0.25 0.25 0.25 0.25
70
70
70
75
75
75
1
1
1
0.8
0.8
0.8
0.4
0.4
0.4
0.3
0.3
0.3
70
70
70
75
75
75
1
1
1
1
1
1
0.6
0.6
0.6
0.5
0.5
0.5
70
70
70
75
75
75
1
1
1
1
1
1
0.8
0.8
0.8
0.9
0.9
0.9
70
70
70
75
75
75
1
1
1
0.9
0.9
0.9
1
1
1
0.9
0.9
0.9
70
70
70
75
75
75
1
1
1
0.9
0.9
0.9
1
1
1
0.9
0.9
0.9
70
70
70
75
75
75
1
1
1
0.8
0.8
0.8
0.7
0.7
0.7
1
1
1
70
70
70
75
75
75
1
1
1
0.7
0.7
0.7
0.4
0.4
0.4
1
1
1
70
70
70
75
75
75
1
1
1
0.6
0.6
0.6
0.2
0.2
0.2
1
1
1
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-M SCHEDULE D
Cha p t e r 5 | Bu il d i ng E n vel o p e
117
118
75
75
70
70
75
75
70
70
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
0.6
0.6
0.6
0.6
0.6
0.9
0.6
0.9
0.6
0.9
0.4
0.6
0.4
0.6
0.5
0.8
0.4
0.4
0.6
0.5
0.5
0.4
0.4
0.6
0.5
0.5
0.5
0.4
0.4
0.6
0.5
0.5
0.5
0.4
0.4
0.6
0.5
0.5
0.5
0.4
0.4
0.6
0.5
0.5
0.5
0.4
0.4
0.4
0.5
0.5
0.5
0.4
0.4
Cooling Set Point
0.6
0.9
0.5
0.8
0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65
0.9
0.7
0.8
0.5
0.4
0.4
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.4
0.5
0.9
0.7
0.8
0.5
0.4
0.4
70
75
70
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.4
0.4
0.9
0.7
0.8
0.5
0.4
0.4
Sun
Sat
0.4
0.4
0.9
0.7
0.8
0.9
0.4
0.4
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.4
0.4
0.7
0.7
0.8
0.9
0.4
0.5
0.4
Mon – Fri
0.4
0.4
0.4
0.7
0.8
0.9
0.4
0.5
0.4
Infiltration
0.4
0.4
0.4
0.7
0.8
0.9
0.4
0.6
0.4
0.4
0.4
0.4
0.7
0.8
0.9
0.6
0.6
0.4
0.4
0.4
0.4
0.5
0.5
0.9
0.6
0.6
0.5
Sun
Sat
0.4
0.5
0.5
0.9
0.6
0.6
0.5
0.4
0.5
0.5
0.9
0.6
0.6
0.6
0.4
0.5
0.5
0.5
0.6
0.6
0.8
Mon – Fri
0.5
0.5
0.5
0.6
0.6
0.8
Plug Loads
0.5
0.5
0.5
0.6
0.6
0.8
0.5
0.5
0.5
0.6
0.6
0.8
0.5
0.5
0.5
0.4
0.5
0.8
Sun
Sat
0.5
0.4
0.4
0.8
0.5
0.4
0.4
0.8
0.5
0.4
0.4
0.8
Mon – Fri
0.4
0.4
0.6
Lighting
0.4
0.4
0.5
0.4
0.4
0.4
0.4
0.4
0.4
Sun
Sat
0.4
Mon – Fri
0.4
0.4
0.4
Occupants
0.4
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-N SCHEDULE E
Cha p t e r 5 | Bu il d i ng E n vel o p e
Standard 90.1 User’s Manual
1
1
1
1
1
1
0.77 0.53 0.53
0.77 0.53 0.53
0.77 0.43 0.43
0.3
0.3
0.2
0.3
0.3
0.2
0.3
0.3
0.2
0.3
0.3
0.2
0.3
0.3
0.2
0.3
0.3
0.2
0.3
0.3
0.3
0.3
Standard 90.1 User’s Manual
Sun
Sat
Mon – Fri
Heating Set Point
Sun
Sat
Mon – Fri
Cooling Set Point
Sun
Sat
Mon – Fri
Infiltration
Sun
Sat
Mon – Fri
Plug Loads
Sat
0.62
0.62
0.9
0.9
0.62 0.29 0.29 0.29 0.29 0.29 0.29 0.29 0.43 0.51 0.49 0.66
0.62 0.29 0.29 0.29 0.29 0.29 0.29 0.29 0.43 0.51 0.49 0.66
0.43 0.43 0.26 0.26 0.26 0.26 0.26 0.26 0.26 0.51 0.51 0.49 0.66
1
1
0.7
0.7
0.7
1
1
0.35 0.11
0.35 0.11
0.35 0.11
0.85 0.41
0.85 0.41
0.89 0.67 0.33
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.3
0.3
0.9
1
1
1
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.11 0.11 0.11 0.11 0.11 0.11
0.11 0.11 0.11 0.11 0.11 0.11
0.11 0.11 0.11 0.11 0.11 0.11 0.62
1
0.53 0.54 0.65 0.65 0.77 0.77 0.77
1
0.53 0.54 0.65 0.65 0.77 0.77 0.77
0.31 0.54 0.54 0.54 0.77 0.77 0.89
0.26 0.26 0.11 0.11 0.11 0.11 0.41 0.41 0.56 0.56 0.41 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.85
1
1
1
Sun
1
1
1
0.26 0.26 0.11 0.11 0.11 0.11 0.41 0.41 0.56 0.56 0.41 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.85
1
1
1
0.22 0.17 0.11 0.11 0.11 0.22 0.44 0.56 0.44 0.44 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.67 0.89
1
1
1
Mon – Fri
Lighting
Sun
Sat
Mon – Fri
Occupants
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-O SCHEDULE F
Cha p t e r 5 | Bu il d i ng E n vel o p e
119
120
0
80
80
60
60
80
80
60
60
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
60
80
60
80
1
1
1
Sun
1
1
1
Sat
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0
0
0
0
0.18 0.18 0.18
0
0
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35
Mon – Fri
Infiltration
0.9
0
0
0
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35
0.9
0
0
0
Sun
Sat
0.9
0
0
0
0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18
0.9
0
0
0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.15 0.15 0.15 0.15 0.15
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.35 0.35 0.35 0.35 0.35 0.35 0.35
Sat
0.9
0
0
0
Mon – Fri
0
0
0
Plug Loads
0
0
0
0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18
0
0
0
Sun
0
0
0
0.18 0.18 0.18 0.18 0.18 0.18 0.18
0
0
0
Mon – Fri
Lighting
0
0
0
Sun
Sat
0
0
Mon – Fri
Occupants
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-P SCHEDULE G
Cha p t e r 5 | Bu il d i ng E n vel o p e
Standard 90.1 User’s Manual
0
0.1
0.2
0.2
0.1
0.6
0.8
0.1
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.8
0.7
0.6
0.2
0.7
0.6
0.2
0.7
0.6
0.2
0.7
0.8
0.2
0.2
0.1
0.1
0
0
0
Standard 90.1 User’s Manual
80
80
60
60
80
80
60
60
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
60
80
60
80
1
1
1
1
Sun
Sat
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
70
80
80
75
1
1
0.3
0.3
0.4
0.3
0.3
0.3
0.5
0.3
0.5
0.3
0.4
0.3
0.5
0.3
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
70
70
70
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
0.05
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
70
70
70
75
75
75
60
60
60
80
80
80
1
1
1
0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.05 0.05
0.5
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
75
0.05
0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.05 0.05
0.4
0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.25 0.05
0.3
0.4
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.3
0.3
0.4
0.3
0.3
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.4
0.05 0.05 0.05 0.05 0.05 0.05 0.05
Mon – Fri
Infiltration
Sat
Sat
0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.1
0.2
0.2
Sun
0.1
0.2
0.2
0.05 0.05 0.05 0.05 0.05 0.05
0
0
0
Mon – Fri
0
0
0
Plug Loads
0
0
0
0.05 0.05 0.05 0.05 0.05 0.05 0.05
0
0
0
Sun
0
0
0
0.05 0.05 0.05 0.05 0.05 0.05 0.35 0.35 0.35 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.25 0.05
0
0
0
Mon – Fri
Lighting
0
0
0
Sun
Sat
0
0
Mon – Fri
Occupants
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-Q SCHEDULE H
Cha p t e r 5 | Bu il d i ng E n vel o p e
121
122
0
85
85
60
60
85
85
60
60
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
60
85
60
85
1
1
1
Sun
1
1
1
Sat
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0.9
0
0
0
0
0
0
0.18 0.18 0.18
0
0
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
60
60
70
85
85
75
1
1
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
60
60
60
85
85
85
1
1
1
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35
Mon – Fri
Infiltration
0.9
0
0
0
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35
0.9
0
0
0
Sun
Sat
0.9
0
0
0
0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18
0.9
0
0
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35
0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.35 0.35 0.35 0.35 0.35 0.35 0.35
Sat
0.9
0
0
0
Mon – Fri
0
0
0
Plug Loads
0
0
0
0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18
0
0
0
Sun
0
0
0
0.18 0.18 0.18 0.18 0.18 0.18 0.18
0
0
0
Mon – Fri
Lighting
0
0
0
Sun
Sat
0
0
Mon – Fri
Occupants
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-R SCHEDULE I
Cha p t e r 5 | Bu il d i ng E n vel o p e
Standard 90.1 User’s Manual
0
0.1
0.2
0.3
0.3
0.3
0.3
0.95 0.95 0.95 0.95
0.1
0.5
0.1
0.1
0.1
0.1
0.95 0.95 0.95 0.95
Standard 90.1 User’s Manual
80
80
60
60
80
80
60
60
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
60
80
60
80
1
1
1
1
Sun
Sat
1
1
Mon – Fri
Infiltration
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
0.3
0.3
0.9
0.3
0.9
0.3
0.9
0.3
0
0
0.05
0
0
0
0.9
0.9
0.9
0.9
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
75
75
1
60
70
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
70
80
80
75
1
1
60
60
60
80
80
80
1
1
1
60
60
60
80
80
80
1
1
1
0.65 0.65 0.65 0.65 0.65 0.25 0.05
0.15 0.15 0.15 0.15 0.15 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.8
0.15 0.15 0.15 0.15 0.15 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.1
0.9
0.3
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.1
0.3
0.3
0
0
0.1
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05 0.05
0.1
0.3
0
0
0.1
Sun
Sat
0.1
0.1
0
0
0.1
0.85 0.85 0.85 0.85 0.75 0.85 0.85 0.85 0.85 0.65 0.65 0.65 0.65 0.65 0.25 0.05
0
0
0.1
0.05 0.05 0.05 0.05 0.05
0.1
0.3
0
0.05
0.3
Mon – Fri
0.05 0.05 0.05 0.05 0.05 0.05
0.1
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0.1
0.1
Plug Loads
Sat
0.1
0
0
0
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
0
0
0
Sun
0
0
0
0.05 0.05 0.05 0.05 0.05
0
0
0
Mon – Fri
Lighting
0
0
0
Sun
Sat
0
0
Mon – Fri
Occupants
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-S SCHEDULE J
Cha p t e r 5 | Bu il d i ng E n vel o p e
123
124
0.5
0.5
1
1
1
1
NA
NA
NA
NA
0.5
0.5
1
1
1
1
NA
NA
NA
NA
Lighting
Mon – Fri
Sun
Plug Loads
Mon – Fri
Sun
Infiltration
Mon – Fri
Sun
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
Sat
Sat
Sat
NA
NA
1
1
0.5
NA
NA
1
1
0.5
0
0
0
0
Sun
Sat
0
0
Mon – Fri
Occupants
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
0.5
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
0.5
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
0.5
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
0.5
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
1
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
1
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
0.5
0
0
0
NA
NA
NA
NA
NA
NA
1
1
1
1
1
1
0.5
0.5
0.5
0
0
0
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-T SCHEDULE K
Cha p t e r 5 | Bu il d i ng E n vel o p e
Standard 90.1 User’s Manual
0.1
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
Plug Loads
Mon – Fri
Standard 90.1 User’s Manual
1
80
80
60
60
1
80
80
60
60
Cooling Set Point
Mon – Fri
Sun
Heating Set Point
Mon – Fri
Sun
Sat
Sat
60
80
60
80
1
Sun
Sat
1
1
1
Mon – Fri
Sun
0
0
Infiltration
0
0
Sat
0.1
0.1
0.1
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
60
60
60
80
80
80
1
1
60
60
60
80
80
80
1
1
0
0
1
0.1
0.1
60
60
60
80
80
80
1
1
0.5
0
0
1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
0.5
0
0
1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
0.5
0
0
1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
0.5
0
0
0.25
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
0.5
0
0
1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
0.5
0
0
1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
0.5
0
0
1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
60
60
60
80
80
80
1
1
0
0
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
0
0
0
60
60
60
80
80
80
1
1
1
0
0
0.1
0.1
0.1
0
0
0
0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.1
0.1
0.6
0
0
0
1.25 1.25
0
0
1
0.1
0.1
0
0
0
0.75 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.75
0
0
0
1.25 1.25
0
0
0.1
0.1
0.6
0
0
Sun
Sat
0.1
0
0
0.1
0
0
0.1
0
0
0
Mon – Fri
0
0
0
Lighting
0
0
0
0
0
0
0
0
0
Sun
Sat
0
0
0
Mon – Fri
Occupants
1am 2am 3am 4am 5am 6am 7am 8am 9am 10am 11am 12am 1pm 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 10pm 11pm 12pm
TABLE 5-U SCHEDULE L
Cha p t e r 5 | Bu il d i ng E n vel o p e
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6 HVAC Systems
Changes to the HVAC Section
 The 2016 edition of the standard adds a new Reference Standard Reproduction Annex 1 that
includes excerpts of climate zone data from ASHRAE Standard 169 for the United States, Canada,
and other international locations.
 A new Climate Zone 0 for extremely hot climates has been added to the standard everywhere that
climate-zone-based requirements are used.
 The requirements for HVAC equipment installed to replace existing equipment have been clarified,
and requirements for controls have been added (6.1.1.3.1).
 The Performance Rating Method, described in Appendix G, has been added as an additional
compliance method (6.2).
 Mandatory minimum efficiency requirements have been added for vapor-compression-based
indoor pool dehumidifiers; electrically operated DX-DOAS units, single-package and remote
condenser, without energy recovery; and electrically operated DX-DOAS units, single-package and
remote condenser, with energy recovery (6.4.1.1).
 Requirements have been added for control of HVAC systems, including temperature set point and
ventilation, in hotel/motel guest rooms (6.4.3.3.5).
 Requirements for vestibules have been expanded to include cooling as well as heating (6.4.3.9).
 Requirements for monitoring of chilled-water plants have been added (6.4.3.11).
 Requirements for fault detection and diagnostics have been added for systems with DX cooling and
air economizers (6.4.3.12).
 Economizer requirements have been modified to replace the term “water economizer” with “fluid
economizer” to allow use of economizers using other fluids, such as refrigerant and water/glycol
mixtures (6.5.1.2).
 A new requirement limits heating of ventilation air to no more than 60°F (16°C) in multiple zone
systems where the majority of zones require cooling (6.5.2.6).
 Control requirements have been added for return and relief fans (6.5.3.2.4) and parallel-flow fanpowered VAV air terminals (6.5.3.4).
 Requirements have been added for design of ventilation (6.5.3.7).
 Requirements have been added for selection of chilled-water coils (6.5.4.7).
 Equipment efficiency tables have been updated (6.8).
These changes are marked with  in the margins of this chapter. For the specific addenda that define
the differences between the 2013 and 2016 editions of Standard 90.1, see Appendix H to the standard.
General (6.1)
Scope (6.1.1)
In general, except where specifically noted by the standard, all new mechanical equipment, systems,
and controls serving a building’s heating, cooling, ventilating, or refrigeration needs must meet the
requirements of Section 6. This is true for new construction as well as additions or alterations,
including equipment replacement, to existing buildings.
However, there are a number of important instances when the standard does not apply to replacement
HVAC&R equipment. In particular, the standard does not apply in these situations (see the exceptions
to Section 6.1.1.3):
• When equipment is repaired but not replaced. The equipment being repaired does not have to meet
the standard’s minimum efficiencies; however, the modifications may not increase the equipment’s
energy use. For instance, if a condenser coil is replaced, the new coil must have an equal or better
heat transfer performance than the coil being replaced.
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• When the replacement of existing equipment with complying equipment requires extensive
revisions to other systems, equipment, or elements of the building, and where the replacement
equipment is a like-for-like replacement.
• When the refrigerant in existing equipment is changed but cannot be replaced with the same
refrigerant due to the phase-out of the existing refrigerant. This may reduce the efficiency of the
existing equipment but is allowed.
• When existing equipment is relocated. For instance, the standard does not apply when an existing
hydronic heat pump is moved to another location within the building.
• When ducts and pipes are located in existing spaces with insufficient space for the code-required
insulation. For example, if the piping in an existing chase needs to be replaced and there is not
sufficient space for the new code-required insulation, the piping may be installed with thinner
insulation.
FYI
Energy Performance and Integrated Design
HVAC&R systems are one of the most significant users of energy in buildings. In typical office and retail
buildings, HVAC&R energy consumption normally accounts for about one-third of the building’s energy
consumption. In hospitals, laboratories, and data centers, the energy intensity of HVAC&R equipment
(on a per square foot [metre] basis) can be significantly larger. In typical high-rise residential
buildings, HVAC&R and domestic water heating are the two largest energy consumers.
HVAC&R system designers can have a major effect on a building’s energy costs and consumption. A
poorly designed and/or commissioned HVAC&R system can significantly increase annual energy
consumption. An efficient system, however, is not merely one that uses efficient equipment. Rather, it
is the integrated system design, proper commissioning, and maintenance that will determine the
overall efficiency and energy bills for the building.
System interactions play a major role in overall system efficiency. For example, the way a system is
controlled—particularly for systems that serve multiple zones—can be a much more important factor
in determining overall HVAC&R system performance than the rated full-load or part-load efficiency of
each piece of equipment. The requirements of Section 6 set minimum standards for the efficiency of
HVAC&R equipment and for the design of HVAC&R systems. Although compliance should ensure
acceptable HVAC&R system performance, designers may wish to consider additional measures for the
specific microclimate and application. Design choices should be made that consider the annual
HVAC&R system performance and not just the full-load design performance. Additionally, operating
efficiency is dependent on proper commissioning and maintenance.
 Compliance Paths (6.2)
There are five approaches to compliance with the standard for HVAC&R systems: the Simplified
Approach Option for HVAC Systems (Section 6.3), the Prescriptive Path (Section 6.5), the Alternative
Compliance Path (Section 6.6), the Energy Cost Budget (ECB) Method (Section 11), and the
Performance Rating Method (Appendix G). These compliance paths are shown in Figure 6-A.
The mandatory provisions (Section 6.4) apply to all systems complying by the Prescriptive Path, the
Alternative Compliance Path, or the ECB Method. Regardless of the compliance path chosen, the
provisions of Section 6.7 Submittals and Section 6.8 Minimum Equipment Efficiency Tables also apply.
Compliance Forms
Compliance forms and worksheets intended to facilitate the process of complying with the standard
are available for download from ASHRAE’s website at http://www.ashrae.org/UM90.1-2016.
Simplified Approach Option (6.3)
The Simplified Approach Option applies only to one- to two-story buildings that are heated or cooled
using single-zone air-cooled HVAC systems. The objective is to provide a simpler approach to
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Standard 90.1 User’s Manual
Cha p t e r 6 | H VAC Sys t e ms
compliance, with all of the HVAC requirements on one or two pages. The building must have a gross
floor area of 25,000 ft² (2300 m²) or less, and the HVAC system must meet a list of criteria specified in
Section 6.3.2.
Prescriptive Path (Section 6.5)
The prescriptive compliance path may be used for any HVAC&R system, but it is primarily used for
larger buildings and buildings with more complex systems for which the Simplified Approach Option is
not applicable. Examples include systems with fume exhaust, multiple-zone systems, and central
hydronic heating and cooling plants. Systems complying using the Prescriptive Path must also satisfy
the mandatory provisions of Section 6.4.
 FIGURE 6-A. COMPLIANCE PATHS
Corresponding section: Compliance Paths (6.2)
Alternative Compliance Path (6.6)
Section 6.6 provides an alternative compliance path for computer room systems that may not be able
to comply with the requirements of Sections 6.3 or 6.5 and cannot be modeled according to the
methodology provided in Section 11. This section currently only applies to computer room systems
and may not be applied to other systems. Systems complying using the Alternative Compliance Path
must also satisfy the mandatory provisions of Section 6.4, 6.7, and 6.8.
Energy Cost Budget Method (Section 11)
The ECB Method is intended for building systems that are unable to meet certain prescriptive
requirements or for designers who want to use the method to explore design alternatives. It allows
trade-offs between various building systems and components. Systems complying using the ECB
Method must satisfy the mandatory provisions of Section 6.4 as well as the ECB requirements in
Section 11. The ECB Method is addressed in Chapter 11 of this user’s manual.
Standard 90.1 User’s Manual
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 Performance Rating Method (Appendix G)
The Performance Rating Method was originally added to the standard to provide a methodology that
designers and energy analysts can use to calculate a proposed building design’s energy savings as
compared to baseline compliance with the standard. The Performance Rating Method is widely used
for LEED projects and for utility incentive programs. Beginning with the 2016 version of the standard,
it can also be used to show compliance with the standard. Systems complying using the Performance
Rating Method must also satisfy the mandatory provisions of Section 6.4.
Simplified Approach Option for HVAC Systems (6.3)
Scope (6.3.1)
The Simplified Approach Option for HVAC Systems can reduce the effort required to show compliance
for HVAC systems serving small buildings. Small buildings—those less than 25,000 ft² (2300 m²)—
represent more than 80% of new building construction in the United States. They generally are served
by simple, single-zone HVAC systems.
Many of the requirements in Section 6 do not apply to these simple systems. Rather than having
designers search the entire section for applicable requirements for small-building systems, these
requirements are grouped into Section 6.3. This approach is intended to be entirely consistent with the
Prescriptive Path so that a system complying by either approach is subject to the same requirements.
The Simplified Approach Option can only be used for buildings and system types that meet the
following criteria:
• The building served by the system must be two stories or less in height.
• The building served by the system must be less than 25,000 ft² (2300 m²) in gross floor area.
• Cooling (if any) must be provided by a unitary packaged or split-system air conditioner that is
either air cooled or evaporatively cooled with efficiency meeting the requirements shown in Table
6.8.1-1 (air conditioners), Table 6.8.1-2 (heat pumps), or Table 6.8.1-4 (packaged terminal and
room air conditioners and heat pumps) for the applicable equipment category.
• Each HVAC system must meet the requirements of Section 6.3.2 Criteria.
Systems with water-cooled direct-expansion (DX), chilled-water systems; multiple-zone systems;
variable-refrigerant-flow (VRF) systems; computer-room air conditioners (CRACs); and
noncompressor cooling systems, such as evaporative cooling, are excluded from this method of
compliance. They must use the Prescriptive Method (Section 6.5), the Alternative Compliance Path
(Section 6.6), or the ECB Method (Section 11) for demonstrating compliance.
Examples 6-A, 6-B, and 6-C address the Simplified Approach Option.
Criteria (6.3.2)
If the basic qualifications are met, and the designer chooses to demonstrate compliance with the
standard by using the Simplified Approach Option, the HVAC&R system must meet the following
requirements (Sections 6.3.2[a] through 6.3.2[s]).
Single Zone (6.3.2[a])
The HVAC&R system must serve a single zone. Systems with any level of subzoning (that is, systems
with any more than one thermostatic control) cannot use this approach for showing compliance. This
applies to both air and hydronic heating systems. Where a hydronic heating system is employed, it can
only serve a single zone if using Section 6.3 for compliance. Buildings with multiple-zone hydronic
systems must use the other paths to compliance (the mandatory provisions of Section 6.4 and either
the Prescriptive Path (Section 6.5) or ECB Method (Section 11).
Variable Fan Airflow (6.3.2[b])
For systems with a cooling capacity above that listed in Section 6.5.3.2.1, the supply fan must follow
the required indoor fan control method. See the Prescriptive Path (6.5) section of this chapter for
further information on this requirement.
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Cooling Efficiency (6.3.2[c])
If cooling is provided, cooling efficiency must meet the requirements shown in Table 6.8.1-1 (air
conditioners), Table 6.8.1-2 (heat pumps), or Table 6.8.1-4 (packaged terminal and room air
conditioners and heat pumps) for the applicable equipment category. See the Mandatory Provisions
(6.4) section of this chapter for further information on equipment efficiency ratings.
Economizers (6.3.2[d])
If the system has mechanical cooling with a capacity that exceeds the limit shown in Table 6.5.1-1, the
system must have an air-side economizer or comply with the incremental cooling efficiency rating
adjustments of Table 6.5.1-3. See the Prescriptive Path (6.5) section of this chapter for an example of
how to use Table 6.5.1-3.
High-limit controls must meet the requirements of Table 6.5.1.1.3; the 2016 edition of the standard,
changes when these controls are used on air-side economizers. The system must also have either a
barometric or powered relief sized to prevent overpressurization of the building when the economizer
is operating. Note that if the static pressure through the relief air path is 0.1 in. of water (24.8 Pa) or
greater, it is recommended that power exhaust be used to maintain building pressure control. See the
Mandatory Provisions (6.4) section of this chapter for the leakage and control requirements for
economizer dampers.
Note that water-side economizers cannot be used for compliance in the Simplified Approach Option.
Heating (6.3.2[e])
For systems with heating capability, the following heating options meet the standard’s requirements:
• Unitary heat pump that meets the efficiency requirements shown in Table 6.8.1-2 (heat pumps) or
Table 6.8.1-4 (packaged terminal and room air conditioners and heat pumps)
• Fuel-fired furnace that meets the efficiency requirements shown in Table 6.8.1-5 (furnaces, duct
furnaces, and unit heaters)
• Electric resistance heater
• Hot-water or steam baseboard convector or radiator connected to a boiler that meets the efficiency
requirements shown in Table 6.8.1-6
See the Mandatory Provisions (6.4) section in this chapter for further information on equipment
efficiency ratings.
Exhaust Air Energy Recovery (6.3.2[f])
Each HVAC system must meet the exhaust air energy recovery requirements of Section 6.5.6.1. The
minimum outdoor air quantity, climate zone, and design supply fan airflow rate are shown in Tables
6.5.6.1-1 and 6.5.6.1-2.
See the Mandatory Provisions (6.4) section of this chapter for further information on the exhaust air
energy recovery requirements.
Thermostat (6.3.2[g])
Each system with both heating and cooling must be controlled by either a manual changeover or dual
set-point thermostat. Almost all thermostats offered as standard options from unitary equipment
manufacturers comply with this requirement.
Heat Pump Auxiliary Heat Control (6.3.2[h])
If heat is provided by a heat pump that is equipped with auxiliary electric resistance heaters installed
within the heat pump, controls must be provided that prevent supplemental heater operation when
the heating load can be met by the heat pump alone during both steady-state operation and setback
recovery.
This requirement differs from the mandatory provisions requirement (Section 6.4.3.5) in that only two
means are acceptable for control of the heat pump:
Standard 90.1 User’s Manual
131
Cha p t e r 6 | H VAC Sys t e ms
1.
A digital or electronic thermostat designed for heat pump use that energizes auxiliary heat only
when the heat pump has insufficient capacity to maintain set point or to warm the space at a
sufficient rate
2. A multistage space thermostat and an outdoor air thermostat wired to energize auxiliary heat only
on the last stage of the space thermostat and when the outdoor air temperature is less than 40°F
(4°C)
Heat pumps that have a minimum efficiency regulated by the National Appliance Energy Conservation
Act (NAECA) (<65 KBtu/h [19 kW] in cooling capacity) must meet the heating seasonal performance
factor (HSPF) rating requirements shown in Table 6.8.1-2; if a heat pump only includes internal
electric resistance heating, it is exempted from this requirement.
Reheat for Humidity Control (6.3.2[i])
The system controls must not permit reheating, recooling, or any other form of simultaneous heating
and cooling for humidity control. If reheat/recool is desired for humidity control in a humid climate,
the Prescriptive Path (Section 6.5) must be used to demonstrate compliance.
Off-Hour Shutoff and Setback (6.3.2[j])
Systems serving spaces other than hotel/motel guest rooms, and other than those requiring
continuous operation, that have both a cooling/heating capacity greater than 15,000 Btu/h (4.4 kW)
and a supply fan motor power greater than 3/4 hp (0.5 kW) must be provided with a time
switch/controller with all the following capabilities:
• Can start and stop the system under different schedules for seven different day-types per week
• Is capable of retaining programming and time setting during a loss of power for a period of at least
10 hours
• Includes an accessible manual override that allows temporary operation of the system for up to two
hours
• Is capable of temperature setback down to 55°F (13°C) during off hours
• Is capable of temperature setup to 90°F (32°C) during off hours
A true seven-day electronic thermostat (typically an option from the unitary equipment manufacturer)
will meet these requirements. However, weekday/weekend (5-2) and weekday/Saturday/Sunday (51-1) thermostats, typically intended for residential applications, do not comply with this requirement.
Hotel/Motel Guest Rooms (6.3.2[k])
Systems serving hotel/motel guest rooms must comply with special control requirements in Section
6.4.3.3.5. Refer to Automatic Control of HVAC in Hotel/Motel Guest Rooms (6.4.3.3.5) of this manual for
more information.
Piping Insulation (6.3.2[l])
Except for piping within manufacturers’ units, HVAC piping must be insulated in accordance with
Tables 6.8.3-1 and 6.8.3-2. Insulation exposed to weather must be suitable for outdoor service (for
example, protected by a cover made of aluminum, sheet metal, painted canvas, or plastic). Cellular
foam insulation must also be protected in the same way or painted with a coating that is water
retardant and prevents ultraviolet degradation. See the Mandatory Provisions (6.4) section of this
chapter for the new insulation requirements for piping systems.
Ductwork Insulation (6.3.2[m])
Ductwork and plenums must be insulated in accordance with Tables 6.8.2-1 and 6.8.2-2 and sealed in
accordance with Section 6.4.4.2.1 Duct Sealing. See HVAC System Construction and Insulation (Section
6.4.4) in this chapter for more details.
Air Balancing (6.3.2[n])
Construction documents must require that all HVAC systems with field-installed ductwork be air
balanced in accordance with industry accepted procedures. Industry accepted test and balance
standards include the following:
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• ASHRAE Standard 111
• National Environmental Balancing Bureau (NEBB) Procedural Standards
• Associated Air Balance Council (AABC) National Standards
Outdoor Air Intake and Exhaust Systems (6.3.2[o])
Outdoor air intake and exhaust systems must meet the requirements of Section 6.4.3.4. See the
Ventilation System Controls (6.4.3.4) section in this chapter for more details.
Simultaneous Heating and Cooling (6.3.2[p])
Where separate heating and cooling equipment serve the same temperature zone, thermostats must be
interlocked to prevent simultaneous heating and cooling. Situations where this requirement may apply
include two or more systems serving a single space, such as a baseboard heating system and an
overhead cooling system. If each of these units has its own thermostat, the controls must be
interlocked to prevent the heating and cooling from operating simultaneously.
Another example is a theater or large meeting room served by two or more units. The thermostats for
each unit will generally prevent the heating and cooling within the unit from simultaneously operating,
but when two units serve the same room, it’s possible for one to be heating and the other to be cooling
unless the thermostats are properly interlocked.
Optimum Start (6.3.2[q])
Systems with a design supply air capacity greater than 10,000 cfm (5000 L/s) must have optimum
start controls. Optimum start controls are defined in Section 3 as “controls that are designed to
automatically adjust the start time of an HVAC system each day with the intention of bringing the space
to desired occupied temperature levels immediately before scheduled occupancy.” Optimum start
routines are usually standard for digital control systems. Start time is computed from the current
outdoor air temperature, space temperature, and a mass/capacity factor that describes how quickly
the system can warm up or cool down the space. This factor is often self-tuned by the controller based
on historical performance.
For installations using electric controls, so-called “intelligent controls” also comply with the standard.
This control logic, which is an option on some electronic thermostats, adjusts the start time based on
the difference between current space temperature and occupied set point. Even though the logic
ignores outdoor air temperature, and the mass/capacity factor is not usually adjustable, this control
logic meets the definition of optimum start controls and thus complies with this section.
Demand Control Ventilation (6.3.2[r])
HVAC systems serving high-density occupancies must be provided with demand control ventilation
(DCV) systems, as described in Section 6.4.3.8 Ventilation Controls for High-Occupancy Areas. In
addition, they must meet the ventilation design requirements of Section 6.5.3.7 Ventilation Design.
See the Mandatory Provisions (6.4) section of this chapter for more information on demand control
ventilation systems.
Door Switches (6.3.2[s])
The system must comply with the door switch requirements of Section 6.5.10. Refer to the Door
Switches (6.5.10) section of this chapter for more information.
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Example 6-A. Simplified Approach Option, Building Area Restriction
Corresponding section: Scope (6.3.1)
Q
A strip shopping mall building contains a series of small stores. Each store is approximately 5000 ft²
(465 m²). The stores are attached to each other and separated only by common demising walls. The
overall contiguous area of the mall is 80,000 ft² (7432 m²). Can the Simplified Approach Option be
used to show compliance for a rooftop packaged air-conditioning unit serving one of the small tenants?
A
Yes. The term “building” is defined in Section 3 as “a structure wholly or partially enclosed within
exterior walls, or within exterior and party walls….” The party walls between tenants in the mall define
each tenant to be a separate building for the purposes of compliance with this standard.
Therefore, the Simplified Approach Option may be used for each tenant that occupies less than 25,000
ft² (2300 m²) in gross area (assuming the other restrictions to this approach are met).
Example 6-B. Simplified Approach Option, Single-Zone Restriction
Corresponding section: Scope (6.3.1)
Q
A gas/electric packaged air-conditioning unit serves a small office building. The unit is a standard
single-zone unit but it serves multiple zones through a zoning control system using a damper, with
each zone having its own thermostat (often called “VVT,” a trademark of the original control system
manufacturer). Can this system show compliance using the Simplified Approach Option?
A
No. The Simplified Approach Option may only be used for units serving a single HVAC zone, defined in
Section 3 as “a space or group of spaces within a building with heating and cooling requirements that
are sufficiently similar so that desired conditions (e.g., temperature) can be maintained throughout
using a single sensor (e.g., thermostat or temperature sensor).”
While the air-conditioning unit in this example is a single-zone unit, the VVT control system expands
the number of zones beyond one, making it ineligible for the Simplified Approach Option.
Example 6-C. Simplified Approach Option, Example Application
Corresponding section: Scope (6.3.1)
Q
A 7.5 ton (26.4 kW) rooftop gas/electric air-conditioning unit is planned for a new 2500 ft² (232 m²)
single-story retail store in Las Vegas, Nevada (Climate Zone 3B). The unit requires 750 cfm (354 L/s) of
outdoor air and supplies 2600 cfm (1227 L/s) of total supply air with a total load of 84,000 Btu/h
(24.6 kW). Supply and return air ducts are located in a ceiling attic between a suspended ceiling and an
insulated roof. A small 75 cfm (35.4 L/s) exhaust fan serves the employees’ toilet room.
What is required for the system to comply with the standard using the Simplified Approach Option?
A
There is not just one way to comply with the Simplified Approach Option, but the following is a typical
example of how this system could meet the requirements.
Cooling efficiency. According to Table 6.8.1-1, a unit with electric resistance heat must have a
minimum full-load efficiency of 12.1 EER at standard AHRI 340/360 rating conditions. The unit must
also have an integrated efficiency of 12.3 IEER at standard AHRI 340/360 rating conditions. Units with
gas heating must use the heating category “all other” in Table 6.8.1-1. This category adjusts the fan
energy due to the higher pressure drop through a furnace that is larger than that for a cooling-only
unit or unit with an electric heating coil.
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Economizers. An economizer is required for this cooling system because its capacity of 84,000 Btu/h
(24.6 kW) exceeds the value in Table 6.5.1-1 for this climate, which is 54,000 Btu/h (15.8 kW).
As an alternative to the economizer requirement, a high-efficiency air-conditioning unit can be selected
using Table 6.5.1-3. In this example, the efficiency improvement requirement for Climate Zone 3B is
32%, which is applied to the IEER by footnote a. So instead of using an economizer with an IEER ≥ 12.3,
a system without an air-side economizer could be installed if the IEER is ≥ 12.3 × 1.32 = 16.2.
If an economizer is specified for this unit, a high limit control must also be specified. According to Table
6.5.1.1.3, the allowable controls include the following:
• Fixed dry-bulb control (i.e., an outdoor air thermostat), set to shut off the economizer above 75°F
(24°C) outdoor air temperature
• Differential dry-bulb control (a temperature sensor in the return airstream and one in the outdoor
air) that will shut off the economizer when the outdoor air temperature exceeds the return air
temperature
• Fixed enthalpy with fixed dry-bulb control (one enthalpy sensor in the outdoor air) that will shut
off the economizer when the outdoor air enthalpy exceeds 28 Btu/lb (47 kJ/kg) or the outdoor air
dry-bulb temperature exceeds 75°F (24°C)
• Differential enthalpy with fixed dry-bulb control (one enthalpy sensor in the return airstream and
one in the outdoor air) that will shut off the economizer when the outdoor air enthalpy exceeds
the return air enthalpy or the outdoor air dry-bulb temperature exceeds 75°F (24°C)
Of the four allowed controls, the least expensive option is the fixed dry-bulb control. Because Las Vegas
is a dry climate, the benefits of an enthalpy high limit control are small and a dry-bulb or differential
dry-bulb control is a better choice.
Another requirement for systems with economizers is that they must have either barometric or
powered relief sized to prevent overpressurization of the building when the economizer is on (Section
6.5.1.1.5). With most rooftop units, both are standard options. If the static pressure through the relief
air path for the application is large, more than about 0.1 in. of water (24.8 Pa), power exhaust must be
used to prevent excessive building pressure during air-side economizer operation.
If air-side economizers are used, the leakage rate requirements for the outdoor air, relief, and exhaust
dampers that are part of the economizer system must meet or exceed the requirements of Section
6.4.3.4.3 and maximum leakage rates of Table 6.4.3.4.3. In some climate zones there are different
leakage requirements based on building height, because stack effect can cause energy losses in cold
climates. In mild climates, such as Climate Zone 3B, the stack effect is not significant; thus, for this
example, these requirements do not apply. The leakage rate for the motorized economizer outdoor
intake dampers is 10 cfm/ft2 (51 L/s·m2), and if nonmotorized barometric relief dampers are used
then the leakage rate is 20 cfm/ft2 (102 L/s·m2). These leakage rates are based on AMCA Standard 500
at 1.0 in. of water (249 Pa). Also, per Section 6.5.1.3(b), the mechanical cooling must have a minimum
of two stages of capacity because the unit has a cooling capacity of 84,000 Btu/h (24.6 kW) and an airside economizer.
Heating. The gas furnace must meet the efficiency requirements shown in Table 6.8.1-5. For this
example, the requirement is in the row labeled “Warm-air furnace, gas fired,” with capacity less than
225,000 Btu/h (<66 kW). In this case (see footnote d in Table 6.8.1-5), the furnace can meet either of
two requirements: 78% AFUE (annual fuel utilization efficiency) or 80% Et (thermal efficiency at full
load). Footnote d also requires a unit with an uninterrupted or intermittent ignition device, maximum
jacket losses not exceeding 0.75% of the input rating, and either a vent or a flue damper.
Thermostat and off-hour controls. A true seven-day electronic thermostat can be specified to meet
several requirements, including those for dual set points, off-hour shutoff, setback, and setup.
Ductwork insulation. In this climate (3B), according to Table 6.8.2 for combined heating and cooling
ducts located in “Unvented Attic w/Roof Insulation,” the supply air duct insulation must have an Rvalue of R-3.5 (R-0.62). This can be met with 1.5 in. (38.1 mm) fiberglass duct wrap or 1 in. (25.4 mm)
of fiberglass or closed-cell foam duct liner. (See Table 6-B under the HVAC&R System Construction and
Insulation (6.4.4) section of this user’s manual for a list of standard duct insulating materials that meet
this R-value requirement.)
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Ductwork sealing. The supply air and return air ducts must be sealed to Seal Class A per Section
6.4.4.2.1 Duct Sealing. See the HVAC&R System Construction and Insulation (6.4.4) section in this
chapter for more details.
Air balancing. A note must be added to the design drawings or the specifications calling for the system
to be balanced according to ASHRAE Standard 111, NEBB standards, AABC standards, or some other
industry recognized standard as defined in Section 6.7.2.3.1.
Shutoff dampers. The toilet exhaust fan is required to have a backdraft damper as defined in Section
6.4.3.4.2.
Fan Control. Because the unit cooling capacity is ≥75,000 Btu/h (22.0 kW), the unit must have
variable- or multiple-speed supply fan airflow per Section 6.5.3.2.1. If a multiple-speed indoor fan is
used, the minimum airflow must not exceed 66% of the design airflow. The low fan speed must be used
to satisfy low cooling loads and ventilation requirements. The two fan speeds must also be used during
the economizer operation, where the low speed is used until the economizer damper is near 100%
open, at which point the fan speed must change to the high speed (the design airflow). Mechanical
cooling must only be used after the economizer is at 100% at high fan speed per the requirements in
Section 6.5.1.3.
Optimum start controls. The system supplies less than 10,000 cfm so it does not need optimum start
controls as defined by Section 6.4.3.3.3.
Energy recovery. Energy recovery is not required. This unit has a design percent outdoor air of
750/2600 cfm (354/1227 L/s) = 29%. Assuming the retail store is open fewer than 8000 hours per
year, Table 6.5.6.1-1 shows that there is no requirement for energy recovery in Climate Zone 3B. If the
building is to be occupied more than 8000 hours per year (continuous occupancy), Table 6.5.1-2 shows
that there is not an exhaust air energy recovery requirement for this building because the supply
airflow rate is less than 19,500 cfm (9203 L/s).
Demand control ventilation. Per Section 6.4.3.8, demand control ventilation (DCV) may be required
because the system has an air-side economizer and serves a space over 500 ft² (46 m²). Depending on
the type of retail space, it may have a design occupancy of 25 people or more per 1000 ft² (93 m²) of
floor area. Assuming more than 75% of the outdoor air is not required for makeup, the system does
not satisfy any of the DCV exceptions. Therefore, the system requires DCV if the occupancy density is
greater than 25 people per 1000 ft² (93 m²) of floor area.
FYI
HVAC&R Cooling Efficiency Rating Metrics
HVAC&R equipment efficiencies have historically been rated at full-load and design ambient conditions
(for example, coefficient of performance [COP], energy efficiency ratio [EER], kilowatt/ton). Some
cooling energy efficiency ratings include an annualized metric such as integrated part-load value
(IPLV) for chillers, integrated energy efficiency ratio (IEER) for commercial rooftops and variablerefrigerant flow (VRF) systems, and seasonal energy efficiency ratio (SEER) for small packaged
equipment. The annualized metrics are a much better representation of the true performance of the
equipment because most commercial equipment seldom, if ever, runs at full load and design ambient
conditions. The industry is gradually moving the emphasis for efficiency metrics from the full-load
metrics to the annualized metrics. The full-load and annualized metrics provide good relative like-forlike equipment comparisons, but because they are determined under specific test conditions they may
not accurately reflect real-world performance. It is important to note that metrics alone cannot be used
to compare the relative performance of different HVAC&R system types. The cooling efficiency rating
for each type of equipment includes different components in the equipment package, making direct
comparisons of efficiency ratings irrelevant. For example, the IEER of a packaged rooftop unit cannot
be compared to the IEER of an air-cooled chiller, as the components each includes in its rating is very
different. Additionally, there are system components required to make each system work, such as
pumps in a chilled-water system, that may not be reflected in the efficiency ratings.
To compare the relative performance of different HVAC&R system types or to more closely estimate
the designed HVAC&R system performance under real-world conditions, a detailed energy analysis
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should be performed. This type of analysis can include the system components not included in the
efficiency ratings and can also account for site-specific conditions not included in the standard rating.
Following are some cooling efficiency rating terms that will be encountered when designing cooling
systems and selecting cooling equipment.
Energy Efficiency Ratio (EER)
An I-P full-load metric that is a ratio of the net cooling capacity in Btu/h to the power input values in
watts at any given set of rating conditions expressed in Btu/W·h. For all air-cooled products, this
includes the condenser fan power. For units with an indoor fan, it also includes the capacity impact of
the indoor fan power and the impact of the indoor fan power in the net capacity. Note that the indoor
fan power is at Air-Conditioning, Heating, and Refrigeration Institute (AHRI) standard rating
conditions and external static, which may not be what the applied static is for your application. This
metric is typically used by the industry for packaged units and for air-cooled chillers.
Full-Load kW/ton
This I-P metric is a ratio of the full-load power divided by the full-load capacity. This metric is typically
used for water-cooled chillers.
Cooling Coefficient of Performance (COPC)
This metric may be used in I-P or SI applications. It is the ratio of the net cooling capacity in watts to
the power input values in watts at any given set of rating conditions, expressed in watts/watts.
Heating Coefficient of Performance (COPH)
This metric may be used in I-P or SI applications. It is the ratio of the heating capacity in watts to the
power input values in watts at any given set of rating conditions expressed in watts/watts, not
including the power for supplemental electric resistance heat.
Seasonal Energy Efficiency Ratio (SEER)
This I-P metric is the total heat removed from the conditioned space during the annual cooling season,
expressed in Btu, divided by the total electrical energy consumed by the air conditioner or heat pump
during the same season, expressed in watt-hours.
Cooling Seasonal Coefficient of Performance (SCOPC)
This SI metric is the total heat removed from the conditioned space during the annual cooling season,
expressed in watts, divided by the total electrical energy consumed by the air conditioner or heat
pump during the same season, expressed in watt-hours.
Heating Seasonal Performance Factor (HSPF)
This I-P metric is the total heating output of a heat pump during its normal annual use period for
heating (Btu) divided by the total electric energy input during the same period.
Heating Seasonal Coefficient of Performance (SCOPH)
This SI metric is the total heating output of a heat pump during its normal annual use period for
heating divided by the total electric energy input during the same period in consistent units.
Integrated Part-Load Value (IPLV)
The IPLV is an annualized metric for chiller performance that combines four points of chiller
performance using weighting factors that represent an average building load profile. This metric is
defined in AHRI 550/590 and in I-P is typically expressed in kW/ton for water-cooled chillers and
Btu/watt for air-cooled chillers. In SI this metric is typically expressed as W/W (see ICOPC). The four
points vary by load, weighting factor, and entering condenser temperature as follows:
% Full Load
Weighing Factor %
Water-Cooled Entering Water
Temperature
Air-Cooled Entering Water Temperature
75%
42%
75°F (24°C)
80°F (27°C)
100%
50%
25%
1%
45%
12%
Standard 90.1 User’s Manual
85°F (29°C)
65°F (18°C)
65°F (18°C)
95°F (35°C)
55°F (13°C)
55°F (13°C)
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In addition to the values in this table, the IPLV is evaluated at prescribed water flow rates, leaving
chilled-water temperatures and fouling factors as defined in AHRI 550/590.
When comparing chiller IPLVs, designers/specifiers should consider the following:
•
•
•
The IPLV is based on single-chiller systems. Each chiller in a multiple-chiller plant may experience
loading conditions quite different from those assumed in determining the IPLV.
The IPLV is based on weighted average United States weather, building types, and building
operations. Your actual design conditions will vary.
The IPLV only includes the input power associated with components included in the chiller
package. For instance, an air-cooled chiller includes the input power required for heat rejection
(the integral condenser coil and fans), where a water-cooled unit does not (condenser water
pumps and cooling tower). Therefore, a comparison of an air-cooled chiller IPLV to a water-cooled
chiller IPLV is of no value.
Nonstandard Part-Load Valve (NPLV)
The nonstandard rating NPLV is a variation of the IPLV for chillers that is at different condenser water and
chilled-water conditions. This rating is intended to be used with centrifugal chillers that have not been
selected to run at AHRI conditions and could go into surge. To adjust for this, Section 6.4.1.2.1 includes an
equation to adjust the minimum efficiency requirements in Table 6.8.1-3 to the alternate conditions. The
equation is complex, but ASHRAE has developed a spreadsheet tool that can be used to calculate the
adjusted NPLV as well as full-load kW/ton. It can be found at http://www.ashrae.org/UM90.1-2016.
Integrated Energy Efficiency Ratio (IEER)
This I-P metric is a replacement for IPLV previously used for rating large unitary equipment. The IEER
is a significant improvement over IPLV as it allows for uniform rating of all products, including singlestage and multistage units. It is based on a weighted average of performance at 100%, 75%, 50%, and
25% of capacity, similar to the IPLV for chillers but based on typical load profiles for buildings that use
packaged equipment. The annualized metric IEER is a more accurate method of representing the
annualized refrigeration performance of commercial unitary equipment and VRF systems. In addition,
the IEER provides part-load rating for equipment down to 65 kBtu/h (5.5 tons) in capacity, whereas
the IPLV was only provided for units 240 kBtu/h (20 tons) and larger. It is a metric for only the
mechanical cooling energy performance and does not include added features such as economizers and
energy recovery, which can be included with these products.
Integrated Coefficient of Performance (ICOP)
The ICOP is the SI version of the IEER and uses units of watt/watt. For a given piece of equipment, the
IEER equals the ICOP times 3.413.
Power Usage Effectiveness (PUE)
PUE is an established metric that was added to Standard 90.1 for 2016. The metric is used specifically
for computer rooms. The metric is the total computer room energy (or power) divided by information
technology (IT) equipment energy (or power) calculated in accordance with industry-accepted
standards (see Informative Appendix E of the standard). Section 6.6 of Standard 90.1 allows the use of
PUE0 or PUE1 for demonstrating compliance. PUE0 is a demand-based metric. It is the peak power of
the entire computer room, including IT equipment and supporting infrastructure divided by the peak
power of the IT equipment. PUE1 is a consumption-based metric. It is the annual energy consumption
(kWh) for the entire computer room, including IT equipment and supporting infrastructure, divided by
annual energy consumption (kWh) of the IT equipment.
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The mandatory provisions apply to all systems complying by the Prescriptive Path (Section 6.5), the
Alternate Compliance Path (Section 6.6), or the ECB Method (Section 11). Except where specifically
noted in the previous section, these requirements do not apply to systems complying by the Simplified
Approach Option for HVAC Systems.
Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Section 6.4.1 addresses cooling and heating equipment minimum efficiencies and labeling
requirements. Most equipment has a full-load minimum efficiency metric that is based on standard
rating conditions at full-load and typical average full-load ambient conditions. Some equipment may
also have an annualized efficiency metric such as integrated part-load value (IPLV) and integrated
energy efficiency ratio (IEER). Other equipment may only have an annualized efficiency metric such as
seasonal energy efficiency ratio (SEER). For equipment classes that have both a peak-load metric and
an annualized metric, both performance metrics must be satisfied to comply with the standard. For
equipment that provides both heating and cooling, there are separate minimum efficiencies specified
for heating and cooling. These metrics are at standard rating conditions and may not be representative
of the installed operating conditions. Some also do not include all the power that would be used in a
building application, such as the pumping power, tower energy, and benefits of added features such as
economizers and energy recovery. But the intent of these efficiency rating metrics is to provide a
standard test procedure for establishing minimum energy performance requirements and to provide a
means of comparing equipment energy efficiency performance.
See Examples 6-E through 6-N for applications of the requirements of Section 6.4.1.
 Minimum Equipment Efficiencies—Listed Equipment—Standard Rating and Operating
Conditions (6.4.1.1)
Section 6.8 contains the equipment efficiency tables used to apply the efficiency and labeling
requirements of Section 6.4. Equipment must meet or exceed the energy efficiencies shown in Tables
6.8.1-1 through 6.8.1-16.
Equipment efficiency requirements apply to all equipment, regardless of the compliance path chosen:
Simplified Approach Option (Section 6.3), Prescriptive Path (Section 6.5), Alternate Compliance Path
(Section 6.6), or ECB Method (Section 11). Therefore, equipment must meet the requirements of this
section even if compliance could be shown with the Alternate Compliance Path, the ECB Method, or the
Performance Rating Method using equipment with lower efficiencies. Also, when the minimum
equipment energy efficiency tables of the standard include a full-load and an annualized energy
performance rating for a class of equipment, both energy performance metrics must be satisfied to
comply. For units with cooling and heating operation, compliance is required for both cooling and
heating minimum energy efficiency performance ratings. Equipment efficiency levels defined in this
section and Tables 6.8.1-1 through 6.8.1-16 are based on industry rating standards such as those of the
Air-Conditioning, Heating, and Refrigeration Institute (AHRI).
Additional Requirements for Furnaces (footnote d to Table 6.8.1-5)
In addition to meeting the heating efficiency requirements listed in the tables, all fuel-fired forced-air
furnaces with input ratings ≥225,000 Btu/h (≥66 kW) must have all the following features:
• An intermittent ignition or interrupted device
• Either power venting or a flue damper (A vent damper is also acceptable for furnaces where
combustion air is drawn from the conditioned space.)
• Jacket losses not exceeding 0.75% of the input rating (Furnaces that are other than gas- or oil-fired,
such as electric resistance forced-air furnaces, and that are not located within the conditioned
space must have jacket losses not exceeding 0.75% of the input rating.)
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Minimum Equipment Efficiencies—Listed Equipment—Nonstandard Conditions (6.4.1.2)
Water-Cooled Centrifugal Chilling Packages (6.4.1.2.1)
The water-cooled centrifugal chiller efficiency requirements in Table 6.8.1-3 apply only to chillers
designed for AHRI 550/590 standard rating conditions (evaporator flow of 2.4 gpm/ton, 44°F leaving
chilled-water temperature; condenser flow of 3.0 gpm/ton, 85°F entering condenser water
temperature) for I-P ratings. For SI, slightly different rating conditions are used: 12°C entering chilledwater temperature with a 7°C leaving chilled-water temperature and a 30°C entering condenser water
temperature with a 35°C leaving condenser water temperature per AHRI 551/591. It is unlikely that a
centrifugal chiller will operate at the standard rating conditions. To determine if a centrifugal chiller
that is selected at nonstandard rating conditions complies with the standard, an equation in this
section adjusts the standard rating condition minimum full-load and annualized efficiencies listed in
Table 6.8.1-3 for the nonstandard application conditions. The factor calculated by the equations is
called Kadj in the standard. What follows is an example of the calculation. The equation is complex, so a
spreadsheet tool has been created to help with the energy performance calculations. The tool can be
found at http://www.ashrae.org/UM90.1-2016.
The following equations are from the I-P edition of the standard. Due to the different units for SI, the
equation is slightly different and can be found following the I-P edition.
where
•
•
•
•
•
FLadj = FL/Kadj
PLVadj = IPLV/Kadj
Kadj = A × B
FL = full-load kW/ton value from Table 6.8.1-3
FLadj = maximum full-load kW/ton rating, adjusted for nonstandard conditions
IPLV = IPLV value from Table 6.8.1-3
PLVadj = maximum nonstandard part-load value (NPLV) rating, adjusted for nonstandard conditions
A = 0.00000014592 × (LIFT)4 – 0.0000346496 × (LIFT)3 + 0.00314196 × (LIFT)2 – 0.147199 ×
(LIFT) + 3.9302
• LIFT = LvgCond – LvgEvap
• LvgCond = full-load condenser leaving temperature (°F) (also called condenser water return
temperature)
• LvgEvap = full-load evaporator leaving temperature (°F) (also called chilled-water supply
temperature)
• B = 0.0015 × LvgEvap + 0.934
The efficiency requirements apply only to chillers with full-load design conditions in the following
ranges:
• Leaving chiller-water temperature: ≥36°F
• Entering condenser water temperature: ≤115°F
• 20°F ≤ LIFT ≤ 80°F
In SI:
where
•
•
•
•
•
FLadj = FL × Kadj
PLVadj = IPLV × Kadj
Kadj = A × B
FL = full-load COP value from Table 6.8.1-3
FLadj = maximum full-load COP rating, adjusted for nonstandard conditions
IPLV = IPLV value from Table 6.8.1-3
PLVadj = maximum NPLV rating, adjusted for nonstandard conditions
A = 0.0000015318 × (LIFT)4 – 0.000202076 × (LIFT)3 + 0.0101800 × (LIFT)2 – 0.264958 × (LIFT) +
3.930196
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• LIFT = LvgCond – LvgEvap
• LvgCond = full-load condenser leaving temperature (°C) (also called condenser water return
temperature)
• LvgEvap = full-load evaporator leaving temperature (°C) (also called chilled-water supply
temperature)
• B = 0.0027 × LvgEvap + 0.982
The efficiency requirements apply only to chillers with full-load design conditions in the following
ranges:
• Leaving chiller-water temperature: ≥2.2°C
• Entering condenser water temperature: ≤46.1°C
• 11.1°C ≤ LIFT ≤ 44.4°C
Chillers whose design operating conditions fall outside of these ranges are not covered by the standard
and are not required to comply with the minimum efficiencies listed in Table 6.8.1-3. Examples include
chillers designed for ice storage systems or chillers used for manufacturing processes.
Positive Displacement (Air- and Water-Cooled) Chilling Packages (6.4.1.2.2)
Positive displacement chillers, which are chillers with reciprocating, scroll, or screw compressors
(both with air- and water-cooled condensers), are required to meet the efficiencies of Table 6.8.1-3
when tested or certified with water at standard rating conditions per the referenced test procedure.
The use of Kadj is not required because positive displacement chillers do not have an issue with
operating at the standard rating conditions, so all metrics are applicable to the standard rating
conditions defined in AHRI 550/590 (I-P) and AHRI 551/591 (SI) standards. These requirements
apply to positive displacement air-cooled chillers when the leaving chilled-water temperature is higher
than 32°F (0°C) and for water-cooled positive displacement chillers when the leaving chilled-water
temperature is higher than 32°F (0°C) and the return condenser water is below 115°F (46°C). If the
application includes brines or glycol-based fluids for ambient freeze protection, then compliance must
be shown to Table 6.8.1-3 when tested with water at the standard AHRI 550/590 or AHRI 551/591
rating conditions. The intent of this requirement is to require products that are protected against
freezing by using a glycol fluid to still be required to meet the requirements of Table 6.8.1-3 by proving
compliance at the standard AHRI rating conditions when using water.
Multiple Chillers
Recently the concept of multiple modular chillers has been introduced to the market. If the chiller
modular system is sold as an assembly with a unique part number, then the efficiency must comply
with the efficiency requirement for the complete assembly of chillers. If it is field assembled using
multiple individual chillers, then it must comply with the rating for each chiller and not the full
assembly. The requirements for chiller/pump isolation defined in Section 6.5.4.3.1 must be used for
separately rated chillers. See Example 6-D.
Example 6-D. Modular Chillers
Corresponding section: Water-Cooled Centrifugal Chilling Packages, Multiple Chillers (6.4.1.2.1)
Q
A chiller plant uses a water-cooled chiller composed of four individual chiller modules combined
together into a single unit. Does Section 6.5.4.3.1 apply to each chiller module individually, or does the
combined chiller count as a single chiller?
A
The standard treats the combined modules as a single chiller provided it is cataloged and tested in the
combined form in accordance with the applicable AHRI standard—in this case, AHRI 550/590. In that
case, the piping shown in the following figure is acceptable.
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Note, however, that to meet AHRI 550/590 part-load requirements while maintaining the 44°F chilledwater temperature mandated by this standard, it is likely that the chiller will have to have internal
factory-installed evaporator isolation valves so that inactive modules can have flow stopped. Isolation
valves on the condenser side would not be required.
If the modules are only tested individually, not as an assembly, then each module counts as a separate
chiller. In that case, to meet Section 6.5.4.3.1, one of the configurations shown in the following two
figures must be used.
Equipment not Listed (6.4.1.3)
Although Sections 6.4.1.1 and 6.4.1.2 include many types of HVAC&R equipment, not every type of
HVAC&R equipment that might be used in a project is covered. This section clarifies that the use of
HVAC&R equipment not covered by these sections does not prohibit compliance with the standard.
Equipment not covered by these sections is not regulated by this standard but may be regulated by
other standards, codes, or federal regulations.
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Verification of Equipment Efficiencies (6.4.1.4)
In most cases, the efficiency of products will be established by certification programs from industry
associations such as the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), the Association
of Home Appliance Manufacturers (AHAM), or the Cooling Technology Institute (CTI). Where such
certification programs exist but a manufacturer chooses not to participate, equipment performance
must be verified by an independent laboratory test. Where there is no industry certification program,
equipment efficiencies must be supported by data furnished by the manufacturer. Field tests of
performance are not required.
Notes Applying to the Equipment Efficiencies Given in Tables 6.8.1-1 through 6.8.1-16
• Equipment not listed. Equipment not listed in the tables has no minimum performance
requirements. These products may be used regardless of their efficiency. Examples include pumps
and electric resistance heaters.
Multiple efficiency requirements. Some equipment has more than one efficiency requirement.
For example, a typical air-conditioning unit with a gas furnace will have a full-load efficiency EER
from Table 6.8.1-1, an IEER from Table 6.8.1-1, and a furnace thermal efficiency (Et) from Table
6.8.1-4. To comply, equipment must satisfy all stated requirements as well as all applicable
footnotes.
• Combined space and water heating equipment. Equipment that provides both space and water
heating must comply with the efficiency requirements of the primary function. For example, a
space-heating boiler that also provides service hot water must comply with the boiler efficiency
requirements in Table 6.8.1-6. A water heater that also provides space heating must comply with
the efficiency requirements in Table 7.8.
• Components from multiple manufacturers. Where components from different manufacturers
are used to field-build a product listed in the tables, the system designer must specify the
performance of each component so that their combined efficiency meets the minimum equipment
efficiency requirements in the tables. The most common example of this is a split-system heat
pump or air conditioner built using an indoor coil and air handler from one manufacturer and an
outdoor condensing unit or heat pump unit from another manufacturer. This is permitted, but the
designer (using data from the manufacturers) must ensure that the combined performance meets
the standard’s requirements.
• Relevant dates. Some of the tables (see, for example, Table 6.8.1-4 for packaged terminal air
conditioner units [cooling mode], standard size) have requirements that change on or after a
specific date. Although it is not explicitly listed in the standard, SSPC 90.1 has interpreted this to be
the date of manufacture. The equipment efficiencies established in federal code (for example, those
for NAECA and EPACT) all reference the date of manufacture not the date of sale or installation. See
for instance U.S. Code, Title 42, Sec. 6295.
FYI
Comparing Equipment Efficiencies
The equipment efficiencies listed in Tables 6.8.1-1 through 6.8.1-16 are for standard rating conditions.
Actual efficiency will vary depending on how the equipment is applied and how it is controlled.
Also, the equipment efficiency data in the tables apply only to the equipment itself and not to any other
equipment that may be required to complete the system. When determining which type of system to
select, it is usually not possible to compare the efficiency of different equipment types simply by
looking at the values in the tables. For instance, consider the following comparisons:
•
Air- vs. water-cooled equipment. Any comparison of air- and water-cooled equipment needs to
include the seasonal variation in dry- and wet-bulb temperatures coincident with the variations in
the loads. Also, the efficiency ratings for water-cooled equipment cannot be directly compared to
those for air-cooled equipment. Water-cooled equipment ratings do not include the energy used by
condenser water pumps and cooling tower fans. Conversely, air-cooled package ratings include
condenser fan energy. The ratings don’t include the energy used by auxiliary systems such as
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•
•
•
•
crankcase heaters, cooler heaters, tower sump heaters, or ventilation/cooling of equipment rooms
required by safety standards such as ASHRAE Standard 15. The ratings also do not account for
strategies used for chilled-water temperature reset, cooling tower condenser water temperature
approach, or condenser water temperature control. The standard efficiency ratings also do not
account for added options such as hydronic economizers. Also, the chiller ratings do not include
the power for indoor air-handler fans or the efficiency benefits from air or hydronic economizers.
DX equipment. The ratings of direct-expansion (DX) equipment don’t account for the operational
changes in efficiency from supply air temperature reset or auxiliary equipment such as crankcase
heaters. The IEER ratings are for mechanical cooling only and do not include energy savings from
air-side economizers and energy recovery.
Condensing vs. packaged or split-system air conditioners. The ratings for condensing units
cannot be directly compared to ratings for packaged or split-system air conditioners. Condensing
unit ratings do not include the energy used by indoor air-handler fans.
Furnaces. Efficiency ratings for different types of furnaces account only for gas use and do not
include the energy used by combustion air fans and indoor air-handler fans that vary from one
furnace to another.
Chilled water vs. DX. The efficiency of chilled-water equipment cannot be compared to a unitary
DX system using standard ratings. Chilled-water equipment efficiency does not include the energy
used by pumps and air-handler fans.
Even a direct comparison of seemingly like energy descriptors may be misleading because of
differences in rating conditions or definitions. For instance, the cooling efficiency of groundwatersource heat pumps may appear higher than standard water-source heat pumps, but this is mostly due
to the differing rating conditions. The groundwater-source heat pumps are rated at 70°F (21°C)
entering water temperature compared to 85°F (29°C) for water-source heat pumps.
A fair comparison between different types of equipment, such as water- versus air-cooled equipment,
requires knowledge of the auxiliary equipment needed for a complete system and the energy they use
at both full and part load. An energy analysis at the level of detail required by Section 11 or Appendix G
using local weather data and the actual building load profile is the only way to make an accurate
comparison. When this is done it is important to include added features such as economizers and
energy recovery and controls strategies such as chilled-water temperature reset and supply air
temperature reset.
Example 6-E. Minimum Equipment Efficiencies, Unitary Heat Pump
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
What are the efficiency requirements for a 25 ton (87.9 kW) unitary air-source heat pump?
A
Table 6.8.1-2 contains the requirements for unitary heat pumps in both the cooling and heating modes.
For cooling, the unit falls into the cooling capacity range of ≥240,000 Btu/h (≥70 kW). Assuming that
the unit has an electric resistance heater for defrost and supplemental heat, it must meet both the fullload EER of 9.5 and the annualized IEER of 10.6.
For heating, the unit falls into the ≥135,000 Btu/h (≥40 kW) cooling capacity range (note this is still
the cooling capacity here, not the heating capacity) and must meet heating COP requirements of both
3.2 at 47°F (8.3°C) dry-bulb temperature/43°F (6.1°C) wet-bulb temperature and 2.05 at 17°F (–8.3°C)
dry-bulb temperature/15°F (–9.4°C) wet-bulb temperature. (Note that Section 6.4.1.1 states that
“Where multiple rating conditions or performance requirements are provided, the equipment shall
satisfy all stated requirements.”)
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Example 6-F. Minimum Equipment Efficiencies, Single-Package Vertical Heat Pump
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
What are the efficiency requirements for a 3 ton (10.6 kW) single-package vertical heat pump?
A
Table 6.8.1-4 contains the requirements for single-package vertical heat pumps. The unit falls into the
cooling capacity range of <65,000 Btu/h (<19 kW). Therefore, the single-package vertical heat pump
must have a minimum cooling EER of 10.0 (2.93 COP) and a minimum heating COP of 3.0.
Example 6-G. Minimum Equipment Efficiencies, Equipment That Was Stored
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
A 6 ton (21.1 kW) packaged air-cooled heat pump with no backup heating manufactured in 2015 has
been in storage and is to be installed in a building in 2016. It has a cooling EER of 11.0 and an IEER of
11.2 with heating COPs of 3.3 at 47°F (8.3°C) dry-bulb temperature/43°F (6.1°C) wet-bulb
temperature and 2.25 at 17°F (–8.3°C) dry-bulb temperature/15°F (–9.4°C) wet-bulb temperature.
Does the heat pump comply with the minimum efficiency requirements?
A
Yes.
A heat pump would not comply, because Table 6.8.1-2 requires a minimum cooling IEER of 12.2.
Equipment is required to meet all efficiency ratings applicable to the equipment to comply with the
standard.
Example 6-H. Minimum Equipment Efficiencies, Equipment Date of Manufacture
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
How do I find the date of manufacture for a piece of equipment?
A
Check the equipment nameplate. If it’s not there, call the supplier or manufacturer. Given the make,
model, and serial number, most manufacturers can provide information on existing equipment.
Example 6-I. Minimum Equipment Efficiencies, Chiller Design for Dual Duty
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
A chiller that is part of an ice storage system is designed both to produce brine at 25°F (–4°C) to make
ice during off-peak periods and to produce normal chilled-water temperatures (40°F to 45°F [4°C to
7°C]) during on-peak and partial-peak periods. Because one of the design conditions is for chilledwater temperatures that are within the range shown in Section 6.4.1.2, must the chiller meet the
efficiency requirements listed in Table 6.8.1-3?
A
No. Chillers that are specifically designed to operate at conditions outside the temperature ranges
listed in Section 6.4.1.2.1—chilled-water supply of ≥36°F (2°C)—are exempt because (1) they may not
be able to operate under the standard conditions or (2) they may operate inefficiently under the
standard conditions because they are designed to operate under other, more extreme design
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conditions that could require compressor and heat exchanger modifications. Also, the unit is charged
with a brine and is set to maintain a leaving water temperature set point below 32°F (0°C), which also
exempts it from the efficiency compliance requirements.
In this example, the chiller must be able to handle the high lift required to produce low-temperature
brine for making ice. This may make the chiller inefficient when producing chilled water within
standard temperature ranges.
Example 6-J. Minimum Equipment Efficiencies, Centrifugal Chiller Design for Nonstandard Conditions
Corresponding section: Minimum Equipment Efficiencies—Listed Equipment—Nonstandard Conditions
(6.4.1.2)
Q
What are the efficiency requirements for a 250 ton water-cooled centrifugal chiller operating at the
following design conditions?
• 45°F leaving chilled-water temperature
• 96°F leaving condenser water temperature
A
Because this centrifugal chiller operates at conditions different from the AHRI 550/590 rating
condition (44°F chilled-water supply, 3 gpm per ton of condenser water flow, and 85°F condenser
water supply), the full-load and integrated part-load efficiencies for this chiller come from Table 6.8.13 as modified by the Kadj factor in Section 6.4.1.2.1.
Assuming this chiller was using the Table 6.8.1-3 Path A efficiencies, the standard condition full-load
requirement from Table 6.8.1-3 is 0.610 kW/ton and the standard condition IPLV is 0.550 kW/ton.
(Note b to Table 6.8.1-3 allows you to use either Path A or Path B for compliance).
At the specified conditions, the requirements at the design conditions are calculated as follows:
• LIFT = 96°F – 45°F = 51°F
• A = 0.00000014592 × (51)4 – 0.0000346496 × (51)3 + 0.00314196 × (51)2 – 0.147199 × (51) +
3.9302 = 0.9862
• B = 0.0015 × 96 + 0.934 = 1.00152
• Kadj = A × B = 0.986 × 1.002 = 0.9876
• FLadj = 0.610/0.9876 = 0.618
• PLVadj = 0.550/0.9876 = 0.557
The chiller complies if it has a full-load efficiency at nonstandard conditions of ≤0.642 kW/ton and the
IPLV at nonstandard conditions of ≤0.603 kW/ton at the design conditions of 45°F leaving chilledwater temperature and 96°F leaving condenser water temperature.
Example 6-K. Minimum Equipment Efficiencies, Part-Load Performance Requirements for Air Conditioner
with a Single Compressor
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
A 7.5 ton (26.4 kW) rooftop air conditioner has a single compressor with no unloading capability. Must
this unit meet the IEER requirement of Table 6.8.1-1?
A
Yes. The equipment must meet the requirements for IEER, which is a measure of part-load efficiency of
unitary equipment as a weighted average of efficiency at different part-load conditions. Note that if the
unit is equipped with an economizer, then per Section 6.5.1.3(b) it must have two stages of mechanical
cooling capacity. If the unit controls space temperature by modulating the airflow to the space, then
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the unit must have three stages of capacity with the minimum stage having a displacement of less than
or equal to 35% as defined in Table 6.5.1.4.
Example 6-L. Minimum Equipment Efficiencies, High Pressure Boiler
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
A gas-fired boiler is designed to provide 125 psig (861 kPa) steam. What efficiency requirements must
be met?
A
None. The term “boiler” is defined in Section 3 to be “low pressure,” which is commonly understood in
the industry to refer to steam at 15 psig (103 kPa) or lower and hot water at 160 psig (1.10 MPa) or
lower. Therefore, boilers designed for higher pressures are not covered by the standard and may be
installed regardless of their efficiency.
Example 6-M. Minimum Equipment Efficiencies, Process Conditioning
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
A 5 ton (17.6 kW) air-cooled upflow split-system computer-room air conditioner (CRAC) unit serves a
large telephone switching equipment room. What efficiency requirements must be met?
A
The minimum energy efficiency ratings for CRAC units are in Table 6.8.1-11. A 5 ton (17.6 kW) upflow
split-system air-cooled CRAC unit must have a SCOP-127* of 2.09 or higher. Note that the scope of
Standard 90.1 covers process equipment for new equipment or building systems specifically identified
in the standard that are part of industrial or manufacturing processes (see Section 2.1[a][4]).
* The sensible coefficient of performance (SCOP-127) is a ratio calculated by dividing the net sensible cooling capacity in watts by
the total power input in watts (excluding reheaters and humidifiers) at conditions defined in ASHRAE Standard 127.
Example 6-N. Minimum Equipment Efficiencies, Data Processing Rooms
Corresponding section: Equipment Efficiencies, Verification, and Labeling Requirements (6.4.1)
Q
An office housing both workers and data processing equipment is cooled by HVAC equipment that is
provided primarily to maintain space conditions for the data processing equipment. Does the
equipment have to comply with the standard or is it exempt because its purpose is primarily to cool
process equipment?
A
CRACs are regulated by the standard. The HVAC equipment must meet the minimum requirements of
the standard. The required efficiency rating will depend on the equipment used to condition the room.
A DX split system must meet different efficiency rating requirements than a CRAC. Which efficiency
rating applies is not based on the space type but rather the features designed into the unit by the
manufacturer specifically for the purpose of conditioning a computer room.
Calculations (6.4.2)
Load Calculations (6.4.2.1)
The designer must make heating and cooling load calculations in accordance with ASHRAE/ACCA
Standard 183 before selecting or sizing HVAC&R equipment. This requirement helps to ensure that
equipment is neither oversized nor undersized for the intended application. Oversized equipment not
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only increases first costs but may also operate less efficiently than properly sized equipment. It can
also result in reduced comfort control due to, for example, lack of humidity control in cooling systems
and fluctuating temperatures from short-cycling. It is important to note that oversizing of items such
as ducts, pipes, and cooling towers can in practice decrease energy use.
Undersizing will obviously result in poor temperature control in extreme weather but can also
increase energy use at other times. For example, an undersized heating system may have to be
operated 24 hours per day because it has insufficient capacity to warm up the building each morning in
a timely manner.
Accurate calculation of expected heating and cooling loads begins with a reliable calculation
methodology. The standard requires that calculation procedures be in accordance with ASHRAE/ACCA
Standard 183. Standard 183 allows all of the standard load calculation methods, including the
following:
• The cooling load temperature difference/cooling load factor (CLTD/CLF) family of methods
• Total equivalent temperature difference/time averaging (TETD/TA) methods
• Transfer function methods (TFMs)
• Radiant time series (RTS) methods
• Heat balance (HB) method
While the standard requires load calculations, it does not require that actual equipment sizes
correspond to the calculated loads (see FYI, Right-Sized Equipment: Applying Reality Checks to Load
Calculations). Also, the standard does not describe how the load calculations requirement is to be
enforced; that is up to the authority having jurisdiction. Because enforcement agencies need only see
that calculations have been performed, they should request only to see a summary of load calculations,
such as a single-page computer printout for the building or system, and should not require that the
entire detailed calculation package be submitted.
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Right-Sized Equipment: Applying Reality Checks to Load Calculations
There is no universal agreement among engineers on a single load calculation procedure, and the
available procedures produce results that vary by 30% or more. This is because the thermodynamic
performance of buildings and HVAC&R systems is so complex that calculation methods and computer
software have simplifying assumptions embedded within them to make them practical to use.
Depending on the application, these simplifications can result in inaccuracies and errors. The designer
should be aware of the limitations of the calculation tool used and apply reality checks to the results,
based on real-life experience, to avoid sizing errors.
While load calculations are required by the standard, there is no requirement that actual equipment
sizes correspond to the calculated loads. In past editions of the standard, sizing equipment consistent
with load calculations was required for compliance using the Prescriptive Path (Section 6.5). However,
it proved very difficult to enforce this requirement given the wide variation in load calculation
methods and differing assumptions regarding internal loads and other parameters. Further, there are
cases where oversizing actually improves energy efficiency (such as oversizing ducts, piping, or cooling
towers), so it is difficult to regulate oversizing without introducing many exceptions and associated
complexity.
So why does the standard require load calculations when there is no corresponding requirement to use
the calculations for equipment sizing? The reason is in part because old rules of thumb used to size
systems may no longer be applicable. Building envelopes continue to improve. Spectrally selective
fenestration reduces solar gain and the cooling load while maintaining good daylight transmission.
Low-emissivity coatings and gas-fill for fenestration, as well as opaque sections with greater insulation
levels and fewer thermal bridges, can reduce heating loads and sometimes eliminate the need for
separate perimeter heating systems. Lighting loads continue to go down, and in many cases office
equipment loads are lower due to more efficient personal computers. However, despite the power
consumption of the individual electronic devices falling, more electronic devices are being used in
buildings, so these loads (often referred to as plug loads) are increasing overall, especially relative to
the other cooling loads, which are decreasing. So it is important to pay attention to the plug loads in a
building.
It is also important to factor in added options such as economizers, dedicated outdoor air systems, and
energy recovery when determining the load and selecting the requirement capacity for a component in
a larger system.
Once load calculations are performed, using them for selecting equipment is at least partly selfregulating due to normal market incentives. For instance, if a load calculation indicates that a 5 ton
(17.6 kW) air-conditioning unit will handle an application, it is not likely that the designer or
contractor will deliberately select a 10 ton (35.2 kW) unit because of its added first costs. On the other
hand, if the equipment had been selected using only rules of thumb without calculations, the larger
unit may have been chosen. The expectation is that most designers will properly size equipment if load
calculations are made.
Pump Head (6.4.2.2)
To prevent pumps from operating inefficiently due to pump oversizing, Section 6.4.2.2 requires that
“the pressure drop through each device and pipe segment in the critical circuit at design conditions
shall be calculated.” It also allows that the methods should follow “generally accepted engineering
standards and handbooks acceptable to the adopting authority.” While performing these calculations is
expected, this language requires pump head calculations to be performed as a mandatory requirement
of the standard to prevent rule-of-thumb sizing.
Controls and Diagnostics (6.4.3)
Zone Thermostatic Controls (6.4.3.1)
An HVAC thermostatic control zone is defined as a space or group of spaces whose load characteristics
are sufficiently similar that the desired space conditions can be maintained throughout with a single
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controlling device. The standard requires that the supply of heating and cooling to each such zone be
individually controlled by a thermostatic controller that senses the temperature within the zone
(Section 6.4.3.1.1).
To meet this requirement, spaces must be grouped into proper control zones. For instance, spaces with
exterior wall and glass exposures cannot be zoned with interior spaces. Similarly, spaces with
windows facing one direction should not be zoned with windows facing another orientation unless the
spaces are sufficiently open to one another that air may mix well between them to maintain uniform
temperatures.
Zoning in this manner does not apply to residential dwelling units. The standard specifically allows an
individual dwelling unit to be served as if it were a single zone. In other words, a single thermostat may
be used to control the supply of heating and cooling to all rooms within the dwelling unit even if they
face different exposures or operate with different occupancy schedules.
Independent Perimeter Systems (Exceptions to 6.4.3.1.1)
The exception for independent perimeter systems applies to perimeter zones that are served by two
independent HVAC systems. One of the two systems, called the perimeter system, is designed to offset
only “skin loads,” those loads that result from energy transfer through the building envelope. Typically,
the perimeter system is designed for heating only. Interior loads, such as those from lights and people,
are controlled by a second system called the interior system. This system may also be designed to
handle skin cooling loads if the perimeter system is heating only.
In the past, perimeter systems were often controlled by outdoor air sensors that would reset the
output of the system proportional to the outdoor air temperature. Because solar loads can offset some
of the heat loss from a space and are different for each exposure, this type of control inevitably causes
overheating by the perimeter system when the sun is shining and subsequent fighting with the cooling
system. Even when this control is improved by solar compensation, it still can result in wasteful
fighting between interior and perimeter systems due to varying internal loads. Therefore, only controls
that respond to temperature within the zones served are allowed. See Examples 6-O and 6-P.
Example 6-O. Independent Perimeter System
Corresponding section: Independent Perimeter Systems (Exceptions to 6.4.3.1.1)
Q
The figure below shows an example of an HVAC system design with a separate perimeter heating
system. The perimeter system consists of a heating-only fan-coil that serves all the exterior exposures.
The interior system consists of a cooling-only variable-air-volume (VAV) system serving the entire
floor, including all exterior exposures as well as interior zones. Does this system comply with the
standard?
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A
Yes. Although this design does not meet the thermostatic control requirements of Section 6.4.3.1.1
because the perimeter system supplies heating to four cooling zones, it is allowed under the exception
to Section 6.4.3.1.1. This system is permitted with the following provisions:
• The perimeter system has a separate zone for each major exposure, which is defined as an exterior
wall that faces 50 contiguous feet (15 contiguous m) or more in one direction. Exterior walls are
considered to have different orientations if the directions they face differ by more than 45 degrees.
• Each perimeter system zone is controlled by one or more thermostats located in the zones served.
Example 6-P. Zoning for Independent Perimeter Systems
Corresponding section: Independent Perimeter Systems (Exceptions to 6.4.3.1.1)
Q
The figure below shows an independent heating system that consists of four fan-coils. A separate VAV
cooling system serves the interior spaces and provides cooling to the exterior zones when required.
Does this system meet the requirements of the standard?
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A
Yes. Each perimeter with an exposure of greater than 50 ft (15 m) in length is served by a separate
system. The shorter exposures on the southeast are all less than 50 ft (15 m), so they may be grouped.
Deadband (6.4.3.1.2)
Zone thermostatic controls that control both space heating and cooling must be capable of providing a
temperature range or deadband of at least 5°F (3°C) within which the supply of heating and cooling to
the space is shut off or reduced to a minimum.
Figure 6-B shows a proportional control scheme that meets this requirement. This might apply to a
variable-air-volume (VAV) zone where the cooling source is cold supply air while heating is provided
by reheat or perhaps an independent perimeter heating system. The point from where the cooling
supply is shut off or reduced to its minimum position to where the heating is turned on is called the
“deadband” and must be adjustable to at least 5°F (3°C).
The deadband requirement is typically met using dual-set-point thermostats, which are essentially two
thermostats built into the same enclosure. One thermostat controls heating and one controls cooling.
The deadband can be achieved by setting the two set points at least 5°F (3°C) apart.
For proportional controls, such as pneumatic controls, that are calibrated so that the thermostat set
point is at the midpoint of the control band, the set points would have to be set apart by at least 5°F
(3°C) plus one throttling range (the temperature difference between full heating and no heating and
between full cooling and no or minimum cooling). For instance, in Figure 6-B, the throttling range
indicated is 2°F (1.1°C), so the deadband would be maintained by a heating set point of 69°F (21°C)
and a cooling set point of 76°F (24°C).
Another type of pneumatic thermostat that would meet the requirement is a so-called deadband or
“hesitation” thermostat. This thermostat is designed to provide a temperature range within which its
output signal is neutral, calling for neither heating nor cooling.
Examples 6-Q through 6-T address deadband controls.
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Exceptions to 6.4.3.1.2
Deadband controls are not necessary for thermostats that require manual changeover between heating
and cooling (Exception 1 to Section 6.4.3.1.2). This is typical of many residential thermostats. The
reason for this exception is that occupants will generally allow the space temperature to swing
considerably before changing the heating/cooling mode, thereby causing an effective deadband.
Thermostats in spaces that have special occupancies, where precise space temperature control is
required, need not have deadband control, when approved by the authority having jurisdiction
(Exception 2). Examples include areas such as hospital operating rooms that house temperaturesensitive equipment or processes or areas such as a museum or art gallery that house sensitive
materials. Buildings that do not fall in this category (that is, buildings where deadband controls are
appropriate) include data processing centers, office buildings, retail stores, schools, and hotels.
Note that data processing centers were exempt from this requirement in editions of Standard 90.1
prior to 2010. However, current guidelines and standards, including ASHRAE’s Thermal Guidelines for
Data Processing Environments, Third Edition, show that electronic equipment can operate under more
relaxed temperature and humidity conditions and the exemption is no longer justified.
FIGURE 6-B. SAMPLE DEADBAND THERMOSTATIC CONTROL
Corresponding section: Deadband (6.4.3.1.2)
Example 6-Q. Deadband Requirement, DDC System
Corresponding section: Deadband (6.4.3.1.2)
Q
A direct digital control (DDC) system using a space sensor and a programmable controller is to be used
to control a VAV box with hot-water reheat. Does it have to meet the deadband requirement?
A
Yes. This system qualifies as a “zone thermostatic control,” although it uses a space sensor and
computer rather than a conventional thermostat to control space temperature. The control logic in the
controller would have to support two separate control loops with individual set points, one for heating
and one for cooling, each with separate output signals connecting to the VAV damper and reheat
control valve, respectively.
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Example 6-R. Deadband Requirement, Single-Setpoint Thermostat
Corresponding section: Deadband (6.4.3.1.2)
Q
A thermostat with automatic changeover and the same temperature set point for both heating and
cooling is proposed to control a VAV box with hot-water reheat. Because the thermostat can be
adjusted in the winter to set points appropriate for heating and then changed in the summer to a set
point 5°F (3°C) higher, does it meet the deadband requirement?
A
No. This does not meet the intent of this section. The deadband must be continuous and automatic. The
deadband requirement saves energy by mitigating continuous cycling between heating and cooling.
The thermostat in this example does not satisfy this requirement.
Example 6-S. Deadband Requirement, Pneumatic Thermostat
Corresponding section: Deadband (6.4.3.1.2)
Q
A single- set-point pneumatic thermostat is proposed to control a fan-coil that has a chilled-water
cooling coil and an electric heating coil. The cooling-coil control valve operates over a 2 to 7 psi (13.8
to 48.3 kPa) range, while the pressure switch for the heating coil is set to turn the heat on at 16 psi
(110 kPa) and off at 14.5 psi (100 kPa). Because there is a 7.5 psi (51.7 kPa) range between the cooling
and heating operating points, does this comply with the deadband requirement?
A
No. Typically, pneumatic thermostat gains are calibrated in the range of 2 to 2.5 psi (13.8 to 17.2 kPa)
per degree. The 7.5 psi (51.7 kPa) deadband would then correspond to about 3°F to 4°F (7.2°C to
2.2°C), not the 5°F (3°C) required. To meet the requirement, the thermostat gain would have to be 1.5
psi per °F (18.6 kPa per °C), which would cause about a 20°F (11°C) swing between full cooling and full
heating, which is not acceptable for comfort.
Thus, while this design could be adjusted to meet the 5°F (3°C) deadband requirement, it would not
maintain reasonable space comfort at the same time. Occupants would be forced to defeat the control
to maintain comfort, reducing or eliminating the associated energy savings. This does not meet the
intent of the standard.
Example 6-T. Deadband Requirement, Dedicated Outdoor Air Supply Systems
Corresponding section: Deadband (6.4.3.1.2)
Q
Heating and cooling from a dedicated outdoor air supply (DOAS) air-handling unit serving several
HVAC subzones (e.g., fan-coil units, chilled beams) is controlled by supply air temperature. Does it have
to meet the deadband requirements of Section 6.4.3.1.2?
A
No. This section applies to zone thermostatic controls, meaning those that control zone (space)
temperature. It therefore does not apply to outdoor air supply equipment controlled by supply air
temperature. Note, however, that Section 6.5.2.6 limits control set points for this type of system. Also
note that the deadband requirement does apply to equipment controlled directly from space
temperature (or by supply air temperature with set-point reset by space temperature), such as a
makeup air system serving a kitchen.
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Set-Point Overlap Restriction (6.4.3.2)
HVAC systems commonly include two or more thermostatic controls serving the same zone. Examples
include the following:
• Dual-set-point thermostats required by Section 6.4.3.1.2 to provide a deadband between heating
and cooling
• Independent heating and cooling control loops in DDC zone controllers, again required by Section
6.4.3.1.2 to provide a deadband.
• Independent heating systems, such as those described above in the exception to Section 6.4.3.1.1,
controlled by a thermostat separate from those controlling the interior system
• More than one air-conditioning unit serving a large single space, such as a large data entry area or
computer room
In each case, it is possible for one control zone to fight with the others if their set points are close to
each other. For instance, in the case of an independent heating system with a separate thermostat, the
heating set point could inadvertently be set to a set point higher than the set point for the interior
cooling system, causing simultaneous heating and cooling supply to each space.
To prevent this inefficiency from occurring, the standard requires that where heating and cooling to a
zone are controlled by separate zone thermostatic controls located within the zone, means must be
provided to prevent the heating set point from exceeding the cooling set point minus any applicable
proportional band. Examples of acceptable means include the following:
• Mechanical stops that prevent set points from being adjusted cooler (for a cooling thermostat) or
warmer (for a heating thermostat) than a given value. This is commonly used on dual-set-point
pneumatic thermostats.
• Mechanical stops that prevent heating set points from being below cooling set points and vice
versa. This is a common approach for electric thermostats using a physical stop on the set point
adjustment levers that prevent the two set point from overlapping.
• Limits in software programming for DDC systems that prevent thermostat set points from
overlapping.
Off-Hour Controls (6.4.3.3)
Most HVAC systems serve spaces that are occupied intermittently. To reduce HVAC system energy use
during off hours, the standard requires that HVAC systems be equipped with automatic off-hour
controls required by Sections 6.4.3.3.1 to 6.4.3.3.4.
Historically, heat pump systems with electric resistance heat were considered less efficient when
operated intermittently because of the increased use of the resistance heat during warm-up. But this
increase is mitigated by the use of proper controls that lock out the auxiliary heat when the heat pump
can handle the load (controls that are required by Section 6.4.3.5).
Exceptions to 6.4.3.3
The standard provides two exceptions to the off-hour controls requirement:
a. HVAC systems intended to operate continuously. Examples include hospitals, police stations and
detention facilities, computer rooms, and some 24-hour retail establishments. See Example 6-U.
b. HVAC systems having a design heating capacity and cooling capacity less than 15,000 Btu/h (4.4
kW) that are equipped with readily accessible manual on/off controls. For example, a ventilation
fan and a small electric wall heater in a toilet room would not require automatic off-hour controls,
provided they had accessible manual on/off controls such as wall switches.
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Example 6-U. Off-Hour Controls, Equipment Room Cooling Unit
Corresponding section: Exceptions to 6.4.3.3
Q
An air conditioner serving an elevator equipment room in an office building is controlled by a
thermostat that cycles the indoor supply fan and the compressor on calls for cooling. Does this unit
need off-hour controls so that it shuts off when the building is unoccupied at night?
A
No. The equipment must be maintained at a given temperature at all times and thus qualifies for
Exception 1 to Section 6.4.3.3. Furthermore, when the elevators are inactive at night, the air
conditioner will automatically shut off because there is no load in the space.
FYI
Off-Hour Controls for Hotel Guest Rooms
Prior to the 2007 edition of Standard 90.1, hotel guest rooms were exempted from this off-hour control
requirement, but several approaches are now available for hotel designers to meet the requirement.
The simplest system is a stand-alone unit that resets temperature and fan levels on the HVAC unit
when the guest leaves the room. There are three main components to this system: a door switch, a
“people detector,” and a relay. The people detector is both an occupancy sensor and a logic device. In
combination with the door switch, it runs through a protocol after a delay to evaluate whether
someone has left the room. If so, then it resets the temperature to a preset level. This level is
determined by management and is preprogrammed into the control at installation time.
Advanced systems may also be used that take action based on occupancy or opening and closing of
doors. These can offer hoteliers many benefits. In addition to serving as a control system for HVAC and
lighting, these systems can be integrated with the reservation system to monitor or control guest-room
locks, minibar access, and other room features.
Automatic Shutdown (6.4.3.3.1)
Other than systems exempted by the exceptions and qualifications to Section 6.4.3.3, all HVAC systems
must be equipped with at least one of the following controls with the capability to automatically shut
down the system when the spaces served are not expected to be occupied:
a. Time switch or scheduling controls that are able to start and stop the system under different time
schedules for seven different day types per week (only two different time schedules per week are
required for residential occupancies), are capable of retaining programming and time setting
during loss of power for a period of at least ten hours, and include an accessible manual override,
or equivalent function, that allows temporary operation of the system for up to two hours.
b. An occupant sensor capable of shutting the system off when no occupant is sensed for a period of
up to 30 minutes.
c. A manually operated timer capable of being adjusted to operate the system for up to two hours.
d. An interlock to a security system that shuts the system off when the security system is activated.
The most common control option is the time switch or scheduling control. For unitary systems, a true
seven-day electronic thermostat generally provides the minimum capabilities listed above. However,
weekday/weekend (5-2) and weekday/Saturday/Sunday (5-1-1) thermostats, commonly used for
residential applications, do not comply with this requirement except where applied to systems serving
residential occupancies (per the exception to Section 6.4.3.3.1).
For larger systems controlled by DDC systems, the standard scheduling capabilities of these systems
will generally meet the above requirements; however, a means of manual override must be provided.
Common solutions include push buttons on zone temperature sensors used with zone-level DDC,
override buttons in common areas, telephone interfaces that allow occupants to use a phone’s keypad
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to request off-hour HVAC operation, and Web interfaces that can connect occupants to Web-based
control systems via the Internet.
Occupant sensors are commonly used as lighting controls, but the same sensors can easily double as
HVAC off-hour controls by adding an interlock (either hardwired or transferred via a network
interface from the lighting control system to the DDC system) as an input to the zone controller
controlling the associated HVAC zone. This contact would be programmed to temporarily operate the
system in the same way that a local override button on the zone temperature sensor would.
Manual wind-up timers are perhaps the least common off-hour control option. They might be
appropriate for seldom-used conference rooms or meeting rooms.
Interlocking the HVAC system to a security system is simply a way of allowing the security system’s
scheduling or occupant presence controls to indirectly control the HVAC system.
Setback Controls (6.4.3.3.2)
Setback controls include controls that provide temperature setback for heating systems and setup for
cooling systems. These controls allow a system that is shut off during off hours to automatically restart
and temporarily operate in order to maintain the space at a setback or setup temperature set point.
Setback control requirements are as follows.
• Heating system setback. Setback controls are required for heating systems. The setback set point
must be an adjustable set point not less than 10°F (5.6°C) below the occupied heating thermostat
set point.
• Cooling system setup. Setback controls are required for mechanical cooling systems. The setback
set point must be an adjustable set point not less than 5°F (2.8°C) above the occupied cooling
thermostat set point.
Limitations on setback controls prevent spaces from becoming so cold or so hot during off hours that
damage may occur to materials within or that the HVAC system will not be able to bring them back to a
comfortable range in a reasonable period of time. In humid climates, the cooling is required for
dehumidification as well as cooling, so the cooling setback is limited to avoid high-humidity issues.
When they are not employed in extreme climates, experience has shown that operators will simply
configure the systems to run 24 hours at least part of the year to avoid undercooled or underheated
spaces. This wastes energy.
Heating Setback Controls (Exceptions to 6.4.3.3.2)
While setback controls are required to have the set-point capabilities listed above, these setback set
points may not be the optimum for all applications. Buildings using radiant heating rely on both the
direct radiant heat from the radiant source and the indirect radiation from the building and the object
within it to maintain occupant thermal comfort. When a radiant system heating set point is set back,
the building and the objects within it cool as well. The associated thermal mass of the building and its
contents extends the time required for the heating system to respond to thermostat set point changes.
For systems such as this, large set point changes can increase energy consumption rather than reduce
it. Therefore, the standard requires radiant heating systems to be configured with an adjustable
setback set point not less than 4°F (2.2°C) below the occupied heating thermostat set point. The
inherently slow pickup capability of these heating systems makes only a slight setback possible.
For buildings that are massive, or where heating or cooling capacity is marginal, it may be more energy
efficient to set back temperatures only slightly from occupied set points. The best way to determine
optimum set points is by trial and error once the building and system installations are complete. The
standard does require that setback be configured at startup, and properly set up optimum start
controls (see Section 6.4.3.3.3) can make setback effective, even with marginal heating or cooling
capacity. Computer simulations are also possible but not always accurate because of the very complex
ways that energy is transferred into and out of and stored within building mass.
Optimum Start Controls (6.4.3.3.3)
The simplest time switch or scheduling controls start systems each day based on the time of day. To
ensure that the space is comfortable prior to occupancy, these controls are typically scheduled to start
two or three hours prior to the expected occupancy time to allow for warm-up or cooldown. But this
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amount of warm-up/cooldown time is not always required. For instance, during mild weather, the
space may be very near comfort conditions and require little or no warm-up/cooldown period. To
eliminate this unnecessary system operating time, optimum start controls were developed.
The standard requires optimum start controls for individual heating and cooling systems configured
with setback controls and a DDC system.
Ideally, optimum start controls will start the system to provide just enough warm-up or cooldown time
to bring the spaces served by the system to occupied set point temperatures at exactly the occupied
hour—no sooner and no later. In practice, this ideal control is not possible but can be approached
depending on the sophistication of the control algorithm.
The standard requires that the control algorithm must, at a minimum, be a function of the difference
between space temperature and occupied set point, the outdoor air temperature, and the amount of
time prior to scheduled occupancy. The inclusion of outdoor air temperature in the optimum start
algorithm allows the system adequate time to provide warm-up or cooldown during extreme weather
conditions. Mass radiant floor systems are required to include additional controls that include the floor
slab temperature in the optimum start algorithm. Although not required by the standard, more
sophisticated algorithms include an adjustable or self-tuned mass/capacity factor that reflects the
thermal mass of the building and the capacity of the heating and cooling systems.
Zone Isolation (6.4.3.3.4)
Large central systems often serve zones that are occupied by different tenants or user groups and may
be occupied at different times. When only a part of the building served by the system is occupied,
energy is wasted if unoccupied spaces are conditioned. To minimize this waste, the standard requires
that systems serving zones expected to operate nonsimultaneously be divided into isolation areas that
can be operated as if they were independent systems.
Isolation areas can be as small as one zone, but more practically zones will be grouped together into a
single isolation area. For offices, zones may be grouped into a single isolation area, provided the area
neither exceeds 25,000 ft² (2300 m²) of conditioned floor area nor includes more than one floor. For
all other occupancies, zones may be grouped into a single isolation area, provided the area does not
exceed 25,000 ft² (2300 m²) of conditioned floor area or it serves only one tenant.
Each isolation area must be equipped with isolation devices and controls that allow each zone to be
shut off or set back individually. See Example 6-V through Example 6-X.
Each isolation area must include individual automatic shutdown controls meeting the requirements of
Section 6.4.3.3.1 as if it were a separate HVAC system. This allows each isolation area to automatically
operate on different time schedules. This is typically done using separate time switches or scheduling
programs for each isolation area. Separate off-hour timed override capability must also be provided for
each zone.
Each isolation area must be equipped with isolation devices capable of automatically shutting off the
supply of conditioned air and outdoor air to and exhaust air from the area. Figure 6-C shows an
example of conditioned supply air control. The figure is a schematic riser diagram of a central VAV fan
system serving several floors of a building, each assumed to be less than 25,000 ft² (2300 m²).
Isolation of each floor is required if they are to be occupied by occupants that can be expected to
operate on different schedules, or if occupant schedules are unknown. Isolation of floors or zones may
be easily accomplished by any one of the methods depicted schematically in Figure 6-C and described
as follows:
• Direct digital controls. On the lowest floor in the figure, individual zones are controlled by DDC. If
the DDC software can be programmed with a separate occupancy time schedule for each zone or for
a block of zones, isolation can be achieved without any additional hardware. The boxes are simply
programmed to shut off or control to setback set points (including zero minimum airflow set
points) during unoccupied periods.
• Normally closed zone boxes. On the next floor up, zone boxes are shown to be normally closed
(which means when control air or control power is removed, a spring in the box actuator causes the
box damper to close). This feature can be used as an inexpensive means to isolate individual
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tenants or floors. The control source to each group of boxes is switched separately from other
zones. When the space is unoccupied, the control source is shut off, automatically shutting off zone
boxes. A separate sensor in the space can restore control to maintain setback or set-up
temperatures.
• Motorized damper. On the next floor up in Figure 6-C, isolation is achieved by simply inserting a
motorized damper in the supply duct.
• Combination fire/smoke damper. On the top floor, the cost of this damper is saved or reduced by
using a combination fire/smoke damper at the shaft wall penetration. Smoke dampers are usually
required by life-safety codes to limit smoke transfer or control floor airflow for pressurization.
These dampers may serve as isolation devices at virtually no extra cost, provided they are wired so
that life-safety controls take precedence over off-hour controls. (Local fire officials generally allow
this dual use of smoke dampers and often encourage it because it increases the likelihood that the
dampers will be in good working order when a real life-safety emergency occurs.)
Note that on all floors in Figure 6-C, shutoff is not shown on floor return openings. This is because the
wording of the standard requires only that supply of conditioned air and outdoor air to and exhaust air
from the area be shut off. In addition, with a plenum return system, the amount of air drawn off an
unconditioned floor will be negligible compared to the occupied floors that have positive air supply
since the latter will be pressurized.
Note also that a positive means of zone shutoff or setback is required. Shutoff VAV boxes (boxes with
no minimum volume setting) cannot be assumed to close automatically by their thermostats during
unoccupied periods due to low loads, because there may be 24-hour internal loads (such as personal
computers, idling copy machines, or emergency lighting) and envelope loads are continuous. Either the
VAV boxes must be forced closed or the thermostat set points must be set back/set up and airflow
minimum set point forced to zero during unoccupied periods, as described above.
Figure 6-C doesn’t show required shutoff for exhaust systems. With some exceptions (see the list that
follows), exhaust air from isolation areas must be shut off along with supply. This is particularly
important in humid climates because operating the exhaust without the supply will draw moist air into
building cavities and the building itself, often leading to microbial growth.
Because exhaust systems seldom have VAV boxes that can be used for shutoff, complying with this
section will require the use of smoke dampers or added shutoff dampers interlocked to the supply air
serving the zone. Depending on the type and size of the exhaust fan, some type of duct static pressure
control, such as variable-speed drives (VSDs), may be required as well.
Simply providing means for central-system zone isolation does not end the design task. Central
systems and plants must be designed to allow stable system and equipment operation for any length of
time while serving only the smallest isolation area served by the system or plant.
Experience has shown that almost any fan with a VSD for static pressure control can operate stably to
near-zero flow. This is true even for large centrifugal fans, which will eventually pass into the surge
region of their fan curves as load reduces, provided this occurs when the fan is operating below about
FIGURE 6-C. ISOLATION METHODS FOR A CENTRAL VAV SYSTEM
Corresponding section: Zone Isolation (6.4.3.3.4)
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50% speed and static pressure set points are less than about 2 in. of water (500 Pa). Under these
conditions, fan power is reduced to the point where surge pulsations will generally have too little
energy to cause objectionable noise or damage to duct systems.
Large axial fans with variable-pitch blades may also be able to operate at low flows without
overpressurizing ductwork. Fan curves at minimum blade pitch should be reviewed to ensure shutoff
pressures are below duct design pressures.
Where fans cannot be selected to operate safely at low loads, large fans can be replaced by multiple
smaller fans in parallel with operation, staged so that only one fan operates at low loads.
The same considerations must be applied to central chiller plants. The plant must be able to operate at
low loads for extended periods. If frequent chiller cycling is not acceptable, either multiple or staged
chillers can be used. Variable-speed-driven chillers are also a very efficient and effective option. With
VSDs, chillers can operate very efficiently at very low loads if condenser relief is available. Where
possible, hot-gas bypass should be avoided, as it can significantly increase energy costs and will not be
as efficient as multiple staged chillers or variable-speed chillers.
Exceptions to 6.4.3.3.4
Isolation devices and controls are not required for the following:
1. Exhaust air and outdoor air connections to isolation areas when the fan system to which they
connect is 5000 cfm (2360 L/s) and smaller. In other words, for exhaust fans or outdoor air
ventilation fans 5000 cfm (2360 L/s) or smaller, no isolation devices (such as dampers) or
controls need be installed at the fan or at any exhaust or outdoor air supply to any isolation area
served by the system.
2. Exhaust airflow from a single isolation area of less than 10% of the design airflow of the exhaust
system to which it connects. For instance, if an exhaust fan is 15,000 cfm (7079 L/s), it is not
necessary to install a damper or other device to shut off exhaust flow from any isolation area that
is less than 1500 cfm (708 L/s).
3. Zones intended to operate continuously or intended to be inoperative only when all other zones
are inoperative. For example, isolation would not be required for the entry lobby of a
multipurpose building, because it is occupied when any of the building areas are in operation. This
lobby would not benefit from isolation because it would need conditioning whenever the HVAC
system is on.
Example 6-V. Off-Hour Controls, Radiant Heating and Cooling Systems
Corresponding section: Zone Isolation (6.4.3.3.4)
Q
A space is heated or cooled by running hot or chilled water through radiant ceiling tiles. The heating
and cooling capacities of the radiant heating and cooling in this space are both greater than
15,000 Btu/h (4.4 kW). The space is not continuously occupied and not intended to run continuously.
The space is surrounded by spaces with similar occupancy schedules. A 5 hp (3.7 kW) ventilation fan
provides tempered ventilation air to the space from a system with a 2,000,000 Btu/h (586 kW) heating
capacity. Are off-hour controls required for this system?
A
Yes. There are three systems in this example: a radiant heating system, a radiant cooling system, and a
ventilation preconditioning system. All three systems must meet the requirements of Section 6.4.3.3.
The requirements will affect each system in a slightly different way. As all three systems together
compose the HVAC system, they each must comply with the automatic shutdown requirements
(Section 6.4.3.3.1). Ventilation fans are referred back to the automatic shutdown requirements by
Section 6.4.3.4.4.
The radiant cooling system must setback 5°F (2.8°C) above the occupied cooling thermostat set point.
The radiant heating system must setback 4°F (2.2°C) below the occupied heating thermostat set point
per the exception to Section 6.4.3.3.2. The radiant cooling and heating system must also include
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optimum start controls. These controls must be a function of space temperature, occupied space
temperature set point, outdoor air temperature, and time remaining until the start of the occupied
schedule. Zone isolation applies to air-side systems and therefore does not impact the controls for the
radiant systems.
The ventilation system must shut off during the setback period, as its function is to provide tempered
ventilation air, not maintain space temperature. Additionally, the ventilation system is not required to
follow the optimum start logic (Section 6.4.3.3.3), as it does not have setback set points, nor can it heat
or cool the space. Zone isolation (Section 6.4.3.3.4) for the ventilation system is required. The spaces
served by the ventilation unit all have the same occupancy schedule, so isolation of individual spaces
served by the system is not required unless the total area served by the unit exceeds 25,000 ft2
(2300 m2) or covers more than one floor.
Example 6-W. Off-Hour Isolation Controls, Floor-by-Floor System
Corresponding section: Zone Isolation (6.4.3.3.4)
Q
A speculative office building is designed to have an air-handling system on each floor. What off-hour
isolation control provisions are required?
A
If the floors are less than 25,000 ft² (2,300 m²) of conditioned area, then each floor may be considered
an isolation area. Each fan system must be able to operate on a different time schedule.
If the floors are larger than 25,000 ft² (2,300 m²) and expected to be occupied by different tenants
operating on different schedules, the system will have to be broken into more than one isolation area.
See the discussion in the text regarding Figure 6-C for ideas about how this might be accomplished.
Example 6-X. Off-Hour Isolation Controls, Water-Loop Heat Pump (WLHP) System
Corresponding section: Zone Isolation (6.4.3.3.4)
Q
A 100,000 ft² (9290 m²) speculative office building is served by an HVAC system consisting of
individual hydronic heat pumps for each zone connected to a central condenser water pump, closedcircuit cooling tower, and boiler. A 15,000 cfm (7080 L/s) central dedicated outdoor air system (DOAS)
fan provides ventilation air to each heat pump. What off-hour isolation devices are required?
A
The heat pumps must be grouped into isolation areas, ideally one area for each tenant. Unless they
cover only one tenant or tenants that operate on similar schedules, isolation areas may be no larger
than 25,000 ft² (2300 m²) each and may include zones only from one floor.
Each isolation area must include an individual time control to control the heat pumps within that area.
This might be an individual time switch thermostat for each zone or for only one of the zones in the
isolation area with interlocks to the other heat pumps in the area. Each isolation area control would
need to be interlocked to start the central equipment as required.
The DOAS also needs to include shutoff controls for each isolation area, as it is larger than 5000 cfm.
Although not required by the standard, pressure-independent controls for each isolation area may be
needed to ensure outdoor air to active areas is not oversupplied when inactive areas are shut off.
Condenser water isolation valves are not required for each isolation area because Section 6.4.3.3.4
only requires the isolation of air supply and exhaust. Water could simply continue to flow through
inactive heat pumps. However, automatic isolation valves at each heat pump, interlocked with its
compressor, may be required if the Prescriptive Path (Section 6.5) is used. See the Hydronic System
Design and Control (6.5.4) section later in this chapter.
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 Automatic Control of HVAC in Hotel/Motel Guest Rooms (6.4.3.3.5)
Guest rooms in hotels and motels are often unoccupied for extended periods even when they are
rented, and the unoccupied periods are even longer when the room is not rented. For hotels/motels
with more than 50 guest rooms, automatic controls are required to provide thermostat setup and
setback and shut off ventilation air when the room is unoccupied. The requirements differ based on
whether the room is rented or not and on whether the control is a room occupancy sensor or part of a
networked system. Networked guest room control system is a new definition added to Section 3 in the
2016 edition of the standard.
When the room has been unoccupied for 30 minutes or more, the thermostat set points must be
increased by at least 4°F (2°C) in cooling and decreased by at least 4°F (2°C) in heating. When the room
is unrented, the set points must be 80°F (27°C) or higher in the cooling mode and to 60°F (16°C) or
lower in the heating mode. An exception allows the cooling system to be operated so as to provide
humidity control even if this results in providing a space temperature below 80°F (27°C).
When the room has been unoccupied for 30 minutes or more, the ventilation and exhaust air for the
room shall be shut off. If the room has individual ventilation or exhaust fans, these must be turned off.
If the ventilation or exhaust is provided by a central system, then isolation devices shall be installed to
stop ventilation and exhaust airflow. Isolation devices is a defined term in Section 3 of the standard and
in this context means dampers to close off the airflow. When the room is unoccupied for more than a
day, ventilation can be turned on for up to 60 minutes per day to provide a daily flush out of any air
contaminants that may have accumulated.
Section 6.4.3.3.5.3 specifically identifies captive key card systems as an acceptable method for
determining whether the room is occupied. These systems require the user to insert their key card into
a reader inside the room to show occupancy. Often these systems are used to enable lights and
entertainment systems as well as the ventilation and thermostat controls required by this section of
the standard.
When an occupancy sensor or captive key card system is used, the room is considered to be unrented
when it is unoccupied for 16 hours or more. When a networked guest room control system is used,
rented or unrented status is expected to be determined by the system. If the room status is unrented
and it has been unoccupied for 30 minutes, then the unrented thermostat set points must be applied.
Occupancy sensors are expected to turn ventilation and exhaust systems back on when hotel staff are
servicing the room, regardless of whether the room is rented or unrented. When a networked guest
room control system is used, thermostat set points can be restored to occupied settings up to 60
minutes prior to the time the room is scheduled to be occupied.
Ventilation System Controls (6.4.3.4)
Section 6.4.3.4 covers ventilation system controls, including both mechanical and nonmechanical
systems. The requirements of this section are intended to reduce infiltration into the building when
ventilation systems are off or not required. Infiltration will speed up the natural cooling or warming of
the space during off hours and thereby increase the energy required to maintain setback temperatures
and possibly increase the energy use required to bring the space back to normal occupied
temperatures.
This section has five subsections, each of which is discussed in the following paragraphs.
•
•
•
•
•
Stair and Shaft Vents (Section 6.4.3.4.1)
Shutoff Damper Controls (Section 6.4.3.4.2)
Damper Leakage (Section 6.4.3.4.3)
Ventilation Fan Controls (Section 6.4.3.4.4)
Enclosed Parking Garage Ventilation (Section 6.4.3.4.5)
Stair and Shaft Vents (6.4.3.4.1)
Stair and elevator shaft vents must be equipped with motorized dampers that are capable of being
automatically closed during normal building operation and are interlocked to open as required by fire
and smoke detection systems. Some building codes may restrict the use of dampers in elevator shaft
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vents; if so, the dampers are not required per Section 2.4 (“This standard shall not be used to
circumvent any safety, health, or environmental requirements.”).
Shutoff Damper Controls (6.4.3.4.2)
All outdoor air intake and exhaust systems are required to be provided with dampers that
automatically close when the fan is shut off. Shutoff dampers must be motorized (e.g., electrically or
pneumatically actuated). There are, however, exceptions to the automatic shutoff damper controls:
1. Gravity (nonmotorized) dampers are acceptable for exhaust and relief in buildings less than three
stories in height above grade and for buildings of any height located in Climate Zones 0 through 3.
In addition, ventilation air intakes may use gravity dampers for buildings of any height located in
Climate Zones 0 through 3 (but not for buildings fewer than three stories in height for the other
climates unless they meet one of the other exceptions). The reason for requiring motorized
dampers in tall buildings in cold climates is due to pressure caused by stack effect that can force
gravity dampers open.
2. Nonmotorized gravity dampers are allowed on outdoor intakes or exhausts less than or equal to
300 cfm (142 L/s).
3. Dampers are not required on ventilation or exhaust systems serving unconditioned spaces, such as
most parking garages.
4. Dampers are not required on exhaust systems for Type 1 kitchen exhaust hoods.
In addition, the provisions in Section 2.4 exempt systems such as laboratory fume exhausts, which
health and safety codes prohibit from having dampers.
For ventilation outdoor air supply systems, in addition to shutting the damper when the fan is off, the
outdoor air damper must also shut during preoccupancy building warm-up, cooldown, setup, and
setback, except when ventilation reduces energy costs (for example, during night purge) or when
ventilation must be supplied to meet code requirements. For systems required to have optimum start
(Section 6.4.3.3.3), the controls must be capable of distinguishing between warm-up and cooldown
modes (the period that the system runs prior to scheduled occupancy to bring spaces to comfortable
temperatures after setback/setup operation) and normal occupied mode so that the minimum outdoor
air damper can be closed for the former and opened for the latter. Standard programmable
thermostats and the air-conditioning unit control boards they connect to are generally not able to do
this (of course they usually do not have the capability of performing optimum start logic, either). More
sophisticated controls, such as direct digital controls, will usually be required. This capability is also
required for systems required to have setup or setback controls (Section 6.4.3.3.2) and motorized
outdoor air dampers; they cannot use standard programmable thermostats without some other means
to lock out the ventilation damper when the system runs in setback/setup mode, such as a separate
time switch with a contact wired in series with the normally closed outdoor air damper.
For systems controlled by DDC, programming the damper to operate in this manner is simple. For
pneumatic or electric control systems, the control is more complex. The time switch or scheduling
control would have to distinguish between occupied hours and unoccupied system operation. In
general, two schedules and outputs would be required, one for normal occupied hours to control the
outdoor air damper along with the fan and one for warm-up/cooldown operation to control the fan but
not the outdoor air damper.
Example 6-Y addresses shutoff damper controls.
Damper Leakage (6.4.3.4.3)
Where outdoor air supply and exhaust/relief air motorized or nonmotorized dampers are required per
Section 6.4.3.4.2 (or where economizer supply and return dampers are required per Section 6.5.1.1.4),
they must be designed to have a leakage less than or equal to the requirements shown in Table 6-A
(Table 6.4.3.4.3 in the standard) when the damper is in the closed position. This table presents the
leakage requirements in cfm/ft² at 1.0 in. of water (L/s per m² at 250 Pa) when tested in accordance
with AMCA Standard 500.
The leakage requirements in Table 6-A are based on climate zone, building height, and whether it is an
intake or exhaust/relief. There are leakage requirements for both motorized and nonmotorized
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(gravity) dampers. The requirements shown in Table 6-A have four levels of stringency from leakiest
to tightest:
• 40 cfm/ft² (200 L/s∙m²) for nonmotorized dampers that are smaller than 24 in. (0.6 m) in either
dimension. This leakage requirement can be met by standard dampers. The exception only applies
to small gravity dampers (see footnote 1 to Table 6-A).
Example 6-Y. Automatic Damper for Outdoor Air Intake, Packaged Air Conditioner
Corresponding section: Shutoff Damper Controls (6.4.3.4.2)
Q
A 7.5 ton (26.4 kW), 3,000 cfm (1,410 L/s) packaged air conditioner is to be installed to serve a twostory office space in Chicago, Illinois. It has an outdoor air intake for minimum ventilation designed for
450 cfm (212 L/s). The manufacturer offers a manual outdoor air damper or a motorized damper as
options. Which should be specified?
A
A motorized damper must be specified. The outdoor air intake is designed for more than 300 cfm (142
L/s) so it must have an automatic damper; Exception 2 to Section 6.4.3.4.2 does not apply. The manual
damper will not meet the requirements of this section. Chicago is in Climate Zone 5A so it is not
exempt from the requirements of Exception 1 to Section 6.4.3.4.2. The building is less than three
stories in height; however, this exception only allows nonmotorized gravity dampers for relief and
exhaust. An automated ventilation damper is still required. The designer must either specify a
motorized damper to be provided with the unit or a separate motorized damper to be furnished and
field installed.
TABLE 6-A. MAXIMUM DAMPER LEAKAGE (CFM PER FT²) AT 1.0 IN. OF WATER [(L/S PER M²) AT 250 PA]
Corresponding section: Damper Leakage (6.4.3.4.3)
(This is Table 6.4.3.4.3 in the standard.)
Table 6.4.3.4.3 Maximum Damper Leakage
(cfm per ft2) at 1.0 in. of water
Climate Zone
0, 1, 2
Any height
3
Any height
4, 5b, 5c
Fewer than 3 stories
Three or more stories
5a, 6, 7, 8
Fewer than 3 stories
Three or more stories
Ventilation Air Intake
Exhaust/Relief
Nonmotorized1
Motorized
Nonmotorized1
Motorized
20
4
20
4
10
10
20
NA
20
10
NA
NA
4
4
NA
NA
Dampers smaller than 24 in. in either dimension may have leakage of 40 cfm/ft²
NA = Not allowed
1
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20
10
20
NA
4
4
10
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Table 6.4.3.4.3 Maximum Damper Leakage
(L/s per m²) at 250 Pa
Climate Zone
0, 1, 2
Any height
3
Any height
4, 5b, 5c
Fewer than 3 stories
Three or more stories
5a, 6, 7, 8
Fewer than 3 stories
Three or more stories
Ventilation Air Intake
Exhaust/Relief
Nonmotorized1
Motorized
Nonmotorized1
Motorized
100
20
100
20
NA
NA
50
50
100
NA
50
50
100
NA
NA
50
20
20
Dampers smaller than 0.6 m in either dimension may have leakage of 200 L/s per m²
NA = Not allowed
1
100
100
NA
50
20
20
• 20 cfm/ft² (100 L/s∙m²). This requirement is also only applied to gravity dampers where they are
allowed. This requirement can be met by standard gravity dampers with blade seals.
• 10 cfm/ft² (50 L/s∙m²) for motorized dampers where indicated in the table. This will require lowleakage triple-vee-groove dampers with flexible metal compression jamb seals and polyvinylchloride-coated polyester blade seals. (Polyurethane foam or similar blade seals will not likely
provide acceptable performance.)
• 4 cfm/ft² (20 L/s∙m²) for motorized dampers where indicated in the table. This will require an
ultra-low-leakage damper. This is typically a damper with airfoil-shaped blades, neoprene or vinyl
edge seals, and flexible metal compression jamb seals. For larger dampers (those greater than 3 ft
[1 m] or so in width), a vee-groove type blade damper with blade and jamb seals may work.
Manufacturers publish leakage requirements in conformance with AMCA Standard 500. The onus is on
the designer to verify that the dampers meet or exceed these requirements.
Ventilation Fan Controls (6.4.3.4.4)
Fans with motors greater than 3/4 hp (0.5 kW) must have automatic controls complying with Section
6.4.3.3.1 that are capable of shutting off fans when they are not required. HVAC systems intended to
operate continuously are exempt from this requirement.
Enclosed Parking Garage Ventilation (6.4.3.4.5)
Ventilation systems for enclosed parking garages must automatically detect contaminant levels and
stage fans or modulate airflow rates to 50% or less of design capacity, provided acceptable
contaminant levels are maintained. Typically, carbon monoxide (CO) and nitrogen dioxide (NO2) are
the primary contaminants to be monitored.
There are three exceptions:
1. Garages smaller than 30,000 ft² (2787 m²) with ventilation systems that don’t use mechanical
cooling or mechanical heating.
2. Garages that have a garage area to ventilation system motor nameplate kilowatt ratio that exceeds
1500 ft²/hp (187 m²/kW) and do not utilize mechanical cooling or mechanical heating.
3. Where not permitted by the authority having jurisdiction.
Heat Pump Auxiliary Heat Control (6.4.3.5)
The heating capacity of air-source heat pumps will decrease as outdoor air temperatures fall. To make
up for this deficiency, auxiliary heaters are typically installed to augment the heat output from the heat
pump. With an electric resistance heater (with a COP of 1), the efficiency of the system is significantly
reduced as compared to the heat pump operating alone (with a COP typically greater than 2). The
standard, therefore, requires that controls be provided that prevent auxiliary heater operation when
the heating load can be met by the heat pump alone, other than during outdoor coil defrost cycles.
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Of primary concern is morning warm-up when the space may be well below set point even during
relatively mild weather. The heat pump could warm the space sufficiently quickly by itself, but typical
thermostatic controls would cause the auxiliary heater to operate as well, wasting energy.
The best way to resolve this problem is to use an electronic thermostat designed specifically for use
with heat pumps. This thermostat can sense if the heat pump is raising space temperature during
warm-up at a sufficient rate or maintaining space temperature during normal operation and only
energize the auxiliary heat if required.
More traditional electric controls can also be used, as demonstrated by Examples 6-Z and 6-AA. The
following are required (and shown schematically in Figure 6-D):
• A two-stage thermostat must be used, with the first stage wired to energize the heat pump and the
second stage wired to bring on the auxiliary heat.
• An outdoor thermostat must be provided and wired in series with the second stage so that the
auxiliary heat will only operate if both the second stage of heat is required and the outdoor air is
cold (below set point).
• The outdoor thermostat set point must be set to the temperature at which heat-pump capacity will
be insufficient to warm up the space in a reasonable period of time, e.g., 40°F (4°C).
This section only applies to electric resistance auxiliary heaters, as they will reduce the overall COP of
the system. If auxiliary heaters such as gas furnaces are installed (generally at greater cost than
resistance heaters), it is presumed that the user does so consciously and will operate the auxiliary heat
to minimize utility costs.
FIGURE 6-D. HEAT PUMP AUXILIARY HEAT CONTROL USING TWO-STAGE AND OUTDOOR AIR
THERMOSTATS
Corresponding section: Heat Pump Auxiliary Heat Control (6.4.3.5)
Heat pumps whose minimum efficiency is regulated by NAECA and whose heating seasonal
performance factor (HSPF) rating both meets the requirements shown in Table 6.8.1-2 and includes all
use of internal electric resistance heating are also exempted, because the use of auxiliary electric heat
has already been accounted for in the equipment rating.
Humidification and Dehumidification (6.4.3.6)
In systems where zone humidification is actively used to increase the space relative humidity (RH),
independent of zone temperature, through the use of fossil fuels or electricity, the zone humidity may
not be increased above 30% rh in the warmest zone. Conversely, a dehumidification system may not
use fossil fuels or electricity to lower the humidity below 60% rh in the coolest zone. These limits are
set to reduce energy associated with mechanical cooling and reheat associated with dehumidification
and energy consumption associated with humidification methods.
Where a zone is served by a system or systems with both humidification and dehumidification
capability, means must be provided to prevent simultaneous operation of humidification and
dehumidification equipment. Acceptable means include mechanical stops on humidistats to prevent
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overlapping set points; electrical interlocks to prevent humidification equipment from operating when
dehumidification systems are on and vice versa; and, for DDC systems, software programming to
prevent overlapping set points. For multiple computer-room units with self-contained controls serving
the same data center, a special network is usually required that ties the unit controls together and is
programmed to prevent units from humidifying while others are dehumidifying.
Example 6-Z. Heat Pump Auxiliary Heat Control, Two-Stage Thermostat
Corresponding section: Heat Pump Auxiliary Heat Control (6.4.3.5)
Q
Will a simple two-stage thermostat, wired to bring on the auxiliary heat as the second stage, meet the
requirements of Section 6.4.3.5?
A
No, because it will still cause auxiliary heat to be brought on during warm-up even when outdoor
temperatures are mild and the heat pump has adequate capacity by itself. One of the acceptable control
options listed must be provided.
Example 6-AA. Heat Pump Auxiliary Heat Control, Two-Stage Thermostat with Outdoor Air Temperature
Lock Out
Corresponding section: Heat Pump Auxiliary Heat Control (6.4.3.5)
Q
Will an outdoor thermostat, wired to lock out auxiliary heat operation during mild weather, meet the
requirements of this section?
A
Yes, if it is used in conjunction with a two-stage thermostat and if it is wired properly (see Figure 6-D).
Many manufacturers’ installation diagrams show outdoor thermostats wired to provide an additional
thermostat stage while using only a single-stage thermostat. It is wired so that electric heat operates
with the heat pump when outdoor temperatures are cold (below the outdoor thermostat set point).
This design (using a single-stage thermostat and outdoor air thermostat) does not comply with Section
6.4.3.5, as it may cause the auxiliary heat to operate when it is not required because the heat pump
may be able to meet the load even during cold weather.
There are three exceptions to this requirement:
1. Desiccant cooling systems used with direct evaporative cooling in series. With these systems,
air is heated and dried with the desiccant, sensibly cooled with a heat exchanger or mechanical
cooling, then evaporatively cooled to achieve an outlet condition of nearly saturated air at
conditions very near what would leave a conventional air-conditioning unit. Technically, this
process both dehumidifies and humidifies the air, which is why this exception is provided.
2. Systems serving zones where specific humidity levels are required, such as museums and
hospitals, and approved by the authority having jurisdiction or required by accreditation
standards. This exception allows additional control of humidity but explicitly requires a 10% rh
deadband be maintained where no active humidification or dehumidification will occur. In most
cases this can be achieved by good design. For instance, in a hospital, cooled and dehumidified air
may be provided to all zones in the hospital by a central system while local humidifiers add
moisture back into the air to achieve a high humidity level that may be desired in the operating
suite. In this example, the minimum dehumidification set point (e.g., 60% rh) for all zones served
by the same system must be at least 10% higher than the maximum humidification set point (e.g.,
50% rh) for the operating rooms. In this scenario, the VAV air handler will dehumidify to 60% rh,
which is not low enough to initiate the humidification of the operating rooms, avoiding
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3.
simultaneous dehumidification and humidification. This exception is designed to allow humidity
control set points between 30% and 60% rh but to explicitly not allow simultaneous
dehumidification and humidification, such as systems that cool to a constant low dew point then
rehumidify for each zone, as might be required for tight humidity control (±5%) as discussed
below. Unless there is a specific code or accreditation requirement for tight humidity control, the
control system should be designed so that dehumidification and humidification are not
simultaneously employed. Where primary air in a VAV system is cooled for temperature control
using the resets required elsewhere in the standard and then reheated and humidified at the zone
level, the coincident dehumidification that occurs would not violate the requirements under
Section 6.4.3.6 as long as the primary air temperature is controlled based on temperature rather
than dehumidification requirements. (Note that computer rooms do not necessitate the tight
humidity requirements and are therefore not included in this exemption. For more information,
see the ASHRAE book Thermal Guidelines for Data Processing Environments, Third Edition.)
Systems where tight humidity control is required. Humidification and dehumidification
systems may be exempt from the requirements of Section 6.4.3.7 when they are required to
maintain zone humidity levels within ±5% rh in order to satisfy the local code, the authority
having jurisdiction, or accreditation standards. The tight humidity requirements of this exemption
may be required for specific critical-environment applications or to maintain occupant safety.
Freeze Protection and Snow/Ice Melting Systems (6.4.3.7)
Freeze protection heating systems are commonly provided on piping and equipment located outdoors
or in unconditioned spaces to prevent freezing in the winter. Perhaps the most common example is
electric resistance heat tracing wound around piping through which a current is run, thus heating the
piping above 32°F (0°C). When outdoor air temperatures are above freezing, it is obviously wasteful to
continue to operate the freeze protection heaters. This is doubly true for heat tracing on chilled-water
and condenser water piping because the heating energy from the tracing becomes a cooling load if the
system is operating at these conditions.
To avoid this waste, the standard requires that freeze protection heating systems include automatic
controls capable of shutting off the systems when outdoor air temperatures are above 40°F (4.4°C) or
when the conditions of the protected fluid will prevent freezing. For example, heat tracing of piping
can simply be shut off by an outdoor air thermostat set to 40°F (4.4°C) or lower. Note that this
requirement applies even if the heat tracing is so-called “self-regulated,” which means that its output is
automatically reduced as the temperature of the heat tracing increases. It is a common
misunderstanding that self-regulated heat tracing reduces its heat output to zero at temperatures
above freezing; this is not the case. While the heat output reduces at warm temperatures, it never
drops completely to zero.
Snow- and ice-melting systems must include automatic controls capable of shutting off the systems
when the pavement temperature is above 50°F (10°C) and no precipitation is falling. This will require
a pavement temperature sensor (generally located midway between two pipes or heating cables) as
well as a snow or precipitation detector. In addition, in order to ensure that the system does not run in
warm weather, an automatic or manual control is required to allow shutoff when the outdoor
temperature is above 40°F (4.4°C).
Ventilation Controls for High-Occupancy Areas (6.4.3.8)
Spaces with high design occupant densities offer an excellent opportunity for demand control
ventilation (DCV) systems because these spaces are seldom occupied at their design occupancy. DCV
systems modulate the amount of outdoor air supplied to a space as a function of the number of people
present, providing significant energy savings when spaces are only partially occupied. The standard
requires DCV for all zones that are larger than 500 ft² (50 m²) and that have an average design
occupancy density of 25 people per 1000 ft² (100 m²) or greater where they are served by an HVAC
system that meets one or more of the following conditions:
• Design outdoor air capacities greater than 3000 cfm (1400 L/s).
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• The system includes an air-side economizer. Air-side economizers require the ability to modulate
the flow rate of outdoor air to in order to meet the required supply air temperature. Thus, the
system-level hardware necessary for DCV exists.
• The system includes automatic modulating control of the outdoor airflow rate. As with air-side
economizers, the hardware necessary for DCV at the system level exists.
The DCV requirements typically include assembly spaces, such as theaters, meeting rooms, and
ballrooms. Note that the occupant density threshold includes a significant number of space types that
may not have required DCV to comply with Standard 90.1 prior to the 2016 edition.
Exceptions to this requirement include the following:
• Systems provided with exhaust air energy recovery systems complying with Section 6.5.6.1.
• Multiple-zone systems that do not have DDC to the zones (for example, systems with pneumatic
zone thermostats).
• Spaces where the design outdoor airflow rate is less than 750 cfm (375 L/s).
• Spaces where more than 75% of the space design outdoor airflow is either exhausted from the
space or transferred from the space as makeup air to an adjacent space. An example of this
exception is a restaurant where the air from the dining area is transferred to the kitchen as makeup
for the kitchen hoods.
• Spaces that contain any one of the following occupancy categories as defined by ASHRAE Standard
62.1: correctional cells; daycare sickrooms; science labs; barber, beauty, and nail salons; and
bowling alley seating. These spaces have a high per-area ventilation rate, so there is less
opportunity for ventilation reduction based on occupancy. The bowling alley seating occupancy is
unusual in that sensing occupancy is difficult because these areas are not separated from adjacent
areas with primarily area-based ventilation requirements.
Although this section specifically lists multiple exempt occupancy categories, the provisions in Section
2.4 exempt spaces such as hospital wards, vivariums, and laboratories where minimum air change
rates are required by health and safety codes. Where DCV systems are provided, they must maintain
ventilation rates in accordance with local codes. Because most ventilation codes prescribe outdoor air
rates proportional to the number of people in a space for at least one component of the ventilation
rate, it follows that a DCV system should modulate outdoor air as a function of the number of people.
The most commonly used indicator of the ventilation required to dilute people-related pollutants is
carbon dioxide (CO2) concentration. People give off CO2 and other bioeffluents (including body odor) at
a rate roughly proportional to their activity level. Hence, CO2 concentration is a good indicator of
bioeffluent concentration and is therefore ideal for DCV.
Most ventilation codes also dictate minimum ventilation for dilution of contaminants from the building
materials or products used in the space (for example, hair spray in a barber shop). Again, the
provisions of Section 2.4 mandate that the control of the DCV comply with these minimum
requirements at all times.
For those jurisdictions that use ASHRAE Standard 62.1 as the ventilation code, Appendix A of 62.1
User’s Manual has a recommended procedure for designing and controlling DCV systems.
 Heated or Cooled Vestibules (6.4.3.9)
This section applies to heating and cooling equipment used for conditioning entrance vestibules or
integrated with an air curtain. Typically vestibules are heated by a small cabinet unit heater or fan-coil.
Air curtains are frequently used for large roll-up doors. These systems must be equipped with controls
that automatically turn off the heating when the outdoor air temperature is above 45°F (7°C). When
the outdoor air temperature is below this, vestibule heaters must be controlled by a thermostat located
within the vestibule. The thermostat set point must be limited (hardware or software limit) to 60°F
(16°C) or below. Cooling systems must have a set point of 85°F (29°C) or more. The ability to sense the
outdoor air temperature is not a feature typically included with a vestibule system or an air curtain.
Buildings equipped with DDC do not consistently incorporate unit heaters and cabinet unit heaters
into the control system. For this situation or other situations where adding an outdoor air temperature
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sensor is an additional cost, the maximum vestibule space temperature set point can be limited to no
more than 45°F (7°C) to comply with the outdoor air temperature requirement.
When the ventilation rate of a building exceeds the mechanical exhaust airflow rate, the central airhandling equipment relieves the excess air. In this situation, a portion of the excess air can be
transferred to the vestibule for conditioning. A small fan pressurizing the vestibule with air that would
otherwise be exhausted from an adjacent lobby is an example. This type of vestibule heating is exempt
from the heating control requirements of this section, as this transfer air must be relieved regardless of
the presence of the vestibule. In addition, systems that use site-recovered energy are exempt.
Direct Digital Control (DDC) Requirements (6.4.3.10)
Section 6.4.3.10 covers requirements for direct digital control (DDC). DDC systems are discussed in
many other areas of this user’s manual, as they are an integral part of most modern HVAC&R systems.
The definition of “DDC” is a type of control where controlled and monitored analog or binary data (e.g.,
temperature, contact closures) are converted to digital format for manipulation and calculations by a
digital computer or microprocessor, then converted back to analog or binary form to control physical
devices. The control and monitoring abilities of these systems make them priceless for adjusting and
troubleshooting HVAC&R systems. These advantages give sufficient cause for mandating their
inclusion in HVAC&R systems.
A DDC system must be included in HVAC&R systems, as required by Table 6.4.3.10.1, with the
exception of projects following the simplified approach. Table 6.4.3.10.1 divides projects into either a
new building or an alteration or addition. Within each project status are specific HVAC&R applications
and qualifications that determine if a DDC system is required. For example, a new building is being
constructed with a 500,000 Btu/h (147 kW) central heating water plant providing heating water to ten
zones. This HVAC system must have a DDC system for the control of the heating water plant, heating
water coils, and terminal units.
For new buildings, DDC is required when any of the following systems are present:
• Air-handling systems serving four or more zones with a fan system input power of 10 hp (7.46 kW)
or larger
• Chilled-water plants and all coils and terminal units served by any system serving four or more
zones and with a design cooling capacity of 300,000 Btu/h (88 kW) or larger
• Hot-water plants and all coils and terminal units served by any system serving four or more zones
and with a design heating capacity of 300,000 Btu/h (88 kW) or larger
For alterations and additions to buildings or systems, DDC is required when any of the following
systems are present:
• Zone terminal units, such as VAV boxes, where the existing zones served by the same air-handler,
chilled-water, or heating water system already have DDC
• Air-handling systems or fan-coils where the existing air-handling systems and fan-coils served by
the same chilled- or hot-water plant already have DDC
• New air-handling systems and all new zones served by an existing system with individual fan
systems with an input power of 10 hp (7.46 kW) or greater and supplying four or more zones and
more than 75% of the new zones
• New or upgraded chilled-water plants where all the chillers are new and the plant design cooling
capacity is 300,000 Btu/h (25 tons, 88 kW) or larger
• New or upgraded hot-water plants where all the boilers are new and plant heating design capacity
is 300,000 Btu/h (88 kW) or larger
Where a DDC system is required it must include the following capabilities:
a. The control system must monitor system- and zone-level demand such as fan pressure, pump
pressure, coil valve positions, and terminal box damper positions. This feedback provides the
information necessary for the control system to react. The system must monitor fan pressure,
pump pressure, heating demand (from zones and coils), and cooling demand (from zones and
coils).
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b.
The control system must be able to communicate the demand signals from the zone-level controls
to the system-level controls such that the system can adjust to satisfy zone-level requirements. For
example, adjusting the outdoor airflow rate at the system for DCV in a VAV system is difficult and
likely inaccurate without feedback from the terminal boxes.
c. The control system must be capable of identifying zones that are preventing set points from
resetting and generate an alarm or an indicator to notify the system operator of the issue. For
example, the control valve for an undersized terminal heating coil remains 100% open under low
load conditions. This valve prevents the boiler plant control logic from resetting the supply water
temperature set point. If the situation persists, the operator must be notified of the issue.
d. The HVAC&R system operator must be able to remove problem zones from the control logic. Given
the example in the previous item, two responses to the problem exist. The zone heating capacity
can be increased by a hardware change, or the coil can be removed from the heating water
temperature reset logic. Depending on the space the coil serves, it may not be necessary to
increase the capacity. If the coil serves a storage room where maintaining a precise space
temperature is not necessary, removing the coil from the logic allows the water temperature to
reset. The space temperature set point may not be satisfied, but if the space temperature is
maintained within a tolerable range this likely is not a problem.
In new buildings where a DDC system is required, the DDC system must include the ability to trend
(record) input and output values. Most control systems can modify the frequency of recording values.
For monitoring general energy performance and system health, many DDC systems will record values
four times per hour. For troubleshooting, it may be desirable to record at shorter intervals to better
understand how the system is operating. The DDC system must also be able to graphically display the
input and output values. This ability may come from a control panel provided by the HVAC&R
equipment manufacturer, or it may belong to a workstation connected to the DDC control network.
 Chilled-Water Plant Monitoring (6.4.3.11)
Section 6.4.3.11 requires monitoring central chiller plant efficiency and electricity consumption in
large electric-motor-driven chilled-water plants. Monitoring is required for plants that have a capacity
equal to or greater than a capacity that is based on whether the plant is water-cooled or air-cooled and
on the climate zone, as shown in Table 6-B.
TABLE 6-B. PEAK COOLING CAPACITY OF CHILLER PLANTS THAT REQUIRE MONITORING
Corresponding section: Chilled-Water Plant Monitoring (6.4.3.11)
Climate Zone
0, 1, 2, 3A, 3B, 4A, 4B
3C, 4C, 5, 6, 7, 8
Plant Cooling Method
Water Cooled
1000 tons (3517 kW)
1500 tons (5275 kW)
Air Cooled
570 tons (2005 kW)
860 tons (3024 kW)
Section 6.4.3.11.2 requires that electrical energy use and efficiency must be recorded at intervals of no
more than 15 minutes, and the system must store all data for at least three years. The system must be
able to display the data graphically, including hourly, daily, monthly, and annual data.
Working with plant operators to design trend graphs that show the efficiency over various time
intervals, such as a day, week, or month, can provide feedback to the operators on how the plant is
performing. This feedback can help the operators improve the efficiency of the chiller plant. Other
graphics can be designed that might include efficiency versus cooling output or efficiency at various
outdoor temperatures. These approaches can create a continuous commissioning environment.
This monitoring does not require separate system and sensors. A monitoring system can use sensors
and measurement devices that are included in associated equipment such as chillers and pump and fan
speed drives. For additional background and information, see ASHRAE Guideline 22, Instrumentation
for Monitoring Central Chilled-Water Plant Efficiency.
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 Economizer Fault Detection and Diagnostics (FDD) (6.4.3.12)
Section 6.4.3.12 provides requirements for systems that use air-cooled DX cooling. Where economizers
are installed in accordance with Section 6.5.1, the system must have sensors and controls that will
allow it to automatically detect any of a list of specific faults. The required sensors must monitor the
temperature of the outdoor air, supply air, and return air. The control system must display these
temperatures, as well as a list of five specific economizer status variables. Finally, the system must
allow the operator to manually put the economizer into each operating mode to allow testing and
verification of system operation.
HVAC&R System Construction and Insulation (6.4.4)
Insulation (6.4.4.1)
General (6.4.4.1.1)
Required piping and ductwork insulation must be installed in accordance with industry-accepted
standards such as those described in the Midwest Insulation Contractors Association’s (MICA)
Standards Manual, 6th Edition.
In addition, insulation that is subject to damage must be protected. For instance, insulation that may be
damaged by workers maintaining equipment (for example, if it has to be walked on or over to access
equipment) must be protected from damage, such as by enclosing it in a plastic or metal jacket or a
canvas wrap.
Insulation located outdoors where it may be subject to damage from sunlight, moisture, and wind must
be suitable for outdoor service. For instance, fiberglass insulation must be protected by an aluminum,
sheet metal, painted canvas, or plastic cover. Cellular foam insulation must be similarly protected or
painted with a coating that is water retardant and provides shielding from solar radiation that can
cause the material to degrade.
Insulation must also be installed to prevent condensation from occurring within the insulation or on
the covered duct or piping surfaces. Many insulation types, notably fiberglass, will lose much of their
insulating properties if they become soaked with water from condensing moisture. Even if the
insulation is not damaged from the moisture directly, moisture can lead to microbial growth that can
cause material degradation and produce noxious odors. To prevent condensation damage, the
standard requires that chilled-water piping, refrigerant suction piping, or cooling ducts located outside
the conditioned space must include a vapor retarder located outside the insulation, unless the
insulation is inherently vapor retardant. All penetrations and joints of the vapor retarder must be
sealed.
Duct and Plenum Insulation (6.4.4.1.2)
All supply and return ducts and plenums installed as part of an HVAC&R air distribution system must
be thermally insulated. The insulation R-values, as installed and excluding film resistance, must be
equal to or greater than the values shown in Tables 6.8.2.
The table lists duct insulation requirements as a function of the duct application (cooling-only supply
duct, heating-only supply duct, return air duct, and heating and cooling supply duct); climate
(characterized by climate zones, which are listed in Reference Standard Reproduction Annex 1 of the
standard); and the following duct or plenum locations:
• Exterior. Includes ducts and plenums exposed to outdoor air, attics above insulated ceilings,
parking garages, and both ventilated and nonventilated crawlspaces.
• Unconditioned space and buried ducts. Includes unconditioned rooms such as equipment rooms
(provided they do not qualify as indirectly conditioned spaces) and ducts located within the
ground. Ground temperatures a few feet below grade are cool and remain relatively constant year
round. See Chapter 5 for definitions of indirectly conditioned and unconditioned spaces.
• Indirectly conditioned space. Includes return air plenums, shafts, and mechanical rooms that are
wholly or mostly enclosed by adjacent conditioned spaces. (See Chapter 5 for a definition of
indirectly conditioned space.) In all climates and duct applications, the requirement is R-1.9
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insulation. This level of insulation is often applied to avoid condensation issues, regardless of the
requirements of Table 6.8.2.
The required minimum thicknesses in Table 6.8.2 do not consider water vapor transmission and
possible surface condensation. Therefore, even if the R-value in the table is low or zero, thicker
insulation may be required to prevent condensation on duct surfaces or within the insulation.
Where exterior walls are used as plenum walls, wall insulation must be as required by the most
restrictive condition of Section 6.4.4.1.2 or Section 5. In most and perhaps all cases, the Section 5
insulation requirements will be more restrictive because it is less expensive to insulate walls than
ducts, allowing higher R-values to be cost-effective.
Examples 6-BB and 6-CC address duct insulation.
Exceptions to 6.4.4.1.2
1. Factory-installed plenums, casings, or ductwork furnished as a part of HVAC&R equipment tested
and rated in accordance with Section 6.4.1 are exempt from the requirements of Section 6.4.4.1.2.
This exception is intended to apply to casings around tested equipment, the energy losses through
which are accounted for in the energy performance tests and ratings. Although they are not always
included in performance testing, optional casings such as economizer sections should also be
exempted from insulation requirements if they are insulated to the same extent as the equipment
to which they are attached. This is a practical requirement because designers seldom have the
option to specify casing insulation levels.
This exception does not apply to air handlers and field-fabricated plenums that do not have
efficiency ratings. These systems must be insulated according to the requirements for ductwork in
this section. For instance, a custom air handler must be insulated as an exterior return air duct
from the return air inlet to the coil section and insulated as an exterior supply air duct from the
coil onward.
2. Ducts or plenums located in heated spaces, semiheated spaces, or cooled spaces are not required
to be insulated. Heat losses and gains from these ducts usually have little or no energy impact.
3. For run-outs less than 10 ft (3 m) in length to air terminals or air outlets, the rated R-value of
insulation need not exceed R-3.5 (R-0.6). This exception is intended to allow standard flexible duct
with 1 in. (25 mm) insulation to connect terminal units even where a greater insulation thickness
may be required for other ducts.
4. Backs of air outlets and outlet plenums exposed to unconditioned or indirectly conditioned spaces
with face areas exceeding 5 ft² (0.5 m²) need not exceed R-2 (R-0.4); those 5 ft² (0.5 m²) or smaller
need not be insulated. This exception is intended to allow 1/2 in. (13 mm) liner for plenums and
duct boots, which is generally the only option for factory-made plenums for linear diffusers, and to
obviate the need to insulate the backs of standard 2 × 2 ceiling diffusers.
Table 6-C shows the thicknesses of common materials that deliver the installed R-values listed in
Tables 6.8.2-1 and 6.8.2-2. This table is not meant to limit the use of other insulation materials that
meet the minimum R-value requirements.
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TABLE 6-C. R-VALUES FOR COMMON DUCT INSULATION MATERIALS
Corresponding section: Duct and Plenum Insulation (6.4.4.1.2)
Installed R-Value1
(h∙°F∙ft²)/Btu
1.9 (0.34)
6.0 (1.06)
8.0 (1.41)
12.0 (R-2.12)
Typical Material Meeting or Exceeding the Given R-Value2
1/2 in. (12.7 mm) mineral fiber duct liner per ASTM C 1071, Type I
1 in. (25.4 mm) mineral fiber duct wrap per ASTM C 1290
1.5 in. (25.4 mm) mineral fiber duct liner per ASTM C 1071, Types I and II
1.5 in. (25.4 mm) mineral fiber duct board per UL 181
1.5 in. (25.4 mm) mineral fiber board per ASTM C 612, Types IA and IB
2 in. (50.8 mm), 2 lb/ft³ (32 kg/m³) mineral fiber duct wrap per ASTM C 1290
2.5 in. (63.5 mm), 0.6 to 1 lb/ft³ (9.6 to 16.0 kg/m³) mineral fiber duct wrap per ASTM C 1290
2.5 in. (63.5 mm) insulated flex duct per UL 181
2 in. (50.8 mm) mineral fiber duct liner per ASTM C 1071, Types I and II
2 in. (50.8 mm) mineral fiber duct board per UL 181
2 in. (50.8 mm) mineral fiber board per ASTM C 612, Types IA and IB
3 in. (76.2 mm), 3/4 lb/ft³ (12 kg/m³) mineral fiber duct wrap insulation per ASTM C 1290
3 in. (76.2 mm) insulated flex duct per UL 181
3 in. (76.2 mm) mineral fiber board per ASTM C 612, Types IA and IB
Listed R-values are for the insulation only as determined in accordance with ASTM C 518 at a mean temperature of 75°F (23.9°C) at the
installed thickness and do not include air film resistance.
2 Consult with manufacturers for other materials or combinations of insulation thickness or density meeting the required R-value.
1
Example 6-BB. Duct Insulation, Example System
Corresponding section: Duct and Plenum Insulation (6.4.4.1.2)
Q
The figure below shows an HVAC system for a building in downtown Chicago, Illinois (Climate Zone
5A). What are the insulation requirements for each duct location shown in the figure?
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The insulation requirements for each duct location are as follows:
1. Heating or cooling unit casings and plenums. Exception 1 to Section 6.4.4.1.2 exempts casings
and plenums in equipment that has an energy rating such as EER or COP, because the methods
used to measure this energy performance include casing losses. Therefore, if this unit is a unitary
product, no duct insulation requirements apply for the unit itself. For air handlers and other
nonrated products, the unit casing would have to be insulated as if it were duct exposed to the
outdoors.
2. Exhaust ductwork. Exhaust ductwork need not be insulated, because it is not covered in Table
6.8.2. In most applications, insulating exhaust ducts will have no impact on building energy use.
3. Supply and return ducts in attic. These ducts are located in an attic that is above an insulated
ceiling. This location is considered to be exterior when using Table 6.8.2. For an HVAC unit that is
cooling only, ducts in this location (including both supply and return ducts) require R-8 (R-1.41)
insulation. For a heating-only system or a system that provides both heating and cooling, the
supply and return ducts require R-12 (R-2.12) insulation.
4. Supply and return ducts on the exterior of the building. Exposed ductwork insulation
requirements for Chicago are R-12 (R-2.12) for heating-only and heating/cooling ducts and R-8 (R1.41) for cooling-only ducts. Again, the requirements are identical for both supply and return
ducts.
5. Supply and return ducts in unconditioned space. The shaft, as shown in the figure, is an
unconditioned space, because the shaft wall between the shaft and the conditioned spaces is
insulated while the outside shaft wall is not. Hence, according to the table, R-6 (R-1.06) insulation
is required for all ducts.
6. Supply and return ducts in an attic with roof insulation. Here the ducts are located within an
attic with roof insulation on the top. This makes the plenum an indirectly conditioned space.
According to the table, R-1.9 (R-0.34) is required for all ducts.
7. Supply and return ducts in indirectly conditioned ceiling space. In this case, the ducts are
located in a ceiling attic that is not significantly exposed to the outside and therefore qualifies as an
indirectly conditioned space. R-1.9 (R-0.34) is required for these ducts.
8. Exterior wall of return plenum. In this area, the ceiling space is being used as a return plenum.
The exterior walls of the space are effectively return duct walls exposed to the outside. This wall
must be insulated to the more restrictive requirement of either the building envelope, as in Section
5, or the requirements for ducts located in the exterior of the building (see #4 in this example). In
most cases, the Section 5 building envelope requirements will be more stringent.
9. Supply outlet in the return plenum. The plenum surrounding the air outlet is part of the supply
duct system and therefore should be insulated the same as the supply ducts (see #11 in this
example). However, Exception 4 allows this plenum to be minimally insulated to R-2 (R-0.4) if
more than 5 ft² (0.5 m²) in area and to be uninsulated if 5 ft² (0.5 m²) or less in area.
10. Supply run-out in the return plenum. According to Exception 3, a run-out of up to 10 ft (3 m) to
a terminal device (supply outlet or VAV box) need only be insulated with R-3.5 (R-0.6). This is
intended to allow the use of standard flexible duct with 1 in. (25 mm) insulation, which has an Rvalue of about R-4.0 (R-0.7). Flexible duct with 2 in. (51 mm) insulation is not commonly available.
This exception holds even if the supply ducts are required to have R-6.0, R-8.0, or R-12.0 (R-1.06,
R-1.41, or R-2.12) insulation.
11. Supply ducts in the return plenum. Return air plenums qualify as indirectly conditioned spaces
because of the large amount of air being drawn through them. This is so even when they are
exposed to a roof above, similar to #6 in this example. Ducts located in return plenums need only
be insulated to R-1.9 (R-0.34).
12. Supply and return ducts in conditioned space. According to Exception 2, supply and return
ducts located in the conditioned space do not require insulation. From a practical viewpoint,
insulation may be desirable on cooling ducts to prevent condensation if the duct passes near local
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areas of high humidity, as might occur in a kitchen. For typical spaces, condensation will generally
not occur even at very low supply temperatures, because the space relative humidity will be
lowered correspondingly by the dry air supply.
13. Supply and return ducts in a vented crawlspace. Vented crawlspaces are considered to be
exterior (see #4 above).
14. Supply and return ducts below grade. For this climate, ducts located underground must be
insulated with R-6 (R-1.06).
Example 6-CC. Duct Insulation, Outdoor Air and Exhaust Louvers
Corresponding section: Duct and Plenum Insulation (6.4.4.1.2)
Q
How must the ductwork shown in the figure below be insulated when it is exposed to a conditioned
space for a building in Chicago, Illinois?
A
Table 6.8.2 does not address outdoor air ducts or exhaust ducts. For exhaust ducts, when the HVAC
system is on, heat transfer from the duct to or from the space served will have little or no overall
energy impact. This is also true of outdoor air intake ducts in many applications; because the outdoor
air would eventually be conditioned by the HVAC system, heat losses or gains to conditioned spaces
from the outdoor air in the duct would have a small net energy impact.
But what happens when the HVAC system is off? Outdoor air will infiltrate through the louver up to the
shutoff damper required by Section 6.4.3.4.3. Consequently, the duct between the damper and the
louver is essentially part of the exterior envelope. But Section 5 does not have a wall classification for
“ductwork walls.” The insulation requirements in Section 5 were based on how practical it is to
improve the insulation for each type of wall versus how much energy is saved. The insulation
requirements in Table 6.8.2 were similarly developed for ductwork. It therefore makes sense to
insulate this exterior wall as a duct in accordance with Table 6.8.2 as opposed to insulating the duct as
if it were a wall in accordance with Section 5. The requirements for ducts exposed to the exterior
should be used. For Chicago, the duct would therefore have to be insulated to R-12 (R-2.12).
Piping Insulation (6.4.4.1.3)
All piping associated with HVAC systems must be thermally insulated in accordance with Table 6.8.3-1
(for heating and hot-water systems) and Table 6.8.3-2 (for cooling, brine, and refrigerant systems). The
values in these tables are minimum thicknesses of insulation having a conductivity falling in the range
listed, when tested at the mean rating temperature listed, for each fluid design temperature range
category. These conductivities are typical of fiberglass and most elastomeric foam insulation, which
are the most commonly used insulation materials.
If a less common insulation product is to be used, such as cellular glass or calcium silicate, then the
thicknesses listed in Tables 6.8.3-1 and 2 must be adjusted using the following equation:
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𝑡𝑡 𝐾𝐾 ⁄𝑘𝑘
𝑇𝑇 = 𝑟𝑟 × ��1 + �
𝑟𝑟
Cha p t e r 6 | H VAC Sys t e ms
− 1�
(6-A)
where
T = the minimum insulation thickness, in inches (millimeters), for alternative material with a
conductivity K
t = the insulation thickness, in inches (millimeters), from Tables 6.8.3-1 and 6.8.3-2
r = the actual pipe outside radius, inches (millimeters). This is generally not equal to half of the
nominal pipe diameter; except for piping 14 in. (350 mm) and larger, actual outside diameter
(OD) will be larger than the nominal diameter and depends on the piping material selected.
Actual ODs can be found in standard piping tables.
K = the conductivity of alternative material, in Btu∙in./(h∙ft²∙°F), when measured at the mean
temperature indicated in Table 6.8.3 for the applicable fluid design temperature range
k = the upper value of the conductivity range listed in Tables 6.8.3-1 and 6.8.3-2 for the applicable
fluid design temperature range
Examples 6-DD, 6-EE, 6-FF, and 6-GG address piping insulation.
Exceptions to 6.4.4.1.3
Insulation is not regulated in the following cases:
• Factory-installed piping within HVAC equipment tested and rated in accordance with Section 6.4.1.
The intent here is to exempt piping within equipment whose energy performance is tested and for
whom piping losses are ostensibly accounted for in the ratings.
• Piping that conveys fluids having a design operating temperature range between 60°F and 105°F
(16°C and 41°C), inclusive, such as typical condenser water piping.
• Piping that conveys fluids that have not been heated or cooled with fossil fuels or electricity (such
as roof and condensate drains, domestic cold-water supply, or natural gas piping).
• Where heat gain or heat loss will not increase energy use. Examples include liquid and hot-gas
refrigerant lines on air-conditioning units and liquid lines on heat pumps.
• Strainers, control valves, and balancing valves in piping ≤1 in. (25 mm) in size (nominal). This
allows easy access to these devices.
Sensible Heating Panel Insulation (6.4.4.1.4)
Where radiant heating panels are provided, all thermally ineffective heating surfaces, including backs
of panels, U-bends, and piping headers, must be insulated to a minimum of R-3.5 (R-0.6). For the
purpose of this requirement, insulation in the surface to which the panel is mounted counts toward the
requirement.
Radiant Floor Heating (6.4.4.1.5)
Where radiant floor heating is applied, the surfaces of the floor below, the radiant heating must be
insulated to a minimum of R-3.5 (R-0.6). As with the previous requirement, insulation integral to the
floor can be counted toward this requirement. An exception is provided for heated slab-on-grade floors
that incorporate radiant heating. For these systems, refer to Chapter 5 for the insulation requirements
for the floor.
Example 6-DD. Piping Insulation, Chilled-Water Return Piping
Corresponding section: Piping Insulation (6.4.4.1.3)
Q
A chilled-water system is designed for a chilled-water supply temperature of 44°F (7°C) with a 16°F
(9°C) range. Is insulation required on the return piping?
A
No. The chilled-water return temperature will be 60°F (16°C) at design conditions, so this piping
would fall under Exception 2 to Section 6.4.4.1.3, and no insulation is required by the standard.
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However, return water temperatures will often be lower at part load and will often be lower than
ambient dew-point temperatures, as well. Therefore, while the standard does not require insulation,
minimal insulation is required from a practical standpoint to prevent condensation, and it may be costeffective because it reduces chiller load.
Example 6-EE. Piping Insulation, Condenser Water System with Water-Side Economizer
Corresponding section: Piping Insulation (6.4.4.1.3)
Q
A high-rise office building has a water-side economizer. Under cooling conditions, the condenser water
operates in the range of 65°F to 95°F (18°C to 35°C), depending on outside conditions and cooling load.
But during the winter, the cooling tower is controlled to cool water evaporatively down to 45°F (7°C).
Does this piping require insulation?
A
No. The standard regulates piping insulation based on fluid “design” operating conditions, which refers
to the fluid state at peak cooling or peak heating design conditions. In this case, it could be argued that
the condenser water is only of concern in the cooling mode, because at design heating conditions,
which will occur during building morning warm-up, the water economizer will be inactive. Therefore,
the condenser water piping would not have to be insulated. (In this case, insulation would have very
little if any energy impact anyway. However, insulation may be desirable in some locations to prevent
condensation.)
Example 6-FF. Piping Insulation
Corresponding section: Piping Insulation (6.4.4.1.3)
Q
In some parts of the country, it is common to leave some elements of the heating system piping system
uninsulated, such as hot water pumps and piping between heating coils and its shut off valves. Does
this comply with Section 6.4.4.1.3?
A
No. The standard defines “piping” as: “the pipes or tubes interconnecting the various parts of a fluid
distribution system, including all elements that are in series with the fluid flow, such as pumps, valves,
strainers, and air separators, but not including elements that are not in series with the fluid flow, such
as expansion tanks, fill lines, chemical feeders, and drains.” So all parts of the circulating system must
be insulated, except as allowed by the exceptions. An example of where the exceptions apply include
piping to heating coils that are exposed to the condition space they serve: the insulation between the
control valve and the heating coil need not be insulated, because heat loss from the piping when the
control valve is open will not increase energy use, as losses are to the conditioned space.
Example 6-GG. Piping Insulation, Calculating Thickness of Cellular Glass Insulation
Corresponding section: Piping Insulation (6.4.4.1.3)
Q
Cellular glass piping insulation is proposed for 10 in. (250 mm) chilled-water lines running outdoors.
(This insulation material is often preferred for outdoor installations because it is very durable and will
not absorb water like fiberglass, which effectively destroys its insulating properties. There is then less
concern about the quality of insulation weatherproofing.) The design chilled-water supply
temperature range is 44°F to 54°F (7°C to 12°C). What pipe insulation thicknesses are required?
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A
From the manufacturer’s catalog, cellular glass has a conductivity of 0.33 Btu∙in./(h∙ft²∙°F) (0.048
W/m∙°C) at 75°F (24°C) mean temperature. This conductivity is outside the 0.21 to 0.27
Btu∙in./(h∙ft²∙°F) (0.030 to 0.039 W/[m∙°C]) range listed in Table 6.8.3-2. Therefore, the minimum
insulation thickness must be determined using the following equation:
𝑇𝑇 = 5.375 × ��1 +
1.0
5.375
�
T = 1.24 in.
0.33⁄0.27
− 1�
(6-B)
where
r = 5.375 in. (136.6 mm) (from Table 6-C for steel pipe)
t = 1.0 in. (25 mm) (from Table 6.8.3)
K = 0.33 (0.048 SI) (from manufacturer’s catalog)
k
= .27 (0.039 SI) (upper value of range from Table 6.8.3-2)
The next largest standard size is 1.5 in. (40 mm) insulation, which is the thickness specified in this
application.
Ductwork and Plenum Leakage (6.4.4.2)
Duct Sealing (6.4.4.2.1)
Ducts and all plenums with pressure class ratings must be constructed to seal class A. This includes
sealing of all transverse joints, longitudinal seams, and other connections, including but not limited to
spin-ins, taps, other branch connections, access doors, access panels, and duct connections to
equipment. See Figure 6-E for illustrations of the terms transverse and longitudinal.
Seal class A also requires that duct wall penetrations be sealed. Openings for rotating shafts must be
sealed with bushings or other devices that seal off air leakage. Sealing that would void product listings
is not required. Spiral lock seams need not be sealed.
All duct pressure class ratings must be designated in the design documents. Static pressure
classifications are determined by the design engineer and establish the duct construction
characteristics, such as metal thickness and reinforcing requirements.
Duct seal classes are consistent with those defined in the Sheet Metal and Air Conditioning Contractors’
National Association’s (SMACNA) HVAC Duct Construction Standards—Metal and Flexible (listed in
Appendix E of the standard). They establish which joints must be sealed but not how the joints are
sealed. Any combination of adhesives, gaskets, and tapes, including pressure-sensitive tapes, may be
used. For seal class A, pressure-sensitive tape must not be used as the primary sealant unless it has
been certified to comply with UL-181A or UL-181B by an independent testing laboratory and the tape
is used in accordance with that certification.
While the seal class definitions are consistent with SMACNA, Section 6.4.4.2.1 of Standard 90.1
requires different classes of duct sealing than SMACNA does for similar duct applications. Therefore,
simply specifying that ducts be constructed “in accordance with SMACNA” will not ensure compliance
with Standard 90.1.
Note also that the required seal classes in Standard 90.1 may not be consistent with the
recommendations in ASHRAE Handbook—HVAC Systems and Equipment. The seal levels in the
handbook were established in part based on considerations other than energy savings, such as
reducing unattractive smudging that can occur at leaks in ducts that are visible in the conditioned
space. The stringency levels in the standard are based on minimal energy cost-effectiveness without
consideration of other application issues.
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FIGURE 6-E. DUCTWORK SEAMS AND JOINTS
Corresponding section: Duct Sealing (6.4.4.2.1)
Duct Leakage Tests (6.4.4.2.2)
Leakage testing is another requirement of the standard that goes beyond the SMACNA standards.
Testing is only required for ducts with a design duct pressure class rating (the maximum pressure
under which the ducts are designed to operate) in excess of 3 in. of water (750 Pa) and for all ductwork
located outdoors (regardless of pressure class). Requirements for these duct sections are as follows:
• They must be identified on the design documents. This might be done by a general note stating, for
instance, “All ducts downstream of the supply fan and upstream of the VAV boxes shall be 4 in. of
water (1000 Pa) pressure class.” Alternatively, ductwork sections may be tagged on the plans with
their design duct pressure class rating. They must be tested in accordance with industry-accepted
test procedures, such as those outlined in Sections 5 and 6 of the SMACNA HVAC Air Duct Leakage
Test Manual. To reduce costs, the entire duct system need not be tested; tests may be made for only
representative sections, provided that these sections represent at least 25% of the total installed
duct area for the tested pressure class.
• The maximum leakage rate when the duct is tested at a pressure equal to the design duct pressure
rating must be less than Lmax as determined by the following equation:
(6-C, I-P)
𝐿𝐿𝑚𝑚𝑚𝑚𝑚𝑚 = 𝐶𝐶𝐿𝐿 × 𝑃𝑃0.65
where
Lmax =
=
CL
P
=
Lmax
CL
P
=
=
=
the maximum permitted leakage in cfm per 100 ft² of duct surface area
duct leakage class, fixed at 4 cfm/100 ft² of duct surface area
test pressure, which must be equal to the design duct pressure class rating in in. of water
(6-D, SI)
𝐿𝐿𝑚𝑚𝑚𝑚𝑚𝑚 = 𝐶𝐶𝐿𝐿 × 𝑃𝑃0.65 ⁄1000
the maximum permitted leakage in L/s∙m² of duct surface area
duct leakage class, mL/s∙m² at 1 Pa (6 for round/flat oval sheet and rectangular ducts)
test pressure, which must be equal to the design duct pressure class rating in Pa
Walk-In Coolers and Walk-In Freezers (6.4.5)
Walk-in coolers and freezers that are assembled on site must comply with the requirements of this
section. This section does not apply to factory-assembled coolers and freezers that leave a factory as a
complete cooler or freezer. It applies to coolers and freezers that have been delivered to the building
site as individual components (insulated panels, condensers, evaporator, lights, etc.) and assembled on
site.
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All site-assembled stand-alone walk-in coolers and walk-in freezers less than 3000 ft2 (280 m2) must
meet all the criteria in Section 6.4.5 of the standard. If a walk-in cooler or walk-in freezer has been
assembled so the combined single enclosure exceeds 3000 ft2 (280 m2), it is exempt.
Opaque walls, doors, and ceilings must have an insulation value of not less than R-25 (R-4.4) for
coolers and not less than R-32 (R-5.6) for freezers. This requirement does not apply to glazed portions
of doors or structural members.
Although walk-in coolers are not required to have an insulated floor, walk-in freezers must have a floor
with and insulation value not less than R-28 (R-4.9).
All doorways into a cooler or freezer must have a means of minimizing heat and air transfer between
the unit and the surroundings in order to reduce the load on the cooler or freezer. This includes strip
doors or spring-hinged doors. Other methods of minimizing infiltration when the doorway is not in use
are acceptable, provided they meet the intent of reducing air and heat transfer.
Where a cooler or freezer has a door that is less than 3 ft 9 in. (1.1 m) wide and 7 ft (2.1 m) tall, the
door must be equipped with an automatic closer. This closer must be able to fully close the door from
any open position within 1 in. (25 mm) of being closed.
Frequently walk-in freezers and coolers have transparent reach-in doors or windows in doors for
product display. In these applications, the glazing must be triple-pane. The space between each of the
panes must be filled with an inert gas (such as argon) to reduce the heat transfer between the panes.
Many inert gasses are available, but when selecting the appropriate gas, long-term performance should
be weighed against initial performance and cost. As an alternative to including inert gas between the
panes, air may be used, provided the glass has been made heat-reflective to reduce radiant heat gain
from outside the unit. For walk-in coolers only, an additional alternative exists. In lieu of triple-pane
glass, double-pane glass may be used, provided it includes both inert gas between the panes as well as
heat-reflective glass.
The evaporator fan in the walk-in cooler or freezer is responsible for circulating the air within the unit.
If the fan motors are less than 1 hp (0.75 kW) and operate on voltage less than 460 V, they must be
electronically commutated motors (brushless direct-current motors) or three-phase air-conditioning
induction. This improvement in motor efficiency (as compared to a shaded-pole motor) is especially
important for evaporator fan motors, as the heat loss from the motor adds to the refrigeration load.
Although some walk-in cooler and freezer condensing units are water cooled, many are air cooled.
Where the condensing unit is air cooled and a condenser fan motor is less than 1 hp (0.75 kW), the
motor must be an electronically commutated motor, permanent split capacitor-type motor, or threephase motor. All these motor options are much more efficient than shaded-pole motors.
To prevent condensation or frost from forming on structural members of coolers and freezers,
antisweat heaters may be installed within these components to raise their surface temperature above
the ambient air dew point. The standard requires these antisweat heaters to either be power limited or
be controlled to limit their runtime rather than allow them to run continuously. This requirement is
met one of two ways:
1. Controls must be provided that limit the antisweat heater use and only allow the heater to run
when the surface temperature of the structural component or glass falls below the dew point of
the air outside the cooler or condensation is present on the inside of the door glass.
2. If the controls are omitted, the total power of antisweat heaters must be limited. For door rail,
glass, and frames, the heater power draw is limited to a maximum of 7.1 W/ft2 (76 W/m2) of door
or window area for walk-in freezers. Walk-in coolers are limited to a maximum of 3.0 W/ft2 (32
W/m2) of door or window area.
Walk-in freezers must have controls to limit the runtime of defrost cycles. The controls must first limit
the defrost cycle to an upper temperature limit. Once this limit has been met, the cycle must end. If the
upper temperature limit has not been met within a designated time, the cycle must end regardless of
the resulting temperature. The upper temperature limit and cycle time limit are not specified by the
standard and are left to the designer or operator to determine based on operating conditions. Where
high humidity and frequent door operation are present, defrost cycles may take longer or require
higher temperatures.
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All lighting within the walk-in coolers and freezers must have an efficacy of no less than 40 lumens per
watt, where the wattage includes inefficiency losses incurred within any ballasts or drivers required to
convert voltage or current in order to operate the lamp. Where the lighting efficacy is less than
required above, lighting controls must be implemented to turn off the lights after the cooler or freezer
has been unoccupied for 15 minutes.
Refrigerated Display Case (6.4.6)
Refrigerated display cases, as the name implies, are used to display products that must be refrigerated
for quality preservation. These units are factory assembled rather than site assembled. They are
typically delivered to the site in an operational (or near operational) condition. Products stored in
these cases can be placed into and removed from the case by reaching into the case as opposed to
walking into the case as with walk-in coolers or walk-in freezers. However, many energy-efficiency
requirements are similar to walk-in units.
Refrigerated display cases must meet all of the following requirements.
• All display cases must meet the requirements of Section 6.4.1.1, Minimum Equipment Efficiencies,
and Tables 6.8.1-1 through 6.8.1-13.
• Display cases that use antisweat heaters must be provided with controls to limit their operation.
Unlike walk-in coolers and freezers, there is no option for eliminating these controls. The controls
must limit the operation of the antisweat heaters based on dew-point controls and glass surface
temperatures as with the walk-in coolers and walk-in freezers.
• Low-temperature display cases that maintain temperatures below freezing must include defrost
controls like those with walk-in freezers. The controller must limit the maximum temperate as well
as the maximum cycle time of the defrost cycle. But as with walk-in freezers, these set points are
left to the designer or manufacturer.
• Lighting within display cases must be operated by either an automatic time switch or motion
detector. The time switch must turn the lights off inside the case during nonbusiness hours. A
temporary override of the timer is allowed for turning the lights on during nonbusiness hours.
However, the temporary override must automatically return the lights to the off position after no
more than one hour has passed. If the motion sensor option is chosen, the motion sensor must
control 50% of the lights within the display case. After three minutes have passed without sensing
motion, the controller must turn off the lights it controls.
Prescriptive Path (6.5)
The prescriptive compliance path may be used for any HVAC&R system, but it is primarily used for
larger buildings and buildings with more complex systems and where the Simplified Approach Option
(Section 6.3) is not applicable. Systems complying using the Prescriptive Path have to meet the
requirements of Section 6.5 as well as the mandatory provisions of Section 6.4.
Economizers (6.5.1)
Commercial buildings generally require cooling even during cool- or cold-weather outdoor air
temperatures. Interior zones—zones not adjacent to the exterior window wall—require cooling year
round. Some exterior zones with large expanses of glass, particularly if facing south or west, will
require cooling during cool, sunny weather because low wintertime sun angles increase solar loads on
the building. Other exterior zones may require cooling during cold weather because of high internal
cooling loads from lights, people, and office equipment such as computers and copiers (plug loads).
In response to this characteristic of commercial buildings, Section 6.5.1 requires that cooling systems
have either an air or a “fluid” economizer, which is more commonly known as a water economizer.
Economizers are systems that use outdoor air as a source of cooling in place of or to supplement
mechanical cooling. The standard uses the term “fluid” because the medium used to transfer energy
from the inside load (e.g. cooling coil) to outside sink (e.g. cooling tower or dry-cooler) may be fluids
other than water, such as glycol mixtures or refrigerants. An important change in 2016 is that now
fluid economizers are required for chilled-water-based systems that do not use fans, such as passive
chilled beams and radiant systems, or systems that induce airflow, such as induction units and active
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chilled-beam systems, with an exception for systems in Climate Zone 1A or with a capacity less than
1,000,000 Btu/h in Climate zones 0, 1B, 2, 3, and 4 or with capacity less than 1,400,000 Btu/h in
Climate Zones 5 through 8.
Exceptions to 6.5.1
The effectiveness of economizers depends on the load characteristics of the building, the type of the
HVAC system, and the local climate. Economizers will always save energy regardless of the size of the
system, but smaller systems may not have an economic justification for including an economizer.
Accordingly, the standard provides the following exceptions to the economizer requirement:
1. Weather and capacity. The cooler the climate, the more hours there will be when outdoor air can
provide free cooling. The size of the cooling system is also a factor, because the cost of economizer
dampers and controls is not proportional to cooling capacity, whereas the energy savings from the
economizer is proportional to cooling capacity. Air-side economizers do not work as well in hot
and humid climates because the number of hours they operate is limited. However, even in most
hot and humid climates (except Climate Zones 0 and 1) there are enough economizer hours over
the life of the system to justify the economizer. The standard requires an economizer only if the
capacity of the individual fan cooling unit is equal to or larger than the capacity listed in Table
6.5.1-1 for the applicable climate. So the requirement is based on the fan-coil unit and not the
capacity of a central chilled-water plant or VRF system condensing unit capacity.
Table 6.5.1-1 does not require economizers for Climate Zones 0A, 0B, 1A, and 1B because these are
hot and humid climates where the outdoor air enthalpy is rarely low enough to justify an
economizer even over the life of the system. In all other climate zones, economizers are required
for fan cooling systems with a cooling capacity greater than or equal to 54,000 Btu/h (16 kW),
which is about 4.5 tons. See Example 6-HH.
2. Air quality. In areas where the outdoor air quality is poor, designers may opt to clean the air
before introducing it into the building for ventilation. ASHRAE Standard 62.1 suggests that when
the outdoor air quality does not meet the National Ambient Air Quality Standards (NAAQS)
established by the U.S. Environmental Protection Agency, the air should be cleaned to reduce
contaminants to the NAAQS limits. For particles (PM10 and PM2.5), this is very easily done; a
particle filter that is MERV 6 (PM10) or MERV 11 (PM2.5), when rated in accordance with ASHRAE
Standard 52.2, will in most cases reduce particle concentrations to below the NAAQS limits.
However, gas-phase air cleaners, such as those used to remove ozone or nitrogen oxides, are
relatively expensive to install and to operate. Because of this high cost, the standard does not
require economizers on systems for which gas-phase air cleaning has been installed to meet
Section 6.2.1 of ASHRAE Standard 62.1. This means that such systems only need to provide air
cleaning for the minimum ventilation rate, not 100% of the fan’s supply air capacity.
Note that even though this exception exempts systems with gas-phase air cleaning from installing
economizers, the designer should still consider using water economizers for these applications.
Water economizers do not increase outdoor air intake rates and therefore will not increase the
cost of gas-phase air-cleaning systems, but they still provide more energy savings than mechanical
cooling.
3. Process humidification. Humidification loads are proportional to the amount of outdoor air the
system supplies. Therefore, while air economizers reduce cooling energy use, they can increase
humidification loads and corresponding energy use. The standard does not require economizers
for systems for which 25% or more of design supply air capacity is to be supplied to spaces
designed to be humidified above 35°F (2°C) dew-point temperature to satisfy process needs. Note
that this exception only applies when spaces are required to be humidified by process needs, such
as printing facilities.
The standard is explicit that this exception does not apply to data centers. It also does not apply if
the building is being humidified only for comfort purposes. See also Humidification Systems
(Section 6.5.2.4) in this chapter.
In hospitals or ambulatory surgeries, this exception only applies if more than 75% of the air
supplied by the system is required to be humidified above 35°F (2°C) to comply with codes or
accreditation standards.
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4.
5.
6.
7.
8.
Condenser heat recovery. Economizers reduce energy use by using cool outdoor air to reduce
cooling energy demand. Heat recovery works on the opposite principle: it reduces heating energy
use by transferring heat rejected from spaces requiring cooling (such as interior zones) to offset
the heating demand from spaces requiring heating (such as perimeter zones) or from domestic hot
water. When economizers are provided, cooling equipment does not run (or runs at reduced load)
in cold weather, so no heat (or little heat) is available for recovery. For this reason, the energy
savings from condenser heat recovery will be significantly reduced if economizers are also used.
Therefore, the standard exempts systems that include a condenser heat recovery system required
by Section 6.5.6.2 from having economizers.
Note that Section 6.5.6.2 only requires heat recovery for the purpose of domestic hot-water use. It
does not require heat recovery for space heating. However, most systems capable of heating
domestic hot water can also be configured to provide space heating as well. Energy studies
indicate that, in cold weather, heat recovery systems can be significantly more efficient than
economizers; in mild weather, economizer systems are more efficient.
The system that is best on an annual basis depends on the building’s load characteristics (how well
the envelope is insulated, how large the cooling loads are in the winter), energy rates (the fuel
source for primary heating may be different from that for cooling, both of which have different
costs), and most significantly, the local climate. However, if there is a large heating load, even
during mild and hot weather, such as for the domestic hot-water heating system described in
Section 6.5.6.2, heat recovery will probably outperform economizer systems on an annual basis. A
detailed computer analysis would be required to evaluate the two design options in this
application. See Example 6-KK.
Residential. Residential buildings seldom have the high internal loads common to commercial
buildings. They therefore tend to need heating when the outdoor air is cool or cold. This reduces
the energy savings and cost-effectiveness of economizers. Therefore, the standard does not
require economizers for systems that serve residential spaces where the system capacity is less
than five times the requirement listed in Table 6.5.1-1. This last clause referring to Table 6.5.1-1
essentially means that this exception does not apply to very large residences that behave more like
commercial buildings than residential buildings.
Envelope-dominated space. Economizers are not required for systems that serve spaces whose
space-sensible cooling load at design conditions, not including transmission or infiltration loads, is
less than or equal to transmission plus infiltration loads calculated at 60°F (16°C) outdoor air
temperature. For such envelope load-dominated spaces, economizers will not be significant energy
savers because cooling loads will not occur in cold weather. To demonstrate the applicability of
this exception, simply recalculate space-cooling loads at 60°F (16°C) outdoor air temperature with
all other design conditions unchanged. If solar and internal loads are offset by the heat losses
through the envelope and by infiltration, then the system serving the space need not have an
economizer.
Few operating hours. Systems that serve spaces expected to operate fewer than 20 hours per
week, such as some places of worship, are not required to have economizers. The few hours of
operation reduce the energy-saving potential of the economizers, which reduces their costeffectiveness.
Supermarket refrigeration. Economizers are not required if they adversely affect open freezer
casework, such as that in grocery stores and supermarkets. When the space dew-point
temperature is above freezer casework surface temperatures, water vapor condenses on case
walls, causing frost. Frost partially insulates the walls from the products in the casework and from
the air surrounding the product, which then requires the casework refrigeration system to operate
at lower temperatures and therefore lower energy efficiency. Frost buildup also increases the
frequency at which the freezer must be defrosted. Economizers exacerbate this problem by
bringing in outdoor air during intermediate weather when outdoor air humidity is above the dew
point of the casework surfaces. The energy losses due to frost buildup reduce the savings from
economizers and therefore reduce their cost-effectiveness. This exception does not apply to
refrigerators and casework that operate above freezing, because economizers will not adversely
affect their operation.
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Higher-efficiency equipment for comfort cooling. For comfort cooling applications,
economizers are not required on cooling equipment that meet or exceed the efficiency
requirements determined using Table 6.5.1-3. The efficiency improvements in Table 6.5.1-3 were
determined from computer simulations of typical buildings to provide equivalent energy
performance to minimum-efficiency air-conditioning units with air economizers. Note that if the
product efficiency metric includes an annualized metric like IPLV or IEER, the improvement
should be determined by increasing these values by the percentage improvement in Table 6.5.1-3.
Also note that the requirement for efficiency improvements can be used to eliminate both air and
water economizers. See Example 6-LL.
10. Systems serving computer rooms. Systems that primarily serve computer rooms are exempt
from the economizer requirements where they meet one of the following:
a. The total cooling load of all computer rooms in the building is less than 3,000,000 Btu/h (880
kW or 250 tons), and the building is not served by a central chilled-water plant.
b. The room total cooling load is less than 600,000 Btu/h (180 kW or 50 tons), and the building
is not served by a central chilled-water plant.
c. The local authority does not allow cooling towers. This would rule out water-side
economizers.
d. Less than 600,000 Btu/h (180 kW or 50 tons) of computer-room cooling load is being added
to an existing building.
The purpose of these four exceptions is to only require economizers where a chilled-water plant
either exists or could reasonably be cost-effective. For facilities with chilled-water plants, waterside economizers typically pay back in less than three years. In mild climates, they have been
retrofitted with simple paybacks as short as six months. Water-side economizers can be applied to
either water-cooled or air-cooled chilled-water plants; for an air-cooled plant, a dedicated tower
or closed-circuit fluid cooler can be used for the economizer.
Air-side economizers can also be used to comply with the computer room and data center
economizer requirements of the standard, but if the computer room or data center requires
humidity control, the incremental energy requirements for humidification at low outdoor air
temperatures should be considered.
11. Systems primarily serving mission-critical computer rooms. Dedicated systems for computer
rooms are exempt where a minimum of 75% of the design load serves the following spaces:
a. Spaces classified as “essential facilities.” Definitions for essential facility and computer room
have been added to Section 3 of the 2016 edition of the standard.
b. Those spaces having a design of Tier IV as defined by TIA-942.
c. Spaces permitted under NFPA 70 Article 708, Critical Operations Power Systems (COPS).
d. Banking institutions that are defined as core clearing and settlement risks in “The Interagency
Paper on Sound Practices to Strengthen the Resilience of the US Financial System, April 7,
2003.”
The purpose of this exception is to not require air-side economizers for the 5% to 10% of facilities
most critical for national security and defense. This exception is not intended for use by
commercial data centers; it is intended only for those data center facilities whose failure would
cause widespread economic damage or a breach in national security if they went out of service.
Examples include the stock exchanges, command and control centers such as the North American
Aerospace Defense Command (NORAD), and facilities controlling the electric grids or nuclear
missile sites. See Example 6-II.
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Example 6-HH. Economizer Exception, Small Systems Serving a Data Center
Corresponding section: Exceptions to 6.5.1
Q
A data center in Los Angeles, California, is served by a 60,000 Btu/h (18 kW or 5 ton) air-conditioning
unit. Is an economizer required?
A
Yes. Los Angeles is in Climate Zone 3B, and Table 6.5.1-1 specifies that an economizer is required for
systems with a cooling capacity greater than 54,000 Btu/h. However, Exceptions 11(a) and 11(b) to
Section 6.5.1 say that economizers are not required if the total cooling load of all computer rooms in
the building is less than 3,000,000 Btu/h and the building is not served by a centralized chilled-water
plant, or is less than 600,000 Btu/h and the building is served by a centralized chilled-water plant.
Example 6-II. Economizer Exception, Data Centers
Corresponding section: Exceptions to 6.5.1
Q
A commercial colocation data center has an emergency generator and uninterrupted power supply
system for power backup. Backup power is part of the requirements for both TIA-942 Tier IV and
NFPA 70 Article 708, Critical Operations Power Systems (COPS). Does this comply with the exception
for essential facilities as defined in Exception 12(a) to Section 6.5.1?
A
No. This data center does not meet the essential facilities for the following reasons: (1) it must meet all
of the requirements from either TIA-942 Tier IV or NFPA 70 Article 708, and (2) it serves a commercial
process that is not critical for national security.
Example 6-JJ. Economizer Exception, Small Systems Serving a Commercial Building
Corresponding section: Exceptions to 6.5.1
Q
A commercial building in Los Angeles (not a data center) has two 50,000 Btu/h (15 kW or 4.2 ton) air
handlers supplying air to a common discharge plenum. Does this qualify as one system (total capacity
of 100,000 Btu/h [30 kW or 8.4 ton]), in which case an economizer is required? Or does it qualify as
two individual systems, each at 50,000 Btu/h (15 kW or 4.2 tons), in which case economizers are not
required?
A
Los Angeles is in Climate Zone 3B. Therefore it is not exempt from the economizer requirement in
Table 6.5.1-1 (for comfort cooling). An economizer is required if the fan system cooling capacity is
greater than or equal to 54,000 Btu/h (16 kW or 4.5 tons).
The rationale for the small size exception is that the energy savings cannot justify the cost of the
economizer dampers, plenums, and controls. In this example, despite sharing a common discharge
plenum, each air handler is considered an individual system and economizers are not required. If this
situation were to arise, installing two cooling systems without an economizer, each sized for 50% of
the cooling load, is likely more costly than using a single system with an economizer sized for 100% of
the cooling load. It is unlikely the decision to split a single system into two systems would be driven by
any cost savings associated with eliminating the economizer.
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Example 6-KK. Economizer Exception, Systems with Condenser Heat Recovery
Corresponding section: Exceptions to 6.5.1
Q
A condenser heat recovery system is installed to preheat the peak service water draw to 80°F (27°C)
(equivalent to 50% of the peak heat-rejection load at design conditions) for a water-cooled system
with 6,500,000 Btu/h (1905 kW) of total installed heat-rejection capacity. The facility operates 24
hours a day. The design service water heating load is 1,200,000 Btu/h (352 kW).
Is the economizer exempt for this system?
A
No, the economizer is required. The minimum required heating capacity of the heat recovery system is
60% of the peak heat-rejection load at design conditions, and the heat reclaim system is only
recovering 50%, so it does not comply with the exception.
Air Economizers (6.5.1.1)
Air economizers use controllable dampers to increase the amount of outdoor air drawn into the
building when the outdoor air is cool or cold and the system requires cooling.
The standard has specific requirements for all the major elements that compose air economizers:
• How the economizer dampers are modulated
• How the economizer is shut off when the weather is warm and no longer conducive to free cooling
• Damper characteristics
• How air is relieved from the building to prevent overpressurization
These components are shown in Figure 6-F and are discussed in the following sections.
Design Capacity (6.5.1.1.1).
Air economizer systems must be capable of modulating outdoor air and return air dampers to provide
up to 100% of the design supply air quantity as outdoor air for cooling.
Control Signal (6.5.1.1.2)
It is essential for economizer dampers to sequence properly with mechanical cooling so that
economizer savings can be maximized. To ensure proper sequencing, the standard requires that the
mixed-air temperature not control the economizer. Instead, the dampers must be controlled by the
same controller or control loop used to control the mechanical cooling, typically controlling the airhandler supply air temperature as shown in Figure 6-F.
There are two reasons why mixed-air temperature should not be used to control the economizer:
• Improper sequencing. Having two control loops controlling the economizer and mechanical
cooling is more likely to result in improper sequencing. For instance, if the economizer were
controlled to maintain a mixed-air temperature set point while the cooling was controlled to
maintain a supply air temperature set point, the two set points would have to be coordinated for
proper sequencing. Because of fan heat, the mixed-air temperature set point would have to be
lower than the supply air temperature set point. On VAV systems, fan heat varies, so maintaining
coordination between the two set points is difficult. If set point reset strategies were used, these too
would have to be coordinated.
• Difficult to measure. Mixed-air temperature is very difficult to measure. Even with a serpentine
averaging sensor, stratification and radiant effects from the chilled-water coil can cause sensor
errors. ASHRAE-sponsored research project RP-1045 documented that air from a mixing box can
remain stratified even after it has passed through the filters, coils, and fan.
Mixed-air temperature is acceptable for controlling the economizer for systems controlled from space
temperature, such as single-zone systems. This is allowed because these systems typically do not have
a supply air temperature sensor from which to control the economizer; controlling the economizer
from the space thermostat alone could lead to low entering air temperatures, which could lead to coil
freezing.
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FIGURE 6-F. ECONOMIZER SCHEMATIC
Corresponding section: Air Economizers (6.5.1.1)
Figure 6-G shows how outdoor air and return air dampers are typically sequenced. Sequencing can be
done using a single controller by selecting sequenced spring ranges, as is typically done with
pneumatic control systems. With digital control systems, sequencing is usually done through software.
For VAV systems, fan energy savings can be enhanced if the dampers are sequenced rather than
overlapped as shown in Figure 6-H; that is, the outdoor air damper is fully opened before the return air
damper is closed. This will reduce the pressure drop through the mixing assembly during most
operating conditions, which, for variable-volume systems with fan volume controls, will reduce fan
energy.
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FIGURE 6-G. SEQUENCED ECONOMIZER SEQUENCING FOR VAV SYSTEMS
Economizer Damper Controls (6.5.1.1.2)
FIGURE 6-H. TYPICAL OVERLAPPING ECONOMIZER SEQUENCING
Economizer Damper Controls (6.5.1.1.2)
High-Limit Shutoff (6.5.1.1.3)
As the outdoor air warms up, there will be a point where outdoor air intake will increase rather than
decrease the energy use of the cooling coil. At this point, the economizer must be shut off and the
system operated at the minimum outdoor air volume required for ventilation. The controller that
causes this to occur is called the economizer high-limit control or high-limit switch. See Example 6-LL.
In Table 6.5.1.1.3, the following control settings are specified:
• The set point levels for fixed dry-bulb temperature have been revised to provide the maximum
cooling benefit, assuming that integrated economizers are used. The use of fixed dry-bulb
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temperature is allowed in all climate zones, but enthalpy options should be considered for humid
climate zones when using constant-volume systems where the supply air temperature floats. This
floating supply air temperature may not provide adequate dehumidification when an economizer
operates at warmer ambient temperatures. This is not an issue for VAV systems because the supply
air temperature is maintained to a lower set point. An integrated economizer will use the cooling
coil during these warmer ambient temperatures to provide dehumidification. Thus, fixed dry-bulb
temperature is effective in all climate zones for VAV systems.
• The differential dry-bulb temperature is still allowed in the table and can provide slightly better
energy savings as compared to a fixed dry-bulb temperature control.
• Electronic enthalpy using the old D, C, B, A curves is not allowed; a new option using fixed enthalpy
and dry-bulb temperature control should be used. The addition of the fixed dry-bulb temperature
prevents introducing warm, dry outdoor air into the space, which would increase energy use.
• Differential enthalpy with a fixed dry-bulb temperature is also allowed. It provides slightly better
energy savings than fixed enthalpy and dry-bulb temperature, but it does require accurate and
good-quality enthalpy sensors to be used because sensor drift and error can cause losses in the
actual savings.
• The dew-point and dry-bulb temperature control option previously allowed has been eliminated.
Sensor accuracy and reliability is important to the high-limit control options defined above because
sensor error and failure can negate the anticipated savings or potentially increase energy
consumption. Section 6.5.1.1.6 Sensor Accuracy provides minimum sensor accuracy requirements.
Example 6-LL. Economizer Controls with Packaged Air-Conditioning Units
Corresponding section: High-Limit Shutoff (6.5.1.1.3)
Q
A 20 ton (70 kW) packaged unit with a gas furnace is proposed for an office building in Phoenix,
Arizona, which is in Climate Zone 2B. What are the economizer requirements?
A
Per Table 6.5.1-1, units with a capacity greater than 54,000 Btu/h or 4.5 tons (16 kW) are required to
have an economizer. The capacity of this unit is 20 tons (70 kW) and an economizer is required.
However, Exception 9 to Section 6.5.1 allows a more efficient unit to be used in lieu of an economizer.
Table 6.5.1-3 is referenced, and for this climate a unit must exceed the minimum requirement by 21%.
From Table 6.8.1-1, a 20 ton (70 kW) packaged unit with a gas furnace has minimum cooling efficiency
requirements of 11.4 EER and 13.79 IEER. Footnote (a) to Table 6.5.1-3 instructs the user to apply the
efficiency improvement to the IEER, so the unit would have to have a minimum IEER of 13.79 (11.4 ×
1.21) (ICOPC of 4.04).
If the unit is equipped with an economizer, the economizer would have to be integrated with the
mechanical cooling (Section 6.5.1.3) so that the economizer can operate concurrently with mechanical
cooling as required by the load up to the high-limit setting. The high-limit settings depend on the type
of changeover control (see Table 6.5.1.1.3). Because this is Climate Zone 2B, all high-limit shutoff
control types are allowed. However, since Climate Zone 2B is a dry climate, fixed dry-bulb temperature
or differential dry-bulb temperature will provide reliability and good economizer control.
For this example, let’s assume that a fixed dry-bulb changeover control is selected. Per Table 6.5.1.1.3,
the high-limit changeover setting should be 75°F (24°C). This means that the unit controls should
allow for the economizer to be used up to a 75°F (24°C) outdoor ambient, and if the load is exceeded
using just the outdoor air, it must be supplemented by mechanical cooling to satisfy the load without
reducing the economizer capacity.
Dampers (6.5.1.1.4)
Return air, exhaust/relief air, and outdoor air dampers are required to meet the damper leakage
specified in Section 6.4.3.4.3. Outdoor air dampers and exhaust air dampers are required to have low
leakage characteristics to prevent air infiltration and exfiltration during off hours. Table 6.4.3.4.3
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defines the maximum allowable leakage requirements for the outdoor air, exhaust air, and relief air
dampers as a function of climate zone, building height, and means of actuation.
Relief of Excess Outdoor Air (6.5.1.1.5)
Economizers can introduce a significant amount of outdoor air into the building, and without a means
to relieve the air, the building will more than likely become overpressurized, causing exterior doors to
stand open and causing whistling at elevator and stair doors. When these problems occur, operators
are apt to disable the economizer. For this reason, the standard requires that systems with air
economizers provide a means to relieve excess outdoor air as required to prevent overpressurizing the
building.
The standard does not specify precisely what type of economizer relief system is to be provided. Relief
systems are discussed in FYI, Common Options for Relieving Excess Outdoor Air. Also see Figure 6-F
and Return and Relief Fan Control Requirements (6.5.3.2.4).
FYI
Common Options for Relieving Excess Outdoor Air
Barometric relief. Barometric relief uses the slightly positive building pressure to push excess air out
of the building through a backdraft damper. The damper is like a check valve; it only allows air to leave
the building. Schematically, relief dampers are shown in Figure 6-F near the supply fans, but they may
be anywhere in the building in contact with the space or the return air path. Barometric relief systems
require no control, although sometimes a shutoff damper is mounted behind the relief damper. This
shutoff damper closes off the system to prevent exfiltration and associated infiltration due to stack
effect when the system is off (see the discussion on Section 6.4.3.3.3). While they are simple and the
least-expensive economizer relief system option, barometric relief systems can only be used if the
relief air path has a sufficiently low pressure drop to prevent overpressurization. This can be difficult
to achieve in most large buildings, so barometric relief is used mostly in single-story buildings.
Designers are encouraged to thoroughly review the performance criteria of barometric relief dampers
(pressure as a function of flow) to ensure that they do not need more than about 0.1 in. of water
(25 Pa) at full design flow. Many projects have pressurization problems when the systems are at 100%
outdoor air due to undersized relief. This is often the case with the reliefs provided by the airconditioning unit manufacturers. This can be very costly to fix in the field.
Exhaust fans. When barometric relief is not practical, powered relief using exhaust fans (also called
“relief fans”) may be used. These fans can be controlled based on damper position or the building
pressure. It is important that they are not just on/off. To control building pressure, it is important that
exhaust fans have some degree of modulation or staging to match the modulating outdoor airflow rate
of the air-side economizer.
Return fans. Return fans are an alternative to exhaust fans. Exhaust fans are generally less expensive
than return fans, may be placed anywhere in the return system (return fans must be placed close to the
supply fans), take up less space, and are simpler to control. Exhaust fans are also more energy efficient
when return static pressure drop is low (less than about 1 in. of water [250 Pa]), because during
noneconomizer operation, the supply fan handles the return static pressure; it is usually a more
efficient fan than the return fan due to the latter’s low pressure requirement (generally, the higher the
static pressure, the more efficient the fan). Therefore, return fans should only be used in place of
exhaust fans if the return system has a high pressure drop—for example, if it is ducted over long runs
or with return-air-volume control boxes.
Another important consideration in the design of the relief air system is the possibility of
reentrainment of relief air back into the outdoor air intake. The standard requires that the exhaust air
outlet be located to minimize recirculation into the building. No prescriptive requirements for how to
achieve this goal are provided in the standard, but the following should be avoided:
• Relief air outlets located within the same hood as the outdoor air intake. It is not uncommon
in some small packaged equipment for the barometric relief damper to be located behind the same
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screen and hood as the outdoor air intake. Recirculation is almost guaranteed with this design. To
resolve this problem, a separate barometric relief hood should be used, located as far as practical
from the air-conditioning unit intake.
Sensor Accuracy (6.5.1.1.6)
Section 6.5.1.1.6 specifies minimum sensor accuracy for use in air-side economizer controls. Sensors
located in the outdoor air, return air, mixed air, and supply air must be calibrated within the tolerances
specified by this section. As economizer energy savings are dependent upon good equipment control,
the accuracy of the sensors used can have a drastic impact on the energy savings realized.
Specifying an allowable calibrated tolerance ensures energy is not wasted due to inaccurate
measurement. Although this section provides accuracy requirements, these accuracies are required
within the typical air-side conditions. While some sensors may be required to occasionally operate
outside these parameters, they are not currently required to maintain the same accuracy outside that
range. Not all sensors have the same accuracy or construction. Consideration should be given to the
accuracy and service of the sensor when selecting sensors.
Outdoor air, return air, mixed air, and supply air sensors must be calibrated within the following
accuracies:
a. Dry-bulb and wet-bulb temperatures must be accurate to ±2°F (1.1°C) over the range of 40°F to
80°F (4.4°C to 27°C).
b. Enthalpy and the value of a differential enthalpy sensor must be accurate to ±3 Btu/lb (±5 kJ/kg)
over the range of 20 to 36 Btu/lb (35 to 63 kJ/kg).
c. Relative humidity must be accurate to ±5% over the range of 20% to 80% rh.
 Fluid Economizers (6.5.1.2)
Air-side economizers use cool outdoor air directly to reduce cooling load, whereas fluid economizers
(most frequently water or water-side economizers) reduce the cooling load by using outdoor air first
to cool a fluid, which then cools supply air through a cooling coil. The fluid is most often water, but
systems have been developed that use refrigerant or glycol mixtures. When evaporative cooling is used
as in an open cooling tower, the system has the advantage that the water temperature can approach
the wet-bulb temperature, which in dry climate zones can be 30°F to 40°F (17°C to 22°C) below the
dry-bulb temperature. However, fluid economizers have the added losses of the heat exchanger
approach, cooling tower fan power, and pump power, which typically results in less energy savings
than an air-side economizer. Fluid economizers do have other potential advantages relative to air-side
economizers:
• Fluid economizers provide free cooling to systems where air-side economizers are not feasible or
would require much larger ductwork.
• Fluid economizers do not require building pressure control.
• Fluid economizers do not reduce the indoor humidity levels during low ambient temperatures.
• Fluid economizers do not introduce additional poor-quality air into the building in nonattainment
areas.
Design Capacity (6.5.1.2.1)
To comply with the standard, fluid economizers must be able to satisfy the system’s entire expected
cooling load when outdoor air temperatures are 50°F (10°C) dry-bulb/45°F (7.2°C) wet-bulb and
below. This design criterion is specified because, unlike air economizers that use cold outdoor air
directly for cooling, the performance of water economizers depends greatly on the selection of
components such as cooling towers and heat exchangers.
There are three exceptions to this requirement:
• Systems primarily serving computer rooms in which 100% of the expected system cooling load can
be met at the dry-bulb and wet-bulb temperatures listed in Table 6.5.1.2.1 by an evaporatively
cooled fluid economizer. These lower design temperatures reduce the size of the evaporative heatrejection device required, improving the controllability of the system, especially during the first few
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years of operation when most computer rooms are lightly loaded. Higher design temperatures can
be used to increase the hours of economizer operation if appropriate for a given computer room.
• Systems primarily serving computer rooms with air-cooled fluid economizers (also known as a “dry
cooler”) for which 100% of the expected system cooling load can be met at the dry-bulb
temperatures listed in Table 6.5.1.2.1. An air-cooled system is less efficient because it does not use
evaporation, but may be necessary in areas with constrained water use or poor water quality.
These lower design temperatures reduce the size of the dry cooler required, improving the
controllability of the system, especially during the first few years of operation when most computer
rooms are lightly loaded. A higher dry-bulb temperature can be used to increase the hours of
economizer operation if appropriate for a given computer room.
• When a fluid economizer is used in situations where dehumidification requirements cannot be met
using outdoor air temperatures of 50°F (10°C) dry-bulb/45°F (7.2°C) wet-bulb. These systems
must satisfy the entire expected cooling load at 45°F (7.2°C) dry-bulb/40°F (4.4°C) wet-bulb. This
exception might apply to systems with either very low inside humidity requirements or relatively
high internal latent loads. It will not apply to most office or computer-room applications.
FYI
Common Types of Water Economizers
There are three common types of water economizers: strainer-cycle or chiller bypass, water
precooling, and air precooling.
Strainer-Cycle or Chiller-Bypass Water Economizer
This type of economizer, shown in Figure 6-I, has control valves that can divert condenser water from
the cooling tower and run it directly into the normal chilled-water piping loop, bypassing the chiller.
This bypass configuration will occur as long as the tower can cool the condenser water sufficiently to
handle the cooling load, usually around 45°F to 50°F (7.2°C to 10°C).
The term strainer-cycle, as this type of economizer is commonly called, started as a trade name of a
type of in-line water filter intended to clean the dirty open-circuit tower water before it flows into the
clean (normally closed-circuit) chilled-water circuit.
Because chilled-water control valves and coils can easily become clogged, it is essential to install good
water filtration and treatment systems with this type of economizer. For this reason, strainer-cycle
economizers are rarely used. To resolve this problem, a heat exchanger can be used to isolate the
tower and chilled-water circuits. However, this will considerably increase first costs as well as reduce
energy savings due to higher pump heads and a nonzero heat exchanger approach.
Note that the chiller-bypass water economizer is nonintegrated, meaning the chiller cannot operate
when the condenser water is in the bypass arrangement, so the economizer either provides all of the
cooling load or none of it. Because of this characteristic, this economizer design may not meet the
requirements of Section 6.5.1.3. (See the Economizer Integration [6.5.1.3] section.)
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FIGURE 6-I. STRAINER-CYCLE WATER ECONOMIZER
Water-Precooling Water Economizer
This type of economizer, shown in Figure 6-J, uses cold tower water when it is available to precool the
chilled-water return (through a heat exchanger) before it enters the chiller.
A major advantage of this type of economizer over the strainer-cycle type is that it is integrated,
meaning it can provide free cooling, even when the chillers are operating, by reducing chilled-water
return temperatures. It also isolates the open-circuit tower system from the chilled-water system with
the heat exchanger, reducing fouling problems caused by the poor water quality of the open circuit. But
the heat exchanger reduces the cooling energy savings because the water leaving the heat exchanger
cannot be as cold as the tower water, and it increases pump energy during economizer operation
because of the pressure drop of the heat exchanger.
While the system shown in Figure 6-J can comply with the standard, this type of economizer works
best when chilled-water return temperatures are kept high, which improves the heat exchanger
effectiveness and allows precooling at warmer tower-water temperatures. This can be achieved by
using two-way valves at cooling coils, as depicted in Figure 6-K. With two-way valves, return water
temperatures will actually rise above design levels at part load due to the characteristics of cooling coil
heat transfer.
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FIGURE 6-J. WATER-PRECOOLING WATER ECONOMIZER WITH THREE-WAY VALVES
FIGURE 6-K. WATER-PRECOOLING WATER ECONOMIZER WITH TWO-WAY VALVES
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Air-Precooling Water Economizer
The air-precooling water economizer requires an additional cooling coil upstream of the normal
mechanical cooling coil, as shown in Figure 6-L. Water from the cooling tower first passes through the
economizer coil, precooling or fully cooling the supply air, then goes on to remove condenser heat from
the mechanical cooling system, with water flow modulated or bypassed to maintain head pressures (a
control required due to the cold water temperatures).
The three-way control valve shown in Figure 6-L operates so that if the tower water is warmer than
the return air, the water bypasses the coil to avoid warming the air and increasing the cooling load.
This is similar to the high-limit control used with air economizers.
Because the economizer and mechanical cooling can operate concurrently with this type of
economizer, it is “integrated” and meets the requirements of Section 6.5.1.3 (discussed in the
Economizer Integration [6.5.1.3] section). This scheme is very popular when water-cooled air
conditioners are used, because the condenser water must be piped to the units anyway, so the only
expense of the water economizer is the added coil and controls.
FIGURE 6-L. AIR-PRECOOLING WATER ECONOMIZER
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FYI
Refrigerant Economizers
While most fluid economizers use water (or water and glycol mixtures for air-cooled economizers) as
the medium to transfer energy from the cooling coil to the outdoors, it is also possible to use
refrigerant. This is usually part of the DX refrigeration system as shown in Figures 6-M and 6-N. Note
that these systems must be designed to meet
•
•
the fluid economizer capacity requirements in Section 6.5.1.2.1 and
the integration requirements of Section 6.5.1.3. This will generally require at least two separate
refrigerant circuits (e.g., one each of the circuits shown in Figures 6-M and 6-N) with coils in series
with each other in the airstream. This allows one circuit to operate in economizer mode to provide
partial cooling while the other operates in DX mode to provide the remainder of the load.
FIGURE 6-M. REFRIGERANT ECONOMIZER – ECONOMIZER MODE
FIGURE 6-N. REFRIGERANT ECONOMIZER – DX MODE
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Maximum Hydronic Pressure Drop (6.5.1.2.2)
Unlike air-side economizers, fluid economizers have parasitic energy losses that reduce the cooling
energy savings. One of these losses comes from increases in pumping energy. To limit the losses, the
standard requires that precooling coils (Figure 6-L) and fluid-to-water heat exchangers used as part of
a fluid economizer system either
• must have a water-side pressure drop of less than 15 ft (4.6 m) of water or
• a secondary loop must be created so that the coil or heat exchanger pressure drop is not seen by
the circulating pumps when the system is in the normal cooling (noneconomizer) mode, as shown
in Figure 6-J.
Example 6-MM. Water-Side Economizer, Performance Verification
Corresponding section: Fluid Economizers (6.5.1.2)
Q
A system serving an office building is designed to use the water economizer depicted in Figure 6-K.
How is compliance demonstrated with the standard’s requirement that the economizer provide 100%
of the expected cooling load at outdoor air temperatures of 50°F (10°C) dry-bulb/45°F (7.2°C) wetbulb and below?
A
Because it requires knowledge of the system’s performance at off-design conditions, the calculations
required to demonstrate compliance are rather complicated. Following are suggested approaches for
different design conditions:
Heating and cooling loads. Recalculate heating and cooling loads just as they were calculated for
design loads, except change the outdoor air temperature to 50°F (10°C) dry-bulb and 45°F (7.2°C) wetbulb. All other parameters must remain at design conditions. The economizer must be able to meet the
cooling load calculated in this manner without supplemental chiller operation. Note that the winter
load for an office building is typically 20% to 40% of the peak summer design load.
Supply air temperature. Determine the supply air temperature of air handlers at the load calculated
above. For VAV systems, the supply air temperature should be reset upward to enhance economizer
performance.
Coil airflow rates. Determine the coil airflow rates using the reset supply air temperature.
Chilled-water supply temperature. Using manufacturer’s coil selection charts or programs,
determine the highest chilled-water supply temperature that will meet these supply air conditions,
assuming design water flow rates. Note that a higher chilled-water supply temperature will increase
the possible hours of economization, which may be advantageous. Note that Section 6.5.4.4 requires
chilled-water reset on systems with a design capacity exceeding 300,000 Btu/h (25 tons) (87.9 kW).
Chilled-water return temperature. The coil selection chart or program will also determine the
chilled-water return temperature. If there are many cooling coils, either
• assume conservatively that all coils will operate as required by the worst-case coil (the one
requiring the lowest chilled-water temperature) or
• redetermine the water flow rate required and leaving chilled-water temperature of all other coils
assuming the chilled-water supply temperature of the worst-case coil. Determine the actual return
water temperature based on the gallons-per-minute-weighted average of each coil’s return water
temperature.
Condenser water supply and return temperatures. Have the heat exchanger manufacturer
determine the required cooling tower supply and return water temperatures using the following
information: the chilled-water supply and return temperature, the chilled-water flow rate, and the
design tower-water flow rate.
Cooling tower performance. Verify that the cooling tower can meet the tower-water flow rate and
supply and return water temperatures determined above at a wet-bulb temperature of 45°F (7.2°C).
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Do this either by using manufacturer’s catalog or selection program data (if available at low wet-bulb
temperatures) or by having the manufacturer check performance using factory data.
If the tower can meet these conditions, the water economizer design complies with the standard. If not,
change the tower, heat exchanger, cooling coil, or air-side designs and repeat the process. Note that if
the winter load is the controlling case for the tower sizing, the tower will typically operate at a lower
fan speed and/or produce colder condenser water during the summer months, saving energy.
Integrated Economizer Control (6.5.1.3)
The standard requires that economizers be integrated. Integrated economizers can reduce the cooling
load while the remainder of the load is met by mechanical cooling. Economizers that cannot operate
simultaneously with the mechanical cooling system are called nonintegrated economizers.
Integration can greatly extend economizer operation, which reduces cooling energy costs. For instance,
a nonintegrated air economizer will only be able to reduce cooling energy when outdoor air
temperatures are below the supply air temperatures of 55°F to 60°F (13°C to 16°C) for a VAV system
(Figure 6-O), depending on required supply air temperatures. Above those temperatures, mechanical
cooling is required, so the nonintegrated economizer is shut off. If the economizer were integrated
(Figure 6-P), it could continue to operate, reducing mechanical cooling energy use even though it
cannot provide the entire cooling load. The integrated economizer can continue to operate until the
high-limit set point is reached, around 65°F to 75°F (18°C to 24°C), depending on the climate. In some
climates, the outdoor air temperature is in this range for hundreds or even thousands of operating
hours.
An example of a nonintegrated water economizer is shown in Figure 6-O; this economizer may only be
used if one of the exceptions exempts the system from this requirement. The water economizers
shown in Figures 6-J, 6-K, and 6-L are integrated economizers, because the economizer and mechanical
cooling may operate concurrently; these economizers comply with Section 6.5.1.3. Also see Examples
6-LL and 6-NN.
Where economizers are required, they must use integrated economizer control. It is important that the
unit have proper controls to integrate the economizer and mechanical cooling such that the mechanical
cooling is used with the economizer and does not totally override the economizer. Prescriptive control
requirements include that the mechanical cooling to be interlocked with the air-side economizer such
that the outdoor air damper is 100% open before mechanical cooling may be used. To prevent coil
freezing on DX systems due to minimum compressor runtime, the air-side economizer damper may
not begin to close until the leaving air temperature is less than 45°F (7.2°C). When this occurs, the
economizer damper must not close but may modulate to minimize very low supply air temperature.
The controls should minimize the time that this is used to maximize the benefits of the economizer. In
dry climates, resetting the supply air temperature up will help this and increase the benefits of the
economizer. Also, using a lower supply air temperature set point for mechanical cooling control and a
higher supply air temperature set point for air-side economizer control can improve the operation of
the integrated economizer function.
As part of the integrated economizer control, the standard includes a minimum number of DX cooling
stages when an integrated economizer is used. Where systems control DX cooling capacity based
directly on the space temperature (rather than the supply air temperature) and have a capacity of
65,000 Btu/h or more, the DX system must have at least two stages of cooling capacity. This can be
achieved with multiple compressors, a single two-stage compressor, or variable-capacity compressors.
DX systems that do not control capacity based directly on the space temperature must comply with the
requirements of Table 6.5.1.3. These are typically VAV units that control capacity based on leaving air
temperature and control the space temperature by modulating the supply airflow. Table 6.5.1.3
provides the minimum number of capacity stages required for the DX cooling, as well as the maximum
capacity of the lowest cooling stage as a percentage of the full-load compressor displacement. The
compressor displacement is equivalent to the mechanical cooling at the full-load rating conditions.
For example, a 120,000 Btu/h (35.2 kW) rated cooling capacity unit would require a minimum of three
stages. If the stages are not achieved with variable displacement compressors, the capacity of the
smallest compressor must not be greater than 0.35 × 120,000 = 42,000 Btu/h (0.35 × 35.2 = 12.3 kW).
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The staging requirements of this section do not necessarily require the number of compressors to
match the number of stages. Many options exist for providing the required cooling stages. One
possibility of achieving three cooling stages is to use two unequally sized compressors (1/3 and 2/3
capacity). The smallest compressor would run for the first stage, the larger compressor for the second
stage, and both for the third. Alternatively, two equally sized compressors could be used where one
compressor is a variable-speed compressor and the second is constant speed. This configuration
allows the cooling capacity to modulate through almost the entire cooling capacity range by
modulating the variable-speed compressor and staging the second constant-volume compressor.
Other combinations and types of compressors than those above could be used to meet the
requirements of Section 6.5.1.3. This includes two-stage compressors, unloading compressors, and
variable-speed compressors.
The fan control requirements of Section 6.5.3.2 include a minimum of two-speed fan control during
economizer operation. This allows the fan to run on low speed when the economizer capacity
requirements are low and open the economizer damper further. Significant fan energy is saved as the
fan power will decrease to the cube of the speed reduction. Refer to Section 6.5.3.2 for details on the
fan control.
FIGURE 6-O. NONINTEGRATED ECONOMIZER (ONLY ALLOWED FOR UNITS <65,000 BTU/H)
Corresponding section: Integrated Economizer Control (6.5.1.3)
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FIGURE 6-P. INTEGRATED ECONOMIZER
Corresponding section: Integrated Economizer Control (6.5.1.3)
Example 6-NN. Economizer Integration, Strainer-Cycle Water Economizer
Corresponding section: Integrated Economizer Control (6.5.1.3)
Q
When can the strainer-cycle water economizer shown in Figure 6-I be used?
A
This economizer is nonintegrated because the chiller cannot operate at the same time as the
economizer, so it does not meet the requirement of Section 6.5.1.3. It is only allowed if compliance is
shown via the Energy Cost Budget Method (Section 11).
Economizer Heating System Impact (6.5.1.4)
The standard requires that the HVAC system and economizer design and controls be such that
operation of the economizer does not increase building heating energy costs during normal operation.
This requirement has many implications that can significantly limit HVAC system selection and design.
For instance, the single-fan/dual-duct system or multizone system shown in Figure 6-Q would not
meet this requirement with an air economizer. This is because economizer operation lowers the
temperature of the air entering the hot-deck heating coil, increasing its energy use. In order to use this
type of system, a water economizer must be used, or the system must meet one of the economizer
exceptions and have neither type of economizer. (Another resolution is to use a dual-fan/dual-duct
system where the hot-deck fan supplies only return air or return air plus minimum ventilation air. This
system is often less expensive and easier to control than a single-fan/dual-duct system.)
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This requirement will not affect three-deck multizone or “Texas” multizone systems, because they
cannot work with an air economizer in any case (it would make the neutral deck a cold deck).
An exception to the heating impact requirement is provided for economizers on VAV systems that
cause zone-level heating to increase due to a reduction in supply air temperature. Reducing supply air
temperature on a cooling VAV system will reduce fan energy (particularly if the system has a variablespeed drive), offsetting the energy lost due to increased reheat energy.
Economizer Humidification System Impact (6.5.1.5)
Humidification systems used in conjunction with outdoor air economizers can waste energy because
the introduction of dry outdoor air in the winter adds to the humidification load. To minimize these
losses, the standard requires that systems that have both hydronic cooling and humidification systems
designed to maintain inside humidity at greater than 35°F (2°C) dew-point temperature must use a
water economizer if an economizer is required by Section 6.5.1.
Note that this requirement is limited to hydronic cooling systems. It does not apply to DX cooling
systems because hydronic systems are more readily fitted with a water economizer than DX systems.
For humidified buildings that are not adiabatically humidified, it is possible under cool, dry outdoor air
conditions for an air economizer to use more humidification energy cost than it conserves in sensible
cooling energy cost. One of the most common examples is a hospital in a dry climate zone. The
determination of the lower economizer boundary should take into account mechanical cooling and
humidifier performance and the typical cooling and humidification loads during cool, dry periods.
Users in dry climates who wish to save additional energy may also add a temperature- and humiditybased lockout or consider a water-side economizer.
FIGURE 6-Q. DUAL-DUCT OR MULTIZONE SYSTEM
Economizer Heating System Impact (6.5.1.4)
Simultaneous Heating and Cooling Limitation (6.5.2)
Zone Controls (6.5.2.1)
When air-conditioning system designs were developed in the late 1950s and early 1960s, energy costs
were a minor concern. The systems were designed primarily to provide precise temperature control
with little regard for energy costs. Several techniques were used to achieve zone temperature control:
reheating cold supply air (constant-volume reheat system), recooling warm supply air (such as
perimeter induction systems), or mixing hot and cold air (constant-volume dual-duct and multizone
systems).
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While these techniques provided fine temperature control, they did so by using a great deal of energy.
To reduce this type of energy waste, Section 6.5.2.1 requires that zone thermostatic controls must be
capable of sequencing the supply of heating and cooling to each space. These controls must prevent the
following:
• Reheating
• Recooling
• Mixing or simultaneous supply of air that has been previously mechanically heated and air that has
been previously cooled, either mechanically or by economizer systems
• Other simultaneous operation of heating and cooling systems to the same zone
See Examples 6-OO through 6-SS for a variety of examples that apply the requirements of Section 6.5.2.
Single-zone systems will inherently meet these requirements, provided their controls are capable of
sequencing typical heating and cooling. However, most common multiple-zone systems require the use
of simultaneous heating and cooling to satisfy ventilation and zone temperature control. This can also
be a concern where more than one system is used to serve a zone. The standard provides allowances
for simultaneous reheating, recooling, or mixing when appropriate controls are in place to limit
simultaneous heating and cooling. In general, simultaneous heating and cooling is allowed when
minimum airflow controls are in place, ventilation rates exceed the minimum zone airflow, laboratory
exhaust exceeds the minimum zone airflow, and reheating energy uses site-reclaimed energy. The
following sections cover the requirements of these exceptions.
Variable-Air-Volume Systems (Exceptions 1 and 2 to 6.5.2.1)
This section covers both Exceptions 1 and 2 to Section 6.5.2.1, as they are very similar. These
exceptions allow reheating, recooling, or mixing, but only after the airflow has been reduced to a
minimum level, and will often be used for standard multiple-zone systems.
Exception 1 is intended to be applied to the following:
• Fan-powered VAV terminals, parallel or series with or without DDC
• Dual duct VAV terminals with or without DDC
• Single duct VAV terminals with reheat and controls other than DDC, e.g., pneumatic electronic
Exception 2 is intended to be applied to single-duct VAV terminals with reheat and DDC.
The minimum measurable airflow rate is a typical limiting factor in VAV box control. VAV boxes
typically use a pressure differential airflow measurement station for control. Pneumatic controls use
this to mechanically adjust the damper position in the VAV box. DDC uses a pressure sensor to
calculate the flow from the resulting pressure differential. Differential pressure airflow measurement
works well at air velocities high enough to produce a pressure differential between total static
pressure and velocity pressure. As the velocity decreases, the differential pressure becomes difficult to
measure, limiting the control’s ability to interpret the airflow. Thus there is a minimum realistic
airflow at which a VAV box can reliably control airflow. VAV boxes with pneumatic controls can
typically control to airflows as low as 30% of the maximum allowable flow, while VAV boxes with DDC
are typically capable of airflows as low as 10% to 20% of the maximum.
Simultaneous heating and cooling is allowed by Exceptions 1 and 2 if it is minimized by limiting the
airflow rate that is being reheated, recooled, or mixed to a rate not greater than the larger of the
following:
• VAV system without DDC, 30% of the zone design peak supply airflow rate.
• VAV systems with DDC, 20% of the zone design peak supply rate.
• The volume of outdoor air necessary to meet the ventilation requirements of ASHRAE Standard
62.1 for the zone. (Note that this refers to the actual minimum outdoor airflow rate without any
adjustment for recirculated air, if applicable. See Example 6-TT.)
• Any higher rate that can be demonstrated, to the satisfaction of the authority having jurisdiction, to
reduce overall system annual energy usage by offsetting reheat/recool energy losses through a
reduction in outdoor air intake in accordance with the multiple space requirements defined in
ASHRAE Standard 62.1. This exception is provided to allow system designers to optimally solve the
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equations in Section 6.2.2 of Standard 62.1. These equations show that the amount of outdoor air
required for a system is a function of how much air is supplied to the critical zone in the system.
The higher the supply air rate to the critical zone, the less outdoor air is required at the air handler.
The designer would determine which is more energy efficient, increasing outdoor air intake and
minimizing reheat at the critical zone or increasing the supply air rate and reheat energy required
at the critical zone and minimizing the outdoor air rate. This decision may also be made
dynamically in real time by the DDC system. The system would dynamically solve Equation 6-1 in
Standard 62.1 using actual operating data (e.g., the supply air rate to the critical space and the
overall system supply air rate) and would reset minimum volume set points and outdoor air intake
rate set points as required to minimize energy use.
• The airflow rate required to comply with applicable codes or accreditation standards. This includes
zones where special pressurization relationships occur, there are cross-contamination
requirements, or there are code-required minimum circulation rates. This exception might apply to
some areas of hospitals, such as operating rooms and patient rooms, and to laboratories that must
be maintained at positive or negative pressures to prevent contaminants from entering or escaping.
VAV systems have been successfully used in these applications to reduce energy costs but typically
require precise airflow measuring and control. The risk of a failure of these controls, such as the
possible release of dangerous chemicals or bacteria, must be balanced against the potential energy
savings.
When compared to pneumatic controls, direct digital controls are able to implement more complex
control logic. Exception 2 to Section 6.5.2.1 uses this ability to further decrease reheat in DDCcontrolled systems by including the following additional control requirements in heating mode:
• If a zone becomes too cool (reaching the heating set point) and the zone VAV box is at the minimum
airflow rate and minimum supply air temperature, a first stage of reheating is allowed. This first
stage must maintain the minimum airflow rate while modulating the heating coil up to the
maximum supply air temperature. The maximum temperature is limited by Section 6.5.2.1.1 for
overhead heating systems to 20°F above the space temperature set point, e.g., 90°F when the space
temperature set point is 70°F.
• If a zone remains at or below the heating set point and the first stage of reheat has reached the
maximum supply air temperature, a second stage of reheat is allowed. The second stage of heat
allows the VAV box to increase the supply airflow rate while maintaining the maximum supply air
temperature. The airflow rate may be increased up to the maximum heating airflow rate, which
shall be no greater than 50% of the design maximum airflow rate.
Note that this advanced sequence requires the use of a supply air temperature sensor, which is not
required by systems complying using Exception 1. This sensor is required to ensure that supply air
temperature may be limited, as required to meet Section 6.5.2.1.1, to limit excessive stratification.
Figure 6-R through Figure 6-V illustrate typical zone control sequences.
Example VAV Reheat Control Sequences
Figure 6-R presents a control diagram that can be used to comply with Exception 1 to Section 6.5.2.1. It
is the traditional control diagram for a VAV box with a hot-water reheat coil and typically applies only
to terminals with controls other than DDC, such as pneumatic controls, as explained below. It
maintains a constant airflow rate in heating and does not require a supply air temperature sensor to
achieve this sequence. The horizontal axis represents the control demand signal. The left section of the
diagram represents operation during heating. As the space temperature drops (moving from right to
left in the “Heating Signal” section of the diagram), the heating coil valve is modulated from fully closed
to fully open.
The middle section of Figure 6-R represents the deadband where the space temperature is between
the heating and cooling set points. In this section, the supply air is held at the minimum, and the
heating-coil control valve is fully closed.
The right section of Figure 6-R represents cooling. As the space temperature rises (moving left to right
in the “Cooling Signal” section of the diagram), the supply air is increased from minimum to cooling
maximum by modulating the VAV box damper.
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As noted above, the minimum set point used in the heating mode of this sequence can be no larger than
20% of the design maximum for terminals with DDC. In most cases, for typical single-duct VAV reheat
terminals, 20% is not sufficient to heat the space, particularly for overhead supply air systems where
supply air temperature is limited per Section 6.5.2.1.1. So Exception 1 is effectively not an option for
single-duct VAV reheat terminals; they must use the more sophisticated and energy-efficient logic
required by Exception 2.
Figure 6-S presents a control diagram that can be used to comply with Exception 2 to Section 6.5.2.1. It
represents a dual maximum (one maximum airflow set point for heating and one for cooling) control
diagram for a VAV box with a hot-water reheat coil and DDC. As with Figure 6-R, the horizontal axis
represents the space control demand signal.
The deadband and cooling function in Figure 6-S is the same as in Figure 6-R, but the heating function
differs. As the space temperature drops (moving from right to left in the heating section of the
diagram), the supply air temperature set point (or heating-coil valve) is increased from minimum to
the design heating temperature. As the space temperature continues to drop, the airflow is increased
from the minimum to the heating supply maximum.
A similar approach used in the past is to simultaneously increase the heating airflow and supply air
temperature set points from minimum to heating maximum. However, this logic is not allowed by
Exception 2 to Section 6.5.2.1 due to higher fan and reheat energy use relative to the staged operation
previously discussed.
FIGURE 6-R. EXAMPLE VAV REHEAT SEQUENCE THAT COMPLIES WITH EXCEPTION 1 TO SECTION 6.5.2.1
Corresponding section: Exceptions 1 and 2 to 6.5.2.1
FIGURE 6-S. EXAMPLE VAV REHEAT SEQUENCE THAT COMPLIES WITH EXCEPTION 2 TO SECTION 6.5.2.1
Corresponding section: Exceptions 1 and 2 to 6.5.2.1
This dual maximum logic saves energy, as compared to the Figure 6-R logic, in that it reduces the
airflow in the deadband (20% vs. 30%). Having a high box minimum in the deadband often causes the
zone to rapidly overcool, which causes the zone to be pushed into and remain in heating mode.
ASHRAE RP-1515 found that this causes “cold” complaints in warm weather when occupants tend to
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wear lighter clothing. So, in addition to saving energy vs. traditional logic, dual maximum logic
improves thermal comfort.
Figure 6-T presents a control diagram that can be used to comply with the requirements of Section
6.5.2.1. It applies to dual-duct VAV terminals. As with the previous diagram, the horizontal axis
represents the control demand signal.
Heating operation. As the space temperature drops (moving from right to left in the “Heating Signal”
section of the diagram), the hot duct airflow is increased from minimum to the design heating
maximum. During heating, the cold deck damper is fully closed.
Cooling operation. As the space temperature increases (moving from left to right in the “Cooling
Signal” section of the diagram), the cold duct airflow is increased from minimum to the design cooling
maximum. During cooling, the hot-deck damper is fully closed.
The control in the deadband changes depending on whether you are moving from heating to cooling or
cooling to heating. From heating to cooling, the hot deck stays at minimum airflow and the cold deck is
closed. From cooling to heating, the cold deck stays at minimum airflow and the hot deck is closed. This
control logic eliminates reheat because it never has simultaneous heating and cooling.
FIGURE 6-T. EXAMPLE DUAL-DUCT SEQUENCE THAT COMPLIES WITH SECTION 6.5.2.1
Corresponding section: Zone Controls (6.5.2.1)
Dashed line is airflow from the hot duct; solid line is airflow from the cold duct. Changeover occurs at the lower end of the deadband
as you move from cooling to heating and at the upper end of the deadband as you move from heating to cooling.
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Figure 6-U presents a control diagram that can be used to comply with Exception 1 to Section 6.5.2.1. It
represents a series fan-powered VAV box with hot-water heat, with or without DDC. As with the
previous diagrams, the horizontal axis represents the control demand signal.
Series fan-powered terminal boxes mix plenum air with primary air upstream of the terminal box fan
to create a constant supply of airflow to the zone. With this style of VAV box, the fan runs whenever the
zone is in occupied mode.
As the space temperature drops and heating is required, the supply air temperature (or heating-coil
valve) is increased from minimum to the design heating temperature. Throughout heating, the cooling
primary air is kept at design minimum airflow.
When both the heating and cooling temperature set points are satisfied, the cooling primary airflow is
kept at design minimum airflow, and the heating-coil valve is closed.
As the space temperature increases and cooling is required, the heating-coil valve remains closed and
the cooling primary airflow is increased from minimum to the design cooling maximum.
FIGURE 6-U. EXAMPLE SERIES FAN-POWERED VAV REHEAT WITH SEQUENCE THAT COMPLIES WITH
EXCEPTION 1 TO SECTION 6.5.2.1
Corresponding section: Exceptions 1 and 2 to 6.5.2.1
Figure 6-V presents a control diagram that can be used to comply with Exception 1 to Section 6.5.2.1. It
represents a parallel fan-powered VAV box with hot-water heat. As with the previous diagrams, the
horizontal axis represents the control demand signal.
Like series fan-powered VAV boxes, parallel fan-powered VAV boxes use a fan to help distribute the air
to the zone. However, the fan in series fan-powered VAV boxes only circulates return air from the
space. The fan in parallel VAV box only runs when the zone is in heating mode and, in some cases, in
the deadband. When the fan is running, the box mixes plenum air with primary air downstream of the
fan.
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FIGURE 6-V. EXAMPLE PARALLEL FAN-POWERED VAV REHEAT WITH SEQUENCE THAT COMPLIES WITH
EXCEPTION 1 TO SECTION 6.5.2.1
Corresponding section: Exceptions 1 and 2 to 6.5.2.1
As the space temperature drops and heating is required, the supply air temperature set point (or
heating-coil valve) is increased from minimum to the design heating temperature. Throughout heating,
the cooling primary airflow is kept at design minimum.
When both the heating and cooling zone temperature set points are satisfied, the cooling primary
airflow is kept at minimum airflow and the heating-coil valve is closed.
As the space temperature increases and cooling is required, the cooling supply airflow is increased
from minimum to the design cooling maximum. Throughout cooling, the VAV box fan is off.
Laboratory Exhaust Systems (Exception 3 to 6.5.2.1)
Laboratory exhaust systems that comply with Section 6.5.7.3 are one exception. These systems are
typically driven by the exhaust air requirement. The air-handling system must be capable of making up all
of the air exhausted from the laboratory. This may exceed the airflow required for conditioning the space
at the current primary air temperature. In this situation, the airflow requirement must be met. To do so
requires adjusting the air temperature to the space in the form of reheating or mixing. This is allowed by
Exception 4 to Section 6.5.2.1.
The standard provides an exception for zones where at least 75% of the energy for reheating or for
providing warm air in mixing systems is provided from a site-recovered energy source (including
condenser heat) or site solar energy source.
Separating ventilation and thermal requirements—as this system does—usually results in minimal or
no reheat losses, but it does not necessarily result in the best energy performance. A more energyefficient system might be one with an outdoor air economizer that delivers outdoor air in the winter to
cool interior zones and then uses transfer air to ventilate perimeter zones.
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Example 6-OO. Simultaneous Heating and Cooling, VAV System with Separate Outdoor Air Supply
Corresponding section: Simultaneous Heating and Cooling Limitation (6.5.2)
Q
A VAV system serving an office building has a standard cool-air supply duct for handling cooling loads.
It also has a separate supply system that provides preheated and precooled 100% outdoor air. Each
zone has a dual-duct terminal box with a VAV connection to the cooling duct (sized for the space
cooling load) and a constant-volume connection to the outdoor air duct (sized for the space minimum
ventilation requirement). Heating is provided by a separate radiant heating panel system.
Does this design meet the standard’s requirements?
A
The presence of the 100% outdoor air ventilation system does not really have an impact on this
system’s compliance. Conditioning of outdoor air does not constitute reheat unless the air is preheated
to high temperatures in the cooling season and then cooled down by opening the cooling VAV damper.
For this system to comply, the minimum airflow set point on the cooling VAV damper in each zone
would have to meet the criteria of either Exception 1 or 2 to Section 6.5.2.1. In this example, the
radiant heating system does not require any airflow from the VAV system, so the minimum volume set
point on the cooling system could be set to zero. The separate 100% outdoor air system will ensure
that ventilation rates are maintained. This outdoor air supply may first be cooled then reheated by the
zone radiant panels, provided the outdoor air supply rate to each zone does not exceed the minimum
rates required by ASHRAE Standard 62.1, per Exception 2 of Section 6.5.2.1 of Standard 90.1.
Example 6-PP. Simultaneous Heating and Cooling, ASHRAE Standard 62.1’s Multiple-Zone Calculation
Method
Corresponding section: Simultaneous Heating and Cooling Limitation (6.5.2)
Q
For a VAV system, if the required outdoor air ventilation rate based on the ASHRAE Standard 62.1
multiple-zone calculation method (Section 6.2.5 of Standard 62.1) results in excessively high outdoor
air rates, may the minimum zone airflow set points be increased above 30% in order to reduce the
outdoor airflow required at the air handler?
A
Yes, Exception 1(c) and Exception 2 (a)(3) to Section 6.5.2.1 of Standard 90.1 allow the minimum
airflow set point to be increased above the prescribed minimums (30% in Exception 1) for specific
critical zones under certain conditions. These critical zones have relatively high occupancy (and thus
need a lot of outdoor air) yet have relatively low thermal load. Therefore, the outdoor air ventilation
requirement in these zones is a relatively large fraction of the zones’ peak supply airflow. When
thermal loads are low and the zone airflow is at its minimum set point, the outdoor air requirement
can be a very large fraction of the total flow.
In the extreme case, if the outdoor air required for a zone is equal to the minimum VAV box airflow,
100% outdoor air at the air handler is required to meet the ventilation requirements of Standard
62.1’s multiple-zone calculation method. In such cases, Standard 90.1 allows a higher minimum flow
set point for critical zones to avoid the energy penalty caused by increased outdoor air ventilation
rates for the overall system (at the cost of increased reheat energy in the critical zones).
For example, if a zone sized for 1000 cfm (472 L/s) peak airflow has a 200 cfm (94 L/s) outdoor air
requirement, then when the VAV box is at minimum flow of 300 cfm (142 L/s), the outdoor air fraction
needed for this zone is 67%. Then, following the Standard 62.1 multiple-zone calculation, the outdoor
airflow required at the air handler might be on the order of 50% (see 62.1 User’s Manual for details).
However, if the minimum airflow set point for this single zone were increased from 300 cfm (142 L/s)
(30%) to, for example, 600 cfm (283 L/s), then the zone’s outdoor air fraction drops to 33%. As a
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result, the outdoor air required at the air handler might drop from 50% to 40%, a significant savings,
at the cost of a relatively minor increase in reheat energy. The engineer must demonstrate that the
energy savings from the reduced outdoor air rate offset the added energy cost of the higher minimum
set point to the satisfaction of the enforcement official. Typically this is done using an energy
simulation.
Example 6-QQ. Simultaneous Heating and Cooling, Cooling-Only Systems
Corresponding section: Simultaneous Heating and Cooling Limitation (6.5.2)
Q
The interior zones of a VAV system have cooling-only VAV boxes. What limitations does Section 6.5.2.1
place on the minimum volume set points for these zones?
A
None. Section 6.5.2.1 restricts the use of simultaneous heating and cooling. Because these zones have
no heating capability, there is no possibility of simultaneous heating and cooling, so Section 6.5.2.1
does not apply.
Example 6-RR. Simultaneous Heating and Cooling, Packaged Gas/Electric Unit
Corresponding section: Simultaneous Heating and Cooling Limitation (6.5.2)
Q
A packaged gas/electric rooftop unit serves a single zone. What is required of this system to comply
with Section 6.5.2.1?
A
The thermostat must be capable of precluding simultaneous operation of the furnace and air
conditioner. The standard thermostat will do this, so the system complies with this section without any
added features required.
Example 6-SS. Minimum Set Point for Ventilation
Corresponding section: Simultaneous Heating and Cooling Limitation (6.5.2)
Q
A VAV system uses logic as shown in Figure 6-R. The air-handling unit supplies 20,000 cfm (10,000
L/s) of air, 4000 cfm (2000 L/s) of which (20%) is outdoor air for ventilation. One VAV zone requires
400 cfm (200 L/s) for cooling and 100 cfm (50 L/s) of outdoor air per ASHRAE Standard 62.1. Using
the multiple-zone calculation method of Section 6.2.5 of Standard 62.1, it is determined that to keep
from supplying more than 4000 cfm (2000 L/s) of outdoor air, the minimum airflow set point must be
200 cfm (100 L/s). As this is needed for ventilation, is a 200 cfm (100 L/s) minimum airflow set point
acceptable per Exception 2 to Section 6.5.2.1 of Standard 90.1?
A
No. Exception 2 allows only the amount of outdoor air required for ventilation to be reheated, 100 cfm
(50 L/s) in this example. To apply this exception, the minimum would have to be 100 cfm (50 L/s),
which would require that the air handler supply 100% outdoor air or that some other ventilation be
provided, such as transfer air (e.g., with a fan-powered VAV box). This exception does not generally
apply to conventional single-duct VAV systems. Instead Exception 3 may be appropriate.
Hydronic System Controls (6.5.2.2)
Most simultaneous heating and cooling in modern HVAC systems occurs due to air system controls, as
discussed in the previous section. Some energy waste, however, can also occur in hydronic systems.
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Three-Pipe System (6.5.2.2.1)
Hydronic systems that use a common return system for both hot water and chilled water—so-called
three-pipe systems—cause heated water and cooled water to be mixed with each other, increasing
both heating and cooling energy use. These systems are prohibited by the standard.
Two-Pipe Changeover System (6.5.2.2.2)
Two-pipe changeover systems use a common distribution system to alternately supply heated or
chilled water to fan-coils and air handlers. No energy is wasted by this design when being held in
either heating-only or cooling-only mode for a long period of time. However, energy is wasted when
the system changes over from one mode to the other, because this requires heating or cooling the mass
of water in the system. The standard allows these systems as long as they include all the following
measures to minimize the energy impact of changeovers:
• The system is designed to allow a deadband between changeover from one mode to the other of at
least 15°F (8.3°C) outdoor air temperature.
• The system is designed to operate and is provided with controls that allow operation in one mode
for at least four hours before changing to the other mode.
• Reset controls are provided that allow heating and cooling supply temperatures at the changeover
point to be no more than 30°F (17°C) apart.
Example 6-TT addresses two-pipe changeover systems.
Example 6-TT. Simultaneous Heating and Cooling, Two-Pipe Changeover System Requirements
Corresponding section: Two-Pipe Changeover System (6.5.2.2.2)
Q
A two-pipe changeover system is proposed for a hotel. Each guest room will have a two-pipe fan coil
with controls to change the control action of the thermostat based on water temperature. What is
required for this system to comply with Section 6.5.2.2?
A
The following is an example of how this system might comply with Section 6.5.2.2 and maintain guest
comfort.
Small electric heating coils could be provided in each fan coil. This will provide heat in mild weather,
allowing the two-pipe system to stay in the cooling mode until the outdoor air temperature drops
sufficiently low that it can be assured no guest rooms require cooling. For instance, below 45°F (7°C),
the system could operate in the heating mode. As the outdoor air temperature rises above 60°F (16°C),
the system would switch to the cooling mode. A timer would be provided that would prevent a
changeover from occurring if the outdoor air temperature changed from below 45°F (7°C) to above
60°F (16°C) in less than four hours.
The chilled-water temperature would have to be reset based on outdoor air temperature, for example
from 45°F at 60°F (7°C at 16°C) outdoor air temperature up to 60°F at 45°F (16°C at 7°C) outdoor air
temperature. Similarly, the heating system would have to be reset, for example from 170°F at 0°F
(77°C at –18°C) down to 90°F at 45°F (32°C at 7°C) outdoor air temperature and further down to 75°F
at 60°F (24°C at 16°C) outdoor air temperature.
Hydronic (Water Loop) Heat-Pump Systems (6.5.2.2.3)
Hydronic heat pumps are typically connected to a common condenser water loop, as shown in Figure
6-W. Also connected to the loop are devices to add heat to the loop (a boiler) should its temperature
fall too low in the winter and to remove heat from the loop (a cooling tower) should its temperature
rise too high. To limit the unnecessary use of these central heating and cooling sources, the standard
requires that these systems be designed as follows.
a. Controls must be capable of providing a heat-pump water supply temperature deadband of at least
20°F (11°C) between initiation of heat rejection and heat addition by the tower and boiler. For
instance, the boiler may come on when the heat-pump water supply temperature drops below
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60°F (16°C), while the tower must be capable of being set to come on at a minimum of 20°F (11°C)
higher, which would be 80°F (27°C).
The deadband may be reduced by system loop temperature optimization controllers that
determine the most efficient operating temperature based on real-time demand and capacity
conditions. Note that this section’s 20°F (11°C) deadband requirement only establishes the
capability of the control system, not the actual set points. For some systems with high-efficiency
cooling towers and heat pumps whose efficiency drops significantly at lower water temperatures,
a lower set point—for example, 60°F to 70°F (16°C to 21°C)—may be optimal. To determine the
optimal set point, an hourly simulation program, such as discussed in Chapter 11, should be used.
b. In Climate Zones 3 through 8, the standard requires that the system be designed to limit the heat
loss from the heat-rejection device (cooling tower) as follows:
• If a closed-circuit tower (fluid cooler) is used, either an automatic valve must be installed (see
Figure 6-W) to bypass flow of water around the tower, or low-leakage positive-closure
dampers must be provided on the inlet or discharge of the fluid cooler to minimize natural
convection across the heat exchanger due to stack effect. If a valve is installed, a minimal
amount of water may be circulated through the heat exchanger to prevent freezing. Note that
bypassing flow around the tower is much more effective than dampers due to damper leakage
and radiant heat losses from the heat exchanger.
•  If an open-circuit tower is used directly in the heat-pump loop, an automatic valve must be
installed to bypass all heat-pump water flow around the tower. Freeze protection can be
provided by sump heaters or by temporarily draining the tower. Note that using an open-circuit
cooling tower without an isolation heat exchanger can lead to the introduction of fouling and
contaminants into the heat-pump condensers, which can seriously degrade their performance
over time. This can be problematic, as the heat-pump condensers are typically tube-in-tube or
brazed plate design, both of which are very difficult to clean.
• If an open-circuit tower is used in conjunction with a separate heat exchanger to isolate the
tower from the heat-pump loop, then shutting down the circulation pump on the cooling tower
loop will control heat loss.
Dehumidification (6.5.2.3)
Most dehumidification in HVAC systems is provided as a part of the normal cooling process. In the
majority of applications in most climates, this uncontrolled, indirect dehumidification provides
acceptable space humidity levels. However, to achieve lower humidity levels in some applications and
in humid climates, active dehumidification is required, controlled by a space or duct humidistat. When
conventional cooling systems are used for active dehumidification control, simultaneous heating and
cooling is usually required: air is first cooled to below its dew point to remove moisture, then the air is
heated so that the space served is not overcooled.
FIGURE 6-W. WATER LOOP HEAT-PUMP SYSTEM
Corresponding section: Hydronic (Water Loop) Heat Pump Systems (6.5.2.2.3)
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Exceptions to 6.5.2.3
To limit the energy used by these systems, the standard allows humidistat controls to cause
simultaneous heating and cooling of the same airstream only if one or more of the following apply:
1. The system is configured to reduce supply air volume to 50% or less of the design airflow rate or
to the minimum ventilation rate specified in Section 6.2 of ASHRAE Standard 62.1, whichever is
larger, before simultaneous heating and cooling takes place. See Example 6-UU.
2. The individual fan cooling unit has a design cooling capacity of 65,000 Btu/h (19 kW) or less and is
capable of unloading to 50% capacity before simultaneous heating and cooling takes place.
3. The individual mechanical cooling unit has a design cooling capacity of 40,000 Btu/h (11.8 kW) or
less.
4. The system serves spaces where specific humidity levels are required to satisfy process needs,
such as vivariums, museums, surgical suites, pharmacies, and buildings with refrigeration systems
(for example, supermarkets, refrigerated warehouses, ice arenas). However, to satisfy this
exemption these buildings must also include either site-recovered energy or site solar energy
capable of providing no less than 75% of the annual energy required for reheat or heating air for
mixing systems. An example of site-recovered energy would be recovered condenser heat from a
refrigeration cycle or any other waste heat that would have normally been rejected to the
atmosphere or earth. A good example of this exception is a standard dehumidifier that uses
condenser heat to reheat supply air. A heat pipe or plate heat exchanger that simultaneously
reheats the air and precools outdoor air should also comply with this exception. The standard
explicitly states that “this exception does not apply to computer rooms.”
5. For buildings not listed in Exception 4, at least 90% of the annual energy for reheating or for
providing warm air in mixing systems is provided from a site-recovered (including condenser
heat) or site solar energy source. The examples listed in Exception 4 are applicable to this
exception but with a higher percentage of recovered or solar energy.
6. Systems where the added heat to the airstream is the result of the use of a desiccant system, and
75% of the heat added by the desiccant system is removed by a heat exchanger, either before or
after the desiccant system, with energy recovery. This exception applies to standard desiccant
dehumidifiers with a heat recovery wheel that uses exhaust air to precool the air that was heated
and dried by the desiccant system. If the heat exchanger removes at least 75% of the heat that was
added by the desiccant, then mechanical cooling may be used to further cool the air as required.
Humidification (6.5.2.4)
Preheating Jackets (6.5.2.4.1)
It is common with steam humidifiers to include a preheat jacket that heats up the steam injection
nozzles to avoid steam condensing in the duct system when the system first starts up. The condensed
steam (water) then can cause damage and may lead to microbial growth in the duct system. But when
humidification is not required, the preheat jacket simply becomes a duct heater, unnecessarily heating
supply air and possibly fighting with the cooling system.
To avoid this energy waste, Section 6.4.3.6 requires that humidifiers with preheating jackets mounted
in the airstream be provided with an automatic valve to shut off preheat when humidification is not
required. Upon a call for humidification, the preheat valve would open, allowing the jacket to warm;
the humidification steam control valve would then be enabled only after a temperature sensor
(typically located in the steam condensate line from the preheat jacket) indicates the jacket is
sufficiently warm to prevent condensation.
Insulated Dispersion Tubes (6.5.2.4.2)
When steam humidification is used, the dispersion tubes located within the ductwork or air-handling
unit reject heat to the air passing over the dispersion tube. This has the unintended consequence of
heating the supply air. In cooling, this leads to an additional cooling load that must be overcome to
maintain the appropriate supply air temperature. To reduce this cooling load, a minimum of R-5 (R1.1) insulation must be provided on all hot surfaces of steam dispersion tubes located in the airstream.
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If humidification will not be used during mechanical cooling or economizer cooling, the insulation is
not required.
Preheat Coils (6.5.2.5)
Preheat coils are often used to temper code-required ventilation air. When used in climates that
frequently experience temperatures below freezing, measures are implemented to prevent coil
freezing. Some designs may cause heating energy to be used when the system is in cooling mode,
particularly when an air economizer is active. This section prohibits using heating, such as opening the
control valve for steam or hot-water coils or activating electric coils, when mechanical cooling or an
economizer is operating.
For hot-water preheat coils, solutions that do not require simultaneous heating and cooling are to
include circulating pumps, antifreeze solutions, or air blenders. If stratification is an issue, pumping the
preheat coil or adding air blenders may be needed to protect chilled water cooling coils. For face-andbypass and integral face-and-bypass (IFB) coils, merely closing the face dampers is not sufficient to
satisfy this requirement because of high heat transfer through “wiping” of the backside of face-andbypass dampers and through “clamshell” dampers in IFB coils. Using antifreeze in chilled-water
cooling coils is acceptable, although this can adversely affect cooling efficiency.
Example 6-UU. Simultaneous Heating and Cooling, Hotel Ventilation System
Corresponding section: Dehumidification (6.5.2.3)
Q
A large 100% outdoor air constant-volume system provides minimum ventilation to hotel guest rooms
in a Florida hotel. The system includes a cooling coil and reheat coil controlled by a supply air dewpoint sensor. Does this system comply with the standard?
A
Yes. Exception 1 to Section 6.5.2.3 allows this system, provided that the supply air rate is equal to the
minimum ventilation rate.
 Ventilation Air Heating Control (6.5.2.6)
Dedicated outdoor air systems (DOAS) use significant heating energy when controlled to provide a
“neutral” supply temperature that matches the space set point. In fact, for humidity control, DOAS air is
often cooled to remove moisture then reheated to a neutral temperature. In addition, when cooling is
required in the building, the neutral air does not contribute to cooling like ventilation, though a single
air system would. Section 6.5.2.6 limits heating the DOAS supply air to 60°F (16°C) when the majority
of the building is expected to require cooling. The determination of when the majority of the building
requires cooling can be established based either on zone conditions or outdoor air temperature. Air
supplied to zones that do not require cooling can provide heating with the zone conditioning system.
Zones that do require cooling will benefit from the lower outdoor air temperature.
Air System Design and Control (6.5.3)
Section 6.5.3 covers fan sizing and efficiency, fan controls, and some specific system controls and
provides some limits on ventilation airflow.
Section 6.5.3.1.1 applies to all air systems having a total fan system power greater than 5 hp (3.7 kW).
Fan system power is the sum of the nominal power demand (the nameplate power) of all fans in a
system that are required to operate at design conditions to supply air from the heating or cooling
source (such as coils) to the conditioned spaces and return it back to the source or exhaust it to the
outdoors.
The following guidelines should be used to determine fan system power:
• One fan system is separate from another if they have different heating or cooling sources. For
instance, if two air handlers, each with separate supply fans and heating and cooling coils, supply a
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large ballroom, they are considered two separate systems even though they both serve the same
room.
• Fans that ventilate only, such as garage exhaust fans or equipment room ventilation fans that
transfer only unconditioned outdoor air, do not qualify as a fan system in this context. Fan systems
must be part of a system with heating or cooling capability. (In any case, fans that only ventilate are
unlikely to have any problems meeting the design requirements of this section, because their
pressure drops are typically very low.)
• Only fans that operate at design conditions need be included. For a heating-only fan system, only
fans that operate at design heating conditions are included, and for cooling-only systems, only fans
that operate at design cooling conditions are included. For systems that have both heating and
cooling capability, the system would be rated by the higher of the power required at heating design
conditions or cooling design conditions.
• Fans need to be included if they supply air from the heating or cooling source to the conditioned
space, return the air from the space back to the source, or exhaust air from the conditioned space to
the outdoors.
Fans that simply recirculate air locally (such as conference room exhaust fans) do not need to be
included. Examples 6-VV through 6-EEE provide further clarification of fan system power issues.
Fan System Power and Efficiency (6.5.3.1)
Fans are one of the largest energy-using components of HVAC&R systems. However, regulating fan
system design to improve performance is made difficult by the wide number of fan applications, from
small fan-coils serving a single zone to large central fan systems serving entire buildings. The fan
power limits in Section 6.5.3.1 apply to fan systems with a nameplate power of more than 5 hp
(3.7 kW) at design conditions. Fan system power is the sum of the nominal power demand (the
nameplate power) of all fans in a system that are required to operate at design conditions to supply air
from the heating or cooling source (such as coils) to the conditioned spaces and return it back to the
source or exhaust it to the outdoors. The fan power limit can be determined in either of two ways:
• Option 1 specifies the maximum nameplate power. This option is simple to apply but does not
consider special filter requirements, heat recovery devices, or other features that would increase
the pressure drop across the fans and thus increase fan power.
• Option 2 specifies the limit in terms of maximum input power at the fan shaft and includes
adjustments to account for special filtering or other devices in the airstream that increase the static
pressure the fan must overcome. Note that fans complying with this option must also comply with
the requirements of Section 6.5.3.1.2, Motor Nameplate Horsepower (Kilowatts), which is covered
later in this user’s manual.
With both options, the power limit applies to all fans that operate at peak design conditions, including
primary supply fans, return fans, exhaust fans, and series-type fan-powered VAV boxes. Parallel-type
fan-powered VAV boxes typically do not operate at fan system design conditions and would not be
included. Different limits apply to the fans in constant-volume and variable-volume systems. Note that
single zone VAV systems use the constant volume criteria.
Option 1
With this option, a limit is placed on the fan system motor nameplate power. The limit depends on
whether the fan system is a constant-volume or a variable-volume fan system. The limit for constantvolume fan systems is 0.0011 times the supply cubic feet per minute (cfm) (0.0017 times the supply
flow in litres per second [L/s]). The limit for variable-volume fan systems is 0.0015 times the supply
volume (in cfm) (0.0024 times the supply flow in L/s).
hp𝑚𝑚𝑚𝑚𝑚𝑚 = CFM𝑠𝑠 × 0.0011
(I-P)
kW𝑚𝑚𝑚𝑚𝑚𝑚 = L/s𝑠𝑠 × 0.0024
(SI)
kW𝑚𝑚𝑚𝑚𝑚𝑚 = L/s𝑠𝑠 × 0.0017
hp𝑚𝑚𝑚𝑚𝑚𝑚 = CFM𝑠𝑠 × 0.0015
Standard 90.1 User’s Manual
(SI)
(I-P)
Equation 6-A,
Constant-volume systems
Equation 6-B,
Variable-volume systems
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where
CFMS (L/sS)
=
the maximum design supply airflow rate to conditioned spaces served by the
system in cubic feet per minute (litres per second)
the maximum combined fan motor nameplate power (kilowatts)
hpmax (kWmax) =
Option 2
With Option 2, the limit is placed on the input power at the fan shaft instead of the nameplate power.
This method is a little more complicated but offers more flexibility for fan systems with special
filtration requirements or other features that increase static pressure.
The input power of the proposed design fan depends on the design airflow (cfm [L/s]), the static
pressure that the fan has to work against, and the efficiency of the fan. Because the limit is applied at
the fan shaft, the efficiency of the motor or the variable-speed drive (VSD) is not considered. For a
given fan, the input power at the shaft in given by the following equations:
bhp𝑖𝑖 =
CFM𝑖𝑖 ×PD𝑖𝑖
Input kW𝑖𝑖 =
where
PDi
bhpi (Input kWi)
CFMi (L/si)
ηi
I
=
=
=
=
=
(IP)
6356 × η𝑖𝑖
𝐿𝐿/𝑠𝑠𝑖𝑖 × PD𝑖𝑖
Equation 6-C
(SI)
101,999 × η𝑖𝑖
the pressure drop across the ith individual fan
the input power of the ith individual fan
the airflow rate of the ith fan at design conditions
the efficiency of the ith fan at design conditions
an index for a particular fan in the system
The total input power for the entire fan system is the sum of the input power of each of the fans that
operate at peak design conditions and is given be the following equation:
∑𝑛𝑛𝑖𝑖=1 bhp𝑖𝑖
= ∑𝑛𝑛𝑖𝑖=1 Input kW𝑖𝑖
bhp𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 =
Input kW𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇
where
bhpTotal (Input kWTotal) =
bhpi (Input kWi)
=
n
=
(IP)
Equation 6-D
(SI)
the total input power for the fan system
the input power of each individual fan
the number of fans in the system that operate at design conditions
The maximum input power permitted by the standard is given by the following equations for constantvolume and variable-volume systems. The first part of the equation gives the basic allowance for input
power. The second part of the equation gives additional input power allowed for special filtration or
devices listed in Table 6-C of this user’s manual. The additional power for these devices is based on the
flow rate of air through the device, not the total supply air flow rate.
𝑎𝑎𝑎𝑎
bhp𝑚𝑚
= CFM𝑠𝑠 × 0.00094 + ∑
kW𝑚𝑚𝑚𝑚𝑚𝑚 = L/s𝑠𝑠 × 0.0015 + ∑
bhp𝑚𝑚𝑚𝑚𝑚𝑚 = CFM𝑠𝑠 × 0.0013 + ∑
kW𝑚𝑚𝑚𝑚𝑚𝑚 = L/s𝑠𝑠 × 0.0021 + ∑
216
CFM𝑖𝑖 × PD𝑖𝑖
4131
𝐿𝐿/𝑠𝑠𝑖𝑖 × PD𝑖𝑖
650,000
CFM𝑖𝑖 × PD𝑖𝑖
4131
𝐿𝐿/𝑠𝑠𝑖𝑖 × PD𝑖𝑖
650,000
(IP)
Equation 6-E,
Constant-volume systems
(IP)
Equation 6-F,
Variable-volume systems
(SI)
(SI)
Standard 90.1 User’s Manual
where
j
bhpmax (kWi)
CFMS (L/ss)
CFMj (L/sj)
PDi (in. of water or Pa)
Cha p t e r 6 | H VAC Sys t e ms
=
=
=
=
=
an index for a particular fan system feature that qualifies for additional
pressure drop
the maximum combined fan system input power (kilowatts)
the maximum design supply airflow rate to conditioned spaces served by
the system in cubic feet per minute (liters per second)
the design supply airflow rate through the jth device, in cubic feet per
minute (liters per second)
the additional pressure drop allowance for certain fan system features
(see Table 6-C).
Exceptions to 6.5.3.1.1
The standard allows the following exceptions:
1. Hospital and laboratory systems. Constant-volume systems are common for hospitals and
laboratories. However, in order to maintain pressure relationships between spaces in these types
of buildings, dampers or other airflow control devices are commonly used where air is exhausted
from each space. If the space needs to be kept to a negative pressure relative to its surroundings in
order to prevent the spread of contaminants, the flow control device would be opened while flow
control devices in surrounding spaces would be closed.
Constant-volume systems that serve hospitals and laboratories that use flow control devices on
exhaust air and/or return air to maintain space pressure relationships necessary for occupant
health and safety or environmental control may use variable-volume fan power limits.
2. Small exhaust fans. Small exhaust fans with a motor nameplate rating 1 hp (0.75 kW) or less need
not be included in the fan power for the proposed design, even though these fans may operate
during fan system design conditions.
Pressure Drop Adjustment Devices
The following paragraphs describe the types of devices listed in Table 6-C that qualify for additional
fan power.
• Return or exhaust systems required by code or accreditation standards to be fully ducted, or
systems required to maintain air pressure differentials between adjacent rooms. The basic
input power allowance is based on the assumption that return air passes through an open plenum
on its way back to the fan system. For systems where all of the return air is ducted back to the
return, an additional pressure drop allowance of 0.5 in. of water (120 Pa) is allowed. This credit
may not be applied for air systems that have a mixture of ducted and nonducted return.
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TABLE 6-B. FAN POWER LIMITATION PRESSURE DROP ADJUSTMENTS
(This is Table 6.5.3.1-2 in the standard.)
Device*
Credits
Return or exhaust systems required by code or
accreditation standards to be fully ducted, or systems
required to maintain air pressure differentials between
adjacent rooms.
Return and/or exhaust airflow control devices
Exhaust filters, scrubbers, or other exhaust treatment
Particulate Filtration Credit: MERV 9 through 12
Particulate Filtration Credit: MERV 13 through 15
Particulate Filtration Credit: MERV 16 and greater and
electronically enhanced filters
Carbon and other gas-phase air cleaners
Biosafety cabinet
Energy recovery device, other than coil runaround loop
Coil runaround loop
Evaporative humidifier/cooler in series with another
cooling coil
Sound attenuation section (fans serving spaces with
design background noise goals below NC35)
Exhaust serving fume hoods
Laboratory and vivarium exhaust systems in high-rise
buildings
Deductions
Systems without central cooling device
Systems without central heating device
Systems with central electric resistance heat
Adjustment
0.5 in. of water (120 Pa)
(2.15 in. of water [535 Pa] for laboratory and vivarium systems)
0.5 in. of water (120 Pa)
The pressure drop of device calculated at fan system design
condition
0.5 in. of water (120 Pa)
0.9 in. of water (220 Pa)
Pressure drop calculated at two times the clean filter pressure
drop at fan system design condition
Clean filter pressure drop at fan system design condition
Pressure drop of device at fan system design condition
For each airstream [(550 × Enthalpy Recovery Ratio – 0.5)] in. of
water (125 Pa)
0.6 in. of water (150 Pa) for each airstream
Pressure drop of device at fan system design condition
0.15 in. of water (37.3 Pa)
0.35 in. of water (87 Pa)
0.25 in. of water /100 ft of vertical duct exceeding 75 ft
(62 Pa per 30.5 m exceeding 23 m)
–0.6 in. of water (–150 Pa)
–0.3 in. of water (–75 Pa)
–0.2 in. of water (–50 Pa)
*See the Pressure Drop Adjustment Devices section of this user’s manual for descriptions of these devices.
• Return and/or exhaust airflow control devices. Some types of spaces, such as laboratories, test
rooms, and operating rooms, require that an airflow control device be provided at both the supply
air delivery point and at the exhaust. The exhaust airflow control device is typically modulated to
maintain a negative or positive space pressure relative to surrounding spaces. An additional
pressure drop and associated input power adjustment are permitted when this type of device is
installed. The credit may be taken when some spaces served by an air handler have exhaust airflow
devices and other spaces do not. However, the credit is taken only for the cfm of air that is delivered
to spaces with a qualifying exhaust airflow device.
• Exhaust filters, scrubbers, or other exhaust treatment. Some applications require that the air
that leaves the building be filtered to remove dust or contaminants. Exhaust air filters are also
associated with some types of heat recovery systems, such as run-around coils. In this application,
the purpose of the filters is to help keep the coils clean, which is necessary to maintain the
effectiveness of the heat recovery system. When such devices are specified and installed, the
pressure drop of the device at the fan system design condition may be included as a credit. When
calculating the additional input power, only consider the volume of air that is passing through the
device under fan system design conditions.
• Particulate filtration credit: MERV 9 through 12. The primary purpose of filters is to keep the
fans, coils, and ducts clean and reduce maintenance costs. A secondary purpose is to improve
indoor air quality. Minimum Efficiency Reporting Value (MERV) ratings are used as the basis of this
credit. These ratings indicate the amount of particulate removed from the airstream. A higher
MERV rating is more efficient and removes more material. (See ASHRAE Standard 52.2 for details
on MERV ratings.) For air handlers that have a MERV rating between 9 and 12, an additional
pressure drop of 0.5 in. of water (120 Pa) may be considered when calculating allowable input
power for the fan system. The credit is calculated for just the cubic feet per minute of air that
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actually passes through the filter; if only part of the air is filtered, then the credit only applies to this
share.
• Particulate filtration credit: MERV 13 through 15. Filters with MERV ratings between 13 and 15
provide 85% or greater filter efficiency; these filters qualify for an additional 0.9 in. of water (220
Pa) of pressure drop. Only consider the volume of air that passes through the filter when
calculating the additional input power allowance.
• Particulate filtration credit: MERV 16 and greater and electronically enhanced filters. The
credit for filters with a MERV rating of 16 and greater and all electronically enhanced filters is
based on two times the clean pressure drop of the filter at fan system design conditions. These
clean pressure drop data are taken from manufacturers’ literature.
• Carbon and other gas-phase air cleaners. For carbon and other gas-phase air cleaners, additional
input power is based on the rated clean pressure drop of the air-cleaning device at fan system
design conditions.
• Biosafety cabinet. If the device is listed as a biosafety cabinet, you can use this credit.
• Energy recovery device. Energy recovery devices exchange heat between the outside air intake
stream and the exhaust airstream. There are two common types of heat recovery devices: heat
wheels and air-to-air heat exchangers. Both increase the pressure drop and require a system with a
larger input power. The fan power allowance for the energy recovery ventilator (ERV) is
determined by the equations in Table 6.5.3.1-1 and the adjustment factor from Table 6.5.3.1-2. The
adjustment factor is a function of the enthalpy recovery ratio. This is intended to encourage
designers to select energy recovery devices that have low pressure drops and high enthalpy
recovery ratios and thus provide a net energy reduction. This allows systems that have trouble
meeting the fan power limit to gain a higher fan power allowance by using larger energy recovery
devices with higher enthalpy recovery ratios.
• Coil runaround loop. The coil runaround loop is a form of energy recovery device that uses
separate coils in the exhaust and outdoor air intakes with a pump in between. The credit is to
account for the increased air pressure of these two coils.
• Evaporative humidifier/cooler in series with another cooling coil. An additional pressure drop
is allowed for systems that provide humidification or evaporative cooling in addition to
conventional cooling coils.
• Sound attenuation sections. Sound attenuation may be needed to help isolate fan noise in
applications where the background noise level goal is below NC35. The type of sound attenuation
section that is credited by the standard is a passive system. A section of the duct is lined with sound
attenuation materials that absorb noise but increase friction. Active sound attenuation sections
(noise-cancelling devices) do not qualify for this section. A credit equivalent to 0.15 in. of water
(37.3 Pa) is allowed for qualifying sound attenuation sections. The credit is allowed per section of
sound attenuator; these are typically sold in 3 to 5 ft (0.9 to 1.5 m) lengths.
• Exhaust systems that serve fume hoods. Exhaust systems that serve fume hoods get an
additional 0.35 in. of water (87 Pa) credit to account for the pressure through the fume hood,
ductwork, and zone valve or balancing devices. This credit applies to the exhaust fans only.
• Laboratory and vivarium exhaust systems in high-rise buildings. This credit addresses fume or
product exhaust systems that exceed 75 ft (23 m) above grade. The credit allows for 0.25 in. of
water (62 Pa) per 100 ft (30.5 m) of vertical duct for the rise above the first 75 ft (23 m) of the duct
system. A 175 ft (53 m) rise of duct serving fume exhaust would get 0.35 in. of water (87 Pa) credit
for the fume hoods and an additional 0.25 in. of water (62 Pa) credit for the 100 ft (30.5 m) of duct
rise over the first 75 ft (23 m) (for a total credit of 0.60 in. of water [149 Pa]). This credit applies to
the exhaust fans only.
While the fan power calculation in the standard grants credits for all of the above devices, it also
already inherently includes many typical devices present in air-handling systems. When these devices
are removed from an air-handling system, the allowable fan power must be reduced in response to the
lower static pressure. The “Deductions” section of Table 6-C includes the required deductions for
devices often removed or excluded from air-handling systems. When calculating the allowable fan
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power in accordance with this section, the corresponding static pressure deduction must be taken for
any of the listed devices that are not present in the fan system. The following describes the deductions
listed in Table 6-C.
• Systems without central cooling device. The standard fan power calculations assume the fan
system includes a cooling coil in the central air handler. When a fan system excludes this device, the
static pressure seen by the fan system must be reduced by 0.6 in. of water (150 Pa) to account for
the excluded device.
• Systems without central heating device. The standard fan power calculations assume the fan
system includes a hydronic heating coil in the central air handler. When a fan system excludes this
device, the static pressure seen by the fan system must be reduced by 0.3 in. of water (75 Pa) to
account for the excluded device.
• Systems with central electric resistance heat. The standard fan power calculations assume the
fan system includes a central hydronic heating coil in the central air handler. When a fan system
excludes the hydronic coil and includes an electric central heating coil, the static pressure seen by
the fan system must be reduced by 0.2 in. of water (50 Pa) to account for the relatively lower static
pressure associated with the electric resistance heating coil.
Motor Nameplate Horsepower (6.5.3.1.2)
Compliance with this section is required only for systems using Section 6.5.3.1.1 Option 2. Systems that
show compliance with Section 6.5.3.1.1 Option 1 are exempt from this section.
When Section 6.5.3.1.1 Option 2 is used, compliance is only shown through comparison of the power
required to turn the fan shaft relative to the power allowed by the standard to turn the fan shaft. The
intention of Section 6.5.3.1.2 is to limit oversizing fan motors when following Option 2. Although motor
electrical power draw is primarily a function of the mechanical load on the shaft, a larger motor has the
ability to support much higher mechanical loads without failure. Fan motor nameplate powers are
frequently selected to prevent motor overloading during balancing or unforeseen operating
conditions. This may require selecting a motor two or three motor sizes greater than that required to
satisfy the design condition. Although this prevents motor failure due to unforeseen operating
conditions, it also enables the fan to potentially operate continuously far outside of design conditions.
To limit wasted fan motor energy, as well as to keep Option 1 results consistent with Option 2 results,
the selected fan motor nameplate power may not be greater than the first available size that exceeds
the fan shaft power. For example, a manufacturer’s fan curve indicates the fan shaft power required at
design conditions is 30.8 bhp (23.0 kW). The allowable motor size is 40 hp (30 kW). However, to
reduce the possibility of motor overloading, Exceptions 1 and 2 to this section allow the motor size to
be increased by one additional size for certain situations.
When the fan shaft power is less than 6 bhp (4.5 kW) but within 50% of the first available motor size,
the next larger size may be used. For fans 6 bhp (4.5 kW) and greater, the shaft power must be within
30% of the first available motor size to use the next larger motor size. For example, a fan shaft power
of 45 bhp (33.6 kW) falls between motor sizes 40 and 50 hp (30 and 37 kW). Without the exceptions, a
50 hp (37 kW) motor must be selected. However, 30% of the fan shaft power is 13.5 bhp (10 kW). The
difference between the fan shaft power and the motor rated power is less than this value. Thus, the
next greater motor size is allowed, 60 hp (45 kW).
Example 6-VV. Fan System Design Requirements, Constant-Volume Hospital System with
100% Outdoor Air
Corresponding section: Air System Design and Control (6.5.3)
Q
A constant-volume air handler serving a hospital wing has a fan system design supply airflow of 10,000
cfm (4719 L/s). The supply fan has a 20 hp (15 kW) (nameplate) supply fan motor that operates at an
input power of 13.9 bhp (10.4 kW). The exhaust fan has a 5 hp (3.7 kW) motor that operates at an
input power of 3.2 bhp (2.4 kW). Flow control devices in the exhaust are used to maintain pressure
relationships between spaces served by the system. The air handler uses MERV 13 filters, and exhaust
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air is completely ducted. The system uses 100% outdoor air and has a run-around heat recovery
system with coils in the supply and exhaust airstreams, each with 0.4 in. of water (100 Pa) pressure
drop at design airflow.
Does this fan system comply with the fan power requirements in Section 6.5.3.1?
A
Yes. Even though this is a constant-volume air handler, because it is a hospital, and flow control devices
are used at the exhaust of each space, the fan power requirements for a VAV system apply per
Exception 1 to Section 6.5.3.1.1.
For this system, Option 2 is required in order to consider the additional pressure drop of the return air
ducts, the MERV 13 filters, and the heat recovery device. From Table 6.5.3.1-1, the allowable system
input power for the system is:
bhp = CFM𝑠𝑠 × 0.0013 + 𝐴𝐴
(I-P)
= 10,000 × 0.0013 + 𝐴𝐴 = 13 + 𝐴𝐴
kW = L/s𝑠𝑠 × 0.0021 + 𝐴𝐴
(SI)
= 4719 × 0.0021 + 𝐴𝐴 = 9.9 + 𝐴𝐴
From Table 6.5.3.1.1-2, the pressure drop adjustment for the MERV 13 filter is 0.9 in. of water (224 Pa),
and the pressure drop adjustment for the fully ducted return is 0.5 in. of water (124 Pa). The pressure
drop adjustment for a run-around loop heat recovery device is 0.6 in. of water (150 Pa) per airstream,
and the pressure drop adjustment for the exhaust flow control device is 0.5 in. of water (124 Pa). The
airflow through all of these devices is 10,000 cfm (4719 L/s), so the additional input power that is
allowed is 5.33 bhp (4.0 kW), as calculated below.
𝐴𝐴 =
CFM𝑀𝑀13 × PD𝑀𝑀13 + CFM𝐷𝐷𝐷𝐷 × PD𝐷𝐷𝐷𝐷 + CFM𝐹𝐹𝐹𝐹 × PD𝐹𝐹𝐹𝐹 + 2 × (CFM𝐻𝐻𝐻𝐻 × PD𝐻𝐻𝐻𝐻 )
4,131
10,000 × 0.9 + 10,000 ×0.5 + 10,000 × 0.5 + 2 × (10,000 × 0.6)
𝐴𝐴 =
= 7.5bhp
4131
L/s𝑀𝑀13 × PD𝑀𝑀13 + L/s𝐷𝐷𝐷𝐷 × PD𝐷𝐷𝐷𝐷 + L/s𝐹𝐹𝐹𝐹 × PD𝐹𝐹𝐹𝐹 + 2 × (L/s𝐻𝐻𝐻𝐻 × PD𝐻𝐻𝐻𝐻 )
𝐴𝐴 =
650,000
4719 × 224 + 4719 × 124 + 4719 × 124 + 2 × (4719 × 150)
𝐴𝐴 =
= 5.6kW
650,000
(I-P)
(SI)
The total allowed input power is 13.0 bhp (9.7 kW) plus 7.50 bhp (5.60 kW), or 20.50 bhp (15.3 kW),
which is greater than the fan system input power of 17.1 bhp (12.8 kW). Therefore, the system meets
the standard’s requirements.
Example 6-WW. Fan System Design Requirements, Laboratory Fume Hoods, Local Exhaust
Corresponding section: Air System Design and Control (6.5.3)
Q
Four laboratories each contain three exhaust fume hoods, and each hood is capable of exhausting air at
the rate of 400 cfm (189 L/s). Supply air is introduced to each laboratory at the rate of 1600 cfm (755
L/s), and a general exhaust of 500 cfm (230 L/s) serves each room. The total supply fan volume is
6400 cfm (3020 L/s), and the total general exhaust volume is 2000 cfm (940 L/s).
Each exhaust hood has a dedicated fan with a 1/2 hp (0.4 kW) motor operating at an input power of
0.30 bhp (0.2 kW). The constant-volume air handler serving the laboratories uses a 5 hp (3.7 kW)
supply fan that operates at 3.2 bhp (2.4 kW) and a 1 hp (0.7 kW) exhaust fan serves the general
exhaust for all of the laboratories and operates at 0.6 bhp (0.4 kW). The system has fully ducted supply
and exhaust.
Does this system comply with the fan power requirements of Section 6.5.3.1?
A
Yes. The hood exhaust fans, which are 1/2 hp (0.4 kW) each, and the general exhaust fan, which is 1 hp
(0.7 kW), are exempt per Exception 2 to Section 6.5.3.1.1. Because the exhaust fans are exempt, they
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cannot use the adjustments for ducted exhausts and fume hoods. Only the supply fan energy counts. As
shown below, the supply fan has a budget of 6.01 bhp (4.5 kW) but a connected input power of 3.2 bhp
(2.4 kW), so it complies.
bhp𝑚𝑚𝑚𝑚𝑚𝑚 = CFM𝑠𝑠 × 0.00094 = 6400 × 0.00094 = 6.01 bhp > 3.2 bhp
(I-P)
kW𝑚𝑚𝑚𝑚𝑚𝑚 = L/s𝑠𝑠 × 0.0015 = 3020 × 0.0015 = 4.5 kW > 2.4 kW
(SI)
Example 6-XX. Fan System Design Requirements, Laboratory Fume Hoods, Central Exhaust
Corresponding section: Air System Design and Control (6.5.3)
Q
If the building in Example 6-WW were served by a common exhaust fan for the hoods and general
exhaust (6800 cfm [3200 L/s] with a motor power of 5 hp [3.7 kW] operating at an input power of 4.8
bhp [3.6 kW]) instead of individual exhaust fans for each laboratory, would the system still comply
with the standard?
A
Yes. Exception 2 no longer applies for the exhaust fan because the fan motor nameplate power is
greater than the threshold. However, the exhaust system is allowed fan power adjustments for ducted
exhaust and fume hoods. From Table 6.5.3.1-2, there is a 2.15 in. of water (530 Pa) credit for the fully
ducted laboratory exhaust and a 0.35 in. of water (87 Pa) credit for each hood. The fan power
limitation is calculated as follows:
bhp𝑚𝑚𝑚𝑚𝑚𝑚 = CFM𝑠𝑠 × 0.00094 + �
bhp𝑚𝑚𝑚𝑚𝑚𝑚 = 6400 × 0.00094 + �
CFM × PD
4131
�
ℎ𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜
400 × 0.35 × 12
4131
�
+�
ℎ𝑜𝑜𝑜𝑜𝑑𝑑𝑠𝑠
CFM × PD
+�
kW𝑚𝑚𝑚𝑚𝑚𝑚 = 3020 × 0.0015 + �
CFM × PD
650,000
�
ℎ𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜
189 × 87 × 12
650,000
�
+�
ℎ𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜
4131
CFM × PD
650,000
+�
�
𝑒𝑒𝑒𝑒ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
6800 × 2.15
bhp𝑚𝑚𝑚𝑚𝑚𝑚 = 6.0 + 0.4 + 3.5 = 9.9 bhp
kW𝑚𝑚𝑚𝑚𝑚𝑚 = L/s𝑠𝑠 × 0.0015 + �
4131
�
�
𝑒𝑒𝑒𝑒ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
𝑒𝑒𝑒𝑒ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
3,200 × 530
650,000
�
𝑒𝑒𝑒𝑒ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎
(I-P)
(SI)
kW𝑚𝑚𝑚𝑚𝑚𝑚 = 4.5 + 0.3 + 2.6 = 7.4 kW
Because the design input power is 3.2 + 4.8 bhp = 8.0 bhp (2.4 + 3.6 kW = 6.0 kW), which is lower than
the allowed 9.9 bhp (7.4 kW), this system would comply with the standard’s fan power requirements.
Example 6-YY. Calculation of Fan Energy, Fan-Coil System
Corresponding section: Air System Design and Control (6.5.3)
Q
A building HVAC&R system consists of 40 fan coils serving individual zones, each with 1/3 hp
(0.25 kW) motors. Does this system need to comply with Section 6.5.3.1?
A
No. Each fan coil is a separate fan system because each has a separate cooling and heating source. The
total fan system power for each fan system is only 1/3 hp (0.25 kW), which is less than the 5 hp
(3.7 kW) threshold in Section 6.5.3.
Example 6-ZZ. Adjustment of Fan Energy, Electronically Enhanced Filter
Corresponding section: Air System Design and Control (6.5.3)
Q
A 20,000 cfm (9400 L/s) supply fan system includes an electronically enhanced filter assembly with a
clean pressure drop of 1.25 in. of water (310 Pa). Using Option 2, how much additional fan input power
is allowed for this filter?
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A
For this type of filter, Table 6.5.3.1-2 allows the rated pressure drop for additional input power to be
two times the rated clean pressure drop of the of the filter. The additional fan power credit (bhp [kW])
is determined as follows:
bhp =
bhp =
kW =
kW =
CFM𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 × 2 × PD𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓
4131
20,000 × 2 × 1.25
(I-P)
= 12.1 bhp
4131
L/s𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 × 2 × PD𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓
650,000
9400 × 2 × 310
650,000
(SI)
= 89.7 kW
Example 6-AAA. Fan System Design Requirements, VAV Changeover System
Corresponding section: Air System Design and Control (6.5.3)
Q
What are the fan system design requirements for a variable-air-volume changeover system (also called
a “variable volume and temperature system”) that includes a bypass damper at the fan?
A
This system is variable volume at the zone level, but the bypass damper will maintain a relatively
constant airflow through the fan. The system is therefore a constant-volume system in this context, and
it must meet the fan power requirements in Table 6.5.3.1-1 for constant-volume systems.
Example 6-BBB. Fan System Design Requirements, VAV Reheat System in Office
Corresponding section: Air System Design and Control (6.5.3)
Q
A VAV reheat system serves a low-rise office building. The building is served by one VAV packaged
rooftop unit with a 10 hp (7.5 kW) supply fan with a VSD. Four parallel fan-powered VAV terminal
units are used on north-facing perimeter offices for heating. Two series fan-powered VAV boxes, each
with a 1/3 hp (0.25 kW) fan with an electronically commutated motor, serve two interior conference
rooms.
The space also uses a local exhaust fan for each of the four bathrooms. Fans for the system are listed
below. Fan performance is as described in the table below.
Is this system in compliance with Section 6.5.3.1?
Quantity
1
2
1
4
4
2
A
Fan Service
Supply fan with variable-speed drive
Condenser fans
Return fan
Bathroom exhaust fans
Parallel fan-powered VAV boxes
Series fan-powered VAV boxes
Design cfm (L/s),
each
12,000 (5700)
9300 (4400)
11,000 (5200)
350 (170)
400 (190)
600 (280)
bhp (kW)
8.7 (6.5)
0.7 (0.5)
4.2 (3.1)
0.16 (0.12)
0.08 (0.06)
0.12 (0.09)
Nameplate Motor,
hp (kW)
10 (7.5)
1.0 (0.75)
5.0 (3.7)
0.2 (0.15)
1/5 (0.15)
1/3 (0.25)
Yes. The fan system can comply with either the nameplate power limitation or the input power
limitation. The nameplate power will be checked in this example.
First, determine which fans to include in the nameplate fan system power calculation:
• The supply and return fans are clearly included in the fan power calculation.
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•
The condenser fans are not included because they circulate outdoor air and do not affect the
conditioned air supplied to the space.
• The toilet exhaust fans are not included because they qualify for Exception 2 to Section 6.5.3.1.1,
which exempts individual exhaust fans with nameplate power of 1 hp (0.75 kW) or less.
• The parallel fan-powered VAV boxes are not included in the fan power calculation because they
operate in heating mode when the supply fan is not operating at design conditions.
• The series fan-powered boxes run continuously and are included in the fan power calculation.
The total nameplate power is 15.7 bhp (11.7 kW), as shown below.
Nameplate Power = 10 + 5 + (2 × 1⁄3) = 15.7 hp
(I-P)
Nameplate Power = 7.5 + 3.7 + (2 × 0.25) = 11.7 kW
(SI)
The total supply air delivered from the air handler is 12,000 cfm (5,700 L/s), and the allowed
nameplate power for a variable-air-volume system is 18 hp (13 kW) as shown below.
Nameplate Power𝑚𝑚𝑚𝑚𝑚𝑚 = 12,000 × 0.0015 = 18.0 hp
(I-P)
Nameplate Power𝑚𝑚𝑚𝑚𝑚𝑚 = 5700 × 0.0024 = 13.7 kW
(SI)
The total nameplate power of 15.7 hp (11.7 kW) is less than the allowed 18.0 hp (13.7 kW), so the fan
system complies with the standard. If the nameplate power exceeded the allowable limit, the system
input power could be checked for compliance.
Example 6-CCC. Fan System Design Requirements, Fan Power Calculation for VAV System
Corresponding section: Air System Design and Control (6.5.3)
Q
A conventional VAV system serves an office building. Fan performance is as described in the table
below. Is the system in compliance with Section 6.5.3.1?
Quantity
2
4
1
1
2
15
120
A
Fan Service
Supply fans with variable-speed drives
Economizer relief fans
Toilet exhaust
Elevator machine room exhaust fan
Cooling tower exhaust fans
Conference room exhaust fans
Series-type fan-powered mixing boxes
Design cfm (L/s), each
75,000 (35,400)
32,000 (15,100)
6750 (3185)
5000 (2360)
unknown
500 (236)
1300 (614) (average)
bhp (kW)
70.5 (53)
3.5 (2.6)
2.7 (2.0)
Unknown
Unknown
240 W
Unknown
Nameplate Motor hp (kW)
75 (56) high efficiency
5 (3.7)
3 (2.2) high efficiency
3/4 (0.56)
15 (11)
—
1/3 (0.25)
First, determine which fans to include in the fan power calculation:
• The supply fans are included.
• The economizer relief fans are not included because they will not operate at peak cooling design
conditions. (Had return fans been used, they would have to be included in the calculation.)
• The toilet exhaust fan is included because it exhausts conditioned air from the building rather than
have it returned to the supply fan, and it operates at peak cooling conditions.
• The elevator exhaust fan is not part of the system because it is assumed in this case that the
makeup air to the elevator room is from the outdoors rather than from the building. Had makeup
air been transferred from the conditioned space, the fan would have been included if it were larger
than 1 hp (0.75 kW).
• The cooling tower fans operate at design conditions, but they also are not part of the system
because they circulate only outdoor air. Note that although the cooling tower fan power does not
contribute to the system fan power, it is required to meet the minimum efficiency requirements in
Table 6.8.1-7.
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•
The conference room exhaust fans are assumed to be transfer fans; they simply exhaust air from
the room and discharge it to the ceiling plenum. Because this air is not exhausted to the outdoors,
the fans are not included.
• The series-type fan-powered VAV boxes are included because they assist in supplying air to the
conditioned space and operate at design cooling conditions. If the boxes were the parallel type,
they would not be included because they would not operate at design cooling conditions.
Second, using Option 1, add up the nameplate power (not input power) of the eligible fans. For this
example, the fans that are included and their motor power requirements are as follows:
Fan Service
Supply fans
Toilet exhaust fan
Fan-powered VAV boxes
Total fan system power
Quantity
2
1
120
Motor hp (kW), each
75 (56)
3 (2.2)
1/3 (0.25)
Total hp (kW)
150 (112)
3 (2.2)
40 (30)
193 (144)
Third, determine the supply air rate. This is the total airflow rate supplied through the heating or
cooling source, which in this case is equal to the total of the two supply fan airflow rates, 2 × 75,000 =
150,000 cfm (2 × 35,000 = 71,000 L/s). The supply rate is not the total of the fan-powered VAV box
airflow rates; although this is the ultimate supply air rate to the conditioned space, this entire airflow
does not flow through the heating or cooling source. The airflow rate from the exhaust fan is also not
included in the supply air rate for the same reason.
Fourth, determine the criteria from Table 6.5.3.1-1. The series fan-powered VAV boxes supply a
constant flow of air to the conditioned space, but the primary airflow—the airflow through the cooling
source—varies as a function of load, so this system meets the definition of a VAV system. Using
Option 1, the maximum nameplate power for the system is 225 hp (168 kW) as shown below.
hp = CFM𝑠𝑠 × 0.0015
(I-P)
hp = 150,000 × 0.0015 = 225 hp
kW = L/s𝑠𝑠 × 0.0024
(SI)
kW = 71,000 × 0.0024 = 170.4 kW
Fifth, compare the allowable fan system power with the proposed power. The actual fan system
nameplate power of 193 hp (144 kW) is less than the 225 hp (168 kW) limit, so this system complies. If
the system did not comply, the designer could consider using larger ducts to reduce static pressure or
shifting to parallel fan-powered VAV boxes.
Example 6-DDD. Fan System Design Requirements, Fan Power Calculation for Floor-by-Floor System
Corresponding section: Air System Design and Control (6.5.3)
Q
A high-rise building has floor-by-floor supply air-handling units but central toilet exhaust fans and
minimum ventilation supply fans. How is the standard applied to this system?
A
Each air handler counts as a fan system. The energy of the central toilet exhaust and ventilation fans
must be allocated to each air handler on a cfm-weighted basis. For instance, if one floor receives 2000
cfm (944 L/s) of outdoor air, and the outdoor air fan supplies a total of 10,000 cfm (4719 L/s) with a 5
hp (3.7 kW) motor, 20% (2000/10,000 cfm [944/4719 L/s]) of the fan power (1 hp [0.75 kW]) is
added to the fan power for the floor’s fan system. Note that the airflow rates from the exhaust and
ventilation fans are not included in the supply rate calculation because these rates do not add to the
airflow passing through the heating/cooling coils in the floor-by-floor air handlers.
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Example 6-EEE. Applying Fan Power Limitation to Dedicated Outdoor-Air System
Corresponding section: Air System Design and Control (6.5.3)
Q
A wing of an elementary school building is served by eight water-source heat pumps, each equipped
with a 3/4 hp (0.56 kW) fan motor and serving a single classroom. Ventilation air is supplied directly
to each classroom by a dedicated outdoor-air system (DOAS). Each classroom requires 500 cfm (236
L/s) of outdoor air, so the DOAS delivers the total of 4000 cfm (1888 L/s) of conditioned outdoor air
using a 5 hp (3.7 kW) fan. Does this system need to comply with Section 6.5.3.1?
A
Each water-source heat pump is a separate fan system because each has a separate cooling and heating
source. The power of the DOAS fan must be allocated to each heat pump on a cfm-weighted basis. For
each classroom, 12.5% (500/4000 cfm [236/1888 L/s]) of the DOAS fan power (12.5% of 5 hp = 0.625
hp [12.5% of 3.7 kW = 0.4625]) is added to the fan power for the heat pump (0.75 + 0.625 = 1.375 hp
[0.56 + 0.4625 = 1.0225 kW]). In this instance, even with the DOAS fan allocated, each heat-pump fan
system is less than the 5 hp (3.7 kW) threshold in Section 6.5.3, so the system does not need to comply
with Section 6.5.3.1.
Fan Efficiency (6.5.3.1.3)
The fan power requirements of Section 6.5.3.1 only limit the fan power and do not specifically require
an efficient fan. A fan system with inefficient fans could potentially comply with the power
requirement of Section 6.5.3.1 through significant reduction of system static pressure.
All fans subject to the minimum efficiency requirements of this section must have a fan efficiency grade
(FEG) of 67 or higher. For one FEG, a range of mechanical efficiencies are required based on the fan
wheel size (see Figure 6-X). The FEG must be based on manufacturers’ certified data as defined by
AMCA Standard 205. In addition to satisfying the FEG requirement, the fan efficiency at design
condition must fall within 15 percentage points of the maximum total efficiency of the fan.
Fan efficiency grades are not required to be certified by an independent testing laboratory. However,
when fans are certified, FEG ratings are desired. Refer to the additional resources at the end of this
section to obtain a list of FEG-labeled fans.
Exceptions to 6.5.3.1.3
The following fans are not required to meet the fan efficiency requirements of Section 6.5.3.1.3:
• Individual fans with motor nameplate power 5 hp (3.75 kW) or smaller that are not part of a group
of fans operated as the functional equivalent of a single fan.
• Multiple fan systems where fans operate in parallel or series as a single fan, provided the sum of the
motor nameplate power is 5 hp (3.75 kW) or smaller.
• Fans that are part of packaged equipment that carries a unit efficiency rating under Section 6.4.1.1,
Minimum Equipment Efficiencies.
• Fans that are part of packaged equipment that bears a third-party-certified seal for air or energy
performance.
• Powered wall or roof ventilators that comprise a centrifugal or axial fan in a weather-resistant fan
housing where the base is designed to mount directly to a roof curb or in a wall opening.
• Fans that are outside of the scope of AMCA Standard 205.
• Fans intended to operate only under emergency conditions—e.g., fans installed specifically for
smoke evacuation, stairwell pressurization, or other emergency applications. This exception does
not apply to air-handling equipment used for space conditioning that also acts as part of an
emergency system.
Additional Fan Efficiency Grade Resources
An online database of fans bearing an Air Movement and Control Association International (AMCA)
FEG label can be found at http://www.amca.org/feg/feg-finder.aspx.
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An article about the use of FEGs in Standard 90.1 titled “Fan Efficiency Requirements for Standard 90.12013,” written by John Cermak and Michael Ivanovich, was published in the April 2016 issue of ASHRAE
Journal. It can be found on the ASHRAE website or at http://www.amca.org/feg/best-practices.aspx.
An article titled “The Role of Fan Efficiency in Reducing Energy Use” by Michael Brendel that describes how
the Standard 90.1 fan efficiency requirement works in conjunction with fan power limits was published in
the May 2016 issue of HPAC Engineering. It can be found at http://www.amca.org/feg/best-practices.aspx.
FIGURE 6-X FAN EFFICIENCY GRADE GRAPH
Above is Figure 3b, Fan Efficiency Grades (FEG) for Fans Without Drives (I-P), as it appears in AMCA Standard 205, Energy
Efficiency Classification for Fans (courtesy of Air Movement and Control Association International)
Example 6-FFF. Determining Minimum Allowable Fan Efficiency from Fan Efficiency Grade (FEG)
Corresponding section: Fan Efficiency (6.5.3.1.3)
Q
A 40 in. (762 mm) housed backward-inclined centrifugal fan that is not part of a packaged piece of
equipment that is rated as a unit has an FEG of 75. According to the fan manufacturer’s data, the fan
efficiency at design conditions is 60%. Does this fan comply with the efficiency requirements of Section
6.5.3.1.3?
A
Yes. The fan complies with both requirements of Section 6.5.3.1.3. The fan’s rated FEG, 75, is greater
than 67, the minimum FEG allowed by Section 6.5.3.1.3. Additionally, the fan efficiency at the design
condition must be within 15% of the peak efficiency of the fan. This fan complies because the efficiency
at design condition is 12% below the maximum rated efficiency of the fan. The FEG rating for a fan is
based only on the fan wheel diameter and the maximum rated efficiency. According to AMCA Standard
205, a 40 in. (762 mm) fan wheel must have a maximum rated efficiency of 72% to achieve an FEG of
75.
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Note that it is possible for a fan with an FEG rating much higher than 67 to fail to comply with Section
6.5.3.1.3 if the efficiency at design conditions is too low. A fan with a 40 in. (762 mm) wheel and an FEG
of 90 (peak rated efficiency of 85%) is designed to operate at an efficiency of 55%. This fan would not
comply with Section 6.5.3.1.3 because the design operating efficiency is more than 15% below the peak
rated efficiency. Conversely, a fan with a 40 in. (762 mm) fan wheel and a rated FEG of 67 (peak rated
efficiency of 64%) operating at a design efficiency of 50% would comply. Although the second fan has
been selected to operate at a lower efficiency, it is likely selected at a better point on the fan’s
performance curve because it is operating closer to its peak efficiency.
Fan Control (6.5.3.2)
Supply Fan Airflow Control (6.5.3.2.1)
This section requires fan controls for the supply fans of HVAC cooling units in response to the cooling
load. Table 6.5.3.2.1 identifies cooling systems that must meet the requirements of this section. These
are systems with DX cooling that have a cooling capacity of 65,000 Btu/h (19 kW) or more, or systems
with chilled-water or evaporative cooling that have a fan motor that is 1/4 hp (0.2 kW) or more.
The system must meet requirement (a) or (b), depending on how the system operates to meet the
cooling load. Requirement (a) applies to systems that control the cooling capacity based directly on
space temperature. Requirement (b) applies to all other systems, which primarily means systems that
vary the zone airflow in response to cooling load.
In addition, requirement (c) applies to all systems that include an air economizer to meet the
requirements of Section 6.5.1. If the system includes an air economizer but is not required to include
one by Section 6.5.1, then compliance with requirement (c) is not required.
• Requirement (a). Systems that vary the cooling load in response to space temperature must be
able to reduce their airflow to the greater of two-thirds of full speed or the minimum speed
necessary to meet the ventilation requirements. At the minimum airflow (low speed), the fan motor
must draw no more than 40% of the full-speed power.
• Requirement (b). All systems that do not vary cooling capacity based on space temperature, such
as those that control space temperature by modulating the zone airflow, must have the ability to
reduce their fan speed to the larger of 50% or the minimum speed necessary to meet the
ventilation requirements. At the minimum fan speed of 50%, the fan motor must draw no more
than 30% of the power at full fan speed.
• Requirement (c). All systems that include an air economizer to comply with Section 6.5.1 will need
to meet requirement (a) or (b) and must also have at least two fan speeds during economizer
operation.
For all systems, full variable-speed fans or staging with more than two stages is also acceptable.
Systems that do not meet the capacity limits in Table 6.5.3.2.1 are not required to meet any of the
requirements (a), (b), or (c). In addition, an exception changes the capacity limit for chilled water and
evaporatively cooled systems to those with a fan motor of at least 1 hp if the system does not supply
ventilation air and the fan cycles with the load.
Example 6-GGG. Part-Load VAV Fan System Efficiency, Size Limit
Corresponding section: Fan Control (6.5.3.2)
Q
A VAV fan system includes a 25 hp (19 kW) supply fan and a 7.5 hp (5.6 kW) return fan. Must both fans
meet the 30% power draw limit at 50% of design airflow?
A
Yes. The supply and the return fan must both have fully modulating fan control.
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FIGURE 6-Y. EXAMPLE CONTROL DIAGRAM FOR A SINGLE-ZONE VAV SYSTEM
Corresponding section: Supply Fan Airflow Control (6.5.3.2.1)
FIGURE 6-Z. GENERIC PART-LOAD CURVES FOR A VARIETY OF FANS
Corresponding section: Fan Control (6.5.3.2)
VAV Static Pressure Sensor Location (6.5.3.2.2)
Where VAV fans are controlled to maintain duct static pressure, the sensor location and set point are
critical to proper fan control. To properly place a static pressure sensor and establish a set point, it is
necessary to understand the purpose. Proper VAV box operation relies on the duct pressure directly
upstream of the box to be greater than the pressure downstream. This pressure differential must be
not less than the pressure drop through the box at the desired flow. If this is not the case, the actual
airflow through the box will be less than requested. VAV systems without advanced control logic are
unable to verify this for every zone box. The solution is to maintain a fixed supply air duct static
pressure at one or more points in the ductwork to prove adequate airflow to all VAV boxes. Choosing
sensor locations is critical because as VAV boxes adjust and redirect airflow within the ductwork, the
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static pressure at duct branches changes. The sensor must be placed in a location remote enough to
prove sufficient static pressure to all boxes but central enough to be subjected to a stable average static
pressure. Based on these requirements the standard requires the duct static pressure sensors be
placed in a location where the set point is not greater than 1.2 in. of water (300 Pa). Due to physical
constraints, some duct systems have major duct splits near the fan. These locations will likely have
static pressures above this value. In this situation the standard requires a sensor be located in each
duct. The set point must be maintained at all sensors to guarantee airflow to all VAV boxes.
DDC systems with zone reset capability meeting the requirements of Section 6.5.3.2.3 are exempt from
this requirement.
VAV Set-Point Reset (6.5.3.2.3)
DDC devices are able to communicate far more data than older controls systems. This facilitates more
advanced control through monitoring numerous points simultaneously. For static pressure set point
reset this means rather than simply maintaining a specified supply air duct static pressure and
assuming the zones downstream are satisfied, the controls can monitor the zones and adjust the static
pressure set point according to actual demand. This not only proves zones are satisfied but uses the
lowest required static pressure set point to do so.
Therefore, according to the standard, the set point must be reset down until one zone VAV box
approaches wide open. The static pressure set point must be reset up if a zone cannot maintain the
temperature set point at wide open. The controls must detect problem zones that regularly prevent the
static pressure set point from resetting down. This can help pinpoint problems and initiate corrective
action. The controls must also include the ability to remove problem zones from the reset logic. For
example, a VAV box serving an unoccupied storage room regularly prevents the static pressure set
point from being reset down. Because this is an unoccupied space, the corrective action in this
situation could simply be to remove the zone from the logic and allow the static pressure to reset
down. The problem zone removed may no longer meet the space temperature set point, but in this
example it likely will not pose a problem, as the space is unoccupied.
Figure 6-AA shows the performance of a fan with a VSD at various static pressure set points. The lower
the set point, the less power the fan consumes. The lowest curve shows the ideal performance where
static pressure is reset based on the zone requiring the most pressure, i.e., the set point is reset lower
until one zone damper is nearly wide open. Note that the static pressure set point only affects the static
pressure downstream of the supply fan. The static pressure of the air handler and return ductwork
follows the system curve. Therefore, when the static pressure set point is 0, the fan still experiences
resistance. Because of the improved performance of this control sequence, the standard requires it to
be implemented for systems that have DDC of individual zone boxes that are capable of reporting zone
information to the central control panel controlling the air handler. For a related example, see Example
6-HHH.
 Return and Relief Fan Control (6.5.3.2.4)
Return and relief air fans are required to be controlled to maintain building pressure as the outdoor air
intake varies due to economizer operation. The fan control can be either
• direct using building pressure sensors or
• indirect based on tracking the airflow of the supply and return airstreams.
The fans must also have variable-speed control or otherwise reduce fan power as flow is reduced such
that the fan power does not exceed 30% of the total design power at 50% of the design airflow.
The standard provides two exceptions to this section: (a) return or relief fans with a total motor size of
0.5 hp or less and (b) staged relief fans with a minimum of four stages.
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FIGURE 6-AA. PART-LOAD CURVES FOR VARIABLE-SPEED DRIVE FAN AT VARIOUS SETPOINTS
Corresponding section: VAV Set-Point Reset (6.5.3.2.3)
Example 6-HHH. Zone Static Pressure Reset
Corresponding section: VAV Set-Point Reset (6.5.3.2.3)
Q
A VAV system has zone-level DDC. The VAV damper is controlled by a floating-point actuator, meaning
the damper is driven open or driven closed by two binary outputs. The actual damper position is not
known with this system, so static pressure set-point reset by zone damper position in accordance with
Section 6.5.3.2.3 does not appear to be possible.
How can this system comply with Section 6.5.3.2.3?
A
Section 6.5.3.2.3 does not mandate knowledge of damper position; it allows “other indicator of need
for static pressure” to be used. Options include (but are not limited to) the following:
• Analog actuator signal
• Position feedback from a floating actuator
• An end switch indicating full-open position for a floating actuator
• Analog output from the terminal airflow control loop used to control the damper
• An alarm indicating airflow is below set point for some period of time
Multiple-Zone VAV System Ventilation Optimization Control (6.5.3.3)
Multiple-zone VAV systems with DDC to the zone level are required to provide an automated means to
reduce outdoor air intake flows below design rates in response to changes in system ventilation
efficiency as defined by ASHRAE Standard 62.1, Appendix A.
Each VAV box controller senses the current primary airflow Vpz and calculates the current zone
outdoor-air fraction Zd. The building automation system totals the primary airflows and required
outdoor airflows Voz from all VAV boxes and determines the highest outdoor-air fraction reported.
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Then the HVAC&R controls logic solves the equations from Appendix A of Standard 62.1, calculating
the system ventilation efficiency Ev and the system-level intake flow Vot that is required at the current
operating condition. This new intake flow set point is communicated to the air-handling unit or rooftop
unit controller, which then adjusts the outdoor air damper accordingly to bring in the required amount
of outdoor air.
In a VAV system that uses communicating DDC, all of the necessary real-time data to implement this
strategy are available digitally.
This control logic is described in 62.1 User’s Manual in Appendix B. Refer to that manual for help with
compliance.
 Parallel-Flow Fan-Powered VAV Air Terminal Control (6.5.3.4)
Parallel-flow fan-powered VAV air terminals must have automatic controls to limit excess fan
operation. The terminal unit fans are only allowed to operate during heating, or when required to meet
for ventilation requirements. This includes turning on the terminal fan as the first stage of heating
before the heating coil is activated. During heating for warmup or setback temperature control, the
controls must either operate the terminal fan and heating coil without primary air or reverse the
terminal damper logic and provide heating from the central air handler through primary air.
Supply Air Temperature Reset Controls (6.5.3.5)
Multiple-zone systems must have automatic supply air temperature reset. To meet this requirement,
the supply air temperature reset can be either in response to zone cooling demand or based on
outdoor air temperature. Systems with DDC to the zone can reset based on the zone that requires the
most cooling, i.e., the zone with the highest cooling-loop output. Where reset is required, the supply air
temperature must be reset by a minimum of 25% of the difference between the supply air temperature
and the room air temperature under design cooling conditions.
Supply air temperature reset increases fan energy but decreases both cooling energy and energy used
for reheat. For systems with either air or water economizers, the use of supply air reset controls shifts
more of the cooling load to the economizers.
Zones with relatively constant loads (such as interior zones and zones serving telecom rooms) must be
designed to operate at the fully reset (warmest) supply air temperature. Failure to do this will cause
these zones to be undercooled if the reset is indirectly controlled by the outdoor air temperature, or
the reset will never occur if it is controlled by the zone demand.
Example 6-III illustrates the requirements of Section 6.5.3.5.
There are three exceptions to the supply air temperature reset requirements:
1. Systems in Climate Zones 0A, 1A, 2A, and 3A are exempt. This is because these climates are hot and
humid, and reset would hamper the system’s ability to control humidity in the building. To
maintain a maximum dew-point temperature in a building, one must maintain a cooling-coil
temperature slightly below the desired dew-point temperature. This control is independent of the
cooling needs of the zones. Supply air temperature reset controls, if enabled, would be overridden
most hours by the dew-point controls.
2. Systems whose design prevents reheating, recooling, or mixing of heated and cooled supply air do
not have to meet this requirement. If the system is designed to prevent reheat and recooling, there
is little energy to save by resetting the supply air temperature.
3. Systems with reheat in which 75% or more of the reheat energy is from site-recovered or site solar
sources are exempt. The logic of this requirement is that the reheat is mostly free. By using a lower
supply air temperature, one could save fan energy on the supply with a lower penalty for reheat, as
the majority of the reheat energy is recovered or from solar.
Fractional Horsepower (Kilowatt) Fan Motors (6.5.3.6)
The efficiency of motor sizes 1 hp (0.75 kW) and greater is regulated by Section 10 of Standard 90.1.
The efficiency of motors installed in packaged equipment rated as a unit is included in the equipment
efficiency rating in Section 6.4.12. This section regulates the efficiency of motors used in fan
applications that are 1/12 hp (62.1 W) or greater but less than 1 hp (0.75 kW). This section states that
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motors in the specified size range must either be electronically commutated motors (ECMs) or
alternatively be at least 70% efficient when rated in accordance with DOE 10 CFR 431.
Regardless of motor type, direct-drive fans must be capable of electronically adjusting the motor speed
(locally or remotely). Belt-driven fans are not required to have motor speed control but must use
sheave adjustments for setting the fan speed. The intent of speed control and sheave adjustment is to
provide a means of reducing the airflow during balancing without adding static pressure to the fan.
Using speed control rather than dampers for balancing reduces pressure on the fan and reduces the
fan power required.
While using speed control for balancing is a requirement of Section 6.5.3.6, it should be noted that
standard balancing practices using airflow restriction devices (dampers) is ineffective for controlling
airflow of ECM fans. The internal electronic controls of the ECMs typically respond to added resistance
by increasing the fan speed to maintain the programmed airflow. This not only fails to balance the
system but also wastes energy.
Where a fan motor runs only for heating purposes and is located in the airstream being supplied to the
heated space, the fan is exempt from the requirements of this section. In this application, heat would be
provided to the space. Any inefficiency of the fan motor is turned into heat and delivered to the space
by the air; thus, the inefficiency is not wasted but is used to reduce the heating required of the heating
coil. For example, a series fan-powered VAV box would require fan speed control because the fan runs
continuously for heating and cooling. A parallel fan-powered VAV box would not require speed control
because the fan motor only runs during heating. In cooling applications, the heat produced by the
motor must be offset by additional cooling.
Example 6-III. Supply Air Temperature Reset
Corresponding section: Supply Air Temperature Reset Controls (6.5.3.5)
Q
A VAV system with a design supply air temperature of 55°F (13°C) at design serves zones that have a
75°F (24°C) cooling set point. What supply air temperature control is required for this system to meet
the requirements of Section 6.5.3.5?
A
As this air-handling unit serves multiple zones, the supply air temperature must be reset to a minimum
of 5°F (2.8°C); this is 25% of 75°F – 55°F (25% of 24°C – 13°C). The supply temperature reset can be
reset based on zone cooling demand, outdoor air temperature, or a combination of the two.
Interior zones and zones with constant loads that are served by this system must be designed for
airflow at the warmest supply air temperature. If this system is designed for reset from 55°F to 60°F
(13°C to 16°C), these zones must be designed to satisfy their loads at the 60°F (16°C) temperature.
 Ventilation Design (6.5.3.7)
The HVAC systems in a building must be designed to supply the minimum outdoor air ventilation
required by the applicable ventilation standard, such as Standard 62.1, Standard 170, or other
applicable code or standard. Where different codes or standards provide different ventilation rates for
a given space, the largest value must be used.
The ventilation system must comply with one of three requirements. They must be designed to either
a) provide no more than 135% of the required minimum outdoor air rate;
b) provide dampers, ductwork, and controls that allow the system to be returned to supplying no
more than the required minimum outdoor air rate with a single set-point adjustment; or
c) include exhaust air energy recovery complying with Section 6.5.6.1.
Option (a) is given because LEED provides credit for buildings that supply 130% of the minimum
required ventilation rate. Option (b) allows increased design outdoor air, if desired, to improve indoor
air quality but to allow operators to reduce outdoor air rates to the minimum in the future without
significant cost to modify the system.
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Hydronic System Design and Control (6.5.4)
Boiler Turndown (6.5.4.1)
Section 6.5.4.1 specifies minimum boiler system turndown. A boiler system is any combination of one
or more boilers and their piping and controls that work together to supply heating water or steam for
delivery to remote heating devices. A single-input boiler is a boiler that is only able to fire at a single
rate and provides its full output capacity or does not fire at all. A modulating boiler operates by
changing its firing rate between its full rated capacity and its minimum turndown firing rate in
response to a varying temperature or heating load. When heating demand is below the minimum
turndown, these boilers cycle between not firing and the minimum turndown capacity. To reduce
boiler cycling, this section requires boiler systems with 1,000,000 Btu/h (293 kW) or greater input to
meet the minimum turndown requirements of Table 6.5.4.1. The turndown requirements of the table
may be met through the use of multiple staged single-input boilers, one or more modulating boilers, or
any combination thereof that results in the required turndown. For example, a 10,000,000 Btu/h
(293 kW) boiler system could meet the requirement with any of the following solutions:
• A single modulating boiler with 4 to 1 turndown
• Two 5,000,000 Btu/h (1465 kW) modulating boilers with 2 to 1 turndown and with the necessary
piping and controls to provide a system with 4 to 1 turndown
• Four 2,500,000 Btu/h (733 kW) single-input stage boilers with the necessary piping and controls to
provide a system with 4 to 1 turndown
TABLE 6.5.4.1 BOILER TURNDOWN
Boiler System Design Input
≥1,000,000 Btu/h (293 kW)
and
≤5,000,000 Btu/h (1465 kW)
>5,000,000 Btu/h (1465 kW)
and
≤10,000,000 Btu/h (2931 kW)
>10,000,000 Btu/h (2931 kW)
Minimum
Turndown Ratio
3 to 1
4 to 1
5 to 1
Other combinations exist and may be appropriate for a given boiler system, provided the minimum
turndown requirement is met. Consideration should be given to controllability when choosing multiple
boiler systems.
In addition to meeting the turndown requirements of this section, all boilers must meet the minimum
equipment efficiency requirements of Section 6.5.
Hydronic Variable Flow Systems (6.5.4.2)
The variable flow requirements of Section 6.5.4.2 apply to chilled-water and hot-water distribution
systems. The standard requires that pumping systems with three or more modulating or two-position
controls must be designed for variable flow. The system must be able to operate at 25% of design flow
or less, unless a higher flow is required for proper operation of the heating or cooling equipment. This
means that two-way rather than three-way control valves must be used for most of the control valves
in a system.
Individual chilled-water pumps serving variable flow systems where the nameplate power of the
motor or combined parallel motors is at least the power shown in Table 6.5.4.2 must have controls or
devices such as variable-speed drives (VSDs) that will result in pump motor demand of no more than
30% of design power at 50% of design water flow. Table 6.5.4.2 has separate requirements for chilledwater pumps and hot-water pumps with size limit varying by climate zone. The threshold for VSDs is
more stringent for chilled-water systems than for hot-water systems because not only do VSDs reduce
pump energy use, this reduction also reduces chiller load and energy use. The same is not true for hotwater pumps where pump energy reduces boiler load; it is simply a form of electric resistance heat.
VSDs on hot-water pumps are only cost effective due to the lower cost per unit energy of fossil fuel
(natural gas) as compared to electricity.
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The controls or devices must be controlled to maintain a desired flow or to maintain a minimum
required differential pressure. In the latter case, differential pressure must be measured at or near the
most remote heat exchanger or the heat exchanger requiring the greatest differential pressure. This
remote location will ensure that the differential set point is as low as possible; as with fans (see Figure
6-AA), the lower the set point, the greater the pump energy savings.
The differential pressure (DP) set point for the pump must not exceed 110% of that calculated to be
required for the critical circuit. Furthermore, systems with DDC at the coils must use demand-based
reset to reduce the pump operating pressure to the minimum required to keep at least one valve at or
near 100% open.
An exception to Section 6.5.4.2 specifies that DP set point reset is not required where valve position is
used to comply with Section 6.5.4.3 Chilled- and Hot-Water Temperature Reset Controls. But Section
6.5.4.3 includes an exception if valve position is used to reset DP set point. So either or both of these
sections must be met at the designer’s option. The reason for this is that resetting water temperatures
improves primary equipment efficiency while resetting DP set point reduces pump energy, and the
most efficient option of the two is not the same for all applications. Simulations indicate the following
in general:
•
•
For chilled-water systems,
o resetting water temperature is more efficient than resetting DP set point for variable-speed
chillers, because their efficiency improves significantly as lift is reduced; and
o resetting DP set point is more efficient than resetting water temperatures for constant-speed
chillers.
For hot-water systems,
o resetting water temperature is more efficient than resetting DP set point for condensing
boilers, because their efficiency improves significantly as return water temperature is
reduced; and
o resetting DP set point is more efficient than resetting water temperatures for noncondensing
boilers.
Additional exceptions specify several applications where variable flow control is not required:
• VSDs are not required on heating water pumps where more than 50% of the annual heat is
provided by electric boilers. As noted above, pump energy in hot-water systems is simply a form of
electric heat, so VSDs do not reduce energy costs on systems with electric boilers (in fact, they
increase costs slightly due to the VSD losses).
• Variable flow is not required on primary pumps in primary-secondary systems. While not required,
VSDs on primary chilled-water pumps, controlled to match the secondary flow down to the
minimum evaporator flow, can be cost effective.
• Variable flow is not required on coil pumps provided for freeze protection.
• Variable flow is not required on heat recovery coil runaround loops.
For chilled-water pumps with motors larger than 5 hp (3.7 kW), the pumps must be controlled by a
system that reduces pump energy by 70% at 50% flow. This requirement is often met with VSDs. It can
also be met with two-speed motors, although VSDs are likely to be less expensive and offer better
control. Other technologies may be used if data are provided to the authority having jurisdiction
demonstrating the required power turndown. For all other pumps (including hot water and condenser
water), the standard does not require any specific type of pump unloading control. Pumps that simply
ride their pump curves—those that are not controlled at all—will still usually use less energy at low
flows than at design flow. However, the higher pressures that occur at low flow may exceed control
valve differential pressure ratings and cause flow rates to exceed those desired. Energy use can be
reduced and differential pressures can be better controlled by using multispeed motors, staged pumps
or, ideally, VSDs.
Some variable flow systems, such as chilled-water systems and some hot-water systems, require that a
constant flow rate, or at least a minimum flow rate, be maintained through the primary
cooling/heating equipment (chiller, boiler). In this case, a primary-secondary system (Figure 6-BB) is
Standard 90.1 User’s Manual
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commonly used. Improvements in chiller controls, which are now more tolerant of variable chiller
flow, also allow the use of primary-only variable flow chilled-water plants, as shown in Figure 6-CC. .
The conventional primary-secondary system still offers significant advantages in control simplicity but
costs more to install and operate compared to a primary-only system with VSDs.
FIGURE 6-BB. PRIMARY-SECONDARY CHILLER PLANT
Corresponding section: Hydronic Variable Flow Systems (6.5.4.2)
FIGURE 6-CC. PRIMARY-ONLY CHILLER PLANT
Corresponding section: Hydronic Variable Flow Systems (6.5.4.2)
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Example 6-JJJ. Variable Flow Requirement, Multichiller Plants
Cha p t e r 6 | H VAC Sys t e ms
Corresponding section: Hydronic Variable Flow Systems (6.5.4.2)
Q
A chiller plant has two chillers piped in parallel with primary-only pumping. Each chiller is sized for
50% of the load, and each has a minimum flow rate that is 50% of the design flow rate through the
chiller. What is the required minimum flow rate for the system?
A
The pumps must be able to provide a minimum flow that does not exceed the minimum flow required
by the chillers. The design flow is the overall system flow, not the flow through each individual chiller.
In this case, 25% of the design flow is sufficient to keep one chiller online. The system could allow the
chillers to be staged with the load so that one chiller could remain online at 50% of design system flow.
The system must be designed for variable flow.
Example 6-KKK. Variable Flow Requirement, Hydronic System
Corresponding section: Hydronic Variable Flow Systems (6.5.4.2)
Q
A hot-water system has two-way valves at most coils, but occasionally three-way valves are provided
at the ends of branches to ensure flow through them. Does this design comply with the standard?
A
Yes, provided the total flow through three-way valves does not exceed 25% of design flow. While these
end-of-line valves are allowed, they are not usually required except perhaps in very large campus
systems. Water piping is generally designed for water velocities that are high enough so that the time it
takes for chilled or hot water to leave the plant and reach the control valve will be seconds or minutes,
a small enough time that the system will not be starved and no discomfort will result. To minimize
energy use in variable flow systems, limit the use of three-way valves to one or two to prevent pump
dead heading.
Chiller and Boiler Isolation (6.5.4.3)
For plants with multiple chillers or boilers, Section 6.5.4.3 requires flow through chillers or boilers
piped in parallel to be stopped when a chiller or boiler is off. Section 6.5.4.3.1 covers chiller plants, and
Section 6.5.4.3.2 covers boiler plants. This can be accomplished in one of two ways:
•
With a pump dedicated to and operated concurrently with each chiller or boiler, as shown in
the upper part of Figure 6-DD for chilled-water pumps serving chillers. The check valves on
the pumps prevent flow through an inactive chiller.
• With headered parallel pumps, automatic isolation valves can be installed on each chiller or
boiler and controlled to close when the chiller or boiler is inactive, as shown in the lower part
of Figure 6-DD for chilled-water pumps serving chillers.
When constant-volume pumping is required to meet equipment minimum flow requirements, the
number of primary chilled-water pumps must be at least equal to the number of chillers. The pumps
should be sized such that staging the pumps matches the staging of the chillers. For example, if you
have two chillers of unequal size, one 400 tons (1407 kW) and one 200 tons (703 kW), a potential
constant-volume pump configuration would be three equally sized constant-volume pumps. One pump
would stage with the 200 ton (703 kW) chiller while two pumps would stage with the 400 ton (1407
kW) compressor. This configuration satisfies the requirements of this section but also simplifies
maintenance.
With respect to Section 6.5.4.3.1, chillers piped in series for the purpose of increased temperature
differential are considered one chiller.
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FIGURE 6-DD. PUMPING ARRANGEMENTS
Corresponding section: Chiller and Boiler Isolation (6.5.4.3)
Chilled- and Hot-Water Temperature Reset Controls (6.5.4.4)
Resetting primary chilled-water or hot-water temperatures at part load improves the efficiency of the
primary equipment and reduces energy losses through piping. Section 6.5.4.4 therefore requires that
chilled- and hot-water systems with a design capacity exceeding 300,000 Btu/h (88 kW) supplying
chilled water or hot water to comfort conditioning systems must include controls that automatically
reset supply water temperatures upward (for cooling systems) or downward (for heating systems) at
low loads.
Reset must be based on system load or outdoor air temperature. The following strategies comply with
the hydronic supply water temperature reset requirement:
• Using actual system demand (that is, the cooling or heating coil that requires the coldest water
for cooling systems or warmest water for heating systems). In other words, supply water
temperature is reset so that the coil control valve that is the farthest open is maintained nearly
wide open. This strategy is both the most energy efficient and the most reliable at ensuring no coil
is starved. If valves are controlled by a DDC system, this method is required.
• Using building load indicators such as return water temperature. This signal should be used
with caution, however, as it provides only an indication of average system requirements. For
instance, if one coil is at near design conditions while all others are at low load, this strategy would
starve the first coil, and comfort levels in the space it served would not be maintained. This strategy
also does not work well if coils are used for dehumidification, because colder supply water may be
required even at low loads. If valves are controlled by a DDC system, this method is not allowed to
be used; the controls must use actual system demand.
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• Using outdoor air temperature. This strategy works well for heating systems because space loads
are almost proportional to the difference between inside and outside temperatures. Aggressive
reset using this strategy usually will not be reliable for cooling systems because the majority of
space cooling loads are independent of outdoor air temperature. If valves are controlled by a DDC
system, this method is not allowed to be used; the controls must use actual system demand.
The standard does not address how much reset must occur. This is left up to the designer (See Example
6-LLL).
The following exceptions to this section apply:
• Where chilled-water supply is already cold, such as chilled water supplied from a district cooling or
thermal energy storage system, such that blending would be required to achieve the reset chilledwater supply temperature. In this situation, resetting the supply temperature will increase
pumping energy with no savings in chiller energy.
• Where a specific temperature is required for a process.
• Water temperature reset is not required where valve position is used to comply with Section
6.5.4.2.
Example 6-LLL. Reset Requirements, Boiler Reset on Outdoor Air
Corresponding section: Chilled- and Hot-Water Temperature Reset Controls (6.5.4.4)
Q
A gas-fired boiler designed for 180°F (82°C) water temperature under peak conditions includes a
controller that resets the boiler hot-water set point proportional to outdoor air temperature. In order
to prevent flue gas condensation on the tubes and flue, hot-water temperatures may not be reset as
aggressively as they might be if a mixing valve were used. Does this design comply?
A
Yes. The standard does not establish how much reset is required. To prevent flue gas condensation,
entering water temperatures should not fall below about 130°F (54°C) or so, depending on the boiler.
The reset schedule for this system might be to provide 180°F (82°C) water during cold weather and
150°F (66°C) water during mild weather, allowing for a 20°F (11°C) temperature drop to maintain
130°F (54°C) boiler entering water temperature. Designs that use a mixing valve can provide even
lower supply water temperatures as delivered to the coils. However, these systems should also include
a boiler-reset controller to improve boiler efficiency by reducing stack and casing losses.
Hydronic (Water-Loop) Heat Pumps and Water-Cooled Unitary Air Conditioners (6.5.4.5)
Variable Flow (6.5.4.5.1)
All water-cooled heat pumps and water-cooled air-conditioning units are required to have twoposition valves that are interlocked to shut off water flow to the unit when the compressor is off. All
condenser water systems serving these heat pumps or air-conditioning units must be variable flow.
There is no exception based on heat-pump or air-conditioning unit size.
There is an exception provided for those heat pumps or air-conditioning units that employ fluid
economizers.
Pump Controls (6.5.4.5.2)
Where the total pump system power exceeds 5 hp (3.7 kW), the pumps must have VSDs (or similar
devices) that will result in pump motor demand of no more than 30% of design power at 50% of
design water flow. As described in Section 6.5.4.2 for chilled-water pumps, this performance
requirement is based on VSDs.
Pipe Sizing (6.5.4.6)
Pipe friction increases pump energy and usually also increases the cooling or heat-rejection load. The
pipe sizing requirements of Section 6.5.4.6 must be used for sizing chilled-water and condenser water
piping. Each piping segment in these systems is to be sized per the requirements in Table 6-c (Table
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6.5.4.6 in the standard). This table presents the maximum allowable flow per pipe section as a function
of the following criteria:
• Pipe size. This is indicated in the left-most column of the table. The table covers pipe sizes 2.5 to 24
in. (75 to 610 mm). Maximum flow-rate values are provided for pipe sizes 2.5 to 12 in. (75 to 315
mm). Maximum velocities are provided for pipe sizes 14 to 24 in. (315 to 610 mm).
• Annual hours of operation. This is the expected annual hours of system operation. For typical
office buildings (especially those with economizers), you would normally use the left-most column
for ≤2000 h/yr. For data centers and hospitals, use the right-most column for >4400 h/yr. These
hours are by branch. If an office building has a data center, the piping to and from the data center
would use the right-most column, and the rest of the office building piping would use the column
appropriate for its intended use.
• System flow and control. Systems that are designed to meet the requirements of Section 6.5.4.2
should use the columns marked “Variable Flow/Variable Speed.” All other systems should use the
columns marked “Other.”
The values in the body of Table 6.5.4.6 are the maximum allowable flow rates per section of pipe.
These values are based solely on energy conservation economics. Other criteria, such as acoustics or
water hammering risks, may require larger pipe sizes to further reduce fluid velocities.
TABLE 6-C. PIPING SYSTEM DESIGN MAXIMUM FLOW RATE IN GPM
Corresponding section: Pipe Sizing (6.5.4.6)
(This is Table 6.5.4.6 in the standard.)
Table 6.5.4.6 Piping System Design Maximum Flow Rate in GPM (I-P)
Operating
Hours/Year
Nominal Pipe
Size, in.
2.5
3
4
5
6
8
10
12
Maximum
Velocity for Pipes
14 to 24 in. Size
Operating
Hours/Year
Nominal Pipe
Size, mm
75
90
110
140
160
225
280
315
Maximum
Velocity for Pipes
355–610 mm
Size
≤2000 Hours/Year
Other
120
180
350
410
740
1200
1800
2500
8.5 fps
Variable Flow/
Variable Speed
180
270
530
620
1100
1800
2700
3800
13.0 fps
>2000 and ≤4400 Hours/Year
Other
85
140
260
310
570
900
1300
1900
6.5 fps
9.5 fps
Other
68
110
210
250
440
700
1000
1500
5.0 fps
Table 6.5.4.6 Piping System Design Maximum Flow Rate in L/s (SI)
≤2000 Hours/Year
Other
8
1
22
26
47
76
114
158
2.6 m/s
Variable Flow/
Variable Speed
11
17
33
39
69
114
170
240
4.0 m/s
>2000 and ≤4400 Hours/Year
Other
5
9
16
20
36
57
82
120
2.0 m/s
Exceptions to 6.5.4.6
There are two exceptions to the pipe sizing requirement:
240
Variable Flow/
Variable Speed
130
210
400
470
860
1400
2000
2900
>4400 and ≤8760 Hours/Year
Variable Flow/
Variable Speed
8
13
25
30
54
88
126
183
2.9 m/s
Variable Flow/
Variable Speed
110
170
320
370
680
1100
1600
2300
7.5 fps
>4400 Hours/Year
Other
4
7
13
16
28
44
63
95
1.5 m/s
Variable Flow/
Variable Speed
7
11
20
23
43
69
101
145
2.3 m/s
Standard 90.1 User’s Manual
1.
2.
Cha p t e r 6 | H VAC Sys t e ms
The flow rates in Table 6.5.4.6 can be exceeded in sections of pipe that are not in the critical circuit
during 30% or more of the system operating hours. The critical circuit is the run of pipe that
determines the pump operating speed, as it has the highest pressure demand. Care must be taken
when reducing pipe sizes so that the shorter runs do not become the critical circuit.
The second exception is for piping systems that have a lower total pressure drop (including
friction loss and fitting coefficients) than steel pipe (on which the values in the table were based).
These pipes can be designed to carry larger flows, provided the total pressure loss does not exceed
that for steel pipe at the maximum flow rate in the table for the nominal pipe size. Calculations
may be required by the authority having jurisdiction.
 Chilled-Water Coil Selection (6.5.4.7)
The standard requires chilled-water coils used in VAV systems to be selected to provide a 15°F
difference between the entering and leaving water temperature with a minimum 57°F leaving water
temperature at design conditions. This requirement will typically result in larger coils but smaller
pumps and piping, generally resulting in lower first costs. The requirements will generally improve
chiller efficiency and reduce pumping energy, although these savings may be partially offset by fan
power increases.
These requirements do not apply to systems with constant air volume or with passive coils (those
without mechanically supplied airflow, or coils in individual fan cooling units with a design supply
airflow of 5000 cfm or less.
The requirement also does not apply if the coil has an air-side pressure drop of more than 0.7 in. of
water when rated at 500 fpm face velocity and dry conditions (no condensation). This exception is
provided to ensure that the coil cleanability requirement of Standard 62.1 can be met.
Further exceptions include coils selected at the maximum temperature difference allowed by the
chiller, coils with a design entering chilled-water temperature of 50°F or higher, and coils with a design
entering air temperature of 65°F or less.
Heat-Rejection Equipment (6.5.5)
General (6.5.5.1)
This section applies to heat-rejection equipment used in cooling systems such as air-cooled
condensers, dry coolers, open-circuit cooling towers, closed-circuit cooling towers, and evaporative
condensers. It does not apply to heat-rejection devices included in the energy ratings for packaged
equipment listed in Tables 6.8.1-1 through 6.8.1-4 of the standard. This would include, but is not
limited to, DX packaged unitary equipment, split-system DX air conditioners, and unitary air-cooled
chillers.
Fan Speed Control (6.5.5.2)
General Speed Control Requirements (6.5.5.2.1)
The fan system on a heat-rejection device powered by a motor or array of motors with connected
power of 5 hp (3.7 kW) or larger, when the service factor of the motor is included, must be able to
operate at variable speed to reduce fan power and control the leaving fluid temperature or condensing
temperature/pressure of the heat-rejection device. Typically this will mean the use of a variable-speed
drive, although other methods are acceptable if they meet the requirement for power reduction.
Figure 6-EE shows the performance of a cooling tower with single-speed, variable-speed, and two
types of two-speed motors. It is clear that the multispeed motors reduce energy costs significantly over
single-speed fans. The best control is achieved with a VSD, but two-speed motors come very close.
The following exceptions to Section 6.5.5.2 apply:
1. Condenser fans serving multiple refrigerant or fluid cooling circuits.
2. Condenser fans serving flooded condensers.
Fan Speed Control Requirements for Multicell Heat-Rejection Equipment (6.5.5.2.2)
Heat-rejection equipment that is composed of multiple cells typically has at least one fan per cell. This
section describes how these fans are to operate when they are equipped with variable-speed controls,
Standard 90.1 User’s Manual
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which include two-speed motors and VSDs. Multiple-cell heat-rejection equipment typically maintains
uniform fluid flow through all cells when the fluid flow rate is within the manufacturer’s recommended
range. Optimal airflow must be maintained across the whole heat-rejection surface area regardless of
load. During periods of low load, the heat-rejection equipment must uniformly reduce airflow across
the entire heat-rejection surface. This allows the maximum amount of heat transfer surface area to be
utilized, significantly reducing the fan energy required as compared to fan staging. The speed of all fans
must be adjusted as required in unison to satisfy the instantaneous heat-rejection load. The minimum
controlled fan speed must not be less than the minimum speed recommended by the heat-rejection
equipment manufacturer for the fan drive system. The fan speed requirement of this section cannot be
satisfied by staging the fans, as this does not maintain uniform airflow across the heat-rejection
surfaces. Note that this requirement applies to all heat-rejection equipment, including dry coolers, aircooled condensers, open- and closed-circuit cooling towers, and evaporative condensers.
Limits on Centrifugal Fan Open-Circuit Cooling Towers (6.5.5.3)
Centrifugal fan open-circuit cooling towers with a combined rating capacity of 1100 gpm (70 L/s) or
greater at the rating conditions of 95°F (35°C) condenser water return, 85°F (29°C) condenser water
supply, and 75°F (24°C) wet-bulb temperatures must meet or exceed the minimum efficiency
requirements for axial fan open-circuit cooling towers as listed in Table 6.8.1-7. Note that this
limitation does not apply to centrifugal fan closed-circuit cooling towers.
FIGURE 6-EE. COOLING TOWER FAN CONTROL PERFORMANCE
Corresponding section: Fan Speed Control (6.5.5.2)
An exception is provided for towers where either the inlet or the discharge is ducted or for towers that
require external sound attenuation. In these applications, the centrifugal fan open-circuit cooling
towers must simply meet the minimum efficiency requirements for centrifugal fan open-circuit cooling
towers as listed in Table 6.8.1-7.
For the same duty, centrifugal fan cooling towers use about twice the energy as axial (or propeller) fan
towers. This is illustrated by the relative minimum efficiency requirements in Table 6.8.1-7: 20
gpm/hp (1.7 L/s∙kW) for centrifugal fan open-circuit cooling towers and 40.2 gpm/hp (3.4 L/s∙kW) for
axial fan open-circuit cooling towers at the same rating conditions. In general, designers should use
axial fan cooling towers when possible. Centrifugal fan cooling towers will likely be required where the
fan will be subjected to additional external static pressure, as axial fans are unable to handle as much
external static. Centrifugal fan open-circuit cooling towers are also used in acoustically sensitive
locations and where a lower tower height is desired for architectural reasons. As an alternative to
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centrifugal fans in these applications, manufacturers of cooling towers offer models with lower-sound
axial fans and low-pressure-drop sound traps.
Open-Circuit Cooling Tower Flow Turndown (6.5.5.4)
Open-circuit cooling towers used for water-cooled chiller condenser heat rejection with either multiple
or variable-speed condenser water pumps must be designed so that all open-circuit cooling tower cells
can be run in parallel with the larger of the flow that is produced by the smallest pump at its minimum
expected flow rate or 50% of the design flow for the cell. This may require the use of weir dams on
open-circuit cooling towers with gravity-flow water distribution systems or special nozzle
arrangements for pressurized water distribution systems. There is no exception for equipment whose
manufacturer does not recommend flow below these rates. Note that this requirement applies to opencircuit cooling towers only as the flow in the condenser loop using a closed-circuit cooling tower is
variable by nature of the closed loop.
Energy Recovery (6.5.6)
Exhaust Air Energy Recovery (6.5.6.1)
The standard requires exhaust air energy recovery on individual fan systems that meet or exceed the
criteria in Table 6-E (Tables 6.5.6.1-1 and 6.5.6.1-2 of the standard). The appropriate table must be
used for the number of hours the ventilation system will operate per year (occupancy). For systems
that require ventilation more than 8000 h/yr (continuous occupancy), heat recovery is required more
frequently. The heat recovery requirements of each table are based on climate zone, percentage of
outdoor air at design condition, and design supply airflow rate.
Equipment used to meet this requirement includes plate heat exchangers (plastic and metal), heat
pipes, run-around coils, and enthalpy wheels, which all must meet the recovery effectiveness discussed
below.
The standard defines exhaust air energy recovery as the process of exchanging heat (sensible and/or
latent) between the exhaust and outdoor airstreams. This reduces energy use in the following manner:
ℎ𝑂𝑂𝑂𝑂 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 −ℎ𝑂𝑂𝑂𝑂 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙
ℎ𝑂𝑂𝑂𝑂 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 −ℎ𝐸𝐸
𝐸𝐸
𝐸𝐸 =
Standard 90.1 User’s Manual
𝑙𝑙𝑙𝑙𝑙𝑙
• During periods of heating, the exhaust air can preheat the cool outdoor air through sensible (dry)
exchange.
• During periods of cooling, the exhaust air can precool the hot outdoor air through sensible (dry)
exchange.
• During periods of cooling, dry exhaust air can be used to dehumidify moist outdoor air through
latent exchange.
• During periods of heating, exhaust air can be used to humidify dry outdoor air through latent
exchange.
Exhaust air energy recovery requirements apply to systems with as low as 10% outdoor air when
operating fewer than 8000 h/yr in cold climates (Climate Zones 7 and 8), moist climates (Climate
Zones 1A, 2A, 3A, 4A, 5A, and 6A), and Climate Zone 6B, which is cold in the winter. Where the
HVAC&R system is required to operate 8000 h/yr or more, heat recovery is required in more climate
zones and at lower outdoor air percentages. For systems that operate more than 8000 h/yr, every
climate zone has a threshold for the inclusion of heat recovery, with the exception of Climate Zone 3C.
Typically office buildings will not meet the minimum requirements for percentage of outdoor air or
hours of runtime per year. However, systems in buildings that require higher volumes of runtime per
year, such as schools, hospitals, and laboratories, will likely require energy recovery in cold or humid
climates despite operating fewer than 8000 h/yr.
Where energy recovery is required per Table 6-E, the exhaust air energy recovery system must have a
minimum 50% enthalpy recovery ratio. This recovery effectiveness must be demonstrated in either
the heating or cooling mode. The enthalpy recovery ratio (E) is defined as shown in Equation 6-G.
≥ 50%
Equation 6-G
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Cha p t e r 6 | H VAC Sys t e ms
where
hOA entering
=
the enthalpy of the outdoor air entering the exhaust air recovery system (Btu/lb∙dry
air or J/kg). Alternatively described as “Supply Air Entering,” “Entering Supply Air,”
and Station 1 or X1.
= the enthalpy of the outdoor air leaving the exhaust air recovery system (Btu/lb∙dry air
hOA leaving
or J/kg). Alternatively described as “Supply Air Leaving,” “Leaving Supply Air,” and
Station 2 or X2.
= the enthalpy of the exhaust air entering the recovery system (Btu/lb∙dry air or J/kg).
hEA
Alternatively described as “Return Air Entering,” “Entering Exhaust Air,” and Station 3
or X3.
When outdoor air load is sensible (heating only, no humidification, for example), the above equation is
still correct (the enthalpy differences between the airstreams will be composed of sensible heat only).
For simplicity in these cases, the designer may replace enthalpy with dry-bulb temperature to calculate
recovery effectiveness.
When energy recovery is applied, it is important to reduce the design loads on the system accordingly.
Heating and cooling equipment should be selected (downsized or right sized) based on the new design
loads with energy recovery.
TABLE 6-D. ENERGY RECOVERY REQUIREMENT
Corresponding section: Exhaust Air Energy Recovery (6.5.6.1)
(This is Tables 6.5.6.1-1 and 6.5.6.1-2 in the standard.)
Table 6.5.6.1-1 Energy Recovery Requirements for Ventilation Systems Operating < 8000 h/yr
Zone
3B, 3C, 4B, 4C, 5B
0B, 1B, 2B, 5C
6B
0A, 1A, 2A, 3A, 4A, 5A,
6A
7,8
≥10%
and
<20%
≥20%
and
<30%
≥28000
(13215)
≥26000
(12271)
≥4500
(2124)
≥26500
(12507)
≥16000
(7551)
≥4000
(1888)
≥10%
and
<20%
≥20%
and
<30%
NR
NR
NR
NR
% Outdoor Air at Full Design Airflow Rate
≥30%
≥40%
≥50%
≥60%
and
and
and
and
<40%
<50%
<60%
<70%
Design Supply Fan Airflow Rate, cfm (L/s)
NR
NR
NR
NR
NR
NR
≥26000
≥12000
(12271)
(5663)
≥11000
≥5500
≥4500
≥3500
(5191)
(2596)
(2124)
(1652)
≥5500
≥4500
≥3500
≥2000
(2596)
(2124)
(1652)
(944)
≥2500
≥1000
>140
>120
(1180)
(472)
≥70%
and
<80%
NR
≥5000
(2360)
≥2500
(1180)
≥1000
(472)
>100
NR
≥4000
(1888)
≥1500
(708)
>120
≥70%
and
<80%
≥80%
NR – Not required
Table 6.5.6.1-2 Energy Recovery Requirements for Ventilation Systems Operating ≥ 8000 h/yr
Zone
3C
0B, 1B, 2B, 3B, 4C, 5C
0A, 1A, 2A, 3A, 4B, 5B
4A, 5A, 6A, 6B, 7, 8
NR – Not required
NR
NR
≥2500
(1180)
>200
NR
≥19500
(9202)
≥2000
(944)
>130
% Outdoor Air at Full Design Airflow Rate
≥30%
≥40%
≥50%
≥60%
and
and
and
and
<40%
<50%
<60%
<70%
Design Supply Fan Airflow Rate, cfm (L/s)
NR
NR
NR
NR
≥9000
≥5000
≥4000
≥3000
(4247)
(2360)
(1888)
(1415)
≥1000
≥500
>140
>120
(472)
(236)
>100
>80
>70
>60
≥80%
NR
≥1500
(708)
>10
>50
>80
NR
>120
>80
>40
Exhaust air energy recovery systems must be installed with bypass or other controls to permit air
economizer operation where economizers are prescribed in Section 6.5.1.1. To meet this requirement,
face and bypass dampers should be provided on the heat recovery heat exchanger on the outdoor air.
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This keeps the energy recovery system from preheating the outdoor air when the economizer is
operating and also eliminates the energy recovery pressure drop during the economizer operation.
Exceptions to 6.5.6.1
There are a number of exceptions to the requirement for exhaust air energy recovery systems:
1. Laboratory systems that meet the requirements of fume hoods in Section 6.5.7.3. This includes
applications of variable-volume fume exhaust systems that reduce design outdoor airflow to 50%
or less and direct (auxiliary) makeup air systems that provide 75% or more of the exhaust air with
tempered air.
2. Systems serving buildings that are heated only and controlled to 60°F (16°C) or less (typically
warehouses).
3. Where more than 60% of the outdoor air heating energy is provided by site-recovered or site solar
energy. Strategies to do this are discussed in FYI, Sources of Site Solar and Site-Recovered Energy.
4 Heating energy recovery systems in Climate Zones 0, 1, and 2 (see Appendix D of the standard for
climate data).
5. Cooling energy recovery systems in Climate Zones 3C, 4C, 5B, 5C, 6B, 7, and 8 (see Appendix D of
the standard for climate data).
6. Where the sum of the airflow rates exhausted and relieved within 20 ft. of each other is less than
75% of the design outdoor airflow, excluding exhaust air that is
a. used for another energy recovery system,
b. not allowed by ASHRAE Standard 170 for use in energy recovery systems with leakage
potential, or
c. of Class 4 as defined in ASHRAE Standard 62.1.
This exception is provided to account for the impracticality of recovering heat from multiple,
widely separated exhaust sources for a single outdoor air intake. An example could be a high-rise
residential facility with a single pressurized outdoor airshaft but half a dozen toilet and kitchen
exhaust risers.
7. Systems requiring dehumidification that employ energy recovery in series with the cooling coil.
This exception recognizes the energy savings inherent in series energy recovery when employed
in dehumidification systems.
8. Systems expected to operate fewer than 20 hours per week at the outdoor air percentage covered
by Table 6.5.6.1-1.
Heat Recovery for Service Water Heating (6.5.6.2)
The standard requires heat recovery from the condenser side of water-cooled systems for preheating
service hot water in large 24-hour facilities. Heat recovery is most effective where the water heating
loads are large and well distributed throughout the day. Typical applications are hotels, dormitories,
mixed-use retail/residential projects, commercial kitchens, and institutions such as prisons and
hospitals. A facility must comply with this heat recovery requirement if all of the following are true:
a. The facility operates 24 hours a day.
b. The total installed heat-rejection capacity of the water-cooled system exceeds 6,000,000 Btu/h
(1758 kW). This equates to roughly 400 tons of electric chiller capacity or 250 to 330 tons of gasfired or thermally fired chiller capacity.
c. The design service water heating load exceeds 1,000,000 Btu/h (293 kW). This equates to a 1000
bed nursing home (at 1.5 gallons per hour [6.7 litres per hour] per bed) or a 75 room hotel.
Where required, the heat recovery system must meet the smaller of two conditions:
a. Sixty percent of the peak heat-rejection load at design conditions. For example, if the chiller plant
were designed to reject 2,000,000 Btu/h (586 kW) at design conditions, the heat recovery system
must be designed to recover 1,200,000 Btu/h (352 kW).
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b.
Preheating of the peak service hot-water draw to 85°F (29°C). This number was selected to be low
enough that single-stage chillers with heat exchangers on the leaving condenser line could meet
the requirement.
There are two exceptions to the requirement of Section 6.5.6.2:
a. Facilities that employ condenser water heat recovery for space heating with a minimum 30%
recovery of the peak water-cooled condenser load at design conditions.
b. Facilities that provide 60% or more of their service water heating from site solar or site-recovered
sources. Examples include heat recovery from cogeneration, condensate subcooling, and solar
panels (see FYI, Sources of Site Solar and Site-Recovered Energy).
Heat recovery systems for water heating can be broadly split into two categories: those that recover
heat from condenser water and those that recover heat directly from the refrigerant. Both types of
system can provide service water temperatures up to 140°F (54°C). However, it may be more energy
efficient to recover condenser heat at temperatures in the 100°F to 110°F (38°C to 43°C) range,
supplemented by booster heaters to provide the desired domestic hot-water temperature, because the
higher the condensing temperature, the lower the chiller efficiency. Some health codes require that
both systems use double-wall heat exchangers to separate potable water from the refrigerant or the
condenser water.
Heat may also be recovered from condenser water systems by using a water-to-water heat pump that
lifts the heat from the condenser water loop and uses it to charge a storage tank (see Figure 6-FF).
These systems have COPs in the range of 4 to 6, depending on the temperatures of both the condenser
and the service hot-water loops. This design can be more efficient than direct condenser water heat
recovery because it allows the chillers to operate at cooler condenser water temperatures. The heat
pumps are placed in the loop upstream of the cooling tower and act as a first stage of heat rejection
when in operation.
Heat recovery systems that extract energy directly from the refrigerant include double-bundle chillers
(Figure 6-GG) and refrigerant desuperheaters (Figure 6-HH). Both of these operate on the same
principle: hot refrigerant gas on the way to the normal condenser is diverted through the auxiliary
water-heating condenser as a first stage of cooling. Refrigerant desuperheater kits are available with a
wide range of controls, capacities, and circuiting options. They can be used with refrigerated casework,
commercial freezers and refrigerators, DX air-conditioning units, and heat pumps.
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FIGURE 6-FF. SERVICE WATER HEATING WITH HEAT RECOVERY HEAT PUMP
Corresponding section: Heat Recovery for Service Water Heating (6.5.6.2)
FIGURE 6-GG. SERVICE WATER HEATING WITH DOUBLE-BUNDLE CHILLER
Corresponding section: Heat Recovery for Service Water Heating (6.5.6.2)
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FIGURE 6-HH. SERVICE WATER HEATING WITH REFRIGERANT DESUPERHEATER
Corresponding section: Heat Recovery for Service Water Heating (6.5.6.2)
FYI
Sources of Site Solar and Site-Recovered Energy
The energy recovery requirements in Sections 6.5.6.1 and 6.5.6.2 provide exceptions where 60% of the
air- or water-heating energy is provided from site-recovered or site solar energy. Three possible
sources are discussed here: cogeneration, solar, and subcooling of steam condensate.
Cogeneration. Cogeneration systems are generally only cost-effective in applications that have large
and rather constant hot-water or steam loads. Service hot-water systems for facilities with large pools,
such as hotels, health care facilities, and sports facilities, are potential candidates. When evaluating a
cogeneration system, carefully consider the following:
• Development of accurate hourly load profiles
• Cost of power conditioning and isolation of sensitive circuits
• Cost of maintenance
• Availability of coincident electrical loads
• Availability of utility excess power purchasing and their requirements for power conditioning
• Economics of other high-efficiency heat-generating alternatives
Solar. Solar heating is best suited to projects where large quantities of low-temperature hot water are
required, coupled with available space for collector arrays. Pools are an excellent application because
the required temperatures are low (permitting the use of low-cost and durable unglazed collectors),
the mass of water in the pool buffers the temperature swings, and freeze protection is accomplished by
draining the collectors back into the pool. Other applications where solar should be considered include
preheat of water for use in locker rooms, low-temperature process heating, and preheat of water for
commercial laundries.
Subcooling of Steam Condensate. In steam systems, a heat exchanger upstream of the condensate
receiver tank can be used for the dual purpose of heating service water and subcooling condensate to
prevent flashing. Without subcooling, a portion of the heat in the condensate will be lost in the form of
flash steam vented from the tank. By subcooling the steam, this energy is captured and put to good use.
As the demands for steam and hot water are not likely to coincide, a storage system is generally
recommended.
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Transfer Air (6.5.7.1)
The standard requires the use of transfer air for exhaust air makeup in most cases. The purpose is to
avoid supply air that requires increased outdoor air intake for exhaust makeup when return or relief
air from neighboring spaces can be used instead. The requirement limits the supply of conditioned air
to not exceed the largest of the supply flow required for space heating or space cooling, the required
ventilation rate, or the exhaust flow minus the available transfer air from conditioned spaces or
plenums on the same floor and within 15 ft and not in different smoke or fire compartments. Available
transfer air does not include air required to maintain pressurization and air that cannot be transferred
based on air class as defined by ASHRAE Standard 62.1.
There are multiple exceptions to 6.5.7.1:
1. Biosafety laboratories classified Level 3 or higher.
2. Vivarium spaces.
3. Spaces that are required by applicable codes and standards to be maintained at positive pressure
relative to adjacent spaces. For spaces taking this exception, any transferable air that is not directly
transferred shall be made available to the associated air-handling unit and shall be used whenever
economizer or other options do not save more energy.
4. Spaces where the demand for transfer air may exceed the available transfer airflow rate and where
the spaces have a required negative pressure relationship. For spaces taking this exception, any
transferable air that is not directly transferred shall be made available to the associated air-handling
unit and shall be used whenever economizer or other options do not save more energy.
Kitchen Exhaust Systems (6.5.7.2)
Two of the requirements of Section 6.5.7.2, rated hood airflows (Section 6.5.7.2.3) and demand
ventilation controls (Section 6.5.7.2.4), are triggered when the total kitchen/facility hood exhaust
exceeds 5000 cfm (2400 L/s). This threshold was determined to separate kitchens in small restaurants
from the national chains and larger sit-down restaurants.
Limitation on Makeup Air in Hoods (6.5.7.2.1)
The untempered makeup air in kitchen hoods is limited to ≤10% of the hood exhaust. Research has
shown that larger quantities of untempered makeup air injected in the hood actually decreases the
hood’s ability to capture the contaminants from the device under the hood.
Limitation on Hood Exhaust (6.5.7.2.2)
Where the total kitchen hood exhaust airflow is greater than 5000 cfm (2358 L/s), all of the hoods
must have an exhaust rate that complies with Table 6.5.7.2.2. Where a single hood is installed over
multiple appliances with different duty ratings, the maximum allowable flow rate for the hood or hood
section must not exceed the Table 6.5.7.2.2 values for the highest appliance duty rating under the hood
or hood section.
The standard refers to ASHRAE Standard 154 for definitions of hood type, appliance duty, and net
exhaust flow rate. To meet these requirements, the hoods have to be rated or “listed” by a
manufacturer. Field-fabricated hoods would not be allowed. Field research has shown that fieldfabricated hoods both use more air (which wastes energy) and are less reliable for containing
contaminants.
An exception is provided where at least 75% of the replacement air is transfer air that would
otherwise be exhausted.
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FIGURE 6-II. MAKEUP AIR EXAMPLE, PARTIAL TRANSFER FOR MAKEUP AIR
Corresponding section: Kitchen Exhaust Systems (6.5.7.2)
FIGURE 6-JJ. MAKEUP AIR EXAMPLE, FULL TRANSFER FOR MAKEUP AIR
Corresponding section: Kitchen Exhaust Systems (6.5.7.2)
FIGURE 6-KK. MAKEUP AIR EXAMPLE, NO TRANSFER FOR MAKEUP AIR
Corresponding section: Kitchen Exhaust Systems (6.5.7.2)
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TABLE 6-E. KITCHEN HOOD EXHAUST FLOW RATES
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Corresponding section: Limitation on Hood Exhaust (6.5.7.2.2)
(This is Table 6.5.7.2.2 in the standard.)
Table 6.5.7.2.2 Maximum Net Exhaust Flow Rate, cfm per Linear Foot of Hood Length (I-P)
Type of Hood
Wall-mounted canopy
Single island
Double island (per side)
Eyebrow
Backshelf/pass-over
Light Duty
Equipment
140
280
175
175
210
Medium Duty
Equipment
210
350
210
175
210
Heavy Duty
Equipment
280
420
280
Not allowed
280
Extra Heavy Duty
Equipment
385
490
385
Not allowed
Not allowed
Light Duty
Equipment
217
433
271
271
325
Medium Duty
Equipment
325
541
325
271
325
Heavy Duty
Equipment
433
650
433
Not allowed
433
Extra Heavy Duty
Equipment
596
758
596
Not allowed
Not allowed
Table 6.5.7.2.2 Maximum Net Exhaust Flow Rate, L/s per Linear Meter of Hood Length (SI)
Type of Hood
Wall-mounted canopy
Single island
Double island (per side)
Eyebrow
Backshelf/pass-over
Demand Ventilation Systems (6.5.7.2.3)
Where the total kitchen hood exhaust rate exceeds 5000 cfm (2400 L/s), the facility must provide one
of the following technologies:
a. Enough transfer air to make up ≥50% of the total kitchen hood exhaust
b. One or more demand ventilation systems on ≥75% of the total kitchen hood exhaust
c. Listed energy recovery devices with a sensible energy recovery ratio of ≥40% on ≥50% of the total
hood exhaust
Option 1—Transfer Air. The design in Figure 6-II. provides 60% of the hood exhaust in transfer air;
this complies with the transfer air requirement. The design in Figure 6-JJ also meets the requirement,
with 100% of the hood exhaust in transfer air.
Option 2—Demand Ventilation System. A demand ventilation system uses a light beam and a photo
detector to detect the presence of smoke. It also has a temperature sensor to detect heat. On detection
of smoke or heat, the exhaust fan speeds up. To comply with the standard, the demand ventilation
system employed must serve ≥75% of the total kitchen hood exhaust and be capable of reducing the
exhaust to a minimum of 50% design flow. To meet the requirement, multiple demand ventilation
systems can be employed in a single kitchen.
Option 3—Sensible Heat Recovery. The earlier Exhaust Air Energy Recovery (6.5.6.1) section
describes the calculation of sensible energy recovery ratio and the devices that can be used in this
design. The one specific additional requirement for compliance with Section 6.5.7.2.4 is that the device
must be listed for use in a hood exhaust.
Laboratory Exhaust Systems (6.5.7.3)
Buildings with laboratory fume exhaust systems having a total exhaust rate greater than 5000 cfm
(2400 L/s) must include at least one of the following features:
a. A combination of variable airflow (supply and exhaust) or heat recovery to precondition the
makeup air that exceeds 50% according to the following formula:
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𝐴𝐴 + 𝐵𝐵 × (𝐸𝐸 ⁄𝑀𝑀) ≥ 50%
b.
c.
Equation 6-H
where
A = percentage of exhaust and makeup airflow rates that can be reduced from the design
conditions
B = sensible energy recovery ratio, converted to a percentage. The sensible energy recovery
ratio is defined as the change in the dry-bulb temperature of the outdoor air supply
divided by the difference between the outdoor air and return air dry-bulb temperatures,
expressed as a percentage.
E = exhaust airflow rate through a heat recovery device at design conditions
M = makeup airflow rate of the system at design conditions
VAV laboratory exhaust and supply systems that comply with the following:
• For spaces with minimum airflow rates or special pressurization requirements, the airflow is
reduced to the minimum rates to comply with the codes or accreditation standards.
• Nonregulated zones reduce both their supply and exhaust to the larger of 50% of the zone
design airflow values or the minimum required to maintain pressurization requirements.
Direct makeup (auxiliary) air supply equal to at least 75% of the exhaust airflow rate, heated no
warmer than 2°F (1.1°C) below room set point, cooled to no cooler than 3°F (1.7°C) above room
set point, no humidification added, and no simultaneous heating and cooling used for
dehumidification control. Auxiliary supply systems can cause drafts at the hoods, reducing their
capture effectiveness, and they cannot maintain close humidity control in the area around the
hood. These systems have therefore fallen out of favor and have been replaced by VAV systems.
Radiant Heating Systems (6.5.8)
Heating Unenclosed Spaces (6.5.8.1)
This section requires that radiant heating must be used when heating is required for unenclosed
spaces. An exception is made for loading docks equipped with air curtains.
Heating Enclosed Spaces (6.5.8.2)
Radiant heating systems that are used as primary or supplemental enclosed-space heating must
conform to the standard’s governing provisions, including, but not limited to, the following:
• Radiant hydronic ceiling or floor panels (used for heating or cooling)
• Combination or hybrid systems incorporating radiant heating (or cooling) panels
• Radiant heating (or cooling) panels used in conjunction with other systems, such as VAV or thermal
storage systems
This section does not require that radiant systems be used for heating enclosed spaces.
Hot-Gas Bypass Limitation (6.5.9)
To limit energy waste, hot-gas bypass, and other evaporator pressure control systems, the standard
allows the use of only hot-gas bypass or other false-loading evaporator pressure control systems for
VAV and single-zone VAV systems designed with multiple steps of unloading or continuous capacity
modulation. The capacity of the hot-gas bypass is then limited to 15% of the cooling capacity for
systems 240,000 Btu/h (70 kW) or smaller and to 10% of the capacity for larger systems. Constantvolume systems may not use hot-gas bypass.
Door Switches (6.5.10)
Doors that open to the outdoors can be a significant source of energy consumption when the door is
propped or held open for an extended period of time. For this reason, the 2016 edition of the standard
requires doors, including doors with more than half glass, that open to the outdoors to be provided
with a means for the HVAC&R controls (a switch) to determine when the door is open or closed. The
HVAC controls use the door’s state to change the HVAC operation in the instance the door is held open.
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When a door to the outdoors has been open for five minutes or more, the HVAC&R controls must
respond by either disabling the space conditioning to the space or setting back the space temperature
set points in the space. If setback is used, the space temperature set point setback must be no greater
than 55°F (12.8°C) in heating and no less than 90°F (32.2°C) in cooling.
A combination of these methods may be used as well. A designer may choose to setback heating for
freeze protection but disable cooling.
If space temperature setback is used for cooling, the mechanical cooling need not be disabled if the
outdoor air temperature is below the space temperature set point. During this condition, there is little
or no energy savings associated with setting back or disabling mechanical cooling.
Exceptions to Section 6.5.10 include the following:
• Building entry doors with automatic closing devices. This includes both electrical and mechanical
door closers.
• Spaces without a thermostat. For example, a corridor with a door may be conditioned by a branch
duct from an adjacent space. The thermostat controlling the air to the corridor is in the adjacent
space and not in the corridor. Although some infiltration will occur, the thermostat will not sense
the temperature change from the open door. Therefore, the door would not need a switch.
• Where existing buildings are being altered, door switches are not required to be added. This
includes adding an exterior door to an existing building or adjusting the HVAC&R system in a space
with a door to the outdoors.
• Loading docks are not required to have door switches, as the doors are frequently open longer for
periods longer than five minutes.
Refrigeration Systems (6.5.11)
Section 6.5.11 refers to refrigeration systems used for display cases, walk-in coolers, and walk-in
freezers with air-cooled condensing coils. These refrigeration systems may have the compressor and
condensing coil in a single package or they may have a compressor with a remote condensing coil. Both
refrigeration system configurations must comply with the requirements of this section. This section
does not apply to refrigerant systems that use a transcritical refrigeration cycle (these systems
typically use CO2 as the refrigerant) or that use ammonia as the refrigerant.
Condensers Serving Refrigeration Systems (6.5.11.1)
While condensing units are designed as a complete system, refrigeration systems are engineered
systems where components are selected based on the specific operating conditions of both the cooler
or freezer and the ambient outdoor air temperatures. If condensers are sized too small, the compressor
must compress the refrigerant to a higher pressure for refrigerant condensation to occur in the
condenser. Section 6.5.11.1 provides guidelines for appropriately selecting a fan-powered condenser
to reduce compression energy. This section applies to fan-powered condensers for use as part of a
refrigerator or freezer. Refrigerators and freezers may have water-cooled condenser heat exchangers
that are not covered by this section.
Air-cooled condensers used in low-temperature refrigeration system applications (freezers) must be
designed such that the saturated condensing temperature is no greater than 10°F (5.5°C) above the
ambient design dry-bulb temperature. Air-cooled condensers used in medium-temperature
refrigeration systems (coolers) must be designed such that the saturated condensing temperature is
no greater than 15°F (8°C) above the ambient design dry-bulb temperature.
To reduce compressor energy consumption, refrigeration systems must be designed such that the
saturated condensing temperature set point is reset based on ambient air temperature conditions. For
air-cooled systems, the set point must be reset based on the ambient dry-bulb temperature. For
evaporatively cooled systems, the set point must be reset based on the ambient wet-bulb temperature.
The set point must be capable of resetting to a value of 70°F (21°C) or lower.
Refrigerant systems may use condenser fans to directly cool the refrigerant passing through a
condensing coil. However, there are multiple other methods frequently implemented. They may use
evaporatively cooled condensers, open-circuit cooling towers, or closed-circuit cooling towers. Note
that cooling towers must meet the requirements of Section 6.5.5 and the minimum efficiency
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requirements for air-cooled condensers and that both open- and closed-circuit cooling towers are in
Table 6.8.1-7. Additionally, minimum efficiencies for axial fan and centrifugal fan evaporative
condensers have been added to this table for the 2016 edition of the standard for both ammonia and
halocarbon applications.
The heat-rejection equipment used in a refrigerator or freezer has a significant effect on the energy
consumed by the compressor. Each form of heat-rejection equipment has advantages and
disadvantages. For example, evaporative condensers increase the energy efficiency of the entire
refrigeration system by lowering the ambient air temperature used for refrigerant condensing. This is
done by evaporating water into the air upstream of the condenser coil. This reduces the dry-bulb
temperature by bringing it closer to the wet-bulb temperature. The lower dry-bulb temperature
reduces the lift on the compressor, reducing its electrical power draw. The lower the relative humidity
of the outdoor air, the more the dry-bulb temperature can be reduced. Conversely, at 100% rh outdoor
air, this system has no advantage. While offering additional energy savings, it comes at the cost of
maintenance. Evaporatively cooled condensers will require more maintenance than air-cooled
condensers, but the degree of additional maintenance will be equipment dependent, as some
equipment may include features that extend periods between maintenance.
Refer to Section 6.5.5 for descriptions of other forms of heat rejection not covered by this section.
To reduce the condenser fan energy, condenser fan motors less than 1 hp (0.75 kW) must have
electronically commutated, permanent split capacitor-type, or three-phase motors. The condenser fans
must also include continuous variable-speed operation. The variable-speed operation must be capable
of reducing the fan motor power consumption to not more than 30% of the design wattage while
moving at least 50% of the design airflow rate. The condenser fan speed must modulate to maintain
the condensing temperature set point. Where multiple condenser fans are used on a single condenser,
they must modulate together rather than stage or cascade.
Compressor Systems (6.5.11.2)
Refrigeration compressor systems for display cases and walk-ins that have either a single variablespeed compressor or multiple compressors must have control logic capable of resetting the suction
pressure temperature set point, and liquid subcooling must be provided.
Compressors that are equipped with crankcase heaters must have control logic to prevent
simultaneous operation of the crankcase heater and compressor.
Alternative Compliance Path (6.6)
Section 6.6 provides an Alternate Compliance Path for computer room systems to achieve compliance
with the provisions of Section 6 of the standard for systems that often have unique requirements,
typically associated with the need for high availability.
Computer Room Systems (6.6.1)
Section 6.6.1 provides a compliance path for computer room systems. Even if this compliance path is
used, compliance with Sections 6.1, 6.4, 6.6.6.1 or 6.6.6.2, 6.6.1.3, 6.7, and 6.8 is still required.
If a computer room system is to show compliance through the Prescriptive Path, it is required to
include either a water-side or an air-side economizer. However, due to potential low humidity, reduced
air quality, or reliability concerns, a designer may wish to exclude the economizer. The Alternate
Compliance Path allows flexibility and promotes developing technologies.
Data center energy is frequently expressed in terms of power usage effectiveness (PUE). It is a ratio of
the instantaneous power or the annual energy consumption of the entire data center relative to the
information technology (IT) equipment. PUE0 is a ratio of instantaneous power (kW) while PUE1 is a
ratio of annual energy consumed (kWh). This section allows compliance with either PUE0 (Section
6.6.1.1) or PUE1 (Section 6.6.1.2). However, both metrics must also satisfy the requirements of Section
6.6.1.3 and must comply with the values listed in Table 6.6.1. The efficiency threshold necessary to
achieve compliance is a function of the climate zone; colder climates are required to meet a more
stringent (lower) PUE. This requires that the energy consumers included in the PUE calculations be
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broken down into at least the following components: IT equipment, power distribution losses external
to the IT equipment, HVAC systems, and lighting for documentation.
Power Usage Effectiveness—Category 1 (PUE1) (6.6.1.1)
PUE1 is based on total energy consumed annually and may not be used for data centers that use
combined heat and power (CHP) systems. For all data centers that do not use CHP systems, Table 6.6.1
provides maximum PUE values allowed for each climate zone. To determine the PUE1 of the proposed
building, the energy simulation methodology of Appendix G of the standard must be used. This path
does not require output results using the PUE metric, so some postprocessing of the results will likely
be needed to align with Section 6.6.1.3. The energy analysis must include all energy consumers in the
building, except the energy for battery charging, including losses from maintaining uninterruptible
power supplies (UPS).
Power Usage Effectiveness—Category 0 (PUE0) (6.6.1.2)
PUE0 is based on power demand while also using Table 6.6.1 for the allowable maximum PUE value.
PUE0 must be calculated at the cooling design outdoor air temperature at both 100% IT equipment
power and 50% IT equipment power and requires that tabulation be made on a design day of the
power draw of all the cooling infrastructure equipment supporting the data center. Because PUE0 is
calculated in a cooling-only scenario, it is limited to electricity-powered systems only. The greatest
resulting PUE0 value for the proposed building must be below that in Table 6.6.1 for the relevant
climate zone.
Power Usage Effectiveness—Documentation (6.6.1.3)
Compliance using either Section 6.6.1.1 or Section 6.6.1.2 requires documentation of the calculations,
including a breakdown of energy consumption or demand by at least the following four components:
IT equipment, power distribution losses external to the IT equipment, HVAC systems, and lighting.
Two example breakdowns are provided in this user’s manual. Example 6-MMM is of an air-cooled
chiller cooling system that does not comply with the Section 6.6.1.1 pathway due to higher-thanallowed annual energy consumption. Example 6-NNN is of a water-cooled chiller that complies using
the Section 6.6.1.2 pathway at both 50% and 100% load.
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Example 6-MMM. Compliance Using Air-Cooled Chiller, Section 6.6.1.1 Pathway, Chicago
(Climate Zone 5A); Design Temperature: 91.4°F dry bulb (0.4%); Maximum PUE for Compliance: 1.36
Corresponding section: Power Usage Effectiveness—Documentation (6.6.1.3)
Equipment Category
Subcomponent
Annual Energy Consumption
@100% Load (kW)
8,760,000
Power distribution losses
Power distribution units
Static transfer switches
Uninterrupted power supply units
Transformers
Line losses
Category subtotal
87,600
43,800
350,400
61,320
87,600
630,720
IT equipment at the rack level
HVAC systems
Lights
Total energy consumption
PUE
Maximum PUE
Compliance achieved?
Rack level load
Air-cooled chiller
Chilled-water pumps
Computer-room air-handling unit
fan energy
Chemical treatment
Category subtotal
1,998,520
96,360
490,560
4380
2,589,820
Category subtotal
65,700
12,046,240
1.375
1.36
No
Example 6-NNN: Compliance Using Water-Cooled Chiller, Section 6.6.1.2 Pathway,
Chicago (Climate Zone 5A); Design Temperature: 77.8°F Wet Bulb (0.4%), 87.8°F Mean Coincident
Dry Bulb; Maximum PUE for Compliance: 1.36
Corresponding section: Power Usage Effectiveness—Documentation (6.6.1.3)
Equipment Category
Subcomponent
Power Draw
@100% Load (kW)
1000
Power Draw
@50% Load (kW)
500
Power distribution losses
Power distribution units
Static transfer switches
Uninterrupted power supply units
Transformers
Line losses
Category total
10
5
75
7
10
107
6
5
40
5
3
59
120
11
10
10
56
57
4
6
4
24
It equipment at the rack level
HVAC systems
Lights
Total load
PUE
Maximum PUE
Compliance achieved?
Rack level load
Water-cooled chiller
Chilled-water pumps
Condenser water pumps
Cooling towers
Computer-room air handling unit
fan energy
Chemical treatment
Category total
Category total
2
209
2
97
15
15
1331
1.331
1.36
Yes
671
1.342
1.36
Yes
Note: Equipment submittals should include manufacturer’s efficiency at the design ambient condition at both 50% and 100% IT load.
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Completion Requirements (6.7.2)
Although they are located in Section 6.7.2, the completion requirements for submittals are mandatory
and apply to all projects regardless of which compliance path is used. An energy-efficient design will
not result in energy-efficient performance unless the system is installed, commissioned, and operated
properly.
Section 6.7.2 addresses the following completion requirements:
•
•
•
•
Drawings (Section 6.7.2.1)
Manuals (Section 6.7.2.2)
System Balancing (Section 6.7.2.3)
System Commissioning (Section 6.7.2.4)
Drawings (6.7.2.1)
The standard requires that construction documents (plans and specifications) call for record drawings
to be provided to the building owner (or owner’s representative) within 90 days of system completion
and acceptance.
At a minimum, the record drawings must show the location and energy-related performance data for
each piece of HVAC&R equipment; the general layout of duct and piping distribution systems, including
duct and pipe sizes; and the air and water flow requirements of all terminal units, such as VAV boxes
and diffusers. See Examples 6-OOO and 6-PPP.
Record drawings are usually the as-built drawings prepared by the contractor showing the system
design as it was installed.
When as-built drawings are not provided, as is common on small projects, the record drawings may be
the engineer’s design drawings updated to show any changes to equipment location or performance.
Manuals (6.7.2.2)
HVAC&R system design documents must require that an operation and maintenance (O&M) manual or
manuals be provided to the owner (or owner’s representative) within 90 days of system acceptance.
The manuals must conform to industry practice. ASHRAE Guideline 4, Preparation of Operating and
Maintenance Documentation for Building Systems, provides information and recommendations for
preparing O&M manuals.
At a minimum, the manuals must include the following:
a. Submittal data. Equipment size and selected options must be stated for each piece of equipment
requiring maintenance. Normally submittals are provided early in the construction of a project for
approval by the designer and for coordination among trades. The standard requires that this
information be made part of the O&M manuals so that all equipment information is in one location
and easily accessible by the operator. (Submittals, such as specifications, tend to disappear shortly
after completion of a project, while O&M manuals are more likely to be retained.)
b. HVAC&R manuals. O&M manuals must be included for each piece of HVAC&R equipment
requiring maintenance that is provided as part of the project. Required routine maintenance
actions must be clearly identified.
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Example 6-OOO. Record Drawings
Corresponding section: Submittals (6.7)
Q
A consulting firm traditionally schedules equipment performance data in specifications rather than
showing these data in equipment schedules on drawings. Does this meet the standard’s requirements?
A
No. Equipment performance must be shown on drawings, not in specifications. This is because
drawings tend to be retained longer than specifications, which increases the chance that equipment
performance information will be available to engineers and contractors years after the system was
built. Specifications, on the other hand, tend to be lost or discarded shortly after construction.
Example 6-PPP. Equipment Substitutions
Corresponding section: Submittals (6.7)
Q
After the design of an HVAC&R system, the installing contractor makes some equipment substitutions
that change their energy performance. Do these changes have to be reflected on the record drawings?
A
Yes. Section 6.7.2.1 requires that the record drawings indicate the actual installation. If substitutions
are made that change the energy of equipment such as air-conditioning units, chillers, towers, etc., the
record drawings must be updated accordingly. To ensure this occurs, consulting engineers should
include a provision in their specifications requiring the contractor to update (or bear the cost of
updating) equipment schedules and plans if contractor-initiated substitutions are made.
c.
d.
e.
Service agency. The name and address of at least one service agency capable of providing system
maintenance must be provided.
HVAC&R control information. HVAC&R controls system maintenance and calibration
information, including wiring diagrams, schematics, and control sequence descriptions, must be
included. For simple systems, such as small individual unitary equipment, a detailed system
description and a control schematic are not necessary to operate the system properly and thus
need not be included in the O&M manuals. For large, complex systems, control sequences and
schematics are essential to proper operation and must be included.
Field-determined set points—those determined after control system design drawings have been
developed—must be permanently recorded on control drawings at control devices, or, for digital
control systems, must be permanently recorded in programming comments.
For example, pressure set points for control of variable-volume fans and pumps, usually
established after construction by the test and balance services company, must be permanently
recorded in a place where they will not be easily lost. For pneumatic or electric controls, the best
location is a label mounted or marked on the control device or next to the pressure gage. For
digital controls, the best location is on graphic system displays or in program comments.
Recording these set points helps ensure that operators will operate the system as intended.
Improper set points, such as a higher-than-required-pressure set point in a variable-volume
system, usually cause the system to operate less efficiently.
Operations narrative. This is a complete narrative of how each system is intended to operate.
This statement of design intent should be written early in the design so that as the design develops
it can be compared to the design intent as a way of ensuring that the design is on track. Once the
system is built, the design intent can be used to help operators understand how to properly
operate the system.
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General (6.7.2.3.1)
Section 6.7.2.3.1 requires that construction documents call for all HVAC&R systems to be balanced in
accordance with generally accepted engineering standards, such as the procedures published by the
National Environmental Balancing Bureau (NEBB), the Associated Air Balance Council (AABC), or
ASHRAE Standard 111. An air balance report must be provided to the building owner or their
representative for all HVAC&R systems serving spaces larger than 5000 ft² (460 m²).
Air System Balancing (6.7.2.3.2)
Air systems must be balanced first in a manner to minimize throttling losses and then by adjusting fan
speed to meet design flow rates. Fan speed adjustment is not required for fans smaller than 1 hp (0.75
kW). See Examples 6-QQQ through 6-SSS for typical applications.
The standard does not specifically address what balancing devices, such as dampers or extractors,
must be included or where they must be located. This is left to the designer’s discretion based on past
experience and on guidance provided in the standards referenced in Appendix E of the standard. Also
see Example 6-RRR for some devices that should be balanced.
Hydronic System Balancing (6.7.2.3.3)
Hydronic systems are balanced in a manner similar to air systems: first, each coil or other device or
terminal is proportionately balanced in a manner to minimize throttling losses; second, the pump
impeller is trimmed or the pump speed is adjusted to meet design flow conditions. Gages or sensors
(or test ports into which handheld gages or sensors may be inserted) should be provided to measure
differential pressure across the pump. This will allow the overall water flow rate to be estimated from
pump curves.
Pump speed adjustment and impeller trimming are not required for pumps with motors 10 hp (7.5
kW) or less (Exception 1) or if throttling results in no greater than 5% of the nameplate power draw or
3 hp (2.2 kW), whichever is greater, above that required if the impeller were trimmed (Exception 2).
Valve throttling alone may be used for balancing such systems. See Example 6-UUU.
As with air systems, the type of valves or other devices required to make the system capable of being
balanced is left to the designer’s discretion. See Example 6-VVV.
Example 6-QQQ. Balancing Requirements, Constant-Volume System
Corresponding section: System Balancing (6.7.2.3)
Q
A constant-volume single-zone system with a 3 hp (2.2 kW) fan serves several rooms, each with its
own supply air and return air grille. How does the standard require the system to be balanced?
A
The standard requires that throttling losses must be minimized. To do this, the fan must be slowed
down until at least one balancing damper is wide open. The other dampers are then adjusted to
provide design airflow rates at the remaining grilles. This is often an iterative process. Finally, the
outdoor air intake damper is adjusted to provide the design minimum outdoor air rate.
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Example 6-RRR. Balancing Requirements, VAV System
Corresponding section: System Balancing (6.7.2.3)
Q
A 10 hp (7.5 kW) variable-air-volume system has pressure-independent VAV box controls. Inlet guide
vanes are used to control duct static pressure. How does the standard require the system to be
balanced?
A
The VAV boxes themselves provide balancing automatically, but the standard still requires that
throttling losses be minimized. To do this, the static pressure set point used to control the inlet vanes
must be set so that at least one VAV box damper is wide open under design flow conditions. If the set
point were higher, then the VAV boxes would pinch down, increasing throttling losses. This set point is
usually determined by the air balancer in the field. In addition, fan speed must be adjusted so that
design airflow conditions can be maintained with the inlet guide vanes wide open. Using the inlet
vanes for balancing is not as efficient as adjusting the fan speed.
Example 6-SSS. Balancing Requirements, VAV Fan with VSD
Corresponding section: System Balancing (6.7.2.3)
Q
A fan serving a VAV system has a variable-speed drive for static pressure control. During balancing, its
full-load fan speed is found to be 20% faster than required. Do fan sheaves need to be adjusted or
changed?
A
No. The standard only requires that the fan speed be adjusted; it does not state how to do this. The
sheaves may be adjusted or changed to meet this requirement, but it is more practical to allow the VSD
to automatically reduce the fan speed as required to meet system static pressure requirements. (This
example applies to variable flow pumping systems as well: it is not necessary to trim the impeller to
balance the system if the pump has a VSD.)
Example 6-TTT. Balancing Requirements, Balancing Valves
Corresponding section: System Balancing (6.7.2.3)
Q
What types of balancing valves does the standard require?
A
The standard requires only that the system be specified to be balanced, which implies that it must be
capable of being balanced. But it does not require that any particular balancing device be used.
Common examples of balancing designs at coils and heat exchangers include the following:
• Calibrated balancing valves
• Automatic system-powered flow control (flow limiting) valves
• Standard ball or butterfly valves along with pressure gages or test plugs that will allow pressure
drop across the coil or heat exchanger flow to be measured so flow can be deduced from
manufacturers’ performance data.
• Pressure-independent control valves.
• Automatic control valves (see further discussion in Example 6-UUU).
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Example 6-UUU. Balancing Requirements, Constant-Volume Pumping System
Corresponding section: Hydronic System Balancing (6.7.2.3.3)
Q
A hot-water heating system serves several heating coils with three-way valves. How does the standard
require the system to be balanced?
A
The standard requires that throttling losses must be minimized. For pumps larger than 10 hp (7.5 kW),
flow through each coil would be proportionally balanced with one balancing valve wide open. If flow
sufficiently exceeds design flow (see Exception 2 to Section 6.7.2.3.3.), the pump impeller would then
be trimmed (or the pump speed reduced) to reduce flow to the design rate. For pumps 10 hp (7.5 kW)
and smaller, impeller trimming or speed adjustments are not required. In this case, flow would be
throttled at each coil to achieve design flow rates; it is not necessary to limit throttling losses.
Example 6-VVV. Balancing Requirements, Variable Flow Pumping System
Corresponding section: Hydronic System Balancing (6.7.2.3.3)
Q
A large chilled-water system serves coils with two-way control valves. How does the standard require
the system to be balanced?
A
As noted in Example 6-UUU, the standard does not state how to provide a balanced system, only that it
be specified to be balanced. Some systems may be designed to be self-balancing via control devices or
system layout. Whether or not the system needs to be balanced in the traditional sense, where flow at
each coil is measured and adjusted, is left to the professional judgment of the engineer. Either system
balancing approach is acceptable from the standard’s perspective.
System Commissioning (6.7.2.4)
The system commissioning process helps to ensure that building systems are designed, installed, and
operating as intended. There are many levels of commissioning, from the simple start-up procedures
that most contractors perform at the end of the project to an elaborate and formal process conducted
by an independent commissioning agent that carries through the entire design and construction
process.
The appropriate level of commissioning varies according to the critical nature or importance of the
project, the owner’s desires, and budget constraints. For a critical project, such as a hospital, a high
level of commissioning is usually appropriate. For a simple, noncritical application such as a small
retail store, standard start-up procedures may be adequate. For most projects, some level between
these extremes is probably the most cost-effective; standard start-up procedures are usually
insufficient, while comprehensive commissioning is probably too time intensive and expensive for the
benefits received.
Specifying the appropriate level of commissioning in a standard such as Standard 90.1 is difficult
because of the wide range of criticality and complexity of systems and applications. It is also very
difficult to assess the energy and operational savings from commissioning and therefore difficult to
assess its cost-effectiveness. For these reasons, the commissioning requirements in Standard 90.1 are
necessarily general and not overly stringent. The requirements are summarized as follows:
• HVAC&R control systems must be tested to ensure that control elements are calibrated, adjusted,
and in proper working condition. This does not necessarily require field calibration of sensors;
ensuring that the sensors have been factory calibrated and that they are in working order is
sufficient.
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• For projects larger than 50,000 ft² (4600 m²) of conditioned area (except warehouses and
semiheated spaces), an HVAC&R system commissioning plan must be developed by the designer
and included in the design documents. The standard does not specify the level of commissioning,
because the appropriate level will vary from project to project. These details are left to the
designer. Guidance for developing commissioning plans can be found in ASHRAE’s Guideline 1.1,
HVAC&R Technical Requirements for the Commissioning Process; NEBB’s Procedural Standards for
Whole Building Systems Commissioning of New Construction; SMACNA’s HVAC Systems—
Commissioning Manual; and the Portland Energy Conservation, Inc. (PECI), Model Commissioning
Plan and Guide Specifications.
 Minimum Equipment Efficiency Tables (6.8)
Section 6.8 of the standard contains the minimum equipment efficiency tables used for compliance
with the requirements of Section 6. The tables are referenced throughout this chapter where
appropriate. See the standard for the complete set of tables.
The following are some highlights and comments regarding the tables.
Overall the tables cover the minimum efficiency metrics for equipment at standard rating conditions
defined by standards from the Air-Conditioning, Heating, and Refrigeration Institute (AHRI);
International Organization for Standardization (ISO); Association of Home Appliance Manufacturers
(AHAM); U.S. Department of Energy (DOE); and Cooling Tower Institute (CTI). Actual performance at
applied conditions in a building may be different due to different operating conditions, different
external static, and options and accessories that may have been added to the units.
Table 6.8.1-1—Electrically Operated Unitary Air Conditioners and Condensing Units—
Minimum Efficiency Requirements
Table 6.8.1-1 covers minimum efficiencies for packaged rooftop, air-cooled, water-cooled, and
evaporatively cooled packaged units, split systems, through-the-wall air-cooled units, and small-duct
high-velocity units.
The following is a summary of the changes that have been made to this table.
The efficiency requirements for single packaged air conditioners will increase from 13.0 SEER to 14.0
SEER (3.81 SCOPC to 4.10 SCOPC) effective 1/1/2015. Note that as per Note b to the table, these
requirements cover only commercial three-phase equipment. Single-phase equipment is defined by
NAECA and is not included in the table. On 1/1/2015 single-phase equipment is scheduled to go to
regional requirements for splits with 13.0 and 14.0 SEER (3.81 and 4.10 SCOPC). Single-phase
packaged units are scheduled to change to 14.0 SEER (4.10 SCOPC), so the change to the table has been
made to match the changes for single phase. Regional requirements will not be implemented for
commercial three-phase equipment, so the commercial three-phase splits will remain at 13.0 SEER
(3.81 SCOPC).
For the larger equipment covered by the table, in 2010 a new annualized metric was introduced. This
metric is called the Integrated Energy Efficiency Ratio (IEER) in I-P and Integrated Coefficient of
Performance (ICOPC) in SI and is a metric that gives a better representation of the annualized energy
use for the unit. It is based on a weighted average of performance at 100%, 75%, 50%, and 25% of
cooling capacity. It was modeled after an average of typical commercial buildings using average
weather data. It is a metric for only the basic refrigeration system and excludes operating hours where
added features like economizers are used. It provides better comparison of the equipment
performance but is not intended to be used to predict building application energy use. For this, a
systems analysis using building simulation tools should be used that take into account the actual
building operating load, the local weather data, and added features such as economizers and energy
recovery devices.
In 2010 the new IEER (ICOPC) metric was introduced with values for all air-cooled, water-cooled, and
evaporatively cooled products with a capacity greater than or equal to 65,000 Btu/h (19 kW). In 2016,
further improvements have been made to the IEER (ICOPC) levels that will go into effect on 1/1/2016.
The efficiency rating increases reflect product improvements as well as the new requirement for
indoor fan speed control defined in Section 6.5.3.2.
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Within each size range of large commercial units, the table includes separate efficiency ratings for units
with electric heat or no heat and those with other heating systems, such as gas heat and hot-water
heat. The EER and IEER (COP and ICOPC) for the units with other heat are 0.2 Btu/W (0.06 W/W)
lower than the electric heat or no heat units due to the added internal static pressure drop and
associated higher indoor fan power.
Also in the table in the 2016 edition of the standard, the efficiency requirements for small-duct highvelocity were increased from 10.0 SEER to 11.0 SEER (2.93 SCOPC to 3.22 SCOPC).
Table 6.8.1-2—Electrically Operated Unitary and Applied Heat Pumps—
Minimum Efficiency Requirements
This table covers minimum efficiencies for the heat-pump versions of the air-cooled packaged rooftop
units, air-cooled split systems, through-the-wall air-cooled units, and small-duct high-velocity units. It
also includes requirements for air-cooled and water-cooled water-source heat pumps. For these
products there are two rating requirements, one for cooling and the other for heating. For the heating
rating, some products include two metrics with one at 47°F (8.3°C) ambient and a second point at 17°F
(–8.3°C). For compliance with this table, all of the metrics must be met or exceeded.
In the table in the 2016 edition of the standard, the following changes have been made:
• The efficiency requirements for single-package heat pump units were increased from 13.0 SEER to
14.0 SEER (3.81 SCOPC to 4.10 SCOPC) effective 1/1/2015. These requirements are only for threephase equipment; single-phase equipment are covered by the NAECA federal requirements.
• Similar to the large commercial cooling-only units, all units now have both an EER and IEER (COP
and ICOPC) for cooling operation. In the 2016 edition of the standard, increased efficiency levels for
IEER (ICOPC) were included with an effective date of 1/1/2016. No changes have been made for
these products to the EER (COPC) and heating COP (COPH) metrics.
• Also in the table in the 2016 edition of the standard, the efficiency requirements for small-duct
high-velocity were increased from 10.0 SEER to 11.0 SEER (2.93 SCOPC to 3.22 SCOPC).
• The efficiency requirements for cooling, as well as heating, for water-source heat pumps and
geothermal units were increased.
Table 6.8.1-3— Water-Chilling Packages—Minimum Efficiency Requirements
Table 6.8.1-3 covers the efficiency requirements for all classes of chillers, which include air-cooled
chillers, water-cooled chillers, and absorption chillers.
For the chillers, there is a full-load metric—for air-cooled chillers, I-P ratings are defined using an EER
(Btu/W), and for water-cooled chillers the practice is to use a kW/ton metric. For absorption units, a
COP (W/W) is used, and ICOP is used for all SI ratings. An important fact to note is that the SI ratings
are based on AHRI 551/591, which is a hard metric standard with slightly different rating conditions
than the I-P ratings, which are based on AHRI 550/590. So conversion of the ratings from SI to I-P and
I-P to SI units should not be used.
For the chillers there is also an annualized metric using the same units of measure as the full-load
ratings. The metric is called the “integrated part-load value” (IPLV). It is based on a weighted average
of performance at 100%, 75%, 50%, and 25% load for an average of commercial buildings using
average national weather data. The IPLV metric is a better representation of the performance of the
chiller because buildings seldom, if ever, run at full load and design ambient temperature but actually
spend all of their time at part load and reduced ambient temperature.
The chiller tables also include two paths, Path A and Path B, where Path A has higher full-load
efficiency metrics and lower IPLV metrics. Path B has slightly lower full-load efficiency metrics and
higher IPLV efficiency metrics. Path A is intended for full-load-intensive applications and Path B is
targeted at part-load-intensive applications. Typically Path B will have systems with variable-speed
compressors driven by inverters that result in slightly lower full-load efficiency but better part-load
efficiency. The use of inverters is not a requirement, and some products may use multiple fixed
compressors or combinations of fixed-speed compressors and variable-capacity compressors.
Compliance with the standard is obtained by complying with either Path A or Path B, but both the fullload metric and IPLV metric must be complied with. Note that the kilowatt/ton output is an inverse
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metric where a lower number is a better efficiency. The choice of Path A or Path B depends on the
application, building load profile, number of chillers being used in parallel or in series, and the ambient
load profile. So when selecting the path, an energy analysis of the building should be performed to help
choose the best path. The metrics are intended only to allow for comparison of like products and are
not intended to predict the energy use of the building. They are also not intended for comparing an aircooled chiller to a water-cooled chiller. In the air-cooled metric, the power of the condenser fans is
included, but, for the water-cooled metric, the cooling tower system power and condenser pumping
power are not included. Also, for a water-cooled chiller the cost and energy associated with the tower
makeup water and treatment should be considered. To understand the energy use in an application, it
is best to use a building simulation program where all the energy can be included. The energy analysis
can also calculate the energy impacts of chiller plant controls, chiller maps, load profiles, and local
weather.
For centrifugal chillers, an adjustment may be required to the efficiencies. Typically centrifugal chiller
speeds and impellers are selected for the application lift. Once this is done, the chiller may not be able
to run at the standard AHRI 550/590 or AHRI 551/591 conditions, so there is an adjustment factor,
Kadj , that adjusts the efficiency metrics in Table 6.8.1-3 for the applied lift. The details of this
requirement are defined in Section 6.4.1.2.1. This is also shown in Example 6-J of this user’s manual.
The Kadj equation is complicated, so Standard 90.1 has developed a spreadsheet tool that can be used to
adjust the efficiency metrics. The tool can be found on the ASHRAE website at
http://www.ashrae.org/UM90.1-2016.
In 2010, significant efficiency improvements were implemented for chillers. In 2016, further
improvements have been made. The following is a summary of the changes that have been
implemented in Standard 90.1-2016 with an effective date of 1/1/2015:
• The efficiency levels for all air- and water-cooled chillers have been increased for both full load and
IPLV.
• Path B was added for air-cooled chillers, which only had Path A in the 2010 edition of the standard.
• The capacity categories for the water-cooled chiller were adjusted to better align between
centrifugal and positive displacement, and also the efficiency requirements were aligned between
the two classes of products.
Table 6.8.1-4—Electrically Operated Packaged Terminal Air Conditioners, Packaged Terminal
Heat Pumps, Single-Package Vertical Air Conditioners, Single-Package Vertical Heat Pumps,
Room Air Conditioners, and Room Air-Conditioner Heat Pumps— Minimum Efficiency
Requirements
Table 6.8.1-4 covers several different products ranging from packaged terminal air conditioners
(PTACs), single-package vertical air-cooled (SPVAC) units, and window units. For these units, there is
only a full-load cooling metric, which is expressed by the EER (Btu/W) in I-P and the COPC (W/W) in SI.
Also for heating there is only a single full-load COP metric at 47°F (8.3°C) ambient. For the PTAC units
there is an efficiency category for standard-size units for use in new construction and a separate
category for nonstandard units that are only for use in replacement markets.
In the 2016 edition of the standard, the efficiency requirements for most of these units were improved:
• The cooling efficiency requirements for the standard-size PTAC unit has been increased and has an
effective date of 1/1/2015.
• The cooling efficiency requirements for the standard-size packaged terminal heat pump (PTHP)
units increased; the new efficiencies went into effect on 1/1/2014. The heating mode efficiencies
also increased.
• The SPVAC unit efficiency requirements increased; the new efficiencies went into effect on
1/1/2014.
The efficiency requirements for room air conditions were not changed, and the cooling efficiency
requirements for the nonstandard-size units were not changed due the space limitations imposed by
the replacement application.
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Table 6.8.1-5—Warm-Air Furnaces and Combination Warm-Air Furnaces/Air-Conditioning
Units, Warm-Air Duct Furnaces, and Unit Heaters—Minimum Efficiency Requirements
Table 6.8.1-5 defines the minimum efficiency requirements for gas- and oil-fired warm-air furnaces
that are often included in unitary products to provide heating. It also defines the requirements for
warm-air gas-fired duct furnaces and oil-fired and gas-fired warm-air unit heaters. For small gas-fired
warm-air furnaces with a capacity less than 225,000 Btu/h (73.3 kW) input rate, the metric for
efficiency is annual fuel utilization efficiency (AFUE); it is the ratio of annual output energy to annual
input energy as developed in accordance with the requirements of DOE 10 CFR Part 430. An alternate
efficiency metric is also allowed—the thermal efficiency Et, which is the ratio of the output capacity
divided by the input capacity of the fuel (gas or oil). In addition to the thermal efficiency, the heating
equipment must also include an interrupted or intermittent ignition device (IID), have jacket losses not
exceeding 0.75% of the input rating, and have either power venting or a flue damper. A vent damper is
an acceptable alternative to a flue damper for those furnaces where combustion air is drawn from the
conditioned space. For the duct heaters and unit heaters, the metric used is the combustion efficiency,
which is the input gas capacity less the fuel losses.For the 2016 edition of the standard, no changes
have been made to the requirements for these products.
Table 6.8.1-6—Gas- and Oil-Fired Boilers—Minimum Efficiency Requirements
Table 6.8.1-6 covers the requirements for gas-and oil-fired hot-water and steam boilers. The efficiency
requirements are established by the type of fuel, the input capacity, and, in the case of steam boilers,
the draft type (natural or forced draft). For small oil- and gas-fired boilers with a capacity less than
300,000 Btu/h (87.9 kW) input rate, the metric for efficiency is AFUE; for larger boilers the metric is
the thermal efficiency Et. (See the discussions of AFUE and thermal efficiency in the previous section.)
Table 6.8.1-7—Performance Requirements for Heat-Rejection Equipment—Minimum Efficiency
Requirements
Table 6.8.1-7 covers the requirements for cooling towers, including open- and closed-circuit towers
with propeller and centrifugal fans. It also covers heat evaporatively cooled and air-cooled heatrejection condensers.
In 2016, several changes were made to the table:
• The minimum efficiency for axial fan open-circuit cooling towers was increased from 38.2 to 40.2
gpm/hp (3.2 to 3.4 L/s/kW) at the rating condition listed in the table. A footnote has also been
added that requires the performance effect of accessories and options to be included in the
efficiency rating when determining compliance with the values in this table.
• The minimum efficiency requirements for axial fan and centrifugal fan evaporative condensers
have been added to the table for both ammonia and halocarbon applications. Evaporative
condensers are used in cold storage warehouses, food processing facilities, supermarkets,
industrial processes, and, to a limited extent, HVAC systems. In addition to being energy-efficient
heat-rejection devices, evaporative condensers increase the energy efficiency of the entire
refrigeration system by enabling a much lower condensing temperature, and thus lower
compressor lift, as compared to air-cooled designs. The test code for evaporative condensers is CTI
ATC-106. There are no certification requirements for evaporative condensers at this time.
Table 6.8.1-8—Heat Transfer Equipment—Minimum Efficiency Requirements
Table 6.8.1-8 was added to Standard 90.1 in 2010 to define a new product category for liquid-to-liquid
heat exchangers. For this new product category, AHRI 400 has been developed and a certification
program has also been started. Currently, efficiency levels are not defined, but they will be added in the
future once a history has been established from the AHRI 400 certification program.
Table 6.8.1-9—Electrically Operated Variable-Refrigerant-Flow Air Conditioners—Minimum
Efficiency Requirements
Table 6.8.1-9 covers air-cooled cooling-only variable-refrigerant-flow (VRF) air conditioners. The
category and table were added in the 2010 edition of the standard. Similar to unitary air conditioners,
VRF systems use a SEER metric for systems with a capacity less than 65,000 Btu/h (19 kW). Also
similar to the unitary products, it uses EER and IEER (COPC and ICOPC) for units with a capacity greater
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than or equal to 65,000 Btu/h (19 kW). In the 2010 edition of the standard, there were two levels of
efficiency for the IEER (ICOPC), with one number before 7/1/2012 and a higher number after
7/1/2012. The only change made for the 2016 edition is to eliminate the efficiency level for IEER
(ICOPC) before 7/1/2012.
Table 6.8.1-10—Electrically Operated Variable-Refrigerant-Flow Air-to-Air and Applied Heat
Pumps—Minimum Efficiency Requirements
Table 6.8.1-10 covers air-cooled, water-cooled, and ground-source VRF heat-pump systems. Also,
within each capacity category for the products, there is a minimum efficiency requirement for VRF
split systems and VRF split systems with heat recovery. The second group is for a product that can heat
and cool simultaneously in different zones conditioned by the VRF system.
Similar to the unitary products, these types of products use EER and IEER (COPC and ICOPC) for units
with a capacity greater than or equal to 65,000 Btu/h (19 kW). In the 2010 edition of the standard,
there were two levels of efficiency for the IEER (ICOPC), with one number before 7/1/2012 and a
higher number after 7/1/2012. The only change made for the 2016 edition is to eliminate the
efficiency level for IEER (ICOPC) before 7/1/2012.
For water-source units, there is only one metric for cooling, the full-load EER (COPC). Because all of the
units are heat pumps, there are also heat COP (COPH) efficiency requirements, with air-source heat
pumps having two efficiency metrics, one at 47°F dry bulb/43°F wet bulb (8.3°C dry bulb/6.1°C wet
bulb) and the other at 17°F dry bulb/15°F wet bulb (–8.3°C dry bulb/–9.4°C wet bulb). For watersource VRF units, there is only one metric at the appropriate condenser water/ground-water standard
rating conditions used for water-source heat pumps.
Table 6.8.1-11—Air Conditioners and Condensing Units Serving Computer Rooms—Minimum
Efficiency Requirements
Table 6.8.1-11 covers computer room air conditioners and condensing units. Because the cooling load
for computer rooms is all sensible, the capacity in the table is the net sensible cooling capacity and not
the total capacity used for all the other HVAC equipment, which includes sensible and latent capacity.
The minimum efficiency requirement measured by SCOP-127 reflects this sensible-only load. The
sensible coefficient of performance (SCOP-127) is a ratio calculated by dividing the net sensible cooling
capacity in watts by the total power input in watts (excluding re-heaters and humidifiers) at conditions
defined in ASHRAE Standard 127.
For the 2016 edition of the standard, there were no changes made.
Table 6.8.1-12—Commercial Refrigerators and Freezers—Minimum Efficiency Requirements
Table 6.8.1-12 covers unitary commercial refrigerators and freezers. It has a maximum energy use
metric, which is the kilowatt-hour/day energy use rated in accordance with AHRI 1200. The metric is a
function of the volume of the refrigerator/freezer, the door construction type, and the unit function
(refrigerator or freezer). The volume of the refrigeration cabinet must be calculated in accordance with
AHRI 1200, Appendix C.
Note that, although the unit of measurement for the performance metric is the same for I-P and SI, the
volume unit of measurement input into the energy use limit equations is not the same and must use
the unit of measurement of the respective system of measurement.
Table 6.8.1-13—Commercial Refrigeration—Minimum Efficiency Requirements
Table 6.8.1-13 covers site-assembled commercial refrigeration. It also has a metric that is the kilowatthour/day energy use. There are many classes of equipment type for which there is a maximum energy
use metric. Some are defined as a function of the volume of the refrigeration cabinet, which is defined
in AHRI 1200, Appendix C. Others are a function of the total display area as defined by AHRI 1200,
Appendix D. They are also further subdivided into categories for the rating temperature, including
medium temperature, low temperature, and ice cream.
Note that although the unit of measurement for the performance metric is the same for I-P and SI, the
area and volume unit of measurement input into the energy use limit equations are not the same and
must use the unit of measurement of the respective system of measurement.
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Table 6.8.1-14—Vapor Compression Based Indoor Pool Dehumidifiers—Minimum Efficiency
Requirements
Table 6.8.1-14 is a new table added in the 2016 edition of the standard to indoor pool dehumidification
equipment. It uses a metric called moisture removal efficiency (MRE). MRE is the rate of moisture
removal in pounds per hour or kilograms per hour, per kilowatt of energy input to the system.
Determination of the MRE must follows AHRI 910 or AHRI 911 for I-P and SI units, respectively.
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7 Service Water Heating
Changes to the Service Water Heating Section
 A requirement was added for insulation of the first 8 ft (2.4 m) of branch piping connected to pipe
that is heated by recirculating hot water, heat tracing, or impedance heated pipe (7.4.3).
This change is marked with  in the margin of this chapter. For the specific addenda that define the
differences between the 2013 and 2016 editions of Standard 90.1, see Appendix H to the standard.
General (7.1)
Scope (7.1.1)
This chapter covers the standard’s requirements for service water heating equipment and systems,
including combination space conditioning and water heating systems. Requirements for space
conditioning boilers and distribution systems are covered in Chapter 6.
Service water heating refers to heating water for domestic or commercial purposes other than space
heating or process requirements. This includes, but is not limited to, the production and distribution of
hot water for
• restrooms,
• showers,
• laundries,
• kitchens,
• pools and spas, and
• living units in high-rise residential buildings and hotels.
Water heating systems and equipment in new buildings must comply with the standard’s
requirements (Section 7.1.1.1). When water heaters are replaced in existing buildings, the replacement
equipment must meet the standard’s requirements (Sections 7.1.1.2 and 7.1.1.3). However, minor
alterations to a water heating system, such as extending the pipes to new fixtures or installing valves,
would not trigger an upgrade to the service water heating system; therefore, the system is not required
to comply with the standard’s requirements.
Residential water heaters are covered by the National Appliance Energy Conservation Act (NAECA).
The efficiency requirements for these water heaters are established by the U.S. Department of Energy
and are shown in Appendix F of the standard.
The federal Energy Policy Act (EPAct) established, among many other provisions, federal minimum
performance requirements for commercial boilers, commercial water heaters, and hot-water storage
tanks. Commercial water heaters covered by EPAct are exempt from any labeling requirements of
Standard 90.1.
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 FIGURE 7-A. COMPLIANCE OPTIONS
 Compliance Paths (7.2)
The majority of the Section 7 requirements are mandatory provisions that must always be satisfied.
The designer can then chose from one of three optional compliance paths. Use of the Prescriptive Path
requires that all of the requirements in Sections 7.5 and 7.7 be met. Alternatively, the Energy Cost
Budget (ECB) Method (Chapter 11) or the Performance Rating Method (PRM) (Appendix G) can be
used to comply with the standard. Compliance paths are shown in Figure 7-A.
Compliance Forms
Compliance forms and worksheets intended to facilitate the process of complying with the standard
are available for download from ASHRAE’ s website at http://www.ashrae.org/UM90.1-2016.
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FYI
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Strategies for Reducing Water Heating Energy Use
For some building types, service water heating can be a major energy consumer. In hotels, for example,
water heating accounts for 25% to 40% of the total energy usage. Fortunately, this component of
energy use is easily controlled by applying some basic, cost-effective design practices as shown in the
figure above. These practices include the following:
•
•
•
•
•
Reducing hot-water use by using flow-limiting or metering terminal devices
Limiting standby losses by using heat traps and thermal insulation.
Reducing distribution losses through thermal insulation and circulation pump controls or
eliminating them through point-of-use heaters
Harnessing waste heat or solar energy to meet part of the load
Increasing efficiency of the water heating equipment
FYI
Improving Water Heating System Performance
Although compliance with the standard ensures a minimum level of water heating system
performance, designers may wish to consider designs that exceed these requirements. Heat recovery,
solar energy, or high-efficiency equipment are options that can be expected to improve system
efficiency.
Noncondensing gas and oil water heaters are available with thermal efficiencies as high as 85%.
Condensing gas water heaters are available with thermal efficiencies over 95%. Gas storage water
heaters are available with low standby losses in the range of 0.40% per hour. Electric resistance water
heaters are available with standby losses as low as 0.06% per hour. (Standby loss is measured as the
percentage of heat stored in the hot water.) Air-source heat pump water heaters are available with
COPs over 3.7.
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Other energy efficiency measures might include installing automatic shutoff devices for lavatories;
selecting low-water-use or low-temperature appliances, including residential and commercial clothes
washers and dishwashers; and recovering heat from gray water.
In addition, designers are encouraged to compare the first and operating costs of large centralized
systems with smaller distributed systems, such as using separate water lines located closer to the
points of use. Centralized systems are likely to be cost-effective in high-use facilities such as hotels and
motels, multifamily residences, dormitories, laboratories, food courts, and prisons. Distributed systems
are best applied in facilities where the hot-water use is low or where the use is widely distributed, such
as office and retail buildings.
Mandatory Provisions (7.4)
All mandatory provisions must be complied with, regardless of whether the designer chooses to follow
the Prescriptive Path (Section 7.5) or the Energy Cost Budget Method (Section 11) of compliance.
Load Calculations (7.4.1)
The standard requires that load calculations be performed to determine the necessary size of water
heating systems, but it does not directly limit oversizing. This same approach is taken for HVAC system
sizing (Section 6.4.2). The standard assumes that if designers and contractors are required to select
equipment based on calculated loads, oversizing will be limited as a result. Oversizing of equipment
generally wastes energy through increased standby losses (due to the larger surface area of bigger
tanks) and reduced heater efficiency (due to cycling).
The standard requires that design loads be calculated using either manufacturers’ published guidelines
or generally accepted engineering standards and handbooks acceptable to the adopting authority. The
designer may also use procedures developed by professional organizations or equipment
manufacturers 1.
In sizing water heating systems, there is a relationship between the storage capacity of the tank and
the output capacity of the heater. A smaller heater can be used if the tank is larger; conversely, a
smaller tank can be used with a larger heater. Most of the calculation procedures consider both storage
capacity and heater size but may not provide assistance in finding the optimum combination.
The right combination depends on a number of factors, including available space, equipment cost, and
concern about standby loss. A smaller tank and larger heater will generally have a smaller footprint, a
higher first cost, and a lower standby loss. A larger tank and a smaller heater will generally have a
larger footprint, a lower first cost, and a higher standby loss. In all cases, premanufactured heaters and
tanks will come in discrete sizes; the first cost will be lowest when you can utilize the total capacity of
both the heater and the tank.
In addition to providing information about sizing heaters and storage tanks, Chapter 50 of the 2015
ASHRAE Handbook—HVAC Applications provides useful information on other aspects of service water
heating systems. These include the following:
• Special considerations for piping for commercial kitchens
• Problems of water quality and protection from corrosion and scale
• Application and design of dual-temperature systems
• Health and sanitation concerns
• Numerous references providing water use temperatures for a range of building and system types
For information about the sizing of distribution piping, consult Chapter 22 of the 2007 ASHRAE
Handbook—Fundamentals.
1
When using a manufacturer’s sizing method, designers are encouraged to compare the results to those obtained from the
applicable method in ASHRAE Handbook—HVAC Applications. Manufacturer’s sizing methods in general will result in larger
equipment capacities and costs.
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Example 7-A. Sizing Service Water Heating Equipment
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Corresponding section: Load Calculations (7.4.1)
Q
What size heater and storage tank is appropriate for an 80-unit apartment building?
A
The graph above is a reproduction of Figure 21 from Chapter 50 of the 2015 ASHRAE Handbook—
HVAC Applications.
Any heater and storage tank combination that falls on the curve for 75 units would satisfy the load. As
shown in the following table, Selection A is the smallest heater with its corresponding storage size, and
Selection B represents a larger heater that allows for a smaller storage tank. Either system would
satisfy the load; the final decision should be based on economics, including energy costs; installation
costs, including the flue and gas line or electric service; and available space.
Selection A
Selection B
Per Unit
Total
Per Unit
Recovery Capacity
2.75 gal/h-apt × 80 apt
= 220 gal/h
5 gal/h-apt × 80 apt
(2.9 mL/s-apt × 80 apt)
(= 231 mL/s)
(5.3 mL/s-apt × 80 apt)
(Does not account for system heat loss. Add system heat loss to loads calculated here.)
Usable Storage
32 gal/apt × 80 apt
= 2560 gal
14 gal/apt × 80 apt
(121 L/apt × 80 apt)
(= 9677 L)
(53.0 L/apt × 80 apt)
Actual Storage
×1.4
(Assuming 70% useful storage capacity)
3600 gal
(13,626 L)
Total
= 400 gal/h
(= 420 mL/s)
= 1120 gal
(= 4239.2 L)
×1.4
1600 gal
(6056 L)
Equipment Efficiency (7.4.2)
The standard has efficiency requirements for water heaters and hot-water supply boilers that are not
covered by NAECA. NAECA is a federal standard that specifies the minimum performance of residential
and small commercial space heating, space cooling, and water heating equipment. NAECA’s efficiency
requirements for water heaters include
• all types of electric heaters at or below 12 kW input (including heat-pump water heaters and
instantaneous heaters),
• fuel-fired storage heaters at or below 75,000 Btu/h (22 kW) input for gas or 105,000 Btu/h (30.8
kW) input for oil,
• fuel-fired instantaneous heaters at or below 200,000 Btu/h (58.6 kW) input for gas or 210,000
Btu/h (61.5 kW) input for oil, and
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• all fuel-fired pool and spa heaters.
Table 7.8 of the standard presents the minimum required efficiencies for water heaters and hot-water
supply boilers. The table references the Federal standards for NAECA equipment, but Table F-2
reprints the efficiency requirements for those items. Most equipment that is not covered by NAECA is
required to have both minimum heater efficiency (thermal efficiency) and a maximum standby loss.
Smaller equipment, which falls under NAECA, is required to meet a minimum energy factor, which is a
combined measure of thermal efficiency and standby loss. Table 7.8 classifies equipment by type
(storage, instantaneous, etc.), fuel, capacity (input rating), input-to-volume ratio, and/or storage size.
Examples 7-B through 7-E demonstrate how these equipment categories are applied.
For all the categories of equipment in Table 7.8 not covered by NAECA or EPAct, it is the
manufacturer’s responsibility to label the equipment as complying with the standard (Section
6.4.1.5.1). The manufacturer will also provide the required data for calculation of the requirements.
Unfired storage tanks are required to be insulated to R-12.5 (R-2.2) in accordance with Table 7.8. The
standby loss requirement is waived on hot-water supply boilers and storage water heaters that meet
all four of the following conditions:
•
•
•
•
Over 140 gal (530 L) measured storage capacity
Tank surface with a thermal insulation of R-12.5 (R-2.2) or more
No standing pilot light
Have a flue damper or fan-assisted combustion if they are gas or oil fired
Example 7-B. Equipment Efficiency Requirements, Hot-Water Supply Boiler
Corresponding section: Equipment Efficiency (7.4.2)
Q
A hot-water supply boiler consists of a gas-fired boiler, circulation pump, and storage tank. The boiler
is rated at 1,825,000 Btu/h (535 kW) input and 1,497,000 Btu/h (439 kW) output. The boiler storage
capacity is 45 gal (170 L), and its standby losses are 4000 Btu/h (1.2 kW). The storage tank has a 400
gal (1514 L) capacity and is insulated to R-16 (R-2.8) with sprayed-on polyurethane foam. Does it
comply with the standard?
A
No. The storage tank is unfired and insulated to R-16 (R-2.8). This complies with the requirement for
Unfired Storage Tanks in Table 7.8 (insulation R-value greater than R-12.5 [R-2.2]). The heater thermal
efficiency and input-to-volume ratio are given by:
𝐸𝐸𝑡𝑡 =
Input to Volume Ratio =
𝑄𝑄𝑜𝑜𝑜𝑜𝑜𝑜
𝑄𝑄𝑖𝑖𝑖𝑖
𝑄𝑄𝑖𝑖𝑖𝑖
=
𝑉𝑉ℎ𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒
Input to Volume Ratio =
1,497,000
1,825,000
=
= 82%
1,825,000 Btu/h
𝑄𝑄𝑖𝑖𝑖𝑖
𝑉𝑉ℎ𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒
= 40,556 (Btu⁄h)⁄gal
45 gal
535,900 W
=
170 L
= 3,147 W/L
(I-P)
(SI)
The boiler falls under the equipment type Hot-Water Supply Boilers, Gas in the subcategory of ≥4000
(Btu/h)/gal (309.75 W/L) and ≥10 gal (37.85 L) of storage. Table 7.8 sets the requirements for the
heater. The minimum allowable thermal efficiency is 80%, and the maximum allowable standby loss is
given by the following:
SL =
𝑄𝑄
800
+ 110√𝑉𝑉 =
SL =
𝑄𝑄
800
1,825,000 Btu/h
+ 16.6√𝑉𝑉 =
+ 110 × �45 gal = 3019 Btu/h
800
534,900 W
800
+ 16.6 × √170 L = 885 W
(I-P)
(SI)
The boiler thermal efficiency complies; its 82% efficiency is greater than the 80% required. However,
the boiler standby loss does not comply; its 4000 Btu/h (1.2 W) loss is greater than the maximum
3019 Btu/h (885 W) permitted. Because the standby loss requirement applies to the unit’s efficiency
performance as provided by the manufacturer and determined by the test procedure specified in DOE
10 CFR Part 430, another unit with a standby loss that complies must be selected.
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Example 7-C. Equipment Efficiency Requirements, Heat Pump Pool Heaters
Corresponding section: Equipment Efficiency (7.4.2)
Q
A heat-pump system is used to heat pool water. The heating capacity is 50,000 Btu/h (14.65 kW), and
the COP is 3.8 tested according to AHRI 1160. Does it comply with the standard?
A
No. The minimum required efficiency is COP 4.0 for all sizes of heat-pump pool heaters.
Example 7-D. Equipment Efficiency Requirements, Electric Resistance Water Heater
Corresponding section: Equipment Efficiency (7.4.2)
Q
An 82 gal (310 L) electric resistance storage water heater has two 7.5 kW heating elements wired for
nonsimultaneous operation. The energy factor (EF) is 0.87. Does it comply with the standard?
A
No. The equipment size category is determined by the nameplate input rating. Because the elements
are wired for nonsimultaneous operation, the input rating of this model is 7.5 kW. If the nameplate
input is ≤12 kW, then the efficiency requirements are found in the size category ≤12 kW (a category
covered by NAECA).
Table F-2 provides the required EF. For electric resistance (and gas) storage water heaters, there are
different requirements for units with a storage volume that is 55 gal (208.2 L) or less and those with a
larger volume. For units with a volume above 55 gal (208.2 L), the minimum EF is
EF𝑚𝑚𝑚𝑚𝑚𝑚 = 2.057 − 0.00113 × 𝑉𝑉 = 2.057 = 0.00113 × 82 gal = 1.964
(I-P)
EF𝑚𝑚𝑚𝑚𝑚𝑚 = 2.057 − 0.0003 × 𝑉𝑉 = 2.057 = 0.0003 × 310 L = 1.964
(SI)
This heater does not comply because its EF is less than the minimum requirement.
With the minimum acceptable EF in this size range being larger than one, a heat-pump water heater is
required to comply. Alternatively, changing the volume to 55 gal (208.2 L) or less, the required EF will
be significantly lower. For example, the required EF for a 50 gal (189 L) water heater with the same 7.5
kW input rating would be 0.95. The EF for the specified water heater still does not comply, but it
should be possible to identify an electric resistance water heater with the required efficiency.
Example 7-E. Equipment Efficiency Requirements, Condensing Gas Water Heater
Corresponding section: Equipment Efficiency (7.4.2)
Q
A gas water heater has the following characteristics: 1,000,000 Btu/h (293,000 W) input, 23 gal (87 L)
storage, 93% thermal efficiency, and 1500 Btu/h (440 W) standby loss. Does it comply with the
standard?
A
Yes. The water heater’s input-to-volume ratio is given by
Input to Volume Ratio =
𝑄𝑄𝑖𝑖𝑖𝑖
𝑉𝑉ℎ𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒
Input to Volume Ratio =
=
1,000,000 Btu/h
𝑄𝑄𝑖𝑖𝑖𝑖
= 43,478 (Btu⁄h)⁄gal
23 gal
293,000 W
𝑉𝑉ℎ𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒
=
87 L
= 3368 W/L
(I-P)
(SI)
As shown in the subcategory column of Table 7.8, an input-to-volume ratio ≥ 4000 Btu/h∙gal (309.75
W/L) puts this heater in the Gas Instantaneous Water Heater category (lower input-to-volume ratios
would make this a Gas Storage Water Heater for rating purposes). For a tank volume ≥10 gal (37.85 L),
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Table 7.8 sets the required minimum efficiency to 80% with the required maximum standby loss given
as follows:
SL =
𝑄𝑄
800
+ 110√𝑉𝑉 =
SL =
𝑄𝑄
800
1,000,000 Btu/h
+ 110 × �23 gal = 1778 Btu/h
800
293,000 W
+ 16.6√𝑉𝑉 =
800
+ 16.6 × √87 L = 521 W
(I-P)
(SI)
The heater complies because its thermal efficiency is greater than the minimum requirement and the
standby loss is less than the maximum limit.
 Service Hot-Water Piping Insulation (7.4.3)
The standard requires insulation on piping of service hot-water distribution systems. Pipe insulation
reduces heat loss from the hot water, which would otherwise have to be made up by the water heater.
Any heat lost from portions of the piping system that is located in conditioned space increases loads on
the cooling system, so pipe insulation has the additional benefit of minimizing these additional cooling
loads.
The standard requires insulation of the following:
• All piping that carries recirculating hot water. This includes the supply and return piping of a
circulating-tank-type water heater.
• The first 8 ft (2.4 m) of supply piping leaving a nonrecirculating storage water heater.
• The first 8 ft (2.4 m) of branch piping connected to piping that carries recirculating water or that
is heat traced or impedance heated.
• The inlet piping between a nonrecirculating storage water heater and the heat trap.
• Any piping that is externally heated, such as pipe that is heat traced or impedance heated.
The insulation required for the piping is specified in Section 6 of the standard in Table 6.8.3-1. This
table specifies insulation thicknesses based on the temperature of the water and the nominal pipe size.
The thicknesses given, however, are only valid for insulation with a conductivity in a range specified in
the table. An equation is provided in the footnotes to the table that converts these thicknesses to
different values for insulation materials with conductivity outside of that range.
Pipe insulation requirements for typical service hot-water applications are summarized in Table 7-A.
No insulation is required for piping that operates below 105°F (41°C).
If the conductivity of the insulation to be used falls outside the range specified in Table 6.8.3-1,
Note (a) to the table provides the following formula that is used to determine the required thickness.
Example 7-F provides a sample calculation.
where
T =
r =
t =
K =
k =
276
𝑇𝑇 = 𝑟𝑟 × ��1 + 𝑡𝑡�𝑟𝑟�
𝐾𝐾�
𝑘𝑘
− 1�
minimum required insulation thickness for proposed material, in. (mm)
actual pipe outside radius, in. (mm)
insulation thickness, in. (mm), specified in Table 6.8.3-1
conductivity of proposed material (Btu∙in./[h∙ft2∙°F]) at 100°F ([W/m∙°C] at 37.8°C)
the upper value of the conductivity range listed in Table 6.8.3-1 of the standard for the
applicable fluid temperature
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FIGURE 7-B. PIPE INSULATION REQUIREMENTS
Corresponding section: Service Hot-Water Piping Insulation (7.4.3)
TABLE 7-A. MINIMUM PIPE INSULATION THICKNESSES FOR SERVICE HOT-WATER SYSTEMS THAT OPERATE
BETWEEN 105°F AND 140°F (41°C AND 60°C). (SEE TABLE 6.8.3-1 FOR COMPLETE INSULATION
THICKNESS TABLE)
Corresponding section: Service Hot-Water Piping Insulation (7.4.3)
Nominal Pipe Diameter
Less than 1.5 in. (40 mm)
1.5 in. (40 mm) and larger
Minimum Pipe Insulation Thickness*
1.0 in. (25 mm)
1.5 in. (40 mm)
* Applicable for insulation with conductivity of 0.22 to 0.28 Btu⋅in./(h⋅ft²⋅°F) [0.032 to 0.040 W/(m⋅°C)]
Example 7-F. Calculation of Required Insulation Thickness
Corresponding section: Service Hot-Water Piping Insulation (7.4.3)
Q
A designer wants to use polyisocyanurate insulation that has a conductivity of 0.19 Btu∙in./(h∙ft²∙°F)
(0.028 W/[m∙K]). What thickness of insulation is required for a 1.5 in. (38 mm) copper hot-water
supply line operating at 120°F (49°C)? The supply line’s outside diameter (o.d.) is 1.625 in. (41 mm).
A
The conductivity is outside the range of Table 6.8.3-1, repeated in Table 7-A. Therefore, the required
thickness has to be calculated using the procedure described in Standard 90.1, Table 6.8.3-1, Note (a).
In this equation, T is the minimum thickness for the cellular glass insulation; t is the insulation
thickness required in Table 6.8.3-1, 1 in. (25 mm); r is the radius to the outside surface of the pipe
0.81 in. (21 mm); K is the conductivity of the polyisocyanurate insulation, 0.19 Btu∙in./(h∙ft²∙°F) (0.028
W/[m∙K]); and k is the upper end of the range of conductivity in Table 6.8.3-1, 0.28 Btu∙in./(h∙ft²∙°F)
(0.040 W/[m∙K]).
The equation is the same for both I-P and SI units as long as the units match for t and r and for K and k.
𝑇𝑇 = 𝑟𝑟 × ��1 + 𝑡𝑡�𝑟𝑟�
𝐾𝐾�
𝑘𝑘
0.19�
0.28
𝑇𝑇 = 0.81 × ��1 + 1�0.81�
𝑇𝑇 = 21 × ��1 + 25�21�
0.028�
0.040
− 1�
− 1� = 0.59 in.
− 1� = 15 mm
(IP)
(SI)
Using insulation with a lower conductivity, the insulation thickness on the hot-water supply line may
be reduced to 0.90 in. (24 mm) from 1.5 in. (40 mm).
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FYI
Reducing the Risk of Legionnaires’ Disease
Designers should be aware that the bacteria that causes Legionnaires’ disease has been found in
building cold-water systems and in service water heating and distribution systems. Legionella bacteria
can colonize water systems that are not properly designed and operated to maintain proper
disinfectant residual levels and/or water at appropriate water temperatures flowing throughout the
entire building water system.
Refer to the ASHRAE position paper on Legionellosis; ASHRAE Standard 188 (2015); and ASHRAE
Guideline 12, Reducing the Risk of Legionellosis Associated with Building Water Systems, for further
information and guidance.
Service Water Heating System Controls (7.4.4)
The standard includes a number of control requirements.
Temperature Controls (7.4.4.1)
Service hot-water systems that store water must have adjustable controls that allow the temperature
of the stored water to be controlled. The control must be able to set the temperature to 120°F (49°C)
or less, unless the equipment manufacturer specifies a higher minimum to avoid condensation and the
resulting corrosion. The control must also be able to set the water temperature as high as is
appropriate for the application.
To comply with this requirement, the water heater must have thermostatic control with an accessible
set point. This set point must be adjustable down to whichever is lower, 120°F (49°C) or the
manufacturer’s minimum recommended setting to prevent condensation. Both standby and
distribution losses will be minimized by designing a system to provide hot water at the minimum
temperature required.
In addition to the potential energy savings, maintaining water temperature as low as possible reduces
corrosion and scaling of water heaters and components. Another important benefit is improved safety
with respect to scalding.
On the other hand, setting the temperature of stored water too low can increase the risk of microbial
contamination, particularly Legionella. Temperatures lower than 120°F, for example, can place the
system within the ideal temperature range for growth of Legionella bacteria.
Temperature Maintenance Controls (7.4.4.2)
When circulation pumps or heat trace are used, the standard requires automatic controls that are
capable of shutting off the pumps or heat trace when hot water is not required. There are primarily
three forms of controls that meet this criterion: time switch control, combination time and
temperature control, and demand control.
• Time switch control. The simplest complying control system is an automatic time switch. This
can be either a stand-alone system or a contact controlled through a building automation system
(BAS). Stand-alone time switches are available with a wide variety of features. The most
important of these is the ability to have multiple schedules, such as a separate schedule for each
day of the week (seven-day time switch) or the ability to program in holidays (programmable
time switch). Most BASs will permit the system to operate on a variety of schedules. Timecontrolled systems are most appropriate for designs where the hot-water use is fairly constant
and predictable. Where hot-water use is not predictable, time-controlled systems tend to waste
energy, both in terms of the pump and heat loss, because they continue to circulate water from
the tank according to the programmed schedule, regardless of the demand.
• Combination time and temperature control. Time and temperature systems improve on the
automatic time switch scheme by using a temperature sensor to shut off the pump whenever the
return water temperature is hot. The system is allowed to sit idle until the return temperature
drops to a predetermined limit. Typical systems will use a 20°F (7°C) deadband and place the
temperature sensor on the return line. With the advances in low-cost variable-speed circulators,
it is also possible to use modulating speed control. These systems reduce line losses 10% to 20%
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by reducing the average temperature of the fluid in the line. They will reduce pump energy by up
to 90%, depending on the frequency of hot-water demand.
• Demand control. Demand-controlled systems use flow sensors to sense the draw of water from
the system. On smaller systems, the sensor will typically be located on the inlet to the storage
tank. On more extensive systems, several flow sensors wired in parallel will be located at each
branch off the main loop. On detection of flow, the circulation pump is initiated. The pump can be
shut off either through an adjustable interval timer or a temperature sensor located on the return
line. Demand-controlled systems will significantly reduce both the line losses and the pump
energy.
Where heat trace tape or other means are used to maintain water temperatures in the hot-water pipes,
time controls must be provided. The piping must also be insulated according to the requirements for
circulating systems. Heat trace is an alternative to circulating systems to maintain temperature in a
domestic hot-water distribution system. Using heat trace eliminates recirculating pipes and the related
heat loss and pumping energy, and may be more conducive to renewable energy use, but uses more
expensive electrical energy and is more difficult to adjust to different temperatures when codes or
guidelines change.
Outlet Temperature Controls (7.4.4.3)
The standard requires that some means of controlling the maximum temperature delivered from
lavatory faucets in public restrooms to no more than 110°F (43°C) be included in the system.
Circulating Pump Controls (7.4.4.4)
The pumps that charge hot-water storage tanks with remote heaters must be controlled with time
controls that limit the operation of circulation pumps after the heater has been shut off. In many
systems, the pump continues to operate at the end of the heating cycle to cool down the heater.
The standard requires these circulating pump controls to provide a maximum of five minutes between
the end of the heating cycle and the shutdown of the circulation pump.
Swimming Pools (7.4.5)
In addition to heaters needing to meet the requirements of Table 7.8 of the standard for minimum
thermal efficiency, there are several additional requirements for pools.
Pool Heaters (7.4.5.1)
Pool heaters must have a manual on/off control that is readily accessible to allow the heater to be shut
off. This must be a dedicated switch or contact. The thermostat set point adjustment may not be used
to satisfy this requirement. The purpose of this requirement is to encourage the occupants or
maintenance personnel to disable the heater when it is not needed. For that reason, the switch must be
readily accessible (see the definition of this phrase in Section 3 of the standard) and easy to use. For
pools in public facilities, the manual on/off switch may be in a locked control panel so that it is not
accessible to the public. However, facility staff must have access to the control panel at all times.
In addition, continuously burning pilot lights are prohibited on natural-gas-fired pool heaters. Either a
pilotless ignition system or an intermittent ignition system will satisfy this requirement.
• Time switch. This must be provided for all pool pumps and pool heaters. Exceptions are
provided for pumps that must operate continuously to meet public health standards and pumps
that use solar or waste heat recovery to heat the pool. Automatic programmable time switches
will meet the requirements and will help reduce energy costs through automatic control.
Pool Covers (7.4.5.2)
Pools lose heat primarily through three mechanisms: radiation, convection, and evaporation. Of these
three, the largest component is generally the evaporation loss, which accounts for 50% to 60% of the
overall heat loss in most cases. The standard requires all swimming pools that are heated to a
temperature of 90°F (32°C) to have covers. This applies to pools located either outdoors or indoors.
Pool covers must be vapor retardant to reduce evaporation losses and have a minimum insulation
value of R-12 (R-2.1). (For information on equipment sizing and heat loss from pools, see Chapter 5 of
the 2015 ASHRAE Handbook—HVAC Applications.)
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Pools that receive over 60% of their energy (computed over an annual operating season) from either
heat recovery or site solar energy do not need covers. Heat recovered from a pool dehumidification
system can be used to meet this requirement. Many pool dehumidification systems have heat recovery
for space or water heating as a standard option.
Note that the 60% figure refers to the heat required by the pool and is not an indication of the
efficiency of the heating source. An analysis consistent with the Energy Cost Budget Method (Section
11) or Performance Rating Method (Appendix G) may be used to demonstrate the percentage of
heating through heat recovery.
Example 7-G. Heat Recovery for Pools, Cogeneration
Corresponding section: Pool Covers (7.4.5.2)
Q
If a pool is heated using waste heat from an on-site electric generating system, are pool covers
required?
A
No, because waste heat is used to heat the pool, it is considered site-recovered energy.
Example 7-H. Heat Recovery for Pools, Dehumidification System
Corresponding section: Pool Covers (7.4.5.2)
Q
An Olympic-size swimming pool receives 80% of its heat (on a yearly basis) through heat recovered
from a dehumidification system. Are pool covers required?
A
No, because the pool is heated through heat recovery. If the dehumidification system could not provide
more than 60% of the annual heating load for the pool, either a cover or additional heat recovery from
another source would be required.
Time Switches (7.4.5.3)
All pool pumps and pool heaters must be provided with time switches. This requirement provides the
capability to automatically shut off pumps and heaters during periods when the pump is not being
used, typically because the facility is closed.
Time switches are not required on pumps that must operate continuously to meet public health
standards and pumps that must operate to use solar or waste heat recovery to heat the pool.
Heat Traps (7.4.6)
A heat trap is a device or arrangement of piping that keeps buoyant hot water from circulating through
a piping distribution system through natural convection. By restricting the flow from the storage tank,
standby heat loss is minimized.
Heat traps are required for storage heaters and storage tanks in noncirculating systems with vertical
piping. Storage heaters with integral heat traps on both inlet and outlet piping satisfy this requirement.
External heat traps must be insulated and should be placed as close as possible to the tank inlet and
outlet fittings.
Heat traps may be configured for inlet and outlet connections on the top (Figure 7-C), bottom (Figure
7-D), and sides (Figure 7-E) of heaters and storage tanks. In all configurations, heat traps can be a 360°
loop of piping (Figure 7-F), a premanufactured device, or some arrangement of piping and elbows that
forms an inverted “U” on the tank fittings. Tanks that have horizontal outlets need only a section of
vertical pipe that turns downward after leaving the tank (an inverted “L”; see Figure 7-E).
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FIGURE 7-C. HEAT TRAP AND INSULATION REQUIREMENTS FOR NONCIRCULATING SYSTEMS
FIGURE 7-D. HEAT TRAPS ON A TANK WITH CONNECTIONS ON BOTTOM
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FIGURE 7-E. HEAT TRAPS ON A TANK WITH CONNECTIONS ON SIDES
FIGURE 7-F. HEAT TRAP THROUGH FLEXIBLE PIPE LOOP
Prescriptive Path (7.5)
Section 7.5 only applies to projects that use the Prescriptive Method of compliance; it does not apply to
projects that use the Energy Cost Budget Method (Section 11). Regardless of which method is used to
demonstrate compliance, all of the mandatory provisions (Section 7.4) must be satisfied. In addition,
the submittals required by Section 7.7, which reference the requirements in Section 4.2.2, are required
in order to use the Prescriptive Method.
Space Heating and Service Water Heating (7.5.1)
Combined space and water heating systems use one or more boilers or water heaters to serve both the
seasonal heating load and the service water heating load. Systems that are covered by these
requirements include the following:
• Combined hydronic heaters
• Indirect service water tube bundles in space-heating water or steam boilers
• Service water heat exchangers using steam from a space-heating boiler
Energy can be wasted in combination systems by using an oversized boiler (sized to concurrent space
and service water heating loads) to perform water heating alone after the heating season is over. The
requirements of Section 7.5.1 are intended to minimize this wasted energy. Systems that serve to heat
both space and water must meet one of three conditions:
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1.
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The single space-heating boiler or component of a modular or multiple boiler system that is
heating the service water has a standby loss not exceeding the following:
SL𝑚𝑚𝑚𝑚𝑚𝑚 =
(I-P)
𝑛𝑛
3.7 × 106 × pmd + 117
SL𝑚𝑚𝑚𝑚𝑚𝑚 =
where
pmd =
13.3 × pmd + 400
(SI)
𝑛𝑛
the probable maximum demand in gal/h (m3/s) as determined using standard
published procedures (see Table 7-B)
n
= the fraction of the year when the outdoor daily mean temperature is greater than 64.9°F
(18.3°C)2
Example 7-I provides a sample calculation of the maximum allowable standby loss.
2. It is demonstrated to the authority having jurisdiction that the combined system will use less
energy than separate space and water heating systems. For instance, a designer may provide
calculations showing that the addition of a heat exchanger to the space-heating boilers to heat
water for a lavatory in a cold climate will use less energy than a dedicated heater due to reduction
in standby losses.
3. The heater input rating of the combined system is <150,000 Btu/h (<44 kW). 2
The standby loss rating in the first requirement is to be determined by a 24-hour test performed either
at the factory or in the field. Section 7.5.1 specifies that the test must be conducted with the following
conditions: the boiler water must be maintained at a minimum boiler water temperature 90°F (32°C)
greater than the ambient temperature, the ambient air temperature must be maintained between 60°F
and 90°F (16°C and 32°C) throughout the test, and the boiler burner must be operated only at its
minimum input rating. The designer must include the test report with the compliance documents.
Requirement 3 is likely to be met by most combination systems serving individual residential dwelling
units.
Example 7-I. Calculation of Allowable Standby Loss for Combination Space and Water Heating Equipment
Corresponding sections: Space Heating and Service Water Heating (7.5.1)
Q
What is the standby loss requirement for a combination space and water heating system for a 40 unit
apartment building in San Francisco, California?
A
From Table 7-B, the probable maximum demand (pmd) is given by
pmd = 40 units × 10
pmd = 40 units × 37.9
L
h∙unit
×
m³
gal
h∙unit
1,000L
×
= 400
h
3,600 s
gal
h
= 421 × 10−6
m³
s
(I-P)
(SI)
Using hourly typical meteorological year (TMY) data from San Francisco, we find that n = 0.65, which is
the fraction of the year when the outdoor daily mean temperature is greater than 64.0°F (18.3°C).
Using these values, the standby loss (SLmax) requirement is given by:
SL𝑚𝑚𝑚𝑚𝑚𝑚 =
SL𝑚𝑚𝑚𝑚𝑚𝑚 =
13.3 × pmd + 400
𝑛𝑛
3.7 × 106 × pmd + 117
𝑛𝑛
=
=
13.3 × 400 + 400
= 8809 Btu/h
0.65
3.7 × 106 × 421 × 10−6 + 117
0.65
= 2576 W
The boiler will comply with requirement 1 of section 7.5.1 if it has a standby loss of less than
8809 Btu/h (2576 W) as determined by a 24-hour test either at the factory or in the field.
2
The value n can be calculated from an hourly weather file. Simply count the number of hourly outdoor dry-bulb
temperature values that exceed 64.9 and divide that number by 8760 hours per year.
Standard 90.1 User’s Manual
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(SI)
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TABLE 7-B. PROBABLE MAXIMUM DEMAND
Corresponding sections: Space Heating and Service Water Heating (7.5.1)
Source: Table 6 from Chapter 50 of the 2015 ASHRAE Handbook—HVAC Applications
Type of Building
Maximum Hourly
Men’s dormitories
3.8 gal (14.4 L)/student
Women’s dormitories
5.0 gal (18.9 L)/student
Motels: Number of unitsa
20 or less
6.0 gal (22.7 L)/unit
60
5.0 gal (18.9 L)/unit
100 or more
4.0 gal (15.1 L)/unit
Nursing homes
4.5 gal (17 L)/bed
Office buildings
0.4 gal (1.5 L)/person
Food service establishments
Type A—full-meal
1.5 gal (5.7 L)/max meals/h
restaurants and cafeterias
Type B—drive-ins, grills,
0.7 gal (2.6 L)/max meals/h
luncheonettes, sandwich
and snack shops
Apartment houses
20 or less apartments
12.0 gal (45.4 L)/apartment
50 apartments
10.0 gal (37.9 L)/apartment
75 apartments
8.5 gal (32.2 L)/apartment
100 apartments
7.0 gal (26.5 L)/apartment
200 or more apartments
5.0 gal (18.9 L)/apartment
Elementary schools
0.6 gal (2.2 L)/student
Junior and senior high schools
1.0 gal (3.8 L)/student
a Interpolate for intermediate values. b Per day of operation.
Maximum Daily
Average Daily
22.0 gal (83.3 L)/student
26.5 gal (100 L)/student
13.1 gal (49.6 L)/student
12.3 gal (46.6 L)/student
11.0 gal (41.6 L)/max meals/day
2.4 gal (9.1 L)/max meals/dayb
35.0 gal (132 L)/unit
25.0 gal (94.6 L)/unit
15.0 gal (56.8 L)/unit
30.0 gal (114 L)/bed
2.0 gal (7.6 L)/person
6.0 gal (22.7 L)/max meals/day
80.0 gal (303 L)/apartment
73.0 gal (276 L)/apartment
66.0 gal (250 L)/apartment
60.0 gal (227 L)/apartment
50.0 gal (189 L)/apartment
1.5 gal (5.7 L)/student
3.6 gal (13.6 L)/student
20.0 gal (75.7 L)/unit
14.0 gal (53 L)/unit
10.0 gal (37.9 L)/unit
18.4 gal (69.7 L)/bed
1.0 gal (3.8 L)/person
0.7 gal (2.6 L)/max meals/dayb
42.0 gal (159 L)/apartment
40.0 gal (151 L)/apartment
38.0 gal (144 L)/apartment
37.0 gal (140 L)/apartment
35.0 gal (132 L)/apartment
0.6 gal (2.2 L)/studentb
1.8 gal (6.8 L)/studentb
Service Water Heating Equipment (7.5.2)
Service water heating equipment used in combination systems must satisfy the minimum performance
requirements of Section 7.4.2. Space heating equipment used in combination systems must satisfy the
applicable minimum performance requirements of Section 6.4.1. The distribution piping, pumps,
controls, and terminal devices for service hot water in combination systems must meet all of the
requirements of Section 7.
Buildings with High-Capacity Service Water Heating Systems (7.5.3)
Section 7.5.3 applies to new buildings with gas service water heating systems with a total installed
input capacity of 1,000,000 Btu/h (293 kW) or more. Section 7.5.3 does not apply to systems where at
least 25% of the annual service water heating load is met by site solar or site recovered energy
(Exception 1). When calculating the total gas water heater input capacity, certain water heaters are not
included:
• Water heaters installed in individual dwelling units (Exception 2)
• Water heaters with an input capacity of 100,000 Btu/h (29.3 kW) or less (Exception 3)
When Section 7.5.3 applies to a system, the water heating units in that system must have a capacity
weighted average thermal efficiency of 90% or more. If the system includes multiple water heaters
with different efficiencies, the capacity weighted average efficiency of the system is calculated as
follows:
Capacity Weighted Average Efficiency =
∑(Input Capacity × Efficiency)
∑ Input Capacity
=
Total Output Capacity
Total Input Capacity
Site Solar or Site-Recovered Energy (Exception 1 to 7.5.3)
Where the input capacity of the gas service water heating system in a new building exceeds the
1,000,000 Btu/h (293 kW) threshold, it is exempt if more than 25% of the annual projected output of
that gas service water heating system is produced on site by solar energy harvesting or by waste
energy recovered on site. Note that this exemption is based on annual energy production rather than
peak power capacity. An annual calculation of the projected gas service water heating consumption
must be compared to the projected solar or site-recovered energy production. Consideration must also
be given to the fact that solar or site-recovered energy production may not coincide with gas service
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water heating demand. Therefore, sufficient storage is required to store the solar or site-recovered
energy until it is required for hot-water production. There may be situations where storage capacity
limits the total energy that can be harvested from the solar or site-recovered source. Credit may not be
claimed for any lost excess energy.
The standard does not limit the site solar energy to a specific type of solar energy technology. The two
primary forms of solar energy production are solar thermal and photovoltaic. Solar thermal converts
the sunlight directly into heat, which is transferred to the load by circulating water or other heat
transfer fluid. Photovoltaic converts the light from the sun directly into electricity, which can be used
for electric resistance heating. While the electricity produced by photovoltaic is easily used for loads
other than water heating, the energy production per area of collector is generally lower than that of
solar thermal. If the solar collectors are to be used solely for offsetting service water production, solar
thermal typically requires less collector area to offset the same service water heating energy. For
additional information on solar systems, refer to Chapter 37 of the 2016 ASHRAE Handbook—HVAC
Systems and Equipment.
Site-recovered energy may use any site-produced waste heat for the production of service hot water.
Examples of waste energy that can be reclaimed would be condenser heat from a refrigeration cycle,
excess heat produced by a manufacturing process, or high-temperature exhaust heat recovery. Other
types of site-recovered energy likely exist and may qualify for this exemption.
Dwelling Unit Service Water Heating (Exception 2 to 7.5.3)
The efficiency requirements in Section 7.5.3 do not apply to water heaters located within dwelling
units. For example, a 20-unit residential tower has a 60,000 Btu/h (17.5 kW) input gas water heater in
each dwelling unit. The total service water heating gas input for the building, 1,200,000 Btu/h
(352 kW), exceeds the input threshold and would indicate the thermal efficiency requirement of this
section must be met. However, none of these water heaters are included when calculating the total
input capacity in the building for comparison to the 1,000,000 Btu/h (293 kW) input threshold
because they are each located in a dwelling unit. For purposes of section 7.5.3, therefore, the total
input capacity for this building is zero, and the efficiency requirement does not apply.
Individual Water Heater Input Capacity (Exception 3 to 7.5.3)
Individual gas service water heaters with an input rating of 100,000 Btu/h (29.3 kW) or less are not
required to meet the requirements of Section 7.5.3. For example, a new 20-unit shopping mall has a
100,000 Btu/h (29.3 kW) input gas water heater in each commercial unit. The total gas water heating
input for the building, 2,000,000 Btu/h (586 kW), exceeds the input threshold and would indicate the
thermal efficiency requirements of this section must be met. However, none of these water heaters are
included when calculating the total input capacity in the building for comparison to the 1,000,000
Btu/h (293 kW) input threshold because none of them have an input capacity that exceeds 100,000
Btu/h (29.3 kW). For purposes of Section 7.5.3, therefore, the total input capacity for this building is
zero, and the efficiency requirement does not apply.
Example 7-J. Determining if a Gas Service Water Heater is Subject to the Requirements of Section 7.5.3
Corresponding section: Buildings with High-Capacity Service Water Heating Systems (7.5.3)
Q
A new building has a daily average gas service water heating design load of 7,500,000 Btu (2198 kW).
This load will be met by using two 83% thermally efficient 500,000 Btu/h (147 kW) gas water heaters.
A roof top solar thermal system provides an annual total of 500,000 kBtu per year. Does this system
comply with the standard?
A
No.
The annual water heating load is 7,500,000 Btu/day × 365 days/yr = 2737 × 106 Btu/yr. The total
annual load met by the solar thermal system is 500 × 106 Btu/yr. This is only 18.3% of the annual
water heating load, so the exception does not apply.
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The total input capacity of the gas boilers in the system is 1,000,000 Btu/h, so the requirements of
section 7.5.3 apply. The minimum thermal efficiency (Et) required for the gas service water heating
system is therefore 90%, and the specified water heaters do not comply.
Example 7-K. Determining if a Gas Service Water Heater is Subject to the Requirements of Section 7.5.3
Corresponding section: Buildings with High-Capacity Service Water Heating Systems (7.5.3)
Q
A new building has a service water heating system containing one electric water heater with an input
of 1,500,000 Btu/h (440 kW) and one 83% thermally efficient gas water heater with an input of
500,000 Btu/h (147 kW) for a total service water heating input of 2,000,000 Btu/h (586 kW). Does this
system comply with Section 7.5.3 of the standard?
A
Yes. The requirements of Section 7.5.3 only apply to the gas service water heating input. Because the
total gas service water heater input is less than 1,000,000 Btu/h (293 kW), the requirements of Section
7.5.3 do not apply.
Example 7-L. Compliant Input Capacity-Weighted Average Thermal Efficiency
Corresponding section: Buildings with High-Capacity Service Water Heating Systems (7.5.3)
Water
Heater
Input
(Btu/h)
Input
(kW)
Thermal
Efficiency
Output
(Btu/h)
Output
(kW)
Notes
1
85,000
25
82%
NA
NA
Exempt - ≤100,000 Btu/h (29.3 kW)
750,000
220
96%
720,000
211
1,250,000
366
90.1%
1,126,000
330
2
90,000
3
125,000
6
200,000
4
5
Total
300,000
26
37
88
59
84%
82%
82%
80%
NA
NA
246,000
160,000
NA
Exempt - ≤100,000 Btu/h (29.3 kW)
NA
Exempt - Individual dwelling unit
72
47
Capacity weighted average efficiency is calculated for only the water heaters that are not exempt
according to Exceptions 2 or 3, and the total output capacity/total input capacity is equal to 90.1%.
This exceeds the 90% weighted efficiency requirement, so this system complies with section 7.5.3.
Example 7-M. Noncompliant Input Capacity-Weighted Average Thermal Efficiency
Corresponding section: Buildings with High-Capacity Service Water Heating Systems (7.5.3)
(Water Heaters are Identical to Example 7-L, except Unit 4 is now 92% efficient)
Water
Heater
Input
(Btu/h)
Input
(kW)
Thermal
Efficiency
Output
(Btu/h)
Output
(kW)
Notes
1
85,000
25
82%
NA
NA
Exempt - ≤100,000 Btu/h (29.3 kW)
750,000
220
92%
690,000
202
1,250,000
366
87.7%
1,096,000
321
2
90,000
3
125,000
6
200,000
4
5
Total
300,000
26
37
88
59
84%
82%
82%
80%
NA
NA
246,000
160,000
NA
NA
72
Exempt - ≤100,000 Btu/h (29.3 kW)
Exempt - Individual dwelling unit
47
Capacity weighted average efficiency is calculated for only the water heaters that are not exempt
according to Exceptions 2 or 3, and the total output capacity/total input capacity is equal to 87.7%.
This does not exceed the 90% weighted efficiency requirement, so this system does not comply with
Section 7.5.3.
286
Standard 90.1 User’s Manual
Cha p t e r 8 | P o w e r
8 Power
Changes to the Power Section
 Feeder conductors and branch circuits are now to be sized together for a maximum of 5% voltage
drop (8.4.1).
 Requirements for transmission to the digital control system and for graphical display of
information have been added for buildings with digital control systems installed (8.4.3.2).
These changes are marked with  in the margins of this chapter. For the specific addenda that define
the differences between the 2013 and 2016 editions of Standard 90.1, see Appendix H to the standard.
General (8.1)
Scope (8.1.1)
The requirements of Section 8 apply to all power distribution systems in buildings that are covered by
the standard. For a review of the standard’s general scope, see Chapter 2 of this user’s manual.
In the case of alterations to existing facilities, when modifications are made to the electric power
distribution system, the requirements of the standard apply to the components that are being modified
or replaced but not to the entire system. However, when an addition or alteration is made to the
system, the voltage drop (VD) analysis must include the parts of the existing system extending to the
point of electrical supply at the transformer or service entrance equipment.
Mandatory Provisions (8.4)
All the requirements in Section 8 are mandatory and must always be met, regardless of whether the
Energy Cost Budget (ECB) Method (Section 11) or Performance Rating Method (Appendix G) are used
to show compliance.
The requirements in Section 8.4 relate to the following:
• Maximum voltage drop in electrical conductors (Section 8.4.1). This requirement saves energy by
directly limiting power loss in the distribution system.
• Automatic receptacle control (Section 8.4.2). This includes requiring that 50% of all 125 V
receptacle outlets (which will typically operate at 110 V to 120 V) in all private offices, conference
rooms, rooms used primarily for printing and/or copy functions, break rooms, classrooms, and
individual workstations be automatically switched off when the space in which they are located is
not occupied.
• Electrical energy monitoring and reporting must be performed for all major electricity consumers
within commercial buildings (Section 8.4.3). Monitoring electrical energy consumption for major
electricity consumers can be invaluable in identifying and eliminating unnecessary electrical
energy consumption.
• Efficiency of dry-type distribution transformers (Section 8.4.4). The standard has efficiency
requirements that are generally greater than 98% for both single-phase and three-phase dry-type
transformers.
 Voltage Drop (8.4.1)
The standard requires electrical feeders and branch circuits be designed such that total voltage drop
does not exceed 5% from the service equipment to the load. In practice, the voltage drop limitation
results in selecting conductors and conduits based on the practice described in Table 9 of the 2011
National Electrical Code® Handbook, which is repeated here as Table 8-A. In this table, the voltage drop
is dependent on the following:
• Circuit type (single-phase or three-phase)
• Number and size of conductors per phase
Standard 90.1 User’s Manual
287
Cha p t e r 8 | P o w e r
• Conduit types (magnetic or nonmagnetic)
• Power factor of the load
• Circuit length
• Load current
Electrical codes may also set minimum wire sizes in some instances. These minimum wire sizes in
certain sections of some codes are usually intended to provide practical trade sizes for electricians and
to match the short-circuit protection provided by overcurrent devices normally installed for the
application. The short-circuit protection is based on the ability of the conductor insulation to
withstand fault currents without destructing. The voltage drop requirements in the standard are
required by the National Electrical Code (NEC).
There are two types of problems caused by significant voltage drop. First, in most electrical circuits the
current increases as voltage at the load drops because the load requires a certain amount of power.
When the current increases, there is an increase in the power loss within the conductor that varies as
the square of the current. Therefore, the voltage drop is an energy-efficiency issue. Second, the voltage
drop in the conductors, if excessive, may result in equipment operation problems or equipment failure.
Note that the voltage drop requirements of the standard do not consider voltage drop through
transformers. The output voltage of a transformer will drop as the load increases or as the power
factor of the load decreases. Therefore, meeting the voltage drop requirements in the standard does
not guarantee proper equipment operation.
Voltage drop calculations are illustrated in Example 8-A for simple single-phase circuits and in
Example 8-B for three-phase circuits. For more details, refer to the National Electrical Code Handbook
published by the National Fire Protection Association (NFPA).
Example 8-A. Voltage Drop Calculation, Single-Phase Circuit
Corresponding section: Voltage Drop (8.4.1)
(Example adapted from the 2011 National Electrical Code® Handbook)
Q
What is the line-to-line voltage drop in a 240 V, two-wire, single-phase heating circuit with a load of
50 amp? The circuit consists of type THHN copper conductors size 6 AWG, and the one-way circuit
length is 100 ft (30.5 m). Does the voltage drop meet the requirements of the standard?
A
The voltage drop equation for a single-phase circuit with 100% power factor (PF) is
VD =
2 × 𝐿𝐿 × 𝑍𝑍 × 𝐼𝐼
1000
where
VD = voltage drop (based on conductor temperature of 167°F [75°C])
L = one-way length of circuit (ft [m])
Z = conductor effective impedance in ohms/1000 ft (ohms/km) (from the National Electrical Code
Handbook, Chapter 9, Table 9, repeated as Table 8-A in this user’s manual)
I
= load current accounting for PF (amperes)
For this conductor, the impedance listed in Table 8-A depends on the conduit type (polyvinyl chloride
[PVC], aluminum, or steel). The impedance also depends on the PF of the load. Neither the conduit type
nor the PF is specified above. In this case it is reasonable to use the values in the column labeled
“Effective Z at 0.85 PF for Uncoated Copper Wires,” which is intended to apply to a typical situation.
For conduit type, a reasonable assumption is the worst-case condition (i.e., steel). Based on these
assumptions, the impedance is 0.45 ohms/1000 ft (1.48 ohms/km).
Substituting values for this example into the equation, the voltage drop is determined to be 4.50 V.
288
VD =
2 × 100 ft × 0.45 × 50
1000
= 4.50 V
(I-P)
Standard 90.1 User’s Manual
VD =
2 × 30.5 m × 1.48 × 50
1000
Cha p t e r 8 | P o w e r
= 4.50 V
Next, find the approximate voltage drop expressed as a percentage of the circuit voltage.
Percentage VD𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑡𝑡𝑡𝑡 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 =
4.50 V
×
240 V
100% = 1.88%
(SI)
Because the voltage drop is less than 5%, it meets the requirements of the standard.
Example 8-B. Voltage Drop Calculation, Three-Phase Circuit
Corresponding section: Voltage Drop (8.4.1)
(Example adapted from the 2011 National Electrical Code® Handbook)
Q
A 270 amp continuous load is present on a feeder. The circuit consists of a single 4 in. (102 mm) PVC
conduit with three 600 kcmil XHHW/USE aluminum conductors supplied from a 480 V, three-phase,
three-wire source. The conductors are operating at their maximum rated temperature of 170°F (75°C).
If the power factor (PF) is 0.7 and the circuit length is 250 ft (76 m), does the voltage drop meet the
requirements of the standard?
A
Step 1: Using Table 8-A, column “XL (Reactance) for All Wires,” select PVC conduit and the row for size
600 kcmil. A value of 0.039 ohms/1000 ft (0.128 ohms/km) is given as this XL. Next, using the column
“Alternating-Current Resistance for Aluminum Wires,” select PVC conduit and the row for size 600
kcmil. A value of 0.036 ohms/1000 ft (0.118 ohms/km) is given for this R.
Step 2: Find the angle representing a PF of 0.7.
Find the arccosine (cos-1) of 0.7, which is 45.57 degrees. For this example, we will call this angle Φ. For
Step 3, also calculate the sine of 45.57 degrees, which is 0.7141.
Step 3: Find the impedance Zc corrected to 0.7 PF.
𝑍𝑍𝑐𝑐 = (𝑅𝑅 × cos Φ) + (𝑋𝑋𝐿𝐿 × sin Φ)
𝑍𝑍𝑐𝑐 = (0.036 × 0.7) + (0.039 × 0.7141) = 0.0531 ohms to neutral
𝑍𝑍𝑐𝑐 = (0.118 × 0.7) + (0.128 × 0.7141) = 0.1740 ohms to neutral
Step 4: Find the approximate line-to-neutral voltage drop.
Circuit length
× Circuit load
1000
250 ft
VD𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑡𝑡𝑡𝑡 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 = 0.0531 ohms ×
× 270 amp = 3.584 V
1000 ft
76.2 m
VD𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑡𝑡𝑡𝑡 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 = 0.1740 ohms ×
× 270 amp = 3.584 V
1000 m
VD𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑡𝑡𝑡𝑡 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 = 𝑍𝑍𝑐𝑐 ×
Step 5: Find the approximate line-to-line voltage drop.
VD𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑡𝑡𝑡𝑡 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 = VD𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑡𝑡𝑡𝑡 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 × √3
VD𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑡𝑡𝑡𝑡 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 = 3.584 V × 1.732 = 6.208 V
(I-P)
(SI)
(I-P)
(SI)
Step 6: Find the approximate voltage drop expressed as a percentage of the circuit voltage.
Percentage VD𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑡𝑡𝑡𝑡 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 =
6.208 V
× 100%
480 V
= 1.29%
Because the voltage drop is less than 5%, it meets the requirements of the standard.
Automatic Receptacle Control (8.4.2)
The standard requires that 50% of all 125 V, 15 and 20 amp receptacles (which will typically operate
at 110 V to 120 V) in private offices, conference rooms, printing/copying rooms, break rooms,
classrooms, and individual workstations be automatically switched off when the space in which they
are located is not occupied. Individual workstations created with modular furniture are required to
have at least 50% of the receptacles in each workstation automatically controlled. Where the modular
furniture is not shown in the construction documents, no less than 25% of the branch circuits serving
Standard 90.1 User’s Manual
289
Cha p t e r 8 | P o w e r
the modular furniture must be automatically controlled. This provides a means to control 50% of the
receptacles integral to the modular furniture. If all the workstation receptacles accessible to the
occupant reside in the modular furniture, at least 50% of them must be automatically controlled. If
automatically controlled receptacles are located in the floor or wall but serve the workstation, these
may be counted toward the automatically controlled percentage.
TABLE 8-A. ALTERNATING-CURRENT RESISTANCE AND REACTANCE
Corresponding section: Voltage Drop (8.4.1)
For 600 volt Cables, Three-Phase, 60 Hz, 167°F (75°C)—Three Single Conductors in Conduit*
Ohms to Neutral per 1000 Feet
XL (Reactance) Alternating-Current
for All Wires Resistance for
Uncoated Copper
Wires
Effective Z at 0.85 PF Effective Z at 0.85 PF
for Uncoated Copper for Aluminum Wires
Wires
Aluminum
Conduit
Steel Conduit
PVC Conduit
Aluminum
Conduit
Steel Conduit
PVC Conduit
Aluminum
Conduit
Steel Conduit
PVC Conduit
Aluminum
Conduit
Steel Conduit
14
0.058
0.073 3.1
3.1
3.1
-
-
-
2.7
2.7
2.7
-
-
-
14
10
0.050
0.063 1.2
1.2
1.2
2.0
2.0
2.0
1.1
1.1
1.1
1.8
1.8
1.8
10
12
8
6
4
3
2
1
1/0
2/0
3/0
4/0
250
300
350
400
500
600
750
1000
290
0.054
0.052
0.051
0.048
0.047
0.045
0.046
0.044
0.043
0.042
0.041
0.041
0.041
0.040
0.040
0.039
0.039
0.038
0.037
PVC Conduit
Steel Conduit
Size
(AWG or
kcmil)
PVC, Aluminum
Conduits
Size
(AWG
or
kcmil)
Alternating-Current
Resistance for
Aluminum Wires
0.068 2.0
0.065 0.78
0.064 0.49
0.060 0.31
0.059 0.25
0.057 0.19
0.057 0.15
0.055 0.12
0.054 0.10
2.0
0.78
0.49
0.31
0.25
0.20
0.16
0.13
0.10
0.052 0.077 0.082
0.051 0.062 0.067
0.052 0.052 0.057
0.051 0.044 0.049
0.050 0.038 0.043
0.049 0.033 0.038
0.048 0.027 0.032
0.048 0.023 0.028
0.048 0.019 0.024
0.046 0.015 0.019
2.0
0.78
0.49
0.31
0.25
0.20
0.16
0.12
0.10
0.079
0.063
0.054
0.045
0.039
0.035
0.029
0.025
0.021
0.018
3.2
1.3
0.81
0.51
0.40
0.32
0.25
0.20
0.16
0.13
0.10
0.085
0.071
0.061
0.054
0.043
0.036
0.029
0.023
3.2
1.3
0.81
0.51
0.41
0.32
0.26
0.21
0.16
0.13
0.11
0.090
0.076
0.066
0.059
0.048
0.041
0.034
0.027
3.2
1.3
0.81
0.51
0.40
0.32
0.25
0.20
0.16
0.13
0.10
0.086
0.072
0.063
0.055
0.045
0.038
0.031
0.025
1.7
0.69
0.44
0.29
0.23
0.19
0.16
0.13
0.11
1.7
0.69
0.45
0.29
0.24
0.19
0.16
0.13
0.11
1.7
0.7
0.45
0.30
0.24
0.20
0.16
0.13
0.11
0.088 0.092 0.094
0.074 0.078 0.080
0.066 0.070 0.073
0.059 0.063 0.065
0.053 0.058 0.060
0.049 0.053 0.056
0.043 0.048 0.05
0.040 0.044 0.047
0.036 0.040 0.043
0.032 0.036 0.040
2.8
1.1
0.71
0.46
0.37
0.30
0.24
0.19
0.16
0.13
0.11
2.8
1.1
0.72
0.46
0.37
0.30
0.24
0.20
0.16
0.13
0.11
2.8
1.1
0.72
0.46
0.37
0.30
0.25
0.20
0.16
0.14
0.11
0.094 0.098 0.100
0.082 0.086 0.088
0.073 0.077 0.080
0.066 0.071 0.073
0.057 0.061 0.064
0.051 0.055 0.058
0.045 0.049 0.052
0.039 0.042 0.046
12
8
6
4
3
2
1
1/0
2/0
3/0
4/0
250
300
350
400
500
600
750
1000
Standard 90.1 User’s Manual
Cha p t e r 8 | P o w e r
Ohms to Neutral per Kilometer
XL (Reactance) Alternating-Current
for All Wires Resistance for
Uncoated Copper
Wires
Effective Z at 0.85 PF
for Aluminum Wires
PVC Conduit
Aluminum
Conduit
Steel Conduit
PVC Conduit
Aluminum
Conduit
Steel Conduit
PVC Conduit
Aluminum
Conduit
Steel Conduit
PVC Conduit
Aluminum
Conduit
Steel Conduit
Size
(AWG
or
kcmil)
Steel Conduit
Effective Z at 0.85 PF
for Uncoated Copper
Wires
PVC, Aluminum
Conduits
Size
(AWG or
kcmil)
Alternating-Current
Resistance for
Aluminum Wires
14
0.190
0.240
10.2
10.2
10.2
-
-
-
8.9
8.9
8.9
-
-
-
14
10
0.164
0.207
3.9
3.9
3.9
6.6
6.6
6.6
3.6
3.6
3.6
5.9
5.9
5.9
10
12
8
6
4
3
2
1
1/0
2/0
3/0
4/0
250
300
350
400
500
600
750
1000
0.177
0.171
0.167
0.157
0.154
0.148
0.151
0.144
0.141
0.138
0.135
0.135
0.135
0.131
0.131
0.128
0.128
0.125
0.121
0.223
0.213
0.210
0.197
0.194
0.187
0.187
0.180
0.177
0.171
0.167
0.171
0.167
0.164
0.161
0.157
0.157
0.157
0.151
6.6
2.56
1.61
1.02
0.82
0.62
0.49
0.39
0.33
0.253
0.203
0.171
0.144
0.125
0.108
0.089
0.075
0.062
0.049
6.6
2.56
1.61
1.02
0.82
0.66
0.52
0.43
0.33
0.269
0.220
0.187
0.161
0.141
0.125
0.105
0.092
0.079
0.062
6.6
2.56
1.61
1.02
0.82
0.66
0.52
0.39
0.33
0.259
0.207
0.177
0.148
0.128
0.115
0.095
0.082
0.069
0.059
10.5
4.3
2.66
1.67
1.31
1.05
0.82
0.66
0.52
0.43
0.33
0.279
0.233
0.200
0.177
0.141
0.118
0.095
0.075
10.5
4.3
2.66
1.67
1.35
1.05
0.85
0.69
0.52
0.43
0.36
0.295
0.249
0.217
0.194
0.157
0.135
0.112
0.089
10.5
4.3
2.66
1.67
1.31
1.05
0.82
0.66
0.52
0.43
0.33
0.282
0.236
0.207
0.180
0.148
0.125
0.102
0.082
5.6
2.26
1.44
0.95
0.75
0.62
0.52
0.43
0.36
0.289
0.243
0.217
0.194
0.174
0.161
0.141
0.131
0.118
0.105
5.6
2.26
1.48
0.95
0.79
0.62
0.52
0.43
0.36
0.302
0.256
0.230
0.207
0.190
0.174
0.157
0.144
0.131
0.118
5.6
2.3
1.48
0.98
0.79
0.66
0.52
0.43
0.36
0.308
0.262
0.240
0.213
0.197
0.184
0.164
0.154
0.141
0.131
9.2
3.6
2.33
1.51
1.21
0.98
0.79
0.62
0.52
0.43
0.36
0.308
0.269
0.240
0.217
0.187
0.167
0.148
0.128
9.2
3.6
2.36
1.51
1.21
0.98
0.79
0.66
0.52
0.43
0.36
.322
0.282
0.253
0.233
0.200
0.180
0.161
0.138
9.2
3.6
2.36
1.51
1.21
0.98
0.82
0.66
0.52
0.46
0.36
0.33
0.289
0.262
0.240
0.210
0.190
0.171
0.151
12
8
6
4
3
2
1
1/0
2/0
3/0
4/0
250
300
350
400
500
600
750
1000
Notes:
1. These values are based on the following constants: UL-type RHH wires with Class B stranding in cradled configuration. Wire
conductivities are 100% IACS copper and 61% IACS aluminum, and aluminum conduit is 45% IACS. Capacitive reactance is ignored since
it is negligible at these voltages. These resistance values are valid only at 167°F (75°C) and for the parameters as given but are
representative for 600volt wire types operating at 60 Hz.
2. Effective Z is defined as R cos(Θ) + X sin(Θ), where Θ is the power factor (PF) angle of the circuit. Multiplying current by effective
impedance gives a good approximation for line-to-neutral voltage drop. Effective impedance values shown in this table are valid only at
0.85 PF. For another circuit PF, effective impedance (Ze) can be calculated from R and XL values given in this table as follows:
Ze = R × PF + XL sin[arccos(PF)]
* Reprinted with permission from the 2011 National Electrical Code© Handbook, National Fire Protection Association, Quincy, MA.
This reprinted material is not the referenced subject, which is represented only by the standard in its entirety.
Flexibility is given to the designer in choosing which receptacles within a space must be automatically
controlled. It is common for duplex receptacles (two receptacles in a single yoke) to be split-wired. In a
split-wired duplex receptacle, one receptacle is automatically controlled and the other is powered
continuously. Receptacle outlets with more than two receptacles may also be split-wired to satisfy the
automatically controlled receptacle requirement.
Other wiring configurations are allowed and may be appropriate. For instance, all the receptacles in
each receptacle outlet may be automatically controlled or continuously powered, provided 50% of the
receptacle outlets within the space are automatically controlled. Although allowed, this may not
provide ideal receptacle placement.
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Section 8.4.2 provides multiple control options for automatically controlling the receptacles. The
automatically controlled receptacles may be controlled by a programmable time clock. When
controlled by a time clock, occupant override must be allowed for hours outside the schedule. When
manually overridden by the occupant, each override cycle must last no longer than two hours before
the schedule must be overridden again. The automatically controlled receptacles may alternatively be
controlled by an occupant sensor located in the space. When this control function is used, the control
must turn off the receptacles within 20 minutes of the last occupant leaving the space. Finally, a signal
from another control or alarm system that indicates if the area is occupied may be used to control the
receptacles.
The standard requires that controlled receptacles be visually distinct from noncontrolled receptacles.
The standard does not require a specific means for doing so. Switched receptacles may be identified by
receptacle color, indication lights integral to the receptacle, receptacle labeling, etc. Regardless of the
means of identification, maintaining clear, uniform identification throughout a building will increase
occupant understanding and reduce confusion.
The standard makes exceptions for spaces where switched receptacles would compromise equipment
operation or safety and security. This includes spaces where the receptacles are specifically designated
for equipment that requires 24-hour operation and where automatic shutoff would endanger the
safety or security of the room or occupants.
Electrical Energy Monitoring (8.4.3)
Section 8.4.3 describes electrical energy monitoring, recording, and reporting requirements. The
requirements of this section apply to the following:
• Buildings 25,000 ft2 (2323 m2) or greater
• Individual tenant spaces 10,000 ft2 (929 m2) or greater
• Residential buildings with 10,000 ft2 (929 m2) or more common area
This section excludes the following:
• Dwelling units
• Critical and equipment branches of NEC Article 517.
FYI
Receptacle vs. Receptacle Outlet vs. Outlet Definitions
To correctly understand the requirements of automatic receptacle control it is necessary to correctly
understand the definitions of the terms used in this section. Per the 2011 National Electrical Code®
(NEC), outlets, receptacles, and receptacle outlets all have unique definitions despite how the terms may
be commonly used. The following descriptions of each term are based on the definitions provided in
the 2011 NEC. An outlet is a broad term to describe any point where an electricity-consuming device is
connected to the electrical system. A receptacle is a device within an outlet that provides a single
electrical connection point for an electrical plug. There are typically one or two receptacles per yoke. A
receptacle outlet is a combination of the previous two devices. It is an outlet that includes one or more
receptacles.
To minimize confusion, the following rules and figure may be helpful in remembering these terms:
A receptacle is a single connection point for an electrical plug. A duplex receptacle includes two
receptacles on a single yoke.
All receptacle outlets are outlets that include one or multiple receptacles. Receptacle outlets are often
incorrectly referred to as receptacles or outlets.
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Monitoring (8.4.3.1)
Electrical energy monitoring systems must be installed to monitor all major electricity consumers
separately. At a minimum, the standard requires monitoring of the whole building, HVACR systems,
interior lighting, exterior lighting, and receptacle circuits. Where a building is occupied by multiple
tenants, these systems must be separately monitored for common areas and for each tenant. This does
not include systems that serve both the tenant spaces as well as common spaces.
This requirement places additional constraints on building electrical distribution and circuiting. If
metering is not accounted for during design, appropriate points for monitoring electrical power may
be overlooked. On one extreme, electrical loads can be grouped for metering by the physical
arrangement of the wiring distribution. On the other extreme, extensive use of electrical meters may
be used and the loads grouped by software. Either way, distribution and hardware selection is likely
impacted by metering. Such is the case with separating interior lighting from receptacle loads. When
lighting and receptacles are grouped into one circuit, metering the two loads independently of one
another can become more difficult due to the physical routing of the wiring. The wiring distribution
will dictate the location and quantity of current transducers required to monitor each load. Due to the
number of current transducers required, it may be cost prohibitive, when metering, to group different
loads into a single circuit.
 Recording and Reporting (8.4.3.2)
The data from the Section 8.4.3.1 monitoring requirements must be recorded at intervals no longer
than 15 minutes. This stored data must be reported to key stakeholders in hourly, daily, monthly, and
annual intervals. Where buildings are occupied by tenants, the data must be available to the tenants. In
buildings with a digital control system installed to comply with Section 6.4.3.10, the energy use data
shall be transmitted to the digital control system and graphically displayed.
The collected data must be stored for a minimum of 36 months. After 36 months the data may be
purged. Building automation systems often include a computer on site for accessing the building
controls. This workstation can serve as the data handling and storage device. Alternatively, data can be
transferred for off-site storage or analysis, provided it is available for retrieval.
FYI
Electrical Monitoring Configurations
This sidebar includes two examples of one-line diagrams of electrical measurement that satisfy the
requirements of the standard. Each has its own advantages and disadvantages.
In the first one-line diagram, the loads are physically arranged such that there are panel boards
dedicated to each specific load. This allows a single meter to be located on the panel board feeders.
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This reduces the number of meters but may create a complicated wiring distribution. Additionally, this
method provides energy information for the group but not for individual loads within the group.
In the second one-line diagram, the branch circuits to each load are each metered. The metered loads
can be grouped within the energy monitoring software to sum the energy consumption of each load
type. This method provides wiring distribution flexibility, but it comes at the cost of additional meters.
However, as an added benefit to the owner, the additional metering provides additional energy
information that may be valuable in troubleshooting.
Although metering of individual branch circuits is becoming more feasible due to increased availability
of equipment to facilitate such metering, it is likely that many projects will still use a combination of
these two types of metering. The distribution in most cases can be modified slightly to include feeders
for central metering. For equipment or branch circuits located in remote locations, it may be
appropriate to individually meter the equipment branch circuit. This allows the remote equipment to
be powered by the nearest panel.
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Example 8-C. Automatic Receptacle Control, Labeling Receptacles
Corresponding section: Automatic Receptacle Control (8.4.2)
Q
Duplex receptacles in a new school are consistently wired such that the top outlet is always on and the
bottom outlet is controlled by a combination of occupant sensors and programmable time clocks. The
building has thousands of duplex outlets. Is it necessary that each one be labeled so that occupants
know which outlet is switched and which is not?
A
Yes. The standard requires that the controlled receptacles be visually distinct. Although the method of
identification is not specified, it is recommended that automatically controlled outlets be consistently
identified in some visual manner to avoid confusion and misuse.
Example 8-D. Automatic Receptacle Control, Portable Power Strips with Occupant Sensors
Corresponding section: Automatic Receptacle Control (8.4.2)
Q
A building owner proposes to provide tenants with portable power strips to meet this provision. The
devices have a wireless occupant sensor that can be placed in the room. When the space is vacant for a
period of ten minutes, the wireless sensor sends a signal to the power strip, and power is cut off to
devices plugged into the power strip. Does this approach comply with the standard?
A
No. The control system has to be part of the building; plug-in devices may not be used to comply with
this provision.
Example 8-E. Automatic Receptacle Control, Office Furniture
Corresponding section: Automatic Receptacle Control (8.4.2)
Q
A permit application is filed for tenant improvements in an existing office building. The improvements
include replacement of existing mechanical, electrical, and lighting and adding new finishes, thereby
triggering the requirements of Section 8.4.2. The building envelope will not be modified.
The proposed design includes a few conference rooms and private offices, but most of the space
consists of an open-plan design with the intent of installing furniture system panels, which are not
shown on the construction documents. The private offices and conference rooms will have half of each
duplex outlet switched with the lighting system and using the same override system as the lighting
system.
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Each cubicle in the proposed furniture system has an integrated occupant sensor that controls half of
the duplex outlets in the cubicle. The other half is powered continuously. The cubicles are wired
directly to a circuit breaker panel on each floor.
Does this configuration comply with the requirements of Section 8.4.2?
A
No. At least 25% of the circuits feeding furniture systems not shown on the drawing must be
controlled.
Example 8-F. Automatic Receptacle Control, Override System for Receptacle Control
Corresponding section: Automatic Receptacle Control (8.4.2)
Q
The electrical design for an office includes an independent time clock system for receptacle control
that switches off the controlled outlets at the close of business and activates the outlets at the start of
business the next morning. Outlets are activated only for business days, generally Monday through
Friday, excluding holidays. Is it necessary to have an override system so that the receptacles can be
activated when workers stay late or work on the weekend?
A
Yes. Occupants must have a means to manually override the schedule during nonbusiness hours. The
automatic control must limit each override cycle to no more than two hours. However, the occupant
may start an additional override cycle when the previous override cycle expires.
Low-Voltage Dry-Type Distribution Transformers (8.4.4)
The standard has efficiency requirements for both single-phase and three-phase dry-type
transformers. Dry-type transformers are generally smaller in size than conventional transformers and
are named such because the coils and core are not submerged in oil, as they are in conventional
transformers. The class of transformers covered by this requirement has an input less than 600 V and
is rated for an alternating current frequency of 60 Hz.
The standard’s requirements are identical to the requirements of the federal Energy Policy Act of 2005
(EPAct). The efficiency requirements are generally greater than 98%. The required efficiency is greater
for larger sizes (see Table 8.4.4 in the standard).
Submittals (8.7)
Drawings (8.7.1)
The construction documents must include a requirement that the owner be provided with record
drawings of the actual installation within 30 days of system acceptance. These drawings must include
• a single-line diagram of the electrical distribution system and
• floor plans showing the location of distribution equipment and the areas served by that equipment.
Manuals (8.7.2)
The construction documents must also include a requirement that the building owner receive manuals
that provide instruction about the operation and maintenance (O&M) of the building’s electrical
distribution system (see Chapter 6 for similar requirements covering mechanical systems and
equipment). The manuals must include at least the following information:
• Submittal data stating equipment nameplate rating. The submittal data must also include optional
and accessory installed equipment.
• Operation manuals and maintenance manuals for each piece of equipment requiring maintenance.
Required routine maintenance actions must be clearly identified. The maintenance instructions
must include all installed equipment requiring scheduled maintenance (for example, corrosion
prevention) and maintenance due to operating conditions (for example, lubrication as a function of
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the load and speed). It is essential that product data sheets, photographs, illustrations, examples,
and other data be marked to indicate the specific equipment supplied. Where the supplier has
product information or O&M instructions available through electronic media, this should also be
provided to the owner.
• Names and addresses of at least one qualified service agency.
• A complete narrative and schematic of the system as it is normally intended to operate. This is
essential for the equipment and facility staff to understand the efficient operation of the system.
Example 8-G. Transformers within Welding Equipment
Corresponding section: Low-Voltage Dry-Type Distribution Transformers (8.4.4)
Q
Do the transformer efficiency requirements of Section 8.4.4 apply to transformers that are part of
welding equipment?
A
No. The requirements only apply to general-purpose transformers. All special-purpose transformers
are exempt from the requirements, including drive transformers, rectifier transformers,
autotransformers, uninterruptable power system transformers, impedance transformers, regulating
transformers, sealed and nonventilating transformers, machine tool transformers, welding
transformers, grounding transformers, and testing transformers.
The efficiency requirements apply only to the class of transformers that are covered by the federal
Energy Policy Act of 2005 (EPAct).
FYI
Enabling the Owner to Operate the Building Efficiently
The standard requires that construction documents (drawings and specifications) include a
requirement that the owner receive information about the building’s electrical system. The intent of
this requirement is to provide the owner with all information that will enable optimal and efficient
operation of the building’s electrical system.
The standard does not designate the person responsible for providing this information, though the
responsibility will normally be assigned to the system installer. The designer is responsible for
including these completion requirements with the drawings and specifications.
The standard recognizes that a building official or inspector cannot be expected to check whether the
owner has received complying documents, but the official does have an opportunity to check that these
completion requirements are part of the construction documents.
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9 Lighting
Changes to the Lighting Section
INTERIOR
 The standard now applies to permanently installed lighting within dwelling units (9.1.1).
 Building area lighting power densities (LPDs) were reduced an average of 12% across all building
types, with LPDs for several building types reduced up to 34% from 90.1-2013 levels (9.5.1).
 Space-by-space LPDs for most interior space types were reduced an average of 26% below 90.12013 levels; LPDs for a few space types were increased based on revised design criteria and
current practice (9.6.1).
 Additional retail interior lighting power allowances were reduced (9.6.2).
 Additional decorative interior lighting power allowances were reduced (9.6.2).
 Exception to manual-on or partial automatic-on requirements for advanced lighting systems in
open-plan offices were added (9.4.1.1[c]).
 High-efficacy lighting requirement was added for dwelling units (9.4.4).
 Stairwells are no longer required to have a local control (9.4.1.1[a]; Table 9.6.1).
 Restrooms are no longer required to have a local control or bilevel control (9.4.1.1[b]; 9.4.1.1[d];
Table 9.6.1).
 A fifty percent power reduction for daylight responsive controls was added for some parking
garage luminaires (9.4.1.2).
EXTERIOR
 Exterior lighting power reduction requirements were increased to 50% for all lighting other than
building façade and landscape (9.4.1.4).
 Exterior lighting power reduction control is now required for all pole-mounted luminaires serving
outdoor parking lots having an input wattage >78 W and a mounting height of <24 ft (9.4.1.4).
ALTERATIONS
 Interior luminaire alterations must now comply with local control, manual-on/auto-on operation,
and bilevel lighting control requirements, in addition to shut-off requirements and meeting LPD
allowances, as applicable to that space (9.1.2).
 Exterior luminaire alterations must comply with requirements for astronomical control and/or
scheduled shut-off control as applicable to that space (9.1.2).
 The threshold for triggering code was increased from modifying 10% to modifying 20% of existing
lighting load (9.1.2).
 Lamp plus ballast/driver retrofits and one-for-one luminaire replacements only have to comply
with LPD limits and automatic shut-off requirements (9.1.2).
These changes are marked with  in the margins of this chapter. For the specific addenda that define
the differences between the 2013 and 2016 editions of Standard 90.1, see Appendix H to the standard.
General (9.1)
 Scope (9.1.1)
As a minimum energy efficiency standard, Section 9 sets the maximum allowable interior and exterior
lighting power allowed for buildings. Additionally, this section includes minimum automatic lighting
control requirements to limit lighting use when it is not required. The standard includes requirements
for interior lighting and exterior building and site lighting. Exterior site lighting not powered by the
building’s electrical service is not covered by this section.
Other exceptions to this section include the following:
• Emergency lighting that is automatically off during normal business hours.
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• As with other requirements of the standard, lighting specifically required for health and life safety
by statute, ordinance, or regulation.
• Lighting that uses gas as its power (fuel) source. Section 9 is intended to regulate the energy
consumption of electrically powered lighting systems.
 Lighting Alterations (9.1.2)
Interior lighting alterations must meet the lighting power density (LPD) requirements of the Building
Area Method in Section 9.5.1 or the Space-by-Space Method of Section 9.6.1 and the control
requirements of Section 9.4.1.1, Subsections (a), (b), (c), (d), (g), (h), and (i), as applicable to the space.
Exterior lighting must meet the LPD allowance of Section 9.4.2 for the appropriate area for which the
lighting is being altered. The altered lighting must also comply with the exterior lighting control
requirements of Sections 9.4.1.4 and 9.4.2. See Examples 9-A and 9-B.
A renovation that affects 20% or less of the installed lighting power in an interior space is not required
to comply with the standard unless such alterations increases the installed lighting power.
Lighting alterations that only involve replacement of lamps plus ballasts/drivers or only involve onefor-one luminaire replacement need only comply with LPD requirement and Section 9.4.1.1(h) or
9.4.1.1(i).
Routine maintenance does not trigger this requirement as long as the replacement lamps or
ballasts/drivers are the same as the old ones.
Installed Lighting Power (9.1.3)
To determine the installed or connected lighting power, one must include not only the lamp but also
the power used by the ballast/driver, the control (when applicable), transformers, and any other
power draws associated with the lighting system. It is often necessary to include only the power used
for the lamp and ballast/driver, as the power required for controls is minimal.
Some lighting applications are exempt and, consequently, their power consumption is not included in
the power calculations. Exempt interior lighting applications are those specifically listed in the
exceptions to Sections 9.1.1, 9.2.2.3, and 9.4.2.
Section 9.1.2 includes lighting applications where multiple lighting systems are not intended to
operate simultaneously. For example, a multifunction room in a hotel might have one lighting system
with incandescent downlights appropriate for ballroom activities and another lighting system to
provide office-level illumination suitable for meetings and conferences. If controls are provided so that
it is not possible to turn on both lighting systems at the same time, then the lighting power calculation
must use the lighting system with the greatest installed power to demonstrate compliance with the
standard.
For more information about calculating the installed exterior or interior lighting power, see the
Exterior Building Lighting Power (9.4.2) and Building Area Method Compliance Path (9.5) sections of
this chapter.
Example 9-A. Renovation of Tenant Space
Corresponding sections: Lighting Alterations (9.1.2), Interior Lighting Controls (9.4.1.1)
Q
A 5000 ft² (465 m²) space in a commercial office building is being leased to a new tenant, and the space
is being renovated to meet the requirements of the new tenant. Existing lighting power is 9800 W. The
tenant proposes leaving most of the lighting unchanged but wants to replace luminaires in the
conference room and several private offices. The luminaires to be replaced require 1600 W, while the
new luminaires use 900 W. How does the standard apply to this lighting improvement?
A
Interior lighting improvements in interior spaces that affect at least 20% of the installed lighting
power must meet the requirements of Section 9.1.2. This includes the lighting power density (LPD)
requirements of the Space-by-Space Method of Section 9.6.1 and the control requirements of Section
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9.4.1.1, Subsections (a), (b), (c), (d), (g), (h), and (i), as applicable to the space. In this example, the
lighting is being replaced in the conference room and private offices, where 100% of the installed
lighting power is affected. The standard applies only to these spaces. It is not necessary for the entire
5000 ft² (465 m²) tenant space to comply.
Example 9-B. Parking Lot Lamp and Ballast Replacements
Corresponding sections: Lighting Alterations (9.1.2), Exterior Lighting Control (9.4.1.4), Exterior Building
Lighting Power (9.4.2)
Q
The lighting system in a 10,000 ft² (929 m²) existing parking lot is illuminated with mercury vapor
lamps. The site is located in an urban area (exterior Lighting Zone 3). The existing lamps and ballasts
are being replaced with a metal halide system; however, the existing poles and luminaires are to
remain. The system is controlled by a conventional time switch and without seasonal adjustments.
How do the exterior lighting requirements apply to this situation?
A
The lighting system that is being altered must comply with the lighting power density (LPD)
allowances of Section 9.4.2 for the area illuminated by the lighting system and the applicable control
requirements of Sections 9.4.1.4.
From Table 9.4.2-1, the lighting power allowance for exterior Lighting Zone 3 is 0.06 W/ft² (0.65
W/m²), so the total allowed lighting power is 10,000 ft² (929 m²) times 0.06 W/ft² (0.65 W/m²) = 600
W. In addition, the standard allows an additional base site allowance of 500 W for Lighting Zone 3,
which may be used for any tradable or nontradeable exterior lighting application. This base allowance
could be used for the parking lot, bringing the total allowance to 1100 W.
The existing time clock would not comply because Section 9.4.1.4 requires exterior lighting to turn off
automatically when there is sufficient daylight. A standard time clock would not offer this functionality.
An astronomical time clock that can calculate sunset and sunrise times and turn lights off during
daytime hours would meet the daytime-off requirements. Alternately, using a photocell control with a
standard time clock could provide this control capability. Additionally, the time clock would need to
retain its programming and time setting for at least ten hours during a power loss.
If the parking lot luminaires are mounted less than 24 ft above the surface of the parking lot and have a
rated input wattage of greater than 78 W, they must be controlled to automatically reduce the power of
each luminaire by a minimum of 50% when no activity has been detected in the area illuminated by
the controlled luminaires for a time of no longer than 15 minutes. No more than 1500 W of power for
parking lot luminaires meeting this criteria may be controlled together.
Interior and Exterior Luminaire Wattage (9.1.4)
Except for incandescent sources, fixture input wattage is not the same thing as lamp wattage. Input
wattage for all discharge sources (which are most common in nonresidential buildings) is determined
by the interaction between lamps, ballast/driver, and fixture construction. It is required that input
wattage be determined. When possible, data supplied by the specified luminaire and/or ballast/driver
manufacturer are preferred. Although performance data from the specified manufacturer are always
best, the use of average product data for similar equipment from multiple other manufacturers may be
used when necessary.
With many types of luminaires, designers may be uncertain about the input watts to use in compliance
calculations. This is particularly true for luminaires that can accept different wattages and for track
lighting where additional luminaires can easily be added. Section 9.1.4 of the standard explains how
the wattage is determined for these special cases.
Incandescent and Tungsten-Halogen Luminaires without Permanently Installed Ballasts/Drivers
This type of luminaire can accept lamps of many different wattages. Compliance calculations must use
the maximum input watts of any permitted lamp shown on the manufacturer’s fixture label. This
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means that a luminaire labeled for 150 W is calculated at 150 W for lighting power allowance
compliance purposes. This applies regardless of whether the specified lamp is 75 W incandescent or
13 W screw-in compact fluorescent, because the luminaire does not contain permanently installed
ballasts/drivers.
In some instances luminaires may not be labeled. This circumstance is not covered by the standard.
However, if no such label is attached to the luminaire, a designer may choose to use the maximum
wattage for any lamp indicated in the luminaire manufacturer’s catalog or website. If the luminaire
manufacturer’s catalog or website does not list the maximum lamp wattage for the luminaire, the
designer may wish to use the wattage indicated in a photometric test report from either a National
Voluntary Laboratory Accreditation Program (NVLAP) accredited photometric laboratory or from an
independent photometric laboratory.
Luminaires with Permanently Installed Ballasts/Drivers
Luminaires with permanently installed or remote ballasts/drivers must use the maximum input watts
of any permitted lamp/ballast/driver combination shown on the fixture label. The wattage input
expressed on the label must be based on the equipment installed in the luminaire. If equipment within
the luminaire is provided by an auxiliary manufacturer, the power information is that provided by the
auxiliary manufacturer. For example, a luminaire manufacturer may produce the body of the luminaire
but not the ballast/driver. In this instance, the ballast/driver determines the power consumption of
the luminaire.
Some ballasts have multiple ballast factors. Ballasts with stepped dimming, or those rated for multiple
lamp configurations, typically have a ballast factor for each configuration. To comply with this
exception, the ballast factor may not be able to be changed by the user. It may be possible for the lowvoltage wiring of a stepped ballast to be changed by maintenance personnel, causing the ballast to
operate at a different power step and associated ballast factor. Thus, this would not meet the
requirement of this exception. However, a ballast rated to work with multiple lamp quantities may
have a ballast factor for each lamp quantity. Because the ballast factor for this ballast is determined by
the number of lamps, it is unlikely the ballast factor could be changed by the user. Provided the ballast
factor cannot be changed, the lighting power calculation for luminaires using ballasts with multiple
ballast factors must use the ballast factor that will be used when the luminaire is installed.
Line-Voltage Track Lighting
Track lighting is a very common lighting technique for display lighting in retail stores and galleries. It
consists of a line-voltage, plug-in busway that allows the addition or relocation of luminaires without
having to change the wiring system. It is very easy to add fixtures to the track after the final occupancy
permit has been issued. When accounting for track lighting that operates at line voltage, the designer
must assume at least 30 W/lin ft (98 W/lin m) of track, the wattage limit of the system’s circuit
breaker, or the wattage limit of other permanent current-limiting devices on the system. If the plans
and specifications show more than 30 W/lin ft (98 W/lin m), the greater installed power must be used
for compliance purposes.
Low-Voltage Track Lighting
Some track lighting systems use a transformer to energize the busway at 12 or 24 V. Examples include
decorative fixtures that have exposed conductors. These systems allow fixtures to be easily added,
removed, or relocated without having to modify the wiring system. When these systems are used for
interior lighting, the wattage used for compliance calculations is the maximum wattage of the
transformer that supplies power to the system.
Other
For all other types of luminaires not specifically addressed above, the wattage must be the specified
wattage of the lighting equipment, taken from the plans and specifications.
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Lighting systems and equipment must always comply with the General (9.1) and Mandatory Provisions
(9.4) sections of the standard. In addition, the building must comply with either the prescriptive
requirements of the Building Area Method (9.5), the prescriptive requirements of the Space-by-Space
Method (9.6), the Energy Cost Budget (ECB) Method (Section 11), or the Performance Rating Method
(PRM) (Appendix G). If either of the two prescriptive methods (Building Area Method or Space-bySpace Method) are used, then project compliance documentation must comply with the requirements
of Section 9.7 Submittals. Section 9.2.2.3 Interior Lighting Power provides a list of exceptions that
identify lighting systems that are not included when using the lighting power allowance requirements
of either of the prescriptive methods. For an outline of the steps for achieving compliance, see FYI,
Compliance Procedures.
Compliance Forms
Compliance forms and worksheets intended to facilitate the process of complying with the standard
are available for download from the ASHRAE website at http://www.ashrae.org/UM90.1-2016.
FYI
Compliance Procedures
The following steps provide a methodology for achieving compliance with the requirements of
Section 9.
Step 1: Sections 9.1.1 and 9.1.2: Determine if the building project under consideration and its
associated lighting systems need to comply.
Step 2: Sections 9.5 or 9.6: Determine the interior lighting power allowance using either the Building
Area Method (Section 9.5) or the Space-by-Space Method (Section 9.6).
i. Section 9.6.2: If the Space-by-Space Method (Section 9.6) has been used, determine how much
additional lighting power allowance is available for decorative lighting and retail accent lighting.
ii. Design the interior lighting accordingly.
Step 3: Section 9.4.2: Determine the exterior lighting power allowance, and design the exterior lighting
accordingly.
Step 4: Sections 9.1.3 and 9.4.2: Determine the installed interior lighting power and the installed
exterior lighting power.
Step 5: Confirm that the installed interior lighting power does not exceed the interior lighting power
allowance and that the installed exterior lighting power does not exceed the exterior lighting power
allowance.
Step 6: Section 9.4: Meet the mandatory provisions, including the following:
i. Section 9.4.1.1: Interior Lighting Controls (local control, restricted to manual on, restricted to
partial automatic on, bilevel lighting control, automatic daylight responsive controls for
sidelighting, automatic daylight responsive controls for toplighting, automatic partial off, automatic
full off, scheduled shutoff)
ii. Section 9.4.1.2: Parking Garage Lighting Control
iii. Section 9.4.1.3: Special Applications (control)
iv. Section 9.4.1.4: Exterior Lighting Control
v. Section 9.4.2: Exterior Building Lighting Power
vi. Section 9.4.4: Dwelling Units (lamp efficiency and luminaire efficacy)
Step 7: Section 9.4.3: Perform functional testing of lighting controls.
Step 8: Section 9.7: Submit all required documentation.
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Prescriptive Requirements (9.2.2)
The prescriptive lighting requirements limit the installed electric wattage for interior building lighting.
As with the other sections of the standard, these lighting requirements are minimum requirements.
Designers working on specific projects may often be able to design more efficient lighting systems.
The mandatory provisions (Section 9.4) discussed below must be met in all instances, but prescriptive
path requirements do not apply when the Energy Cost Budget (ECB) Method (Section 11) or
Prescriptive Rating Method (Appendix G) are used for compliance. In other words, more power can be
used for lighting if other building systems are made more efficient. The opposite is also true—a more
efficient lighting system, for example, might permit a less efficient HVAC system.
Interior Lighting Power (9.2.2.3)
If the ECB Method (Section 11) or the PRM (Appendix G) are not being used for compliance, then there
are two ways to determine the interior lighting power allowance:
• Use the Building Area Method (Section 9.5), which is usually the easiest method and is based on an
entire building or entire occupancy.
• Employ the Space-by-Space Method (Section 9.6), which is based on the individual rooms of a
project; it offers flexibility and, generally, more lighting power than the Building Area Method.
Often, separate permit applications are filed for the lighting systems serving different occupancies in
multioccupancy buildings. In these cases, each building occupancy must separately comply with the
requirements. When a single permit application includes the lighting systems for more than one
occupancy, it is possible to make trade-offs between the occupancies. However, these trade-offs are
possible only when both occupancies use the same method to determine the lighting power allowance.
If one occupancy uses the Space-by-Space Method and the other occupancy uses the Building Area
Method, then trade-offs are not permitted.
Note that these lighting power allowances apply regardless of whether a space, such as a warehouse, is
heated or unheated. Also, be aware that the standard considers covered parking garages to be interior
spaces and includes them in the interior lighting category.
Exceptions to 9.2.2.3
Most interior lighting systems are covered by the standard. However, some specialized lighting
applications are exempt. See the complete list of exceptions in Section 9.2.2.3 in the standard. One of
the exceptions is for areas designed for the life support of nonhuman life forms, which is applicable to
vivariums or horticultural grow facilities.
Also exempt are certain lighting systems or portions of systems required for emergency use. Note that
these are exemptions from the prescriptive requirements in Section 9.2.2. Designs must still comply
with the control requirements and other mandatory provisions in Section 9.4.
• Lighting in shell buildings. Shell buildings are built before the building’s use is known. The space
could become light manufacturing, office, warehouse, or any other use, depending on the tenant’s
requirements. In shell buildings, the lighting system is rarely installed before the space is leased.
Leasing a building to a tenant effectively defines its use and allows for a determination of the
interior lighting power allowance.
• Garages and parking areas. The covered portions of a parking garage are treated as interior space
and are included as part of the interior lighting power allowance. Table 9.6.1 shows the maximum
LPD for a parking garage (Parking Area, Interior) as 0.14 W/ft² (1.5 W/ m²). Open parking lots
(including rooftop parking) are covered by the exterior lighting power allowance requirements; the
allowed lighting power ranges from zero to 0.08 W/ft² (0.86 W/m²), depending on the exterior
lighting zone (Table 9.4.2-2).
Examples 9-C and 9-D illustrate some exemptions to the interior lighting power allowance.
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Portable Lighting
Although the designer cannot prevent users from plugging in portable lighting of their own choosing,
the designer must account for portable lighting, not otherwise exempted, that is intended for the space,
including furniture-mounted task lights and lighting in permanent displays. Even if the project
designer is not responsible for specifying portable lighting, the calculations should include an
allowance for the expected use of this equipment if its consideration is included in the design.
Applying the Standard
For the most part, applying the standard is relatively simple. However, there are a number of specific
cases pertaining to lighting systems where additional information may be helpful in interpreting the
standard’s requirements. These instances are described below.
Example 9-E provides additional information on how to apply the standard.
• Exterior and interior lighting power trade-offs. The standard contains separate requirements
for exterior and interior lighting systems. Trade-offs between the two are not allowed. Trade-offs
are allowed, however, among interior spaces and among the exterior lighting applications listed in
the Tradable Surfaces section of Table 9.4.2-2.
• Calculation methods for interior power allowance. There are two ways to determine the
interior lighting power allowance: the Building Area Method (Section 9.5) and the Space-by-Space
Method (Section 9.6). Both methods may be used in the same building, but trade-offs are not
allowed between different sections of the building that use different methods of determining the
power allowance.
• Lighting in multibuilding facilities. Each building in campus-like facilities must comply
separately with the interior lighting power allowance requirements, even if multiple buildings are
covered under a single building permit. The exterior lighting power allowance, however, applies to
the entire site. Trade-offs are allowed only among the exterior lighting applications listed in the
Tradable Surfaces section of Table 9.4.2-2.
• Lighting in speculative buildings. Speculative buildings are built before the tenants are known.
The initial building permit application usually includes just the shell and core with lighting installed
only in the building’s common areas, such as corridors, toilets, stairwells, and lobbies. Lighting for
tenant spaces is provided later as part of the tenant improvements and is often customized for each
tenant. The interior lighting power allowance for speculative buildings may be determined by using
either the Building Area Method (Section 9.5) or the Space-by-Space Method (Section 9.6);
however, each portion of the building that is permitted must separately satisfy the standard’s
requirements.
• Lighting in shell buildings. Shell buildings are built before the building’s use is known. The space
could become light manufacturing, office, warehouse, or any other use, depending on the tenant’s
requirements. In shell buildings, the lighting system is rarely installed before the space is leased.
Leasing a building to a tenant effectively defines its use and allows for a determination of the
interior lighting power allowance.
• Garages and parking areas. The covered portions of a parking garage are treated as interior space
and are included as part of the interior lighting power allowance. Table 9.6.1 shows the maximum
LPD for a parking garage (Parking Area, Interior) as 0.14 W/ft² (1.5 W/ m²). Open parking lots
(including rooftop parking) are covered by the exterior lighting power allowance requirements; the
allowed lighting power ranges from zero to 0.08 W/ft² (0.86 W/m²), depending on the exterior
lighting zone (Table 9.4.2-2).
Example 9-C. Exempt Interior Lighting, Retail Store Windows
Corresponding section: Exceptions to 9.2.2.3
Q
A proposed retail store in a mall will have display windows on the parking-lot (exterior wall) side and
windows on the mall (interior) side. The parking-lot side window displays will be closed off from the
store interior, but the displays on the mall side are not closed off from inside the store. Is either of
these lighting systems exempt?
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A
All display lighting in the windows on the parking-lot side is exempt because the display area is
enclosed by ceiling-height partitions. However, the display area on the mall side of the store is not
exempt because it is visually connected to the sales area of the store. While the display lighting in the
enclosed show window (parking-lot side) is exempt from the lighting power allowance requirements,
it still must have a separate control (per the exceptions listed for Section 9.2.2.3). Display lighting that
is intermingled in the sales area must also have a separate control.
Example 9-D. Exempt Interior Lighting, Laboratory Test Lights
Corresponding section: Exceptions to 9.2.2.3
Q
A medical laboratory is studying the effect of lighting on a chemical process. Ordinary fluorescent
luminaires are arranged over the test bench and connected to timers. This lighting is separate and
distinct from general lighting used throughout the laboratory. Is either the general lighting or the test
lighting exempt?
A
The general lighting is within the scope of the standard. The test lighting is exempt. The exceptions to
Section 9.2.2.3 require separate lighting control for nonvisual lighting. The lighting arranged for the
test is nonvisual because its purpose is to affect a chemical process, not to enable human sight. The
lighting arranged for the test must therefore have a separate control.
Example 9-E. Applying the Standard to Tenant Spaces
Corresponding section: Applying the Standard
Q
The core and shell of a high-rise office building was completed before the standard’s effective date. The
construction included the building envelope, the base HVAC system, and lighting for the common areas
only. Lighting improvements for each tenant space will be made on a tenant-by-tenant basis when each
space is leased.
Tenant spaces on two floors of the building remain empty and unimproved until they are leased a year
after the standard takes effect. At this time, the tenant files a permit application for the construction of
a lighting system along with other tenant improvements. Does the standard apply to the design of the
lighting system?
A
Yes. The first tenant improvements in a building are considered new construction, and the lighting
standard in effect at the time applies. Either the Building Area Method (Section 9.5) or the Space-bySpace Method (Section 9.6) may be used.
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The lighting system mandatory provisions of Section 9 must be followed regardless of the compliance
path used. Projects that choose to follow the Energy Cost Budget (ECB) Method or Performance Rating
Method (PRM) compliance paths must comply with the mandatory provisions the same as a project
following the either of the prescriptive compliance paths.
Lighting Control (9.4.1)
Limiting lighting power requirements reduces the power requirements of lighting and the
corresponding energy consumed for the time the lighting is used. However, the inclusion of lighting
controls has the ability to reduce both the lighting power requirement as well as the number of hours a
light operates by turning off lights when they are not necessary. Due to the significant energy savings
that can be realized by automatically turning off lighting when it is not required, many lighting controls
are mandated by Section 9.4.1. This section imposes mandatory controls for interior lighting, parking
garage lighting, exterior lighting, and select special applications.
 Interior Lighting Controls. (9.4.1.1)
Section 9.4.1.1 covers all mandatory lighting control that must be included in interior spaces. There are
multiple control functions included in this section, and each applies to specific space types. Table 9.6.1
serves two roles. It specifies the maximum allowable lighting power density (LPD) for the Space-bySpace Method, which is why it is located in Section 9.6. It also specifies the mandatory control
functions that apply to each space type, regardless of compliance path. Not all control functions are
mandatory for every space type. Where a specific control function is required for a space type, it is
indicated by “REQ” in the intersection of the control function column and space type row.
Some mandatory control functions result in similar effects and energy savings. Therefore, a lighting
designer may be allowed to choose one of multiple control functions to satisfy the mandatory
requirement for that function. In Table 9.6.1 these control functions are designated by either “ADD1”
or “ADD2” at the intersection of the control function column and the space type row. However, an
“ADD1” control type may not be chosen in lieu of an “ADD2” control type. One of each must be chosen.
For example, when selecting the controls for the space type “Computer Room,” a designer must
implement either “Restricted to Manual ON” or “Restricted to Partial Automatic ON” control. They
must also implement either “Automatic Full OFF” or “Scheduled Shutoff” control. Implementing only
one of these four control functions does not satisfy the mandatory requirement. In other words, if a
space type has two or more lighting control methodologies labeled “ADD1,” then at least one of those
methodologies must be implemented; likewise for “ADD2.”
Where a space type has a dash in a control function column, there is no mandatory requirement for
that particular control type in that particular space type.
Note that while not every space function may be described by one of the space types listed in Table
9.6.1, the designer must choose a space type that most closely resembles how the space being designed
typically would be illuminated. Where the Space-by-Space Method (Section 9.6) is used for calculating
the allowable lighting power, the same space type must be used for selecting the control functions. A
space may not be designated as one space type for determining the LPD and as a different space type
for determining the required lighting control functions.
The requirements of each control type are described in the following sections. These control
methodologies are to be implemented as dictated by Table 9.6.1.
a. Local control: This requires each space to have at least one means of manual lighting control.
Multiple control areas may be required for a given space. If a space is no greater than 10,000 ft2
(929 m2), each controlled area must be limited to 2500 ft2 (232 m2). For instance, an 8000 ft2
(743 m2) space would need at least four areas of control. If the space is greater than 10,000 ft2
(929 m2), each control area must not be larger than 10,000 ft2 (929 m2).
The manual control device for each respective area must be located where the controlled lighting
can be observed by the user as the control device is operated.
However, for reasons of safety and security, it is permitted for the lighting control device to be
located in a location remote from the area controlled. If the control device is located where the
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b.
c.
d.
e.
controlled lighting area cannot be observed from the control device, the control device must
clearly describe the lighting area it controls and indicate the state of the lights (on/off). The
indicator for the state of the lights may be part of the control device or it may be a separate unit
located adjacent to the lighting control device, provided that association between the two is
obvious to the user.
Restricted to manual ON: Lighting areas that require this type of control may not have any type of
automatic control that turns on any of the lighting without the user consciously turning the lights
on via a manual control device.
As with local control, safety and security take precedence over this lighting control requirement.
Where the implementation of manual-on-only lighting control would create an unsafe
environment or a security risk, it is not required.
Restricted to partial automatic ON: This requirement is typically an alternative option to the
“Restricted to manual ON” requirement. This requirement allows up to 50% of the general lighting
load in a space to automatically turn on without conscious action from an occupant. This automatic
on could be in response to a lighting schedule or occupancy sensors. The remainder of the lighting
in the space must turn on by manual ON control only.
 An exception allows the control to automatically turn on more than 50% of the lighting power in
open-plan office spaces, provided the control zone is 600 ft² (56 m²) or less in area.
This type of control may be desired by some building owners due to the increased automation.
However, implementing this control function may add additional installation cost, relative to the
“Restricted to manual ON” control function, by requiring either additional electrical wiring or a
more sophisticated lighting control package.
Bilevel lighting control: This control function requires the general lighting in a space to have no less
than one intermediate lighting level between 100% and 0%. This can be implemented with
stepped lighting control or with continuous dimming. If stepped control is used, at least one
intermediate step must be between 30% and 70% of the full lighting power. Stepped lighting
control can be accomplished with stepped ballasts, luminaires with dual ballasts for separately
switched lamps, or separately controlled luminaires. Bilevel lighting may be automatic or manually
controlled based on other potential lighting control functions included in the system.
Automatic daylight responsive controls for sidelighting: Most building systems interact with one
another in some way. With some systems, the interaction is more apparent. Such is the case with
daylight responsive controls. Chapter 5 includes fenestration requirements intended to optimize
the energy savings associated with these daylight responsive controls without negatively
impacting the performance of other building systems. As such, these controls are mandatory for
the general lighting for many space types. Daylight responsive controls are not required for accent,
display, or task lighting. The following situations are exempt from the requirements of this section:
• Sidelighted areas that are significantly shaded by a permanent adjacent structure. To use this
exception the height of the adjacent structure above the shaded fenestration must be twice the
horizontal distance from the fenestration.
• Spaces where the total glazing area is less than 20 ft2 (1.9 m2). Energy savings associated with
automatic daylight responsive controls for sidelighting in this situation would typically not
justify the cost of the controls. For example, a private office with a single 3 × 5 ft (0.9 × 1.5 m)
window would qualify for this exemption. An open office space with ten (or two) 3 × 5 ft (0.9 ×
1.5 m) windows would not qualify for this exemption.
• Retail spaces are exempt from the automatic daylight responsive controls for sidelighting for
multiple reasons. Typically, retail spaces have windows intended for display purposes that
prevent daylight penetration into the occupied space, and lights in these areas are typically
dedicated to product display rather than general lighting. Retail spaces are also exempt from
automatic daylight responsive controls for sidelighting because it is thought that general
lighting remaining on near windows signals to customers that a store is open for business,
while lights off near windows implies a store is closed.
When required by Table 9.6.1, spaces with 150 W or more of general lighting within the primary
sidelighted area (as defined in Section 3.2) must include daylight responsive controls for lights in
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the primary sidelighted area. These daylight responsive controls must use photocontrols to change
the general lighting level in the primary sidelighted area to reduce the electric lighting when there
is an adequate contribution of daylight into the space.
Where required by Table 9.6.1, primary and secondary sidelighted areas must be controlled
separately to reduce the electric lighting in any space with 300 W or more of general lighting
within the combined primary sidelighted and secondary sidelighted daylight areas when there is
an adequate contribution of daylight into the space. The primary sidelighted area and secondary
sidelighted area must be controlled independently of one another to maintain each area’s
respective illuminance level.
The controls (and associated lighting hardware) must have the ability to adjust the lighting level to
a minimum of four steps. These steps must include the following percentages of the full lighting
power:
• 100%
• One point between 70% and 50%
• One point between 40% and 20% (or the lowest level the lighting system can achieve )
• 0%
Alternatively, continuous dimming may be used in lieu of stepped dimming.
Part of any daylight responsive control system is calibrating the lighting control system to the
installed environment. The calibration adjustment control must be located no higher than 11 ft
(3.4 m) above the finished floor. The system must allow calibration to occur without a person
being physically present at the daylight sensor, which assures that the person calibrating the
sensor is not self-shading it.
Refer to Section 3 of the standard for definitions of primary sidelighted areas and secondary
sidelighted areas and to Chapter 5 of this user’s manual for additional clarification and examples.
Automatic daylight responsive controls for toplighting: As with sidelighting, Chapter 5 of this user’s
manual includes toplighting requirements for the purpose of optimizing energy savings associated
with daylight responsive controls. Toplighting may be provided by skylights or roof monitors.
Situations that are exempt from the requirements of this section are as follows:
• Daylighted areas that are significantly shaded by a permanent adjacent structure. It must be
documented that the adjacent structure blocks direct sunlight to the skylight more than 1500
daytime hours of the year. The daytime hours are defined by this section as the hours between
8:00 a.m. and 4:00 p.m.
• Daylight areas under skylights where the overall skylight effective aperture for the enclosed
space is less than 0.006. In this situation, there will be little usable light entering the space,
limiting the energy that will be saved by daylight responsive controls.
• When buildings in Climate Zone 8 have individual spaces with less than 200 W of general
lighting power, these spaces are not required to have automatic daylight responsive controls for
toplighting. Due to the high latitude of Climate Zone 8, the daytime hours for a significant
portion of the year are dark, and the energy savings of daylight responsive controls for
toplighting are limited.
When required by Table 9.6.1, spaces with 150 W or more of general lighting within a toplighted
area (as defined in Section 3.2) must include daylight responsive controls. These controls must use
photocontrols to change the general lighting level in the toplighted area to reduce electric lighting
when there is an adequate contribution of daylight.
The controls (and associated lighting hardware) must have the ability to adjust the lighting level to
a minimum of four steps. These steps must include the following percentages of the full lighting
power:
• 100%
• One point between 70% and 50%
• One point between 40% and 20% (or the lowest level the lighting system can achieve )
• 0%
Alternatively, continuous dimming may be used in lieu of stepped dimming.
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g.
h.
i.
Where a building has both toplighting and sidelighting, it is likely that these areas will overlap at
times. Where these areas overlap, the general lighting serving the overlapping areas must be
controlled with the general lighting serving the toplighted area.
As with sidelighting, calibration of the system must not require that a person be physically present
at the daylight sensor.
Refer to Section 3 of the standard for the definition of toplighted area and to Chapter 5 of this
user’s manual for additional clarification and examples.
Automatic partial OFF: To satisfy the requirements of this control function, the lighting power of a
space must be automatically reduced to less than 50% of the full power within 20 minutes of the
last occupant leaving the space. The power reduction may be achieved through switching of lamps,
luminaires, or dimming. If lighting isn’t necessary when the space is unoccupied, automatic full off
may be used to satisfy this control function in lieu of automatic partial off.
Spaces using high-intensity discharge (HID) lighting are exempt from this control function
requirement if they have an LPD of 0.80 W/ft2 (8.6 W/m2) or lower and are capable of reducing
the lighting power by at least 30% within 20 minutes of the last occupant leaving the space. This
may be achieved through turning off at least 30% of the luminaires or dimming all luminaires.
The root of this exception has to do with maintaining safety and security when the lighting power
is reduced. HID lights can take minutes to turn on due to an inherent warm-up period. Lights using
other technologies can turn on with no delay to maintain safety and security where HID cannot.
When using HID lighting at the reduced power level, the lighting must still produce sufficient light
to maintain safety and security. If the LPD is 0.8 W/ft2 (8.6 W/m2) or less, reducing the power by
50% would create an unsafe or insecure environment. Therefore, the minimum required HID
lighting power reduction is 30% when the LPD is 0.8 W/ft2 (8.6 W/m2) or less. If the LPD is higher,
the exception is not met, and the full lighting power reduction must be met regardless of lamp
type.
Automatic full OFF: Similarly to automatic partial off, satisfying the requirements of this control
function requires automatically turning off all lighting within 20 minutes of the last occupant
leaving the space. This includes turning off lighting connected to emergency circuits. Each
controlled area may not be larger than 5000 ft2 (465 m2). For example, a continuous 6000 ft2
(557 m2) space must be divided into no less than two controlled areas.
Exemptions to this control function exist where automatic shutoff would create a safety or security
risk for the occupants of either the specific space or the building in general:
• Spaces where patient care is rendered
• General lighting in shop and laboratory classrooms
• Spaces that operate 24 hours a day/7 days a week (as the space operation would prevent the
controls from turning off the lights)
A one-line diagram representing how this might be implemented is shown in Figure 9-A.
Scheduled shutoff: This control function requires that all lighting within a space, including lighting
connected to emergency circuits, be turned off by a time-of-day control device or a signal from
another control system with a time-of-day schedule (for example, the time-of-day schedule may be
a signal from the building automation system or the security system). The controlled spaces must
be no greater than 25,000 ft2 (2322 m2) and may not affect more than one floor. The time-of-day
control schedule must also account for weekends and holidays. A one-line diagram representing
how this might be implemented is shown in Figure 9-B.
Manual override of the schedule is not mandatory but is allowed, provided each override instance
is no longer than two hours. This maximum override duration limits the potentially wasted energy
in the instance the schedule is overridden in an unoccupied space. Additionally, spaces with
override capability must be smaller than the maximum area allowed by the schedule: although the
maximum space area to be controlled by the time-of-day schedule may be 25,000 ft2 (2322 m2),
each space with override capability may not be larger than 5000 ft2 (465 m2).
Manual override is any means of allowing the occupant to turn the lights on when the schedule
indicates they should be off and is not necessarily a manual line voltage switch. The override
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interface could be a low-voltage wall switch, Web app, telephone, etc., that communicates with the
lighting control system to override the schedule.
Scheduled shutoff control is not required for spaces that operate 24/7, are used for patient care, or
have lighting required for maintaining safety or security. Scheduled shutoff does not need to be
provided for building lighting loads that do not exceed 0.02 W/ft2 multiplied by the gross lighted
area of the building. This exception allows a small amount of wattage for path of egress lighting
and is not required to shut off. Because path of egress often overlaps multiple spaces, it is applied
to the entire building for use wherever path of egress is designed.
Example 9-F. Local Control, Number of Controls
Corresponding section: Interior Lighting Controls, Local Control (9.4.1.1[a])
Q
An open office is 9000 ft² (836 m²). How many local control devices are required for this space?
A
Four, because this space is smaller than 10,000 ft² (929 m²). Each local control can serve a maximum
area of 2500 ft² (232 m²).
Q
An open office is 11,000 ft² (1022 m²). How many local control devices are required?
A
Two, because this space is larger than 10,000 ft² (929 m²). Each local control can serve a maximum
area of 10,000 ft² (929 m²).
Example 9-G. Local Control, Accessibility of Lighting Controls
Corresponding section: Interior Lighting Controls, Local Control (9.4.1.1[a])
Q
Can the lighting controls for public corridors in a mall be physically grouped and switched from a
remote location?
A
Yes, this is allowed for security reasons. In addition, by switching from a remote location, any unusual
appearance or functional discrepancy caused by partial lighting can be avoided. The remote control
must have an indicator light and must be clearly marked to indicate which lighting it controls. Note
that grouped refers to the physical placement in the same area of a number of individual controls.
There is no reduction allowed in the number of controls required.
Q
Do lighting controls in airports, building lobbies, banks, libraries, and department stores need to be
accessible?
A
No, there is an exception to the accessibility requirement for safety and security reasons. The
remotely-located control must have an indicator light and must be clearly marked to indicate the
controlled.
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FYI
Lighting Control Systems
Over time, lighting control systems have evolved significantly as technology has advanced. The most
basic lighting control system is a line voltage wall switch used to operate the lighting. Automatic
lighting control systems automate line voltage switching and can combine this with manual switching.
Simple automatic lighting control systems (line voltage controls) use individual control devices with
built-in transformers and relays to switch the line voltage to the lighting load. The packaged nature
and stand-alone operation of these control devices keeps implementation simple. As the need for
multiple combinations of control functions and switching configurations arises, these types of line
voltage control devices may become more difficult to combine to achieve the desired control logic.
More advanced lighting control systems may have multiple sensors that communicate with a central
lighting control device or controller. The controller uses the input signals from the sensors to
determine how the lights should be controlled. The controller sends the lighting control signal to
addressable power packs, ballasts, lighting drivers, or other line voltage control devices to adjust the
lighting power. These systems often use a digital control to communicate directly with digitally
addressable control devices and are frequently referred to as a digitally addressable control system.
Lighting control systems compliant with the requirements of this standard will include at least one of
the following control devices:
•
•
•
•
Manual wall switch: Allows occupants to manually turn the lighting on or off within a lighting zone
Occupancy sensor: A device that uses sensors to determine the occupancy of a lighting zone
Photosensor (daylight sensor): A device that measures the illuminance within a space
Time clock: A device that uses a full calendar year time-of-day schedule to define when lighting
should be on or off. This time clock may be part of the lighting control system, but it may also be a
signal to the lighting control system from another control system, such as the building automation
system or security system.
Line voltage lighting control systems and digitally addressable lighting control systems can often
achieve similar control functions, but the choice of system is typically project dependent. Many levels
of controls exist and should be explored.
Line voltage lighting control system components are frequently lower cost but offer less control
flexibility and expansion. The control devices that make up this system are typically stand-alone
devices that cannot communicate with other devices and do not require a central lighting control
system to function. As such, the control logic, adjustment, and features are built into each device.
Adjustment and calibration are manually performed at the device with physical switches or buttons.
These lighting control systems may be appropriate for small new projects or retrofits but may not be
an optimal solution for large new projects or major renovations.
Digitally addressable lighting control systems typically have low-voltage sensors and control devices,
and only the luminaires are line voltage. This simplifies the required line voltage lighting wiring.
Additionally, the control logic, adjustment, and calibration of the system can be performed through a
variety of methods that minimize physical interaction with the devices. This allows system-wide
changes in set points and control logic to be made from a central workstation. For further building
integration, these systems frequently have the ability to communicate with other building control
systems. Digitally addressable lighting control system components are often more costly, but they
typically include features that save installation and setup time. On large projects, these savings can
substantially offset the hardware costs.
FYI
Occupant Sensors
Occupant sensors detect the presence of occupants in lighting zones. The lighting controlled by the
occupant sensor may be turned on or off automatically. Most devices can be calibrated for sensitivity
and for the length-of-time delay between the last detected occupancy and extinguishing of the lights.
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While occupancy sensors are capable of automatically turning lighting on and off, the standard
restricts “auto on” and encourages “auto off.”
FIGURE 9-A. AUTOMATIC ON/OFF FUNCTION
Restricted to partial automatic ON, Section 9.4.1.1(c): To comply with this section, only a portion of the
total lighting power may be turned on automatically. The remainder of the lighting power must be
manually turned on. This control function may be achieved with stepped ballasts, dimming ballasts,
separately controlled lamps, or dimmable drivers. Figure 9-A applies only to the automatically
switched lighting using an occupancy sensor and does not illustrate the wiring for the remainder of the
manually switched lighting.
Automatic partial OFF, Section 9.4.1.1(g): Automatic partial off is similar to restricted partial on in that
the occupancy sensor is only switching a portion of the total lighting power in a space. However,
instead of turning a portion of the lighting power on, this control function turns a portion off. When
using this control function, the remainder of the lighting power may be required to be turned off
automatically at the end of the day by a scheduled shutoff. The required control functions are specified
in Table 9.6.1.
Automatic full OFF, Section 9.4.1.1(h): Regardless of how the lighting is turned on, this control function
must turn off the total lighting power after the space is unoccupied for a specified time. This control
function is typically allowed in lieu of the scheduled shutoff control function as denoted by ADD2 in
Table 9.6.1.
There are two major types of occupancy control devices:
•
Wallbox units are designed to fit into a standard wall switch box and operate on the building
voltage; a separate power supply is not required. They are inexpensive replacements for standard
wall switches. Their main limitation is their relatively short range and limited ability to adjust
direction of detection. Consequently, they tend to be used in small offices and meeting rooms.
• Wall and ceiling units typically contain an integrated sensor/controller unit wired (Class 2) to a
power supply relay switch pack containing the relay and power supply. They are far more popular
than wallbox units and have very few application limitations.
With lighting control, four different means of detecting occupancy are used:
•
•
Passive infrared (PIR) sensors perceive and respond to the heat patterns of motion. The chief
advantage of PIR sensors is that they are relatively inexpensive and reliable. They very rarely false
trigger (that is, respond to nonoccupant motion in a space). The major limitation of PIR sensors is
that they are strictly line-of-sight devices, unable to see around corners or partitions.
Ultrasonic (US) detectors radiate ultrasonic waves into a space and then read the frequency of
the reflected waves. Motion causes a slight shift in frequency, which the detector interprets as
occupancy. They are more sensitive than PIR sensors, which is both an advantage and a
disadvantage. They are often used very effectively in partitioned spaces but are also more prone to
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false triggering due to their sensitivity to air movement. Proper design and installation minimizes
this potential problem.
• Microphonic (MP) detectors are equipped with sensitive microphones that listen for sharpedged sounds generally of the type emitted by human activity.
• Dual technology (DT) usually works with a combination of PIR sensors and either US or MP
detectors to ensure extra reliability against false detections. The PIR may function as the on switch
as an occupant enters the space, with the US or MP functioning as the sensor that monitors the
space for continued activity. Alternatively, DT sensors may be set such that both PIR and US are
required for on.
Many occupant sensor manufacturers also offer products that integrate both PIR and US technology
into one package. Typically, these are designed to avoid false triggering by holding the lights off unless
both detectors sense motion in the space.
Although it is difficult to generalize about the amount of lighting energy savings attributable to the use
of occupant sensors, they are consistently cost-effective for many lighting applications. Occupant
sensors are most effective in building spaces where occupancy is sporadic or unpredictable and in
spaces where the lights are likely to be left on inadvertently, such as storage areas. Typical savings
range from 10% in large open offices to 60% in some warehouse applications. In many cases, occupant
sensors pay for themselves in less than a year.
FYI
Automatic Time Scheduling
FIGURE 9-B. SCHEDULING CONTROL
The standard does not require a specific type of scheduling control. However, the prudent designer
will choose a control that permits scheduling detail appropriate for the intended use of the space or
building. For instance, an office operates for different hours on weekdays, Saturdays, Sundays, and
holidays. Restaurants may be open late on Friday and Saturday nights but closed on Mondays. Some
retail stores may be open for the same hours every day of the year, whereas other retail stores may be
open late one night during the week, close early on Saturday night, and open later on Sunday morning.
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An appropriate scheduling control should be capable of knowing the type of day (weekday, etc.) and
using an appropriate lighting schedule for that day type.
In the most basic time-scheduling scheme, a time switch switches lighting circuits on or off based on
programmable schedules. For example, exterior lighting is usually switched on to correspond to
sundown and is switched off again at daybreak. By contrast, time scheduling of interior lighting
systems is based, for the most part, on occupancy schedules.
In some cases, time switches are used to energize additional lighting control systems, such as
daylighting controls, which are held off during unoccupied periods.
Time-scheduling systems employ the following components:
•
A central processor is usually capable of controlling several output channels, each of which may
be assigned to one or more lighting circuits.
• Relays are series-wired to lighting control zones and are controlled by the central processor.
• Overrides are required to accommodate people who use the space during scheduled off hours.
Individuals can activate manual switches, use personal-computer-based software prompts, or use
telephone overrides to regain temporary control of the lights in a given space.
In most cases, Class 2 (low-voltage) wiring links all the components in the system, and the system uses
a flashing warning system to let individuals know that the lights are going off. This allows occupants to
either vacate the space or activate an override to keep the lights on.
The crucial component in any time-scheduling system is the programmable central processor, which is
essentially a multiple-circuit controller. The central processor can be programmed by building
maintenance personnel to schedule on and off loads on each of its output channels. If desired, several
different on-off sequences may be programmed on each channel.
A central processor typically consists of the following components:
•
A programmable microprocessor with an electronic clock is capable of separately scheduling
weekday, weekend, and holiday operation. Astronomical timekeeping ability means that the
processor is able to make seasonal and daylight savings adjustments. (The power of the microchip
now allows for one-time programming of all 365 days in the year.) Typically, the processor has a
built-in battery backup so that the programmed schedule remains in memory during power
outages. The processor is usually able to “sweep” at regular intervals during its off hours. The
processor remembers when overrides have been employed to keep lights on in any particular
area; the processor will then repeat the operation to turn off the lights.
• Switch inputs allow occupants to override the shutoff function of the processor. Usually the
switches and wiring to the controller are low voltage. Inputs may also be wired to photocells or
occupancy sensors for additional flexibility.
• Output channels are required for each lighting control zone. Sophisticated designs sometimes
provide two or more outputs for each control zone. This allows for stepped control of the zone. In
some systems, output channels can be designed to provide a variable signal, allowing for dimming
applications.
Generally, time scheduling is the most effective way to save lighting energy when occupancy patterns
are relatively regular or when lighting operating hours are easy to predict. Exterior lighting controlled
with an astronomical time switch is the best example of this type of application.
Example 9-H. Manual On vs. Partial Automatic On Lighting Control Functions
Corresponding section: Interior Lighting Controls, Restricted to Manual ON (9.4.1.1[b]) and Restricted to
Partial Automatic ON (9.4.1.1[c])
The following example questions apply to the required lighting control functions for a middle school
computer room.
Q
When designing a computer room lighting control system, does the standard allow the full lighting
power to be turned on automatically by a time clock or an occupancy sensor in the space?
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A
No. A computer room may not have automatic controls that turn on the full lighting power. Per Table
9.6.1, a computer room must use one of two control functions for turning on the lights within the
space. The acceptable control function options are designated by ADD1 in the applicable control
function column. The designer must include one of the control functions designated by ADD1 in the
lighting control system. One of the following two control functions must be used to turn the lights on
within a computer room:
1. Restricted to manual ON: The occupants turn on the total lighting power in the computer room by
the use of a manually operated wall switch located in the space. None of the general lighting is
turned on automatically (9.4.1.1[b]).
2. Restricted to partial automatic ON: Up to 50% of the total lighting power in the computer room
may be automatically turned on by the lighting controls. The automatic controls may use a
schedule or an occupancy sensor as the automatic control device. The standard does include
requirements that limit the type of automatic control device. The remaining lighting power may
only be turned on by a manual control device (9.4.1.1[c]).
Q
It is lunchtime on a typical school day and the computer room has been completely unoccupied for 30
minutes. The last person to leave the room did not turn off the lights and the full general lighting
power is on. Are the computer room lighting controls in compliance with the mandatory requirements
of Section 9.4.1.1?
A
The answer depends on the control function used to shut off the general lighting.
The lighting control system is in compliance with the standard if it uses a scheduled shutoff control
function meeting the requirements of Section 9.4.1.1(i). To satisfy these requirements, the lighting
control system must have a means of automatically shutting off all the lighting in the computer room
according to a time schedule. Turning the lighting off in the middle of the day due to occupancy change
is not a requirement of the scheduled shutoff control function.
The lighting control system is not in compliance with the standard if it includes the automatic full off
control function. While this control function may be used in lieu of the scheduled shutoff control
function, it must turn off all general lighting in the space within 20 minutes of the last occupant leaving.
Q
Is an occupancy sensor required in the computer room for the lighting control system to be compliant
with Section 9.4.1.1?
A
No. If automatic lighting control is desired to turn on the lighting with a time schedule and occupancy
sensors are not, the lighting controls may use the automatic partial on control function and the
scheduled shutoff control function. Using these control functions, the general lighting in the computer
room would operate as follows on a typical school day. The schedule would turn on no more than 50%
of the general lighting power at the start of the day. When the teacher enters the classroom, he or she
must manually turn on the remainder of the general lighting power if desired. The manually controlled
lighting will remain on until an occupant turns it off manually or the scheduled shutoff turns it off. The
scheduled shutoff will automatically turn off all of the manually and automatically controlled lighting
at the end of the day.
Although the above logic is allowed by the standard, it may not provide enough occupant control for a
teaching computer room. In a classroom it is often required that all the lights be turned off for the use
of a projector. If this is the case, an additional manual control device can be added to turn off the
automatically controlled lighting. This would allow occupants to turn off all the lights during the
occupied time of the lighting schedule. As before, the automatically controlled lighting will
automatically come on in the morning, and all the lights will be automatically turned off at the end of
the day.
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The computer room with scheduled shutoff is to be used after hours for an extracurricular activity
when the schedule normally has the lighting turned off. The activity will last for three hours past the
scheduled off time. Can the lighting be overridden by the occupants to stay on for the full three hours?
A
Yes, but not without multiple overrides. The maximum allowable single override period without
occupant intervention is two hours. Thus, after the initial schedule has turned off the lights, the
occupants may turn them back on (with a wall switch, telephone, computer, etc.). The maximum
override period will be two hours from that point. After the initial override time has expired, the
occupants must override the scheduled shutoff again if they wish to keep the lighting on. The
occupants may repeat the manual override as many times as they choose. Once the override time
expires and the occupants fail to manually override the scheduled shutoff, all the lights will turn off.
Example 9-I. Primary Sidelighted Area, Uniform Window Configuration
Corresponding section: Interior Lighting Controls, Automatic Daylight Responsive Controls for
Sidelighting (9.4.1.1[e])
Q
An enclosed space at the perimeter of a building has a ceiling height of 10 ft (3 m) and a depth of 24 ft 6
in. (7.5 m). A series of windows are located around the perimeter that are 6 ft (1.8 m) wide and 6 ft 6
in. (2 m) tall and spaced at 10 ft (3 m) on center. The head height of the windows is 9 ft How is the
sidelighted daylighted area defined?
A
The depth of the primary daylighted area is 9 ft (2.7 m), which is the distance from the floor to the
window head (top). Because the width is defined as one half the head height, as long as the horizontal
distance between windows is less than the head height of the windows, the daylighted area is
continuous. The daylighted area stops when it reaches any obstruction with a height above the floor of
5 ft (1.5 m) or greater. The secondary daylight area is the same depth and width as the primary, as long
as there are no obstructions. Repeat the width and depth calculation for each window in the space. The
daylight area is the combined total of all the daylight areas from each window.
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Example 9-J. Primary Sidelighted Area, Nonuniform Window Configuration
Corresponding section: Interior Lighting Controls, Daylighting Controls for Primary Sidelighted Areas
(9.4.1.1[e])
Q
The building facade below has a nonuniform window configuration. What is the primary sidelighted area
created by these windows?
A
The daylighted area is continuous when the distance between window jambs is equal to or less than
the window head height. When the distance between window jambs is greater than the window head
height, which is the case between the narrow windows and the short windows, there is a gap in the
daylighted area.
In this example, if some of the window heads are at a height of 9 ft (2.7 m), then these would create a
primary daylighted area that extends 9 ft (2.7 m) into the space. If other window heads are at a height
of 7 ft (2.1 m), then these would create a primary area that extends 7 ft (2.1 m) into the space. See the
figure below. Note that while the sidelighted areas overlap, the overlapping areas may only be counted
once toward the total daylighted area.
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Example 9-K. Unobstructed Daylight Area under a Single Skylight
Corresponding section: Interior Lighting Controls, Automatic Daylighting Controls for Toplighting
(9.4.1.1[f])
Q
A 4 × 8 ft (1.2 × 2.4 m) skylight with a straight light well is located in a space with a 20 ft (6.1 m)
ceiling. Without considering the impact of adjacent skylights or sidelighted areas, what is the
daylighted area that is created?
A
The band around the horizontal projection of the skylight is 70% of the 20 ft (6.1 m) ceiling height, or
14 ft (4.3 m). The dimension of the daylighted area in the long direction of the skylight is 8 ft (2.4 m)
(the skylight well dimension) plus two times 14 ft (4.3 m) for a total of 36 ft (11 m). The dimension in
the short direction of the skylight is 4 ft (1.2 m) plus two times 14 ft (4.3 m) for 32 ft (9.8 m). The
overall dimensions of the top-daylighted area are therefore 36 ft (11 m) times 32 ft (9.8 m), or 1152 ft²
(107.1 m²).
Example 9-L. Warehouse with Skylights
Corresponding section: Interior Lighting Controls, Automatic Daylighting Controls for Toplighting
(9.4.1.1[f])
Q
A simple warehouse measuring 120 × 120 ft (36.6 × 36.6 m) and with a 20 ft (6.1 m) ceiling height is
shown in the following illustration. Sixteen skylights with an opening of 4 × 8 ft (1.2 × 2.4 m) each are
spaced at 30 ft (9.1 m) centers. The building has no vertical fenestration. Ignoring possible
obstructions in the building’s interior, what is the daylighted area under these skylights?
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A
Before consideration of exterior walls, each skylight creates a 36 × 32 ft (11 × 9.8 m) daylighted area,
as described in the previous example. The daylighted areas created by each skylight overlap because
the 30 ft (9.1 m) skylight spacing is smaller than both the 36 and 32 ft (11 and 9.8 m) dimensions of
the daylighted area. The figure below shows the overlapping daylighted areas under each skylight.
Because the daylighted areas also extend to the exterior walls, the entire 120 × 120 ft (36.6 × 36.6 m)
space is daylighted. An automatic daylighting control is required for all luminaires in the space.
Example 9-M. Warehouse with Skylights, Equipment on Roof
Corresponding section: Interior Lighting Controls, Automatic Daylighting Controls for Toplighting
(9.4.1.1[f])
Q
A design modification in Example 9-L requires that the center four skylights be removed so that
equipment can be installed on the roof. What is the daylighted area in this case?
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The portion of the building that is not daylighted measures 54 × 58 ft (16.5 × 17.7 m), as shown below.
This represents a floor area of 3132 ft² (2.91 m²). The area that is still daylighted is 14,161 ft²
(1316 m²) less 3132 ft² (2.91 m²), or 11,029 ft² (1025 m²). (The original daylighted area is 14,161 ft²
[1316 m²] because the interior floor dimension of the building, minus wall thickness, is 119 × 119 ft
[36.3 × 36.3 m].) Automatic daylighting controls are required for luminaires located in the perimeter of
the space.
Example 9-N. Warehouse with Storage Racks
Corresponding section: Interior Lighting Controls, Automatic Daylighting Controls for Toplighting
(9.4.1.1[f])
Q
Assume that the building described in Example 9-L is used as a warehouse for bulky items and that the
space contains large storage racks. Each storage rack is 10 ft (3 m) wide and 16 ft (4.9 m) high. They
are spaced 10 ft (3 m) apart to enable forklift access to both sides of the storage racks. What is the
daylighted area for this situation?
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A
The daylighted area includes the perimeter around the storage racks plus the two center aisles. The
two outer aisles are not considered daylighted space because light (assumed to be at an angle of 35
degrees from plumb) strikes the top of the storage racks and does not make its way to the two outer
aisles.
Example 9-O. Daylight Responsive Controls to Sidelighting and Toplighting
Corresponding section: Interior Lighting Controls, Automatic Daylight Responsive Controls for
Sidelighting (9.4.1.1[e]) and Automatic Daylighting Controls for Toplighting (9.4.1.1[f])
Q
The example building below consists of one large open office area. It has windows lining one exterior
wall of the building and skylights over one end of this building. The secondary sidelighting area of six
of the windows overlaps with the toplighting area associated with the skylights. The primary
sidelighting area is 9 ft (2.7 m) deep and extends the length of the wall. The secondary sidelighting
area is also 9 ft (2.7 m) deep and extends the length of the wall. The toplighting area is 18 × 60 ft (5.5 ×
18.3 m) and overlaps the secondary sidelighting area by 5 ft (1.5 m). The space contains 28 luminaires
for general illumination, 96 W each, plus four “wall washer” luminaires that highlight artwork, 55 W
each. Based on the figures below, how many daylight responsive zones must there be and how many of
the lights shown below are to be in each zone?
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A
There must be three daylight responsive zones for the area shown. Per Table 9.6.1, open office plan
areas must include both sidelighting daylight responsive controls and toplighting daylight responsive
controls. Only the general lighting in a space requires daylight responsive controls. Accent, display, and
task lighting are not required to have daylight responsive controls, which includes the four “wall
washer” luminaires used to accent artwork.
There are seven luminaires over the primary sidelighted area. Per Section 9.4.1.1(e), these luminaires
must be controlled to the same daylighting responsive control based on a photosensor located in the
primary sidelighted area. The secondary sidelighted area has a total of seven luminaires above it.
However, five of these luminaires are in an area that is both toplighted and sidelighted. The seven
luminaires over the secondary sidelighted area may not be responsive to the same photosensor.
Section 9.4.1.1(f) requires lighting serving areas both toplighted and sidelighted to be controlled with
the lighting serving the toplighted area.
There are ten luminaires above the toplighted area, five of which are also above the secondary
sidelighted area. These ten luminaires must be controlled together in response to the daylight in the
toplighted area. This leaves two luminaires in the secondary sidelighted area. Per Section 9.4.1.1(e),
general lighting luminaires in the secondary sidelighted area, must be controlled independently of
general lighting in the primary sidelighted area.
The total general lighting power of the lighting over the primary and secondary sidelighted areas is
greater than 300 W, so the secondary sidelighted area must be controlled in addition to the primary
sidelighted area. When secondary sidelighted areas are controlled, they must be controlled
independently from the primary sidelighted area, as the amount of daylight in these two areas is
significantly different.
Therefore, the two luminaires must be controlled to the daylight level within the secondary sidelighted
area below the luminaires. The remaining nine luminaires in the space are outside of any daylighting
area and are not required to include any daylight responsive controls.
The four “wall washer” luminaires used to accent artwork are not required to be controlled by daylight
responsive controls but must be controlled separately from the general illumination luminaires.
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Example 9-P. Daylighted Area under Roof Monitor
Corresponding section: Interior Lighting Controls, Automatic Daylighting Controls for Toplighting
(9.4.1.1[f])
Q
What is the daylighted area that is created under the roof monitor shown in this figure?
A
The daylighted area width is equal to the width of the vertical glazing in the monitor plus 2 ft (0.6 m)
on each side of the vertical glazing. The daylighted area depth is equal to the monitor sill height (MSH).
The vertical glazing in the example building are 4 ft (1.2 m) apart a