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ASHRAE Dedicated Outdoor Air Systems

Complete, Up-to-Date DOAS Guidance
Guided by the information in this book, HVAC system designers will be
able to optimally incorporate DOASs into their projects. Architectural
designers, building developers and owners, maintenance professionals,
students, teachers, and researchers may also find the contents useful.
Featuring practical checklists, full-color graphics and psychrometric
charts, and common tips and traps for designers, ASHRAE Design
Guide for Dedicated Outdoor Air Systems is an indispensable guide
for the working HVAC professional with interest in DOASs.
ISBN 978-1-939200-71-6 (ppbk)
ISBN 978-1-939200-72-3 (PDF)
9 781939 200716
Product code: 90304
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Atlanta, GA 30329-2305
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ASHRAE_DOAS_Cover Spread.indd 1
Dedicated Outdoor Air Systems
Experienced DOAS designers will find this guide helpful in dealing
with deviations from the norm, while HVAC designers without DOAS
experience will find a complete guide to implementing a DOAS. The
guide can be read front to back or in parts, depending on the needs
of the designer.
Dedicated outdoor air systems (DOASs) provide HVAC designers
with opportunities for advantages in simplicity, efficiency, and economy.
This book represents the most complete and up-to-date guidance on
the design, installation, and operation and management of DOASs in
nonresidential applications.
Dedicated Outdoor
Air Systems
operation and maintenance
5/18/2017 10:59:44 AM
ASHRAE Design Guide for
This publication was supported by ASHRAE Research Project RP-1712 under the auspices of
TC 8.10, Mechanical Dehumidification Equipment and Heat Pipes.
Updates and errata for this publication will be posted on the
ASHRAE website at www.ashrae.org/publicationupdates.
ASHRAE Design Guide for
ISBN 978-1-939200-71-6 (paperback)
ISBN 978-1-939200-72-3 (PDF)
 2017 ASHRAE
1791 Tullie Circle, NE
Atlanta, GA 30329
All rights reserved
Cover design by Laura Haass
ASHRAE is a registered trademark in the U.S. Patent and Trademark Office, owned by the American
Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
ASHRAE has compiled this publication with care, but ASHRAE has not investigated, and ASHRAE
expressly disclaims any duty to investigate, any product, service, process, procedure, design, or the like
that may be described herein. The appearance of any technical data or editorial material in this publication does not constitute endorsement, warranty, or guaranty by ASHRAE of any product, service,
process, procedure, design, or the like. ASHRAE does not warrant that the information in the publication is free of errors, and ASHRAE does not necessarily agree with any statement or opinion in this
publication. The entire risk of the use of any information in this publication is assumed by the user.
No part of this book may be reproduced without permission in writing from ASHRAE, except by a
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nor may any part of this book be reproduced, stored in a retrieval system, or transmitted in any way
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Library of Congress Cataloging-in-Publication Data
Library of Congress Cataloging in Publication Control Number: 2017017812
Special Publications
Mark S. Owen, Editor/Group Manager of Handbook and Special Publications
Cindy Sheffield Michaels, Managing Editor
James Madison Walker, Managing Editor of Standards
Sarah Boyle, Assistant Editor
Lauren Ramsdell, Assistant Editor
Michshell Phillips, Editorial Coordinator
Publishing Services
David Soltis, Group Manager of Publishing Services
Jayne Jackson, Publication Traffic Administrator
W. Stephen Comstock
Chapter 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . . 1
Reasons for Using DOAS
Role of the Designer
Chapter 2: Outdoor Air and Load Requirements . . . 9
Outdoor Air Requirements
Outdoor Air Design Conditions
Outdoor Air Loads
Chapter 3: System Selection . . . . . . . . . . . . . . . . . . . 29
Common Approaches to Air Distribution
DOAS Equipment Configurations
System Selection Considerations
Chapter 4: Detailed Design Considerations . . . . . . . 49
Codes and Standards
Air Distribution
Dehumidification and Cooling
Air-to-Air Energy Recovery
Energy Recovery Effectiveness
Filtration/Air Cleaning
Chapter 5: Controls . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Control of the DOAS Unit
Humidity Control
Temperature Control
Energy Recovery Control
Frost Control
Outdoor Airflow Control
Building Pressurization Control
Sensors and Instrumentation
Building Automation
Sequence of Operations
Chapter 6: Construction . . . . . . . . . . . . . . . . . . . . . 107
Construction Phase Process
Chapter 7: Operation and Maintenance. . . . . . . . . 119
Operation and Maintenance of DOAS Equipment
Case Study
Appendix A:
Sample DOAS Installation Checklist . . . . . . . . . . . 127
Appendix B:
Sample DOAS Operational Checklist. . . . . . . . . . . 135
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
The purpose of this book is to help technical professionals design dedicated
outdoor air systems (DOAS) for commercial and institutional buildings. If you are
interested mostly in residential or industrial buildings, this book may not meet all
of your needs.
Additionally, this book is focused on design considerations that are specifically relevant to DOAS. It is not intended to be a general guide for designing any
HVAC system. Rather, its intent is to highlight issues that should be considered
when incorporating DOAS into your overall HVAC design.
We assume the reader is an HVAC designer, although not necessarily one
with an engineering degree, and has a basic understanding of HVAC systems, terminology, psychrometry, and the common variables of equations used in designing HVAC systems.
As an overall focus, we write for the system designer who “has to get it done
by Friday.” We expect that the guide may also be useful to others, such as architectural designers, building developers and owners, maintenance professionals,
students, teachers, and researchers. However, where decisions have been made
about technical depth and detail, we have tried to meet the needs of the HVAC
designer first.
This book has been organized into seven main chapters. Chapter 1 is designed
to give the reader some background on DOAS as well as describe the primary
motivations behind developing this Guide. Chapters 2 through 5 contain the core
guidance for designing a successful DOAS. These chapters cover outdoor air and
load requirements, system selection, detailed design considerations, and controls,
respectively. Chapter 6 familiarizes the reader with items to consider during the construction process, and Chapter 7 focuses on DOAS operation and maintenance.
Some of the topics covered in this book are relevant to more than one piece of
the design process. Rather than reiterate the same information in multiple chapters,
choices have been made regarding where to place these subjects. We have tried to
organize the content in this book such that subjects are placed in the chapter most
relevant to them, and the reader is referred to that section when that subject comes
up again in other places.
Readers will get the most use out of this guide if they come to it with a clear
understanding of the goals and requirements of the specific project they are working
on. With these in mind, it will be easier to narrow down the types of systems and
equipment configurations that would be most appropriate for their particular application. It is also important to remain aware of any specific climate considerations for
the project in question.
This book has been organized such that someone designing their first DOAS
could read the book from start to finish and receive DOAS-specific guidance for
each step in the design. Readers who have already started their design or have previous experience with DOAS should skip to the areas of the book that are relevant to
their project or that present DOAS variations they might be less familiar with. We
recommend that all readers pay some attention to controls, construction, and operation, as these are the elements that can make the difference between a marginally
functional DOAS and a fully optimized DOAS.
This work was made possible by the contributions and assistance of many individuals and organizations. We are particularly grateful to John Murphy and Lew
Harriman for their encouragement and support throughout this enormous task. The
entire project monitoring subcommittee (John Dieckmann, Chris Gray, Lew Harriman, Scott McGinnis, John Murphy, and Paul Pieper) was instrumental in providing
insightful guidance and feedback. Many of our best graphics were either borrowed
from or inspired by existing ASHRAE publications, and we thank those authors for
helping to pave the way. We are also deeply appreciative of all of the engineers,
building owners, building managers, and market stakeholders who took part in interviews and/or showed us around their buildings as we prepared to write this guide.
As the use of DOAS becomes more common, the applied experience of our
readers will also grow. This book should be thought of as a starting point for the discussion on DOAS. We encourage any feedback or suggestions from our readers that
could be used to improve this guide. Please send your recommendations to one of
our authors at info@sustaineng.com:
Svein Morner, PhD, PE, CPMP, CPP, LEED AP, Member ASHRAE
Founding Principal, Sustainable Engineering Group, Middleton, WI
Amalia Hicks, PhD, LEED AP, Associate Member ASHRAE
Senior Associate, Cadmus Group, Madison, WI
Manus McDevitt, PE, CPMP, CPP, LEED AP, Member ASHRAE
Founding Principal, Sustainable Engineering Group, Middleton, WI
Figure 1.1 Changing seasons.
George Hodan, The Four Seasons, www.publicdomainpictures.net
For the purposes of this book, all material is based on the following definition
of a dedicated outdoor air system (DOAS):
A dedicated outdoor air system (DOAS) uses separate equipment to condition all of the outdoor air brought into a building for ventilation and delivers it to each occupied space, either directly or in conjunction with local or
central HVAC units serving those same spaces. The local or central HVAC
units are used to maintain space temperature.
As shown in Figure 1.1, outdoor air conditions can vary drastically from season
to season, and even throughout the day. What doesn’t change is that buildings need
to be ventilated with fresh outdoor air. DOAS are designed to ensure that a building
receives the required amount of outdoor air, delivered at conditions that ensure
occupant comfort—regardless of what the weather is doing outdoors.
Why This Book Was Written
The use of DOAS has become increasingly popular throughout the world. However, the early adoption of all new technologies and approaches is often fraught
with challenges. Because of the current lack of DOAS exposure and experience,
many DOAS designed and installed today do not always take full advantage of all
the benefits DOAS can offer. Often, they are designed to achieve only one or two of
their many available functions. They may cool the air efficiently, but not dry it;
they may dry the air, but not recover heat; or they may not vary outdoor airflow in
response to building occupancy, forgoing substantial energy savings. Some use
overly simplified control strategies, improperly apply exhaust air energy recovery,
and/or deliver conditioned outdoor air to the building in a manner that reduces or
limits the benefits of decoupling ventilation from space conditioning.
This guide was developed to assist engineers in designing optimal DOAS.
Although, over time, a sizable body of literature has accumulated pertaining to the
design, installation, and operation of DOAS, it is widely scattered and time-consuming to search out. Such searches are particularly impractical for design engineers with pressing deadlines. In this book, we aim to consolidate much of the
existing material into one volume, focusing on the overview and major issues while
referring the reader to supplemental sources for more specialized or in-depth information.
There are many reasons to use DOAS. Some of the most common drivers are (1)
improving humidity control, (2) reducing energy use, (3) the desire to simplify ventilation design and control, (4) the desire to use heating and cooling equipment that
does not provide ventilation and/or dehumidification (e.g., radiant panels or passive
chilled beams), and (5) reducing installation cost. Here is how these driving factors
have encouraged a rise in DOAS installations over the last few decades.
Humidity Control
In many locations worldwide, for both residential and commercial buildings,
mechanical ventilation is either a code requirement or an industry-standard practice. With the introduction of outdoor air often comes an increase in dehumidifica-
tion loads. In fact, incoming ventilation and makeup air typically carries more than
80% of a building’s annual dehumidification load (ASHRAE 2015a, Chapter 62).
Annual cumulative latent ventilation loads typically exceed sensible ventilation
cooling loads by 3:1 to 5:1 in all but high-altitude and desert climates (Harriman et
al. 1997). Figure 1.2 shows the annual cumulative latent and sensible cooling and
dehumidification load from ventilation for sample climates. In addition, the peak
sensible-cooling conditions are typically not close to the peak latent load conditions, as is shown for a sample climate in Figure 1.3. Ventilation latent loads peak
at more moderate temperatures, not necessarily the peak dry bulb condition. Traditional system designs may be unequipped to handle such high latent loads (Kosar et
al. 1998) over the wide range of ambient conditions. A dedicated dehumidification
component, such as that typically included in a DOAS, can remove the ventilation
latent load, often avoiding the need for the cooling equipment to cool the air too
low for the sensible load to a temperature lower than required for the sensible load,
thereby requiring the need to use extensive reheat.
Figure 1.2 Annual cumulative latent (dehumidification) and sensible-cooling
load from ventilation air.
Harriman et al. (1997)
Because of widespread adoption of light-emitting diode (LED) lighting and low
solar heat gain coefficient windows, internal sensible-cooling loads have been
greatly reduced while the ventilation load has not. The consequence of these technology advancements is that, in many cases, separating the latent cooling from the
majority of the sensible cooling with a DOAS may reduce the cost of the mechanical system (in particular, if using an energy recovery device in the DOAS) because
of downsized sensible-cooling equipment, which can help pay for the cost of dehumidification, cooling, heating, and filtering the ventilation air in the DOAS.
Energy Impacts
The goal of reducing energy consumption in buildings has had a much greater
influence on design decisions in recent years. Due in part to this increased awareness of efficiency, DOAS has gained in popularity and is emerging as an effective,
cost-efficient approach to reducing energy use.
For example, one important way that DOAS contributes to energy savings is by
removing humidity from the outdoor air, which allows the remaining cooling components to operate based solely on dry-bulb temperature. Dry ventilation air can
eliminate or strictly limit the use of reheat energy as required by ANSI/ASHRAE/
IES Standard 90.1 (Murphy 2006).
Another energy benefit of DOAS is that less outdoor air may need to be introduced to the building compared to, for example, a typical mixed-air system that
must meet the multiple-zone recirculating system requirements of ANSI/ASHRAE
Figure 1.3 Design extremes.
Harriman et al. (2000)
Standard 62.1 (ASHRAE 2016a). Reducing excessive outdoor air intake saves
energy by reducing total outdoor air heating and cooling loads.
In addition, DOAS can make it easier to implement demand-controlled ventilation strategies. Traditional designs often ignore the issue of reducing outdoor air
when spaces are unoccupied, which impacts energy consumption (Crowther and
Ma 2016; Persily et al. 2005). With the right components installed, DOAS can
allow outdoor air to be reduced in response to changes in occupancy.
Figure 1.4 shows an example of the potential energy savings available from
implementing demand-controlled ventilation in a DOAS. Annual costs are shown
for an example building in Chicago using a constant-volume (CV) DOAS unit providing ventilation directly to the spaces with no energy recovery, a CV DOAS unit
with energy recovery, and a demand-controlled ventilation (DCV) variable-volume
DOAS unit with energy recovery. It is based on 50% average occupancy during
operating hours and 55°F (12.8°C) summer conditioned outdoor air temperature
reset to 65°F (18.3°C) in winter. Going from no energy recovery to demand-controlled ventilation with energy recovery results in a 72% energy savings and a 59%
cost savings (Crowther and Ma 2016).
Figure 1.4 Demand-controlled ventilation benefits.
Crowther and Ma (2016)
Finally, the centralization of ventilation air conditioning provided by DOAS can
make it easier to recover both heating and cooling energy from exhaust air. From an
installation and operational point of view, DOAS is often the easiest way to provide
the air-to-air energy recovery required for large airstreams by ANSI/ASHRAE/IES
ASHRAE Standard 90.1 (ASHRAE 2016b).
Advances have recently been made toward increasing awareness of the energy
associated with dehumidification. The new metric of moisture removal efficiency
(MRE) provides a means of assessing the energy efficiency of a DOAS unit. The
first implementation of this metric is in the 2016 version of ANSI/ASHRAE/IES
ASHRAE Standard 90.1, which adds minimum equipment efficiency requirements
for direct expansion (DX) DOAS units based on integrated seasonal moisture
removal efficiency (ISMRE) rated in accordance with ANSI/AHRI Standard 920
(AHRI 2015).
Ventilation Control
DOAS offers a simple and elegant way to address outdoor air loads. Because
DOAS airflow is independent of building heating and cooling loads, it is relatively
simple to control and operate, particularly if the DOAS is a constant-volume system. DOAS is also effective at meeting outdoor air requirements under all conditions.
Systems without Ventilation Capabilities
Over the last few decades, improvements in envelope technology, lighting, and
other interior equipment have resulted in generally lower sensible-cooling demands
per unit area than were previously required. This has resulted in the development
and increased use of less traditional cooling and heating equipment that can meet
these comparatively lower cooling loads more efficiently than mixed-air systems.
This includes radiant cooling, chilled beams, and variable-refrigerant-flow systems,
among others. However, because these kinds of equipment typically have very limited dehumidification capacity, they rely on a separate DOAS to provide dry air
ventilation for the building. And, in some cases, this equipment is not capable of
providing any dehumidification (sensible cooling only), so the DOAS must remove
the entire latent load of the building.
First Cost Reduction
It may sound contradictory that adding a system could reduce the first cost of a
project rather than increase it. By addressing outdoor air loads, however, DOAS
can reduce the heating and cooling loads that must be met by other components of
the HVAC system. This can result in the downsizing of these other components
(e.g., terminal units, chillers, boilers, air-handling units, ductwork, and/or piping).
For example, using a DOAS that handles the entire (external and internal) latent
load with an enthalpy exchanger and a cooling coil at a school might allow for
downsizing the tonnage of classroom units; reduce the central heating and cooling
equipment capacities; and reduce the piping, ductwork, and electrical installation.
DOAS can be effectively incorporated into nearly any commercial, institutional,
industrial, or multifamily building. While all building types can benefit from
DOAS, those with strict indoor air quality, ventilation, humidity, or energyefficiency requirements make particularly good candidates, and as the ratio of
outdoor air to the recirculated air increases, DOAS benefits rise accordingly (Kosar
et al. 1998). For example, buildings located in very humid climates provide a good
application for DOAS. Facilities with extended occupancy schedules benefit more
from DOAS than buildings with longer unoccupied periods, obtaining higher
annual savings through use of the technology. Other particularly good candidates
for DOAS are facilities that require more than just standard or code-required
minimum ventilation rates, such as those that handle pollutants that should not be
recirculated to other spaces (e.g., hospitals and laboratories).
It is the design engineer’s job to work closely with other members of the project
team to design and deliver a building that complies with the applicable codes and
satisfies the owner’s objectives and vision. The specific duties of this role include
creating a system that meets owner requirements within the constraints of the project and delivering documentation of that system to help facilitate its successful
installation, operation, and maintenance. These duties are present in any HVAC
design process and will shape the final result of the system whether DOAS is
included or not.
When choosing to incorporate DOAS, the designer must first identify the loads
that the system will be required to meet and note that, in contrast to other loads,
outdoor air loads are highly variable. Consequently, it is important to determine
how the DOAS will interact with other systems, which components the DOAS unit
should include, and how to control those components. DOAS creates new opportunities and present different challenges than more traditional systems.
This guide presents the pros and cons of various options, while remaining
focused on DOAS-specific topics. The designer must decide how best to apply
DOAS within the constraints and opportunities of a specific project.
Figure 2.1 A heavy occupant load.
HAAP Media Ltd. (Melvin Muñoz)
The density and number of occupants in the space shown in Figure 2.1 create
the need to bring in a large amount of outdoor air for ventilation. In addition, the
dancers are giving off heat and humidity, contributing to cooling and dehumidification loads. Building equipment must be sized and selected to handle variations
between this load and the significantly different loads that are present when the
building is much less heavily occupied.
This chapter presents an overview of the variables that should be considered
when determining building loads; it does not provide a detailed walk-through of
load calculation processes. The main purpose of this chapter is to highlight the load
calculation issues that are most relevant to DOAS design. These issues may or may
not be applicable to other types of systems.
The first step in designing any HVAC system is to determine the amounts of
heating, cooling, dehumidification, humidification, and outdoor air required by
individual building spaces. These space calculations are then combined to determine overall system requirements. Building use is an essential piece of information
for determining loads, because it will guide design conditions such as temperature,
humidity, and outdoor air requirements. Defining these early in the process is
highly recommended, as this can reduce costly rework and result in a project that is
ultimately more successful for the owner.
Load simulation software (not to be confused with energy simulation software)
is often used to calculate the heating, cooling, and outdoor air loads for each space,
each system, and ultimately the building as a whole. Be careful to ensure that
humidification and dehumidification requirements are adequately treated by the
software package being used. Humidity-related calculations are also covered in
detail in the ASHRAE Humidity Control Design Guide (Harriman et al. 2001).
One of the defining benefits of a DOAS is that the entire outdoor air load and
portions of the space heating, cooling, and dehumidification load can be addressed
by the DOAS unit, leaving central and local system components free to address the
remaining space heating/cooling loads. In humid climates, DOAS dehumidification
capabilities are particularly advantageous, and often drive the choice to use DOAS
in those locations. A DOAS unit can condition the outdoor air and distribute it at
desired temperature and humidity to central units, local units, plenum spaces, or
directly to occupied spaces. Because of this characteristic DOAS feature, outdoor
air is the main focus of this chapter.
Outdoor air requirements for commercial and institutional buildings are typically
determined by local building codes or industry standards such as ANSI/ASHRAE
Standard 62.1, Ventilation for Acceptable Indoor Air Quality (ASHRAE 2016a).
ANSI/ASHRAE Standard 62.1 is often incorporated into local building codes. The
minimum amount of outdoor air needed by a space is determined by one of three
factors: codes and standards, exhaust, or loads. Outdoor air requirements for all
spaces in a building may not be driven by the same factor; therefore, care must be
taken when determining total outdoor airflow. Brief descriptions of these factors as
they apply to DOAS design are as follows:
• Code-Driven Airflow. The outdoor airflow delivered is based on the minimum
required by building codes or standards.
• Exhaust-Driven Airflow. Certain spaces/buildings require more exhaust air than
outdoor air (for example laboratories or facilities with many bathrooms). To
make up for (or replace) this exhausted air, more outdoor air is brought into the
building, exceeding that required by building codes or standards.
• Load-Driven Airflow. For a DOAS, the types of loads that may result in increasing outdoor airflow typically involve dehumidification or humidification. As an
example, if chilled ceilings or passive chilled beams are being used in a space,
the design engineer may choose to increase the outdoor air delivered to provide
sufficient dehumidification (without having to dehumidify the outdoor air to an
extremely low dew point).
For most applications, the amount of outdoor air is code-driven. The following
section provides an example of how code-driven airflow is calculated. This is based
on ANSI/ASHRAE Standard 62.1-2016, which has been widely adopted as the goto reference for outdoor air design calculations.
Example Calculations
ANSI/ASHRAE Standard 62.1 contains three procedures that can be used to
design a ventilation system: the “Ventilation Rate Procedure” (Section 6.2), the
“Indoor Air Quality (IAQ) Procedure” (Section 6.3), and the “Natural Ventilation
Procedure” (Section 6.4) (ASHRAE 2016a). While all three are allowable procedures for compliance with Standard 62.1-2016, the following example is based on
the “Ventilation Rate Procedure” of ASHRAE Standard 62.1-2016. Stanke (2012)
explains how to use the IAQ Procedure for the design of ventilation systems.
The Ventilation Rate Procedure prescribes the quantity of outdoor air that must
be delivered to each zone based on the expected use of that zone, and then describes
how to calculate the total outdoor airflow that must be brought in at the systemlevel intake. Figure 2.2 shows a simple eight-zone office building served by one
DOAS unit coupled with a water-source heat pump system, as an example to illustrate the calculation process.
Minimum Ventilation Required in Breathing Zone. Table of ANSI/
ASHRAE Standard 62.1-2016 prescribes two ventilation rates for each occupancy
category: one related to the number of occupants, and one related to the floor area
of the zone under consideration. Calculating the amount of outdoor air that must be
delivered to the breathing zone requires determination of its occupancy category,
followed by identification of the corresponding ventilation rates.
The people-related ventilation rate Rp is quantified in terms of cfm per person (L/
s per person) and the building-related ventilation rate Ra has units of cfm/ft2 (L/
s·m2). After obtaining the appropriate ventilation rates from Table 6-1, the number
of people expected to occupy the zone during typical usage Pz, and the occupiable
floor area Az must be identified. Finally, the following equation is used to find the
minimum outdoor airflow required for the breathing zone Vbz:
Vbz = (Rp × Pz) + (Ra × Az)
Zone Air Distribution Effectiveness. In addition to defining the breathing zone
outdoor airflow Vbz, ANSI/ASHRAE Standard 62.1 assigns zone air distribution
effectiveness Ez, which designates the fraction of outdoor air that makes it into the
breathing zone (ASHRAE 2016a). The breathing-zone outdoor airflow Vbz is
divided by this effectiveness Ez to determine the outdoor airflow that must be delivered through the supply air diffusers Voz.
Figure 2.2 Example office building.
Stanke (2005)
Voz = Vbz /Ez
Table 2.1 is an excerpt from ANSI/ASHRAE Standard 62.1 (ASHRAE 2016a),
and provides default values for Ez for common air distribution configurations. It is
based on the placement of supply air diffusers and return air grilles, and the temperature of the air being supplied.
Zone-Level Ventilation Requirements. The example assumes that the building
is served by one DOAS unit that delivers outdoor air directly to the intakes of ceiling-mounted water-source heat pumps (WSHP). The preconditioned outdoor air
Table 2.1 Zone Air Distribution Effectiveness Eza
Configuration of
Air Distribution System
Supply Air Temperature,
Supply from ceiling, return from ceiling
cooler than zone
warmer than zone
≥Tzone + 15°F (8°C)
warmer than zone
<Tzone+ 15°F (8°C)
1.0b (0.8)c
cooler than zone
warmer than zone
cooler than zone
1.2d (1.0)e
warmer than zone
Supply from floor, return from floor
warmer than zone
Outdoor air drawn into the room,
opposite of the exhaust or return outlet
Outdoor air drawn into the room,
near the exhaust or return outlet
Supply from ceiling, return from floor
Supply from floor, return from ceiling
a. Excerpt from Table of ANSI/ASHRAE Standard 62.1-2016
b. Provided that 150 fpm (0.8 m/s) supply air jet reaches to within 4.5 ft (1.4 m) of floor
c. For supply air velocities <150 fpm (0.8 m/s)
d. Provided that vertical throw is 50 fpm (0.25 m/s) at height of 4.5 ft (1.4 m) above floor
e. For vertical throw >50 fpm (0.8 m/s) at height of 4.5 ft (1.4m) above floor level
mixes with locally recirculated air (drawn out of the zone through ceiling-mounted
return air grilles), and this mixture is then either cooled or heated by the WSHP
before it is supplied, via ceiling-mounted diffusers, to the zone. This configuration
has a zone air distribution effectiveness Ez of 1.0 in cooling mode and 0.8 in heating mode.
The amount of outdoor air required in each zone is the sum of the floor area and
occupancy ventilation requirements divided by the zone air distribution effectiveness. Table 2.2 shows the results of cooling mode zone-level ventilation calculations for each of the eight zones in the example office building.
Table 2.2 DOAS Ventilation Calculations for
Example Office Building (Cooling Design)
A z,
(L/s· m2)
conference room
R p,
(L/s· p)
South offices
West offices
East offices
South interior
North interior
North offices
conference room
System totals
(Vot =  Voz)
System-Level Ventilation Requirements. ANSI/ASHRAE Standard 62.1
(ASHRAE 2016a) also defines procedures for calculating the outdoor airflow Vot
needed at the system-level intake to ensure that the required quantity of outdoor air
Voz is delivered to each zone. Which procedure is chosen will depend on the configuration of the ventilation system in question. In the case of this example, per Section 6.2.4 of ANSI/ASHRAE Standard 62.1-2016, the system level intake airflow
Vot delivered by the DOAS unit is simply the sum of calculated zone outdoor airflows Voz:
Vot = sum of Voz
Therefore, at the cooling design condition, the required system outdoor air intake
flow Vot is 3919 cfm (1849 L/s) (Table 2.2). At the heating design condition, supplying warm air to the zones through ceiling-mounted diffusers results in a zone air
distribution effectiveness of 0.8. This has the effect of increasing the total system
intake airflow required in heating mode by approximately 25%. If, however, the
DOAS in this example delivered conditioned outdoor air (near or below zone temperature) directly to zones via separate “ventilation” diffusers (see Figure 3.2), the
zone air–distribution effectiveness would remain 1.0 in both cooling and heating
Other Considerations. In some cases, there may be benefits to supplying more
outdoor air than the minimum required. For example, a building seeking Leadership in Energy and Environmental Design® (LEED®) Green Building Rating System v4 certification can achieve one point for supplying 30% more outdoor air Vbz
to each breathing zone than the minimum required by ANSI/ASHRAE Standard
62.1-2016. Some studies have also shown correlations between increased ventilation, enhanced productivity, and reductions in employee sick days (Kumar and Fisk
2002; Fisk 2000).
For spaces that exhibit high variability in occupancy density (e.g., conference
rooms, auditoriums, and cafeterias) technologies such as occupancy sensors and
CO2 sensors can be used to curtail outdoor airflow when it is not needed. Occupancy sensors allow outdoor airflow to be turned off in unoccupied zones,
whereas a CO2 sensor enables the adjustment of outdoor airflow to levels that
match the current occupancy of a zone. Each of these strategies requires a system
capable of varying the outdoor airflow to the zones in question.
Buildings with predictable occupancy schedules, such as schools and offices,
are ideal candidates for scheduling outdoor airflow. Figure 2.3 shows the dramatic variations in outdoor air requirements over one week for the simple office
building used in the example. Occupied hours for this building are 6 a.m.–7 p.m.
Monday through Friday. On evenings and weekends, when the building is unoccupied, no outdoor air is required. Reducing outdoor air intake to zero when the
building is unoccupied is highly recommended; this practice will result in significant energy savings.
Designing an effective DOAS requires a thorough knowledge of outdoor air
design conditions, including their variations throughout the year. ASHRAE Handbook—Fundamentals (ASHRAE 2013a) provides heating, cooling, dehumidification and enthalpy outdoor conditions for more than 6000 locations worldwide (e.g.,
Figure 2.4).
Deciding how many hours in a typical year that loads above design values will
be risked is usually left up to the owner and engineer. For most commercial and
institutional applications, if the temperature or humidity is slightly higher or lower
than usual for a few hours per year, there will likely be no significant problems.
Therefore, either 1% (88 h/yr) or 2% conditions (175 h/yr) are usually the most
practical choice. The more stringent 0.4% (35 h/yr) conditions are more frequently
used in industrial or medical applications. Generally, the more extreme the outdoor
design condition, the more expensive the equipment will be.
In all cases, it is important for the owner to keep in mind that weather varies and
that every design assumes that some number of hours will occur in which outdoor
air conditions fall outside of those used to estimate peak loads. It is also useful to
remember that most ASHRAE design values are averages based on 25 years of
weather data. This means that during less typical years the number of hours with
outdoor air conditions above and/or below ASHRAE design values will be
Figure 2.3 Occupancy-driven variations in outdoor air requirements.
Sustainable Engineering Group, LLC
In addition to peak conditions, designers must keep in mind the significant variations in outdoor air conditions hour by hour over the course of the year. Hourly outdoor air conditions can vary drastically as day turns to night and as the seasons
change. Far more than internal loads, outdoor air loads have the potential to switch
from heating to cooling and from humidification to dehumidification multiple times
a day. The design of a system and its controls must be able to respond to weatherrelated changes in addition to changes in ventilation and exhaust air requirements.
As an example of these variations, Figure 2.5 illustrates typical latent and sensible outdoor air loads over a 24-hour period in spring, summer, and winter for four
different climates. This figure shows the extreme variations that can occur in outdoor air conditions from hour to hour and season to season, and how the magnitude
of this variability is dependent on climate. Loads are based on 1000 cfm (472 L/s)
Figure 2.4 Dallas-Fort Worth, TX, outdoor design conditions, (a) I-P, (b) SI.
2013 ASHRAE Handbook—Fundamentals (ASHRAE 2013a)
Sustainable Engineering Group, LLC
Figure 2.5a Variability in outdoor air conditions (I-P).
Sustainable Engineering Group, LLC
Figure 2.5b Variability in outdoor air conditions (SI).
of outdoor air. Climate zones represented are Zone 1A—hot and humid (Singapore), Zone 3A—warm and humid (Atlanta), Zone 4—mixed (Beijing), and Zone
6A—cold and humid (Minneapolis).
Energy Recovery. One significant benefit of exhaust air energy recovery is that
it can narrow the wide range of outdoor air conditions encountered over the year
into a much tighter range of conditions. Figure 2.6 shows how a wide distribution
of outdoor air conditions can be consolidated into a more homogeneous set of temperatures and humidities by preconditioning the outdoor air with an enthalpy
exchanger. The green triangles represent temperatures and humidities for each hour
of the year in Dallas-Fort Worth, TX. The blue diamonds show the resulting temperature and humidity after the air has passed through an enthalpy wheel with 75%
sensible effectiveness and 65% latent effectiveness. Exhaust air conditions were
modeled at 70°F–75°F (21°C–24°C) dry bulb, with a 55°F (13°C) dew point in the
summer and exhaust air dew point equal to the outdoor air dew point in the winter.
Using this strategy, however, introduces the question of whether to reduce the size
Figure 2.6 Preconditioning benefits of enthalpy energy exchangers.
Sustainable Engineering Group, LLC
of the cooling, dehumidifying, heating, or humidifying components in the DOAS
equipment based on the expected contributions of preconditioning.
Arriving at an answer to this question can depend on a number of factors. In
many facilities, it may be worthwhile to downsize the cooling, dehumidifying,
heating, or humidifying components in the DOAS equipment to lower initial cost
and potentially increase energy savings. This downsizing may be required by certain energy codes (Sections C403.2.1 and C403.2.2 of the 2012 International
Energy Conservation Code [ICC 2012]).
In facilities where it is critical to maintain consistent conditions at all times,
some level of redundancy may be desired and the engineer may not be comfortable
with reducing the sizes of these components. In this case, however, ensure these
components have sufficient turn down for operation at low load conditions (e.g.,
dehumidification capacity turn down on a mild damp day or heating capacity turn
down on a cool day). Also, if the energy recovery device may not be available at
times, such as a humidified building in a cold climate where the energy recovery is
expected to shut down for frost prevention on a regular basis, then reducing the size
of the heating components may not be advisable.
Figure 2.7 shows a total-energy (enthalpy) wheel in cooling mode. Note that air
filters are essential. These are located upstream of the wheel on both the outdoor
and exhaust air sides. They have been omitted from Figure 2.7, to show other components more clearly. When using an air-to-air energy recovery device, exhaust air
temperature and humidity can also become important design factors, in particular if
preconditioning is taken into account when sizing equipment.
Figure 2.7 Total-energy (enthalpy) wheel in cooling mode.
Harriman and Lstiburek (2009)
In addition to the material presented on this subject in later chapters, designers
interested in taking advantage of heat exchangers should reference Chapter 26 of
ASHRAE Handbook—HVAC Systems and Equipment (ASHRAE 2016c).
DOAS units are particularly well positioned to deal with building dehumidification, because in most climates the bulk of latent load is caused by the outdoor air.
DOAS units can be used to dehumidify outdoor air to a dew point low enough to
compensate for zone (internal) latent loads, thereby satisfying overall building
dehumidification needs. Figure 2.8 shows how a DOAS unit can be used to address
the entire latent load during warm/humid seasons. The cooling systems can then
often operate with a dry coil and at a warmer temperature. In many cases, this will
allow reductions in the size of zone heating and cooling equipment.
For dehumidification load calculations, the designer should use the peak dew
point and/or peak enthalpy conditions for outdoor air displayed in ASHRAE Handbook—Fundamentals (ASHRAE 2013a), and not the peak dry-bulb conditions. All
three groups of data usually appear on the same page.
When an owner wants accurate humidity control rather than simple humidity
moderation, a well-considered load estimate is an essential first step. Ensuring that
the dehumidification load estimate is a collaborative effort provides a useful opportunity to clarify the owner’s expectations compared to his or her construction budget. Major dehumidification load considerations include outdoor design conditions,
target indoor dew point, occupant number and activity level, ventilation and
Figure 2.8 Using DOAS to address latent loads.
Harriman et al. (2001)
makeup air loads, infiltration loads, and door openings. A detailed treatment of
dehumidification load calculations is provided in Chapter 11 of the ASHRAE Guide
for Buildings in Hot and Humid Climates (Harriman and Lstiburek 2009).
In some cases, cooled, dehumidified air from a DOAS unit may cause some
zones to become too cool. In such cases, it is advisable to consider whether some
approach to heat the space or tempering the conditioned outdoor air (e.g., wraparound heat pipe or other energy-recovery-based methods) might be justified. Figure 2.9 shows an example of a wraparound heat pipe, which precools the outdoor
air, then tempers it before it leaves the DOAS unit, avoiding additional energy consumption for reheat.
Example Calculation for the Conditioned Outdoor Air Dew Point. The purpose of calculating indoor latent loads with respect to DOAS is to determine the target leaving air dew point. As discussed previously, the absolute humidity of the air
leaving the DOAS unit should be lower than the target space humidity ratio to offset the latent load in the spaces.
For the hypothetical example office, assuming that occupants are responsible for
the bulk of the space latent loads (at 200 Btu/h/person [58.6 W/person]), and that
the desired space dew point limit is 55°F (13°C) (64.4 gr/lb [9.1 g/kg]), the humidity ratio of the conditioned outdoor air supplied by the DOAS can be calculated
using the following equation:
Figure 2.9 Wraparound heat pipe.
Photo courtesy of Venmar CES. ©2017 Nortek Air Solutions, LLC. All rights reserved.
QL = 0.69 × Voz × (Wspace – Wca)
QL = 3.0 × Voz × (Wspace – Wca)
latent load in the space, Btu/h (W)
zone outdoor airflow, cfm (L/s)
Wspace =
maximum limit for the humidity ratio in the space, gr/lb (g/kg)
required humidity ratio of the conditioned outdoor air, gr/lb (g/kg)
Note: In the equation, 0.69 (3.0) is not a constant but is derived from the density
of air and the latent heat of vaporization at “standard air” conditions. Air at other
conditions and elevations will cause this factor to change.
In our example, the DOAS unit supplies air to the intakes of zone heat pumps.
Using expected occupancies (Table 2.3), the required dew point of the conditioned
outdoor air delivered to each zone can be calculated.
The zone with the lowest required dew-point temperature represents the constraining factor. In this example, the DOAS unit must be capable of supplying air at
a dew-point temperature of 47°F (8°C) to meet the latent load requirements of several spaces. The colder the conditioned outdoor air temperature, the smaller the size
of the zone cooling equipment can be (Mumma 2008). The DOAS may also need
the ability to reset this conditioned outdoor air temperature higher when the zones
served are at part-load cooling conditions (see Chapter 5).
In hot and humid climates, a DOAS unit is typically selected to address latent
loads. In hot and dry climates, there is little or no dehumidification required of the
outdoor air, and the DOAS unit is usually sized on sensible cooling. For such climates, the concepts outlined previously remain applicable; DOAS-treated outdoor
air conditions are based on how dry the conditioned outdoor air must be to offset
space latent loads.
In all climate types, the conditioned outdoor air temperature from the DOAS unit
should be set low enough to allow for partial space cooling, with an ability to reset
the conditioned outdoor air temperature higher under part-load conditions. Taking
the South Conference Room from the previous example, 510 cfm (240 L/s) of
DOAS-conditioned outdoor air delivered at 47°F (8.3°C) translates to approximately 15,000 Btu/h (4.40 kW) of space sensible cooling. Assuming a zone cooling
load of 42,000 Btu/h (12.3 kW), the DOAS unit provides more than one third of the
total space sensible-cooling requirement. As this example illustrates, DOAS contributions to zone sensible cooling allow for downsizing of local cooling units, resulting in first cost and operational savings.
Table 2.3 Required Dew Point of Conditioned OA for
Example Office Zones
Dew Point,
South offices
West offices
conference room
East offices
South interior
North interior
North offices
conference room
If too-low space temperature is a concern, there are a number of options available in addition to resetting the conditioned outdoor air temperature. One possibility is that the HVAC unit serving the space with too low a temperature could
activate its local heat. If this occurs in just a few spaces, the sensible-cooling benefit of delivering cold outdoor air to all remaining spaces may outweigh the heating
energy required in just a few spaces. Another approach could be to implement
occupancy control or demand-controlled ventilation, thereby reducing the amount
of cool outdoor air delivered during unoccupied periods or partial occupancy when
internal loads are low (see Chapter 5 for more detailed discussion).
In cold and dry climates, there may be a need to add moisture to the outdoor air
to meet the space humidity requirements. The amount of humidity required can be
Figure 2.10 DOAS unit used to add humidity to outdoor air during cold/dry
Harriman et al. (2001)
determined using the calculation procedures in ASHRAE Handbook—HVAC Systems and Equipment, Chapter 22 (ASHRAE 2016c). Humidification dispersion
tubes can be either integrated into the DOAS unit or installed in the conditioned
outdoor air duct downstream of the unit. This is discussed further in Chapter 4. Figure 2.10 shows how a DOAS unit can be used to add humidity to outdoor air during
cold/dry seasons.
Depending on climate and selected zone heating equipment, it may sometimes be
necessary to include a heating component in the DOAS unit to address very cold
outdoor air conditions. Even with an air-to-air energy recovery device in the
DOAS, the conditioned outdoor air might at times in some cold climates get too
cold to introduce directly to the space or even too cold to mix with recirculating air,
depending on the ratio and the DOAS-conditioned outdoor air temperature. The airto-air energy recovery device might also need to be protected from frosting in cold
climates. In these cases, a heating coil may be needed. Chapter 4 discusses the
details associated with including a heating coil in the DOAS.
Air Cleaning
In some environments, outdoor air may not be as clean as desired and may
require treatment before entering a building to avoid causing indoor air quality
(IAQ) issues. Motor vehicles, industrial facilities, household combustion devices,
airplanes, aerosol cans, and forest fires are common sources of air pollution.
Figure 2.11 Los Angeles skyline.
HAAP Media Ltd. (Chad Littlejohn)
Air pollution is most common in large cities where emissions from multiple
sources are concentrated. Mountains or tall buildings can prevent air pollution from
dispersing; in these locations it often manifests as a cloud of smog or haze. The
World Health Organization (WHO) 2016 database (WHO 2016) lists the fine particle levels in 3000 urban areas throughout the world. Figure 2.11 shows an example
of a nonattainment time in Los Angeles, a city with outdoor air quality concerns.
In general, it is easier to clean outdoor air prior to mixing it with return air.
Doing this will reduce the total amount of air that needs to be treated, subsequently
minimizing initial cost, maintenance, and energy consumption. Air purification is
discussed in further detail in later chapters, as well as Chapters 29 and 30 of
ASHRAE Handbook—HVAC Systems and Equipment (ASHRAE 2016c).
A thorough consideration of outdoor air requirements and conditions is the key
to correctly sizing a DOAS unit. Outdoor air requirements are typically determined
using the procedures outlined in ANSI/ASHRAE Standard 62.1. Occupancy sched-
ules vary, often predictably, and the DOAS should be designed to accommodate
those variations by adjusting the amount of outdoor air delivered.
Evaluation of DOAS cooling, dehumidification, heating, and/or humidification
requirements begins with calculation of the required dew-point temperature of the
conditioned outdoor air. Dew-point conditions should be targeted at removing
space latent loads as well as outdoor air latent loads. Supplying cool air to zones or
equipment can help offset space sensible-cooling loads and potentially reduce the
size of local HVAC equipment. In some cases, an additional source of heating may
be needed to temper the conditioned outdoor air, for example during partial load
conditions or for applications with consistently high outdoor air requirements.
Figure 3.1 Making choices.
Getty Images (D. Diederich)
As shown in Figure 3.1, the most immediately attractive option may not
always represent the best long-term choice. Similarly, selecting an HVAC system
based on convenience or first cost may negatively affect comfort, operation, or
utility cost in the long run. Consideration of all relevant parameters (i.e., building
function, climate zone, and energy efficiency) enables the design engineer to
select the optimal system for the specific building in question.
This chapter presents a series of options for delivering conditioned outdoor air to
zones and describes common DOAS unit configurations. Decisions regarding air
distribution may affect the choice of DOAS equipment, and vice versa. Therefore,
when a decision has been reached regarding one of these topics, it may be useful to
revisit the material presented on the other.
Readers of this chapter are assumed to have already calculated the amounts of
outdoor air, moisture removal, and cooling required for their project (if not, please
see Chapter 2, Outdoor Air and Load Requirements). It is also assumed that the
designer has chosen the type of heating and cooling system that will be used to satisfy space sensible loads. If this is not the case, some successful system types used
in combination with DOAS are radiant cooling/heating, water-source heat pumps,
fan-coils, DX split systems, packaged terminal air conditioning units (PTACs),
chilled beams, and variable-refrigerant-flow (VRF) systems.
DOAS can be successfully integrated with nearly every type of HVAC system.
To maintain comfortable temperature and humidity conditions for occupants, it is
important to select the correct type of DOAS. A system configuration that works
well in hot and humid climates may be an inappropriate choice for dry, moderate
climates. Likewise, configurations that complement one type of heating and cooling system may produce drawbacks with others. This chapter provides recommendations for successful application over a variety of building types and climates.
DOAS units can be described as independent air handlers that condition the outdoor air entering a building. DOAS units are most commonly be ducted separately
to each zone throughout a building, but there are a number of options regarding
how the outdoor air (OA) is delivered to each zone (directly to the zone, to local
equipment, etc.). The following is an overview of four common DOAS air distribution configurations.
Conditioned Outdoor Air Supplied Directly to Each Zone
In this configuration, the DOAS unit supplies conditioned outdoor air directly to
each zone through a dedicated duct system and independent space diffusers. The
DOAS unit is sized to meet outdoor air and dehumidification loads for the building,
allowing the other equipment (either local or central) to address space sensible
loads only.
One major advantage of this approach is that it is easy to ensure that the required
outdoor airflow reaches each zone, because it can be measured through dedicated
diffusers during the start-up and balancing process. In addition, if outdoor air is
delivered at a cold temperature rather than reheated to close to space set-point temperatures (“neutral”), this configuration offers the opportunity to downsize the local
sensible-cooling equipment.
The main drawback of this configuration is that it requires the installation of
additional ductwork and separate diffusers, resulting in added material first costs.
These costs can be largely offset, however, by downsizing sensible-cooling equipment and capturing utility savings from cycling sensible equipment fans off when
Local Units. This combination consists of a central DOAS and local HVAC
equipment that conditions only recirculated air (Figure 3.2). It is often used when
local units are installed in the occupied space, such as PTACs, fan-coils, watersource heat pumps, VRF terminals, passive chilled beams, or radiant panels. It can
also be applied if local equipment is installed in the ceiling plenum, on the roof, or
in a closet near the space.
This approach affords the opportunity to cycle off or reduce the speed of local
fans when no cooling or heating is needed in a given zone (because they do not supply outdoor air). Similarly, the DOAS can be activated during unoccupied periods
Figure 3.2 Direct to zone—local units.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
Mumma et al. (2013)
RA = recirculated air
without needing to operate local units, for example, to maintain indoor humidity
conditions after hours.
Central AHU. This configuration consists of a central air-handling unit (AHU)
and a central DOAS, with separate duct distribution throughout the building (Figure 3.3). Delivery to the space might be through dual-duct VAV terminal units, separate single-duct VAV terminals for each airstream, or separate single-duct VAV
terminals for SA and CA, as shown in Figure 3.3. The central AHU is sized for the
space heating and cooling loads, and is typically designed for variable air volume
(VAV). Because the duct systems are separate, this configuration is much easier to
Figure 3.3 Direct to zone—central AHU.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
Mumma et al. (2013)
RA = recirculated air
operate than approaches that attempt to combine the two systems (see the Tips and
Traps section, below). In the occupied period, the space VAV terminals can vary
the recirculated supply airflow from 0% to 100% without any concerns about ventilation, which is taken care of by the DOAS. During unoccupied periods, the DOAS
unit can simply switch off, while the central AHU can operate to maintain space set
points as needed.
Tips and Traps
It is also possible to provide conditioned outdoor air from the DOAS to the
intakes of the central AHU to distribute to all zones through the AHU ductwork.
This is often more complicated in particular for VAV AHUs where the airflow will
vary as the heating and cooling loads in the zones change but the outdoor air provided by the DOAS will not vary with these loads. This configuration also makes it
necessary to use multiple-zone recirculating calculations from ANSI/ASHRAE
Standard 62.1 (ASHRAE 2016a), which will most likely increase the total amount
of outdoor air required and therefore increase the size of the DOAS.
Many DOAS units are designed to provide a constant volume of ventilation air at constant pressure. Therefore, use caution when connecting the
DOAS to any part of a system where airflows and pressures will vary. Any
change in pressure at the point of connection will alter the amount of outdoor air delivered by the DOAS unit.
Conditioned Outdoor Air Supplied to Intake of Local Units
This configuration (Figure 3.4) delivers conditioned outdoor air directly to the
intake of each local unit, where it mixes with recirculated air from the zone. The
local unit then conditions the mixture and delivers it to the space, thereby ensuring
that required outdoor airflows are met. This approach is often used when local units
are installed in the ceiling plenum, on the roof, or in a closet. Examples might be
water-source heat pumps, fan-coils, small rooftop units, or VRF terminals.
This approach avoids some of the cost and space required to install additional
ductwork and separate diffusers. However, because the local fan is used to deliver
outdoor air to the zone, it must operate continuously whenever outdoor air is
needed during occupancy. If it cycles on and off, or varies its speed, outdoor air
delivery is compromised because of the pressure variations. And, if the local unit
ever delivers air at a temperature warmer than the space, the designer may need to
increase ventilation to account for Ez < 1.0 (see Chapter 2).
Figure 3.4 Air supplied to intake of local units.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
RA = recirculated air
Mumma et al. (2013)
In addition, when the outdoor air is delivered at a cold temperature, it results in
cool air entering the coil in the local unit. This must be considered during equipment selection, as it may impact cooling coil capacity and the need for reheat at the
local unit. Measurement and balancing are typically more complicated in this scenario than if the outdoor air was delivered directly to the space.
Conditioned Outdoor Air Delivered to Supply Side of Local Units
In this configuration (Figure 3.5), conditioned outdoor air is ducted directly to
the supply side of each local unit, where it mixes with supply air before being delivered to the zone through a common set of diffusers. This approach also ensures that
required outdoor airflows are met, but in this case, the local unit conditions only
recirculated air.
This strategy is typically used when local units are installed in the ceiling plenum, on the roof, or in a closet. Examples might be water-source heat pumps, fan-
Figure 3.5 Air supplied to supply side of local units.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
RA = recirculated air
Mumma et al. 2013
coils, small rooftop units, active chilled beams, or VRF terminals. If the outdoor air
is delivered at a cold temperature, rather than reheated to near space temperature,
this configuration offers the opportunity to downsize the local units.
Measurement and balancing, however, are more difficult in this scenario than if
the outdoor air is delivered directly to the space. Additionally, the local fans typically must operate continuously during occupancy to ensure sufficient outdoor air
delivery. If the local fan cycles off or varies its speed, the pressure in the supply
duct decreases; this can interfere with air balancing of the ventilation system and
may result in backflow.
One solution is to install a pressure-independent damper (such as a VAV terminal unit) in the DOAS ductwork to each zone to respond to changes in pressure,
ensuring that the required outdoor airflow is delivered to the zone whether or not
the local fan is operating. Including this terminal has the added benefit of providing
a means of incorporating some method of demand-controlled ventilation.
Again, if the local unit ever delivers air at a temperature warmer than the space, the
designer may need to increase ventilation to account for Ez < 1.0 (see Chapter 2).
Conditioned Outdoor Air Supplied to Plenum near Local Units
In this approach (Figure 3.6), conditioned outdoor air is delivered to the open
ceiling plenum or closet, near the intake of each local unit. The outdoor air mixes
with recirculated air in the plenum before being drawn in through the intake of the
local unit.
This strategy is sometimes used when local units such as water-source heat
pumps, fan-coils, or VRF terminals are installed in the ceiling plenum or closet.
The primary advantage of this configuration is that it avoids much of the cost and
space needed to install additional ductwork, separate diffusers, or mixing plenums
on the local units.
Figure 3.6 Air supplied to ceiling plenum.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
Mumma et al. 2013
RA = recirculated air
It is difficult, in this configuration, to ensure that the required amount of outdoor
air reaches each zone, because it is not ducted directly. For this reason, ANSI/
ASHRAE Standard 62.1 (ASHRAE 2016a) contains some words of caution about
this approach. The 62.1 User’s Manual (ASHRAE 2016d) provides guidance on
how to (and how not to) design this type of system to minimize this drawback.
For example, the outdoor air duct should deliver air close to the intake of each
local unit and include some means of balancing to ensure that the correct amount of
air is supplied to each unit. Additionally, the conditioned outdoor air typically cannot be delivered at a cold temperature in this configuration. In most cases, it must
be reheated to avoid condensation on surfaces within the plenum.
Because of these drawbacks, it is wise to consult both local codes and ASHRAE
standards before designing a system that uses this configuration.
DOAS units are capable of incorporating a wide range of components, depending on the specific needs of the project. Common equipment components include
the following:
• Supply and/or exhaust fans
• Variable-speed drives
• Air-to-air energy recovery devices (wheels, plate heat exchangers, heat pipes,
and coil loops)
• Desiccant dehumidification wheels
• Cooling coils (DX and chilled-water)
• Heating coils (hot-water, indirect gas-fired, or electric)
• Humidifiers
• Condenser heat recovery (reheat) coils
• Motorized dampers
• Filters or other air cleaning devices
The chosen combination of these components will vary depending on the application and climate. The following sections describe some of the more common configurations in detail, beginning with the most basic. It is assumed that all units
discussed below include, at a minimum, a supply fan, a filter, and a cooling or heating coil. A more in-depth coverage of individual DOAS components is provided in
Chapter 4, Detailed Design Considerations.
Cooling Coil and Reheat
This is one of the simplest and most straightforward DOAS configurations. This
unit dehumidifies entering outdoor air with a cooling coil (either DX or chilledwater) and may include a wraparound heat exchanger, hot-gas reheat coil or heating/reheat coil to prevent some spaces from becoming too cool at low cooling loads
(Figure 3.7). It is typically used in cases where exhaust air energy recovery is contraindicated, such as those in which routing the exhaust air ductwork back to the
DOAS unit is impractical, or exhaust air is contaminated. An example psychrometric chart representing the action of a DOAS unit with cooling coil is shown in Figure 3.7a. Note that the conditioned outdoor air is supplied at a lower humidity ratio
(a lower dew point) than space conditions to offset space latent loads.
For most applications in humid climates, DOAS are designed so that the capacity
of the cooling coil is modulated to maintain the desired dew-point temperature
leaving the unit. However, in following this approach, the resulting conditioned
outdoor air dry-bulb temperature can sometimes become cold enough to cause discomfort for occupants if introduced directly to the space. In these cases, adding a
wraparound heat exchanger or reheat coil downstream of the cooling coil may be
The HVAC designer should evaluate whether the particular project requires
using a reheat coil. For example, office spaces with low outdoor air requirements
may not need any reheat, whereas spaces that require significant amounts of outdoor air, such as hospitals and laboratories, are often appropriate candidates for this
approach (see Chapter 4).
Exhaust Air Energy Recovery
If exhaust air can be designed to exit the building near the outdoor air intake,
installing an air-to-air energy recovery device is an excellent way to precondition
the outdoor air. Such a device reduces heating and cooling loads by transferring
energy between the exhaust airstream, which is close to indoor temperature and
humidity, and outdoor airstream. For simplicity, this section focuses on a totalenergy (enthalpy) wheel (Figure 3.8). Some systems may alternatively use a fixedplate or fixed-membrane heat exchanger, a coil loop, heat pipe, or even a sensible
wheel. These options are covered in greater detail in Chapter 4.
Total-energy (enthalpy) wheels operate by rotating between the incoming and
exhaust airstreams. During hot and humid weather, the outdoor air gives up some of
its moisture to the surface of the wheel, which is coated with a desiccant. The wheel
then rotates into the exhaust airstream, which is dry enough to strip moisture off of
the desiccant and vent it back outdoors. The outdoor air is therefore predried and
precooled by the wheel, greatly reducing the load on the downstream cooling and
dehumidification components. During cold weather, the effect is reversed; the
enthalpy wheel transfers heat and humidity from the exhaust air to the incoming
outdoor air, thereby reducing the load on the downstream heating and/or humidification equipment.
By installing an enthalpy wheel as part of a new system, the peak heating, cooling, and dehumidification loads from outdoor air can be greatly reduced. These
benefits come at the modest expense of the small motor used to spin the wheel, plus
the fan energy used to overcome the pressure drop created by the wheel.
Figure 3.7 Cooling coil and reheat: (a) a DOAS unit containing a cooling coil
and reheat coil and (b) the psychrometric path of outdoor air (OA) as it passes
through the DOAS unit described previously at cooling design conditions.
OA = outdoor air
CA = conditioned outdoor air
Mumma et al. 2013
Figure 3.8 Exhaust air energy recovery: (a) a DOAS unit containing a cooling
coil, reheat coil, and total-energy (enthalpy) wheel and (b) psychrometric paths
of outdoor air (OA) as it passes through the DOAS unit described previously at
both cooling design conditions (blue) and heating design conditions (red).
OA = outdoor air
CA = conditioned outdoor air
OA' = outdoor air pre-treated by wheel
Mumma et al. (2013)
EA = exhaust air
When using an enthalpy wheel, there should also be an active dehumidification
component operating, because the air leaving the supply side of the wheel (shown
in blue as OA' in Figure 3.8) can never be drier than the desired space conditions.
This is often accomplished with a cooling coil located at the discharge of the wheel.
The coil is sized to dry the air enough to handle the latent loads for both the outdoor
air and in the space, as described in Chapter 2. A heating device may be warranted
in cold climates, if the temperature leaving the wheel at low outdoor air temperatures is too cold to discharge directly into the space (shown in red as heating the air
from OA' to CA). Figure 3.8 shows the effects of this configuration on entering outdoor air. Referring to Chapter 2 and Figure 2.5, one way to look at the effect of an
energy recovery device is that it greatly reduces the difference between outdoor and
indoor conditions of the air entering the rest of the DOAS.
Energy Recovery and Sensible Reheat
This configuration uses both a total-energy wheel (as discussed in the previous
section) and a sensible heat exchanger. Sensible heat exchange can be performed
by a fixed-plate heat exchanger, coil loop, heat pipe, or sensible wheel. The following example will use a fixed-plate heat exchanger; alternatives are discussed in
greater detail in Chapter 4.
As seen in Figure 3.9 in cooling mode, the downstream plate exchanger recovers
sensible heat from warm exhaust air and uses it to reheat cold, dehumidified air
from the cooling coil, when desired. Entering outdoor air is precooled and dehumidified by the total-energy wheel. Because the exhaust air has effectively been
precooled by the fixed-plate heat exchanger, the impact of the wheel is enhanced,
allowing it to transfer additional energy and further reduce the load on the cooling
coil. The cooling coil then dehumidifies the air to the desired dew point, and the
plate exchanger reheats the air before it is delivered to the space.
If the system is being designed to deliver nearly space temperature air for a significant part of the dehumidification season, this configuration can be an efficient
choice. However, there is now the resistance of two heat exchangers for the fans to
overcome, so energy costs and benefits should be carefully weighed.
Energy Recovery and Desiccant Wheel
Dehumidification components use a considerable amount of energy. DX or
chilled-water cooling coils not only dry the air, but also cool it. In some cases, this
air may be too cool and must then be reheated.
Desiccant dehumidifiers dry the air but often heat it, so it may require recooling.
Desiccant dehumidifiers use a heat stream to activate the desiccant (dry it out)
enabling it to continuously dehumidify incoming air. There are a number of possibilities for supplying the heat required for desiccant reactivation. Condenser heat is
a highly recommended source, because using waste heat avoids the need to consume additional energy (this approach also complies with ANSI/ASHRAE/IES
Figure 3.9 Energy recovery and sensible reheat: (a) a DOAS unit containing a
cooling coil, reheat coil, fixed-plate heat exchanger, and total-energy wheel and (b)
the path of outdoor air (OA) as it passes through the DOAS unit described above,
at both cooling design conditions (blue) and heating design conditions (red).
OA = outdoor air
CA = conditioned outdoor air
OA' = outdoor air pre-treated by wheel
Mumma et al. 2013
EA = exhaust air
Standard 90.1 [ASHRAE 2016b] requirements). Other potential sources of desiccant reactivation heat are a separate outdoor airstream, exhaust air, a preheat coil,
or gas-fired heating.
Figure 3.10 shows an example of combining both methods of dehumidification,
with a cooling/dehumidifying coil and a passive desiccant dehumidification wheel
in the DOAS configuration. By locating the desiccant wheel downstream of the
cooling coil, it is able to further lower the dew point leaving the unit, something a
sensible heat exchanger cannot do. In this example, the desiccant wheel is configured in series with the cooling coil, but in other configurations it might be in parallel with the regeneration side in the exhaust airstream. Outdoor air is
preconditioned by a total-energy wheel and then moves through the upstream, or
regeneration, side of the desiccant wheel. The air then passes through the cooling
coil and the downstream side of the desiccant wheel. In this configuration, the passive desiccant wheel removes water vapor from the air before it leaves the unit,
transferring it upstream of the cooling coil, where it can be removed by condensing
on the coil.
In addition to first cost, building function, and architectural considerations, there
are a number of factors that will influence a designer’s choice of DOAS. The following sections highlight some important points to consider in determining the
appropriate DOAS configuration for a given project.
Climate Considerations
The first important factor to consider when selecting a DOAS configuration is
climate (Figure 3.11). DOAS design considerations are associated with each of the
following four major climate types. Some locations may fit into two of these categories, such as regions that are hot in the summer and cold in the winter. In these
cases, the DOAS should be designed with the considerations of both applicable climate types in mind.
Humid Climates (Miami, Houston, Singapore). Average daily dew-point temperatures in humid climates frequently top 55°F (13°C). Because typical space conditions (75°F [24°C] and 50% relative humidity) have a dew point near 55°F
(13°C), active humidity control in these regions is essential. DOAS units are ideal
for these climates, because they can successfully address large latent loads that tend
to challenge conventional air conditioners.
Dry Climates (Las Vegas, Los Angeles, Mexico City). As the name implies,
dry climates have very little humidity load. Therefore, cooling coils in these regions
will typically operate dry for much of the year. Some locations may have seasonal
monsoon cycles in late summer, however, so dehumidification capacity should be
designed with these extreme events in mind. Indirect/direct evaporative coolers
may be beneficial to reduce the energy consumption of the DOAS.
Figure 3.10 Energy recovery and desiccant wheel: (a) DOAS unit containing a
cooling coil, preheat coil, desiccant wheel, and total-energy wheel and (b) the
path of outdoor air (OA) as it passes through the DOAS unit described above,
at both cooling design conditions (blue) and heating design conditions (red).
OA = outdoor air
CA = conditioned outdoor air
OA' = outdoor air pre-treated by wheel
Mumma et al. 2013
EA = exhaust air
Mild Climates (San Francisco, Seattle, Copenhagen). Mild climates are characterized by minimal dehumidification loads and moderate temperatures, so a
DOAS unit may operate with cooling and heating off for a significant portion of the
year. For this reason, the energy saving benefits of a DOAS are somewhat diminished in mild climates.
Cold Climates (Chicago, Minneapolis, Moscow). Cold climates are heating
dominated for a large portion of the year. In the coldest regions of this climate zone,
it is advisable to include a preheat coil in the DOAS unit. In slightly warmer areas it
may be possible to produce warm-enough temperatures with an air-to-air heat
exchanger. In that case, if the DOAS unit discharges directly into the space, a
reheat coil may be needed to avoid the perception of cold drafts.
An air-to-air enthalpy exchanger (e.g., enthalpy wheel, fixed-membrane heat
exchanger) will help humidify the incoming outdoor air during cold weather,
when outdoor air holds very little humidity. Frost prevention sequences may be
necessary to prevent the exhaust side of the exchanger from frosting. If there is a
possibility that the air-to-air heat exchanger will frost up at cold outdoor air conditions, it may be prudent to size the heating coil to account for the full heating
load of the outdoor air.
Figure 3.11 World map showing climate regions.
Reproduced with permission of Prof. Dr. Franz Rubel (Rubel and Kottek 2010)
Application Type—New or Retrofit
DOAS units can be applied to both new and retrofit projects. With respect to retrofits, DOAS is particularly well suited for buildings that were originally designed
with little space for air distribution ductwork. In these cases, local units can satisfy
space heating and cooling needs, and outdoor air can be provided by a central
DOAS unit via distribution ducts. The duct size required for outdoor air delivery
only is much smaller than what would be needed for a central AHU.
When retrofitting older buildings that did not previously have air conditioning, a
common practice is to route outdoor air ducts along corridors and hallways.
Ductwork may remain exposed or be hidden behind a “cloud” or a suspended ceiling. Conditioned outdoor air is then delivered to each space using one of the configurations shown previously.
For new construction, the smaller ductwork associated with direct outdoor air
delivery coupled with local heating and cooling units can result in a reduction in
overall building height, as compared to a fully ducted, centralized heating and cooling system.
Additional Considerations
Fan Energy. Fans are responsible for the majority of DOAS energy consumption. A variety of fan types can be used. Design engineers should pay careful attention to fan selection to ensure that the system will operate efficiently and costeffectively. Fan selection is covered in more detail in Chapter 4.
Cooling/Dehumidifying Capacity Modulation and Staging. The number of
compressors (stages) in a cooling/dehumidifying system will affect how close the
system will be able to control to the set-point temperature. The more stages, the
closer the system will be able to be to the set point and as the load changes, the
actual supplied temperature will either be higher or lower than the set point. A
modulating compressor will be able to meet the set point as long as the load is
above the minimum (and below maximum) load. One modulating compressor can
be combined with several nonmodulating compressors to maintain modulation over
a wide range of loads. For DOAS with DX cooling that provides conditioned outdoor air directly to the zone, the lowest possible temperature that can occur with a
given number of stages (or modulation) and the conditioned outdoor air set point
should be determined such that diffusers (or other equipment) types and locations
can be selected to avoid drafts at these conditions and not at the set point.
Variable-Speed Drives. Variable-speed drives are sometimes included in
DOAS equipment to allow fan power consumption to be reduced during some operating conditions. For example, a carbon dioxide sensor located in a classroom can
sense how much (or how little) outdoor air is needed, and adjust the damper in a
VAV terminal accordingly. The DOAS unit responds by reducing the fan speed to
match, resulting in considerable savings in fan energy. Variable-speed drives also
allow motors to “soft-start,” which extends their useful life.
Ductwork Sizing. Distribution ductwork for a DOAS coupled with local heating
and cooling units is typically sized for outdoor air delivery only, and is therefore
smaller than that required for a central air system such as VAV. Ductwork should
be designed to achieve a low pressure drop throughout, which will allow the fan to
use less energy as well as reduce noise. Efficient duct design can prevent the need
for expensive, bulky, and high-pressure-drop sound attenuators.
Filtration/Air Cleaning. DOAS units are typically capable of higher levels of
filtration, which meet or exceed ANSIASHRAE 62.1-2016 requirements. If a project is pursuing LEED v4 certification, fitting the DOAS unit with a Minimum Efficiency Reporting Value (MERV) 13-rated filter can help achieve a LEED point. In
urban locations where air pollution is a persistent problem, higher levels of filtration or other types of air cleaning may be needed; concentrating the filtration in one
unit allows for reduced energy and maintenance costs.
Cost Evaluation. Depending on how the DOAS is designed, it can result in significant first cost and operational savings. DOAS coupled with local heating and
cooling units result in smaller distribution ductwork and therefore a reduced floor
to floor height. Mechanical room space for indoor equipment is also smaller in this
case than that required for a central air system.
Figure 4.1 Getting the details right.
HAAP Media Ltd. (Spencer Britton)
Anything permanent, such as a tattoo (Figure 4.1), should be planned down to
its most minute details before execution. The expected lifetimes of buildings (and
their associated HVAC systems) place them firmly in this category. Getting the
details right the first time can save time and money in the long run, as well as
ensure occupant comfort throughout. From construction to craftwork, careful attention to detail is a crucial part of the design process.
DOAS units are configured to supply 100% outdoor air to a building at conditions that remove both the latent and sensible loads from the outdoor air, as well as
the space latent loads and a portion of the space sensible loads. This is difficult to
accomplish with “off-the-shelf” packaged equipment that has been designed to
blend both return air and a much smaller percentage of outdoor air, which requires
more modest temperature differences across the coils. This type of equipment is
often designed for 300 to 400 cfm/ton (500 to 660 L/s·kW), whereas DOAS units
may need to be designed for 100 to 250 cfm/ton (170 to 420 L/s·kW) to deal with
100% outdoor air at peak dew-point conditions.
DOAS units should be configured to suit the unique requirements for the application. Common components in a DOAS unit include cooling/dehumidification
coils, heating coils, filters, humidifiers, supply/exhaust fans, and air-to-air heat and
energy recovery devices. Selection of each component for the DOAS should
address the extreme temperature and humidity outdoor air conditions for that climate. For example, in hot, humid weather climates this will involve the removal of
large amounts of moisture and for cold weather climates this will involve including
a heating coil as well as a robust mechanism for frost prevention of the energy
recovery device.
As an early step in designing any HVAC system, applicable codes and standards
must be identified. The codes that are most relevant to DOAS design are those that
involve outdoor air or energy use. The International Energy Agency maintains a
database of energy codes for 34 major countries (IEA 2017). Although no equivalent database exists for outdoor air codes, many industrialized countries have minimum outdoor air requirements in place.
The two ASHRAE standards that are most relevant to commercial DOAS design
are ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality
(ASHRAE 2016a), and ANSI/ASHRAE/IES Standard 90.1., Energy Standard for
Buildings Except Low-Rise Residential Buildings (ASHRAE 2016b).
ANSI/ASHRAE Standard 62.1 focuses on determining required amounts of outdoor air, along with filtration requirements and other items related specifically to
indoor air quality. Determination of outdoor air requirements using ANSI/
ASHRAE Standard 62.1 (ASHRAE 2016a) is discussed in Chapter 2, Outdoor Air
and Load Requirements.
ANSI/ASHRAE/IES Standard 90.1 (ASHRAE 2016b) includes a broad array of
energy-related items that might influence the selection of a DOAS unit. Some of
these include the following:
• Minimum equipment efficiencies
Fan power limitations
Use of an economizer
Use of exhaust air energy recovery
Requirements for simultaneous heating and cooling (as applicable to dehumidification)
Although the ASHRAE standards by themselves are not enforceable, many
states and municipal governments adopt ASHRAE standards as the basis for their
energy and ventilation codes. Alternatively, if states or municipalities have a customized set of codes, they often allow ASHRAE standards to be used as an equivalent. It is therefore important to check which code is used in the area where the
system is being installed.
Both ASHRAE Standards 62.1 and 90.1 have user’s manuals that provide more
detailed insight as well as calculation samples that can be helpful for engineers
when tackling some of the practical issues with meeting these standards. For example, 90.1-2016 User’s Manual (ASHRAE 2014) example 6-CCC details how to
apply the fan power limits to a DOAS, and 62.1-2016 User’s Manual example 5-C
shows the calculations required to show whether the relative humidity limit of 65%
will be met for the “dehumidification challenge” condition (at the peak outdoor
dew-point and mean coincident dry-bulb design conditions, and at the design
indoor latent and sensible loads, with solar loads at zero).
Minimum Equipment Efficiency
For ANSI/ASHRAE/IES Standard 90.1 (ASHRAE 2016b) to include a certain
class of equipment in the minimum equipment efficiency tables in Section 6.4.1 of
the standard, there must be an industry standard that defines how to uniformly rate
the efficiency of that class of equipment.
Until recently, an industry rating standard for DX DOAS units did not exist;
however, the 2016 version of Standard 90.1 added minimum efficiency requirements for DX DOAS units tested in accordance with ANSI/AHRI Standard 920
(AHRI 2016).
Meeting ANSI/ASHRAE/IES Standard 90.1
Economizer Requirements
For most climate zones, ANSI/ASHRAE/IES Standard 90.1 (ASHRAE 2016b)
(Table 6.5.1-1 in the standard) requires each cooling system with a fan to include
either an air or water economizer. There are a number of exceptions to this requirement, however. The most notable is Exception 1, which limits the requirement to
fan cooling units of 4.5 tons (16 kW) or greater. With DOASs, local HVAC units
will typically be smaller than this threshold, rendering them exempt from this
requirement. The 90.1-2013 User’s Manual (ASHRAE 2014) example 6-HH discusses this exception further. In addition, if the system uses condenser water heat
recovery, it is also exempt from the need to have an economizer. This might be
applicable in buildings such as hospitals, hotels, or dormitories, which commonly
use DOASs.
When an economizer is required, the design team might consider one of the following potential solutions. First, the DOAS could be oversized to allow for extra
airflow to be delivered when economizing. For most applications, this is probably
not desirable, because it requires much larger ductwork and larger fans. Second, an
additional air path could be created to deliver 100% supply airflow for economizing. This may work for zones that are near a perimeter wall, but it becomes challenging for interior zones. Finally, for water-based systems such as fan-coils,
chilled beams, or radiant cooling, a water-side economizer at the chiller plant may
present the easiest solution.
If none of these approaches are viable, the design team could choose to comply
using the energy cost budget method, rather than follow the prescriptive requirements of ANSI/ASHRAE/IES Standard 90.1 (ASHRAE 2016b). This approach
requires the use of an energy simulation program.
Tips and Traps
One common concern that arises when a DOAS is discussed is whether
the loss of 100% outdoor air economizing will produce an associated loss
in operational savings. A few basic points on this subject are worth mentioning. First, savings obtained via free cooling in mixed-air systems can be
offset by the fan energy savings provided by DOAS (Mumma 2005a, 2006).
Second, a significant fraction of the expected benefits of economizers are
not borne out in practice, because of a combination of improper installation, poor operation, and equipment failure over time. Finally, economizers
that are controlled by dry-bulb temperature (particularly those using conditioned outdoor air temperature reset) run the risk of introducing excess
humidity, which can lead to IAQ problems (Mumma 2005a, 2006).
DOASs frequently use air distribution ducts that are independent from the spaceconditioning system. DOAS air distribution duct design generally follows the same
guidelines as conventional air system duct design. Duct sizing calculations and procedures are explained in detail in ASHRAE Handbook—Fundamentals, Chapter 21,
Duct Design. Air terminal selection and location also follow the same principles as
conventional air systems, and are covered in ASHRAE Handbook—Fundamentals,
Chapter 20, Space Air Diffusion (ASHRAE 2013a). Some unique considerations
for DOAS are addressed in the following sections.
Conditioned Outdoor Air Considerations
The types of air terminals that are commonly used with DOAS include standard
ceiling diffusers, high-induction ceiling diffusers, displacement ventilation grilles,
wall grilles, and underfloor air diffusers. Diffusers should be carefully selected and
located to avoid air velocities greater than 50 fpm (0.25 m/s) in occupied zones. Air
velocities above 50 fpm (0.25 m/s) can result in occupant discomfort, particularly
when conditioned outdoor air temperatures are low.
In cases where zone level heating/cooling is provided via air moving equipment,
placement of DOAS diffusers and heating/cooling diffusers must be carefully coordinated. Ventilation diffusers should be located where they will not create uncomfortable drafts for occupants. Therefore, avoid placement near walls or too close to
other diffusers, and choose diffusers/grilles that will not create air velocities of
greater than 50 fpm (0.25 m/s) in occupied zones.
Attention should be paid to zone air distribution effectiveness Ez when locating
DOAS air terminals as well. As described in Chapter 2, zone air distribution effectiveness (Equation 2, Table 2.1) represents the fraction of ventilation air that
reaches the breathing zone. It is a function of delivery air temperature and the locations of supply air diffusers and return air grilles. Some examples of Ez considerations as applied to common DOAS configurations are as follows:
• If outdoor air (OA) is delivered directly to the space through ceiling-mounted
diffusers, Ez = 1.0 as long as the temperature of the conditioned OA is cooler
than the temperature in the space.
• If the OA is delivered to the intake of a local HVAC unit (a water-source heat
pump or variable-refrigerant-flow terminal, for example) then Ez = 1.0 when the
local unit delivers cool air, but may be 0.8 when it delivers warm air.
• If the OA is delivered directly to the space through floor-mounted grilles or diffusers, Ez may be 1.0 or 1.2 (depending on velocity) as long as the temperature of
the conditioned OA is cooler than the space temperature. If the air is delivered
warm, Ez drops to 0.7.
Demand-Controlled Ventilation
In some cases, it may be desirable to use a demand-controlled ventilation strategy (or dynamic reset of outdoor airflow) such that the outdoor airflow is reduced
when building spaces are partially occupied. Treating and delivering less outdoor
air will result in fan and heating/cooling energy savings. Occupancy sensors
(OCC), CO2 sensors, and/or time-of-day (TOD) scheduling can be used to reduce
or shut off ventilation to partially occupied or unoccupied spaces (Figure 4.2).
ANSI/ASHRAE Standard 62.1-2016 provides further guidance on these strategies.
Figure 4.2 Demand-controlled ventilation with a DOAS.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
RA = recirculated air
Sustainable Engineering Group, LLC
To effectively use demand-controlled ventilation, the system must be capable of
providing outdoor airflows that vary over time and throughout the building, while
maintaining minimum exhaust rates and building pressure. The choice to implement demand-controlled ventilation will therefore impact the design of the DOAS
unit, the supply and exhaust duct distribution system, and the control strategy.
The main impact on the DOAS unit itself is that it will have to have volume control for both exhaust and supply fans. As the demand for outdoor air changes
(whether it is caused by schedules or sensors) the outdoor and exhaust airflows both
must be reduced. Airflow reduction is most often accomplished using variable-frequency drives to modulate fan speed. Airflow measurement devices may be needed
to control the outdoor airflow and possibly the exhaust airflow as well.
To ensure adequate ventilation as required by ANSI/ASHRAE Standard 62.12016, installing a system of dampers and individual space airflow measurement
devices is necessary. Dampers and measurement devices are required on all spaces,
including those that will have constant flow, because both pressure and airflow will
vary based on the amount of outdoor air requested by demand-controlled zones.
Tips and Traps
When the outdoor air is introduced directly to the space, at least one
additional diffuser is needed in addition to the diffuser(s) for the local cooling/heating equipment. In many cases, it is challenging to find a good location for this diffuser while still maintaining good air distribution for the local
cooling/heating diffuser(s). However, it is just as important to locate this
diffuser in a suitable location that will not cause draft or uncomfortable
conditions in the occupied space (which can happen if, for example, the diffuser ends up being too close to a wall or other diffusers) as it is with the
other diffusers. In some cases, this conditioned outdoor air diffuser ends
up being similar to the other diffuser for architectural reasons. These will
often be diffusers that distribute air in four directions. Keep in mind that
two- and three-directional diffusers modified from a four-directional diffuser may have only small restrictions in third/fourth airflow directions,
which can result in significant airflow in undesirable directions, potentially
creating high velocities and comfort issues.
On the exhaust side, there will be a minimum amount of exhaust required by
code for some spaces, such as restrooms and showers. These required exhaust airflows represent the minimum allowed exhaust airflow value, and consequently set
the minimum outdoor airflow value, for the purpose of maintaining building pressure. In addition, if there is both general exhaust (relief air) and code-required
exhaust, a mechanism for reducing the relief airflow while maintaining coderequired exhaust should be included.
Exhaust Air Considerations
DOAS exhaust air is often drawn from toilets, janitor’s closets and similar lowcontaminant areas. ANSI/ASHRAE Standard 62.1-2016 allows most general
exhaust (defined as Class I and II per section 5.16.1) to be routed through the
exhaust air energy recovery components of a DOAS unit. It is important for the
HVAC designer to lay out the exhaust ductwork so that most, if not all, exhaust air
can be routed through the DOAS unit.
Often, the required amount of outdoor air exceeds the amount of exhaust air. To
make up this difference, additional exhaust air can be drawn from the occupied
spaces. If ceiling plenums are used for return air, exhaust air may be drawn directly
from plenums into the DOAS unit (transfer ducts may be needed to create a path for
the exhaust air to return to the unit).
Care should be taken to ensure that the ratio of outdoor air to exhaust air creates
a slightly positive pressurization to the building when it is hot and humid outdoors.
In cold weather, the building pressure should be neutral (Harriman et. al. 2001).
There is not yet a consensus in the building research community on an appropriate
switchover temperature to go from neutral to positive pressure, for climates that are
cold and hot and humid. Harriman et al (2001) suggests that the switchover from
neutral to slightly positive pressure should occur when the outdoor air dew point (or
outdoor air dry-bulb temperature if humidity is not measured) exceeds the indoor
dew-point set point. The designer should always consult local building codes to
check for instructions related to building pressure. Building pressurization using
DOAS is discussed further in this chapter’s section on Space Pressurization and
Energy Recovery.
Louver Placement and Sizing
When DOAS units are located indoors, exterior wall louvers and ductwork are
often used to connect the outdoor and exhaust air paths to the unit. If wall space is
limited, these connections can also be made through the roof using specially
designed roof vents. Outdoor and exhaust air louvers should be placed sufficiently
far apart to minimize cross-contamination between the two airstreams. This minimum spacing is usually specified in local building codes. Depending on what
national code or standard the local code is based on, the required distance between
exhaust and intake varies. Section 501.3.2 of the 2015 International Mechanical
Code (ICC 2015) requires “environmental air” exhaust outlets to be at least 10 ft
(3.0 m) away from any outdoor air intakes, whereas ANSI/ASHRAE Standard
62.1-2016 has no minimum separation distance requirement for Class 1 exhaust air,
or Class 2 and 3 air being exhausted from the same spaces being served by the
DOAS unit. For more hazardous exhaust air that could contain harmful contaminants, this distance may be greater, and prevailing wind directions must also be
considered. Care should be taken to position the outdoor air intake as far away from
other contaminant sources (e.g., loading docks, sewer vents, kitchen exhausts) as
Louvers should be sized to prevent rain and snow from entering the ductwork.
ASHRAE Handbook—Fundamentals, Chapter 21 (ASHRAE 2013a) recommends
an intake louver be sized to a velocity of 400 fpm (2.0 m/s) or less over its gross
area. For exhaust louvers ASHRAE recommends sizing to a gross area velocity of
500 fpm (2.5 m/s) or less.
In many cases, even these intake velocities are high enough to entrain snow and
debris through the louver. ASHRAE Handbook—Fundamentals (2013a) notes that
“if debris can collect on the screen of an intake louver, or if louvers are located at
grade with adjacent pedestrian traffic, louver face velocity should not exceed (a
gross area velocity of ) 100 fpm (0.5 m/s).”
Bird and insect screens should be fixed to the outside face of the louver to help
keep the ductwork clean. These screens can become clogged with airborne seeds,
such as those from cottonwood trees, and need to be regularly cleaned and
Packaged outdoor DOAS units have integral louvers and are designed by manufacturers to minimize cross-contamination, usually by locating the exhaust and outdoor airstreams on opposite sides of the unit. This is a generally accepted practice
Tips and Traps
and is allowed by many codes; however, it is recommended to always check with
the local code authority.
Outdoor air should not be drawn from areas where point sources of
gaseous contaminants are likely, such as building exhaust discharge points,
busy roads, loading docks, and parking decks.
Motorized dampers located at the inlet and outlet of DOAS units are not only
recommended but are often required by code as well. The most common types of
dampers are either parallel or opposed blade. Gravity dampers are sometimes used
when cost is an issue.
Return air dampers between exhaust air and outdoor air paths can be used to
avoid bringing in outdoor air during unoccupied periods if the DOAS unit is used to
control humidity levels in the building after hours. Also, dampers are often used to
bypass air around an energy recovery device when the outdoor air is cool enough to
be used for free cooling of the space.
Air Duct Design Recommendations
Many of the principles that apply to designing conventional ducted air systems
also apply to DOAS. Some of the most important duct design considerations are
listed as follows:
• Outdoor air, supply air, and exhaust air ducts (leaving the DOAS unit) located
indoors should be insulated and have a vapor barrier.
• Insulated duct supports should be used to avoid compressing the duct insulation.
• Exposed supply air ductwork routed within conditioned spaces should be constructed with double-wall insulation.
• Consider ducted distribution rather than using ceiling or floor plenums for supply
and recirculated return air. Plenums are very difficult to seal and clean, and can
lead to problems with condensation.
• Ducts should be sized for minimal pressure drop and duct fittings should include
long radius elbows, smooth transitions, and takeoffs.
Tips and Traps
The cooling coil in a DOAS unit can perform both cooling and dehumidification.
This coil is designed to handle the sensible and latent outdoor air loads as well as
the latent loads for the building (as described in Chapter 2). The coil can either be
fed with chilled water from a chiller plant, or be part of a vapor compression circuit
(commonly referred to as direct expansion, or DX). DX cooling is often used in
packaged equipment when no chilled water is available or the unit is too far from
the chiller plant.
ANSI/AHRI Standard 920 (AHRI 2016) has introduced moisture removal efficiency (MRE) and integrated seasonal moisture removal efficiency (ISMRE) metrics for use with equipment that is designed to dehumidify 100% outdoor air to a
low dew point, such as a DOAS unit. MRE is defined as the unit’s dehumidification
capacity (lbwater/h [kgwater/h]) divided by the power input to the unit (kW); ISMRE
is a weighted calculation of the MRE at four standard operating conditions.
The 2016 version of ANSI/ASHRAE/IES Standard 90.1 adds minimum equipment efficiency requirements for DX DOAS units, based on ISMRE rating in
accordance with ANSI/AHRI Standard 920 (2016).
Conventional packaged DX rooftop units are typically designed to handle a relatively narrow range of coil inlet air conditions and not the wider
range of conditions needed for a 100% outdoor air unit. Wider ranges of
coil inlet air temperatures require different ranges of suction pressures.
Cooling Coil Design Tips
Cooling coils must be carefully selected using outdoor air conditions based on
peak design dew-point temperature and/or peak enthalpy conditions (not peak drybulb temperature, see Chapter 2). Because the required leaving air temperature is
often lower than that used in conventional systems, coil performance parameters
must be carefully reviewed. For DX cooling coils, care should be taken to ensure
that the suction pressure is acceptable at the required air conditions leaving the coil.
In climates with humid weather, the cooling coil in a DOAS unit must be
designed to remove significantly more water vapor (per unit airflow) than in a conventional mixed-air system. Increasing the surface area of the coil allows it to
remove more water vapor. This is often achieved by using deeper coils that are constructed with more rows.
It is also important to properly manage this condensate so that it properly drains
down the coil surface into the drain pan and is piped away by the condensate drain
line. Too high a face velocity through a wet coil can result in some of this condensed water blowing off the fin surfaces onto downstream components in the air
handler or duct system. In some cases, moisture carryover can cause damage to the
HVAC equipment, or contribute to other moisture-related problems in the building.
In addition to air velocity, the design of the fin surface, the fin material, spacing,
and height of the coil are key factors. Using a trusted coil manufacturer’s experience, proven designs, and selection tools—not just rules of thumb—are essential
for avoiding this moisture carryover. Taller coils have more stringent limits on face
velocity to prevent moisture carryover, so it can be helpful to use two coils stacked
on top of each other, with a second drain pan located to catch condensate from the
upper coil.
Desiccant Wheels
Desiccant wheels can either be passive or active. Active wheels use heated air to
strip water vapor from (regenerate) the desiccant, and they dehumidify the air very
thoroughly. Passive desiccant wheels simply use dry air exhaust or return air for
this regeneration. They dry the outdoor air if and when the return air is also dry.
While passive wheels may dry the air less, they do not use any supplemental heat to
accomplish that drying. Although active wheels are highly effective for humidity
control, they can also be more expensive. For most applications, passive wheels can
provide plenty of moisture transfer, which makes them a very popular choice in
DOAS units.
Heating coils may be warranted in DOAS units for either preheating cold outdoor air; reheating cooled, dehumidified air before it is introduced to the occupied
space; or frost protection of air-to-air energy recovery devices. Where the heating
coil is expected to be used for extended periods or when electric demand costs are
high, heat recovery is preferred. Air-to-air energy recovery device, hot-gas reheat,
or heat recovered from the chiller plant are examples of heat recovery. If this is not
possible, a gas-fired heater, a hot-water heating coil, or a DX heating coil (heat
pump) will usually have lower operating costs than electric resistance heaters. Gasfired heaters are common in units mounted outdoors (such as roof-mounted equipment), but can be used in equipment installed indoors if sufficient care is taken in
the design and installation of the combustion process. Electric resistance heat is
common as a means of backup heat (when heating is provided by a heat pump), or
in locations where gas service is not available, electricity costs are low (pay attention to demand [kilowatts], cost in addition to energy [kilowatt-hours], and cost)
and/or when the expected usage is limited and off peak.
Multiple heating coils may be installed in a DOAS unit, depending on the
application. For example in a climate with cold winters and hot/humid summers, both a frost-prevention coil and reheat coil may be needed.
Preheating Cold Outdoor Air
During very cold weather, outdoor air temperatures can drop low enough
that, even after passing through an air-to-air energy recovery device, the leaving air can be too cold to be supplied directly to the space or too cold to pass
through a downstream, chilled-water coil. In these cases a preheat coil is used.
A preheat coil can also serve as a useful backup if the energy recovery device
should fail. The most common types of preheat coils are hot water, steam and
gas fired. For additional freeze protection, hot-water coils may include an antifreeze fluid such as propylene glycol or ethylene glycol.
Reheating Dehumidified Air
DOAS units are designed to remove the latent load of the outdoor air as well
as latent loads generated within the space. To achieve this, the air temperature
leaving the cooling coil can be as low as 45°F (7°C). Supplying air this cold
directly to occupied spaces could in some instances be challenging to achieve
without jeopardizing occupant comfort. When methods such as selecting appropriate diffusers and locating the diffuser in optimal locations fail to result in
acceptable comfort in the occupied zone, some engineers choose to add a reheat
coil downstream of the dehumidification/cooling coil, see Figure 4.3. But even
in such applications that require a very low dew point, reheating this dehumidified air to a dry-bulb temperature near space temperature (“neutral”) at all times
is generally not recommended (see Chapter 5). And preferably, this reheat coil
should use recovered heat, such as an air-to-air energy recovery device, a hotgas reheat coil, or a hot-water coil that uses condenser heat from a chiller plant.
But in some cases, a gas-fired heater or electric resistance is used.
Section 6.5.2 of ANSI/ASHRAE/IES Standard 90.1 (2016b) limits simultaneous cooling and heating (i.e., reheat). For dehumidification systems, Subsection states “Where humidity controls are provided, such controls shall
prevent reheating, mixing of hot and cold airstreams, or other means of simultaneous heating and cooling of the same airstream.” However, there are several
• Exception 1 allows simultaneous cooling and reheating of the same airstream
if “The system is configured to reduce supply air volume to 50% or less of
the design airflow rate or the minimum outdoor air ventilation rate specified
in ASHRAE Standard 62.1 or other applicable federal, state, or local code or
recognized standard, whichever is larger, before simultaneous heating and
cooling takes place.” That is, if the DOAS has been designed for an airflow
Figure 4.3 Neutral air drawbacks—cooling wasted by delivering air to spaces
at near space dry-bulb temperature (“neutral”).
Mumma et al. (2013)
that equals the minimum ventilation rate required by ASHRAE 62.1, it cannot reduce airflow any further, and Standard 90.1 does not prohibit reheat.
(This is addressed by Example 6-SS in the Standard 90.1-2013 User’s Manual.)
• Exception 5 allows simultaneous cooling and reheating of the same airstream
if: “At least 90% of the annual energy for reheating… is provided from a siterecovered or site-solar energy source.” Hot-gas reheat, an air-to-air energy
recovery device, or condenser heat from a chiller plant are all common
sources for site-recovered heat.
Recognizing the inefficiency of reheat in a DOAS, a new requirement was
added to section in the 2016 version of ANSI/ASHRAE/IES Standard
90.1, which prevents heating or reheating the outdoor air to any warmer than
60°F (15.6°C) during the cooling season.
Tips and Traps
Supplying the conditioned outdoor air (CA) to spaces at a dry-bulb temperature near space temperature (“neutral”) at all times is generally not
recommended, because it typically results in higher first cost and increased
operational costs. When a chilled-water or refrigerant coil is used to dehumidify outdoor air, a by-product of that dehumidification process is sensible cooling. The dry-bulb temperature of the air leaving the coil is colder
than the space. If this dehumidified air is then reheated to a dry-bulb temperature that is close to the space set-point temperature, a significant portion of the sensible cooling performed by the DOAS unit is wasted. If cold,
dry air is delivered to the space, however, it addresses a significant fraction
of the space sensible-cooling load. This means that local units can be sized
for less airflow and less cooling capacity than would be required for a system delivering near space temperature air. Therefore, designing a DOAS
that supplies cold air at design conditions results in a need for less total
cooling capacity than a system delivering near space temperature air. The
capacity of the DOAS unit is the same in either case, because it is sized for
the dehumidification load, but delivering cold air to spaces reduces the
required capacity of each local unit (Murphy 2006; 2012). ANSI/ASHRAE/
IES Standard 90.1-2016 added a new requirement to Section that
prevents reheating this air to any warmer than 60°F (15.6°C) during the
cooling season:
Units that provide ventilation air to multiple zones and operate in conjunction with zone heating and cooling systems shall not use heating or
heat recovery to warm supply air above 60°F (15.6°C) when representative building loads or outdoor air temperature indicate the majority
of zones require cooling.
Air-to-air energy recovery can significantly improve the operating efficiency of
a DOAS unit by transferring energy between exhaust air and incoming outdoor air
(Figure 4.4). It also enables the downsizing of cooling and heating plants that
would otherwise be required to condition outdoor air, resulting in an estimated first
cost reduction of 20% to 30% (Harriman and Lstiburek 2009). Using energy recovery is also a requirement of many modern energy standards and codes, such as
ANSI/ASHRAE/ASHRAE 90.1 (ASHRAE 2016b) and the International Green
Construction Code (ICC 2012).
When considering using air-to-air energy recovery in a DOAS unit, it is important to remember the following:
• The greater the outdoor air requirements, the more operational savings will be
achieved. This is true for applications with significant ventilation requirements
as well as those with longer operating hours.
Figure 4.4 Energy recovery wheel.
Mumma (2014)
• During humid weather, a dehumidification device is still needed. Even technologies that include latent heat exchange are not substitutes for a dehumidifier; they
simply reduce and stabilize the dehumidification load.
• Consider downsizing major cooling and heating plants. (In critical facilities such
as operating rooms, the effects of reducing the size of cooling and heating equipment should be carefully reviewed.)
• Consider installing bypass dampers. Depending on climate, there may be many
hours during the year when outdoor air temperatures are cooler than the exhaust
air but not so cold that heat is required; at these times, operating a heat exchanger
could increase cooling loads. To address such conditions, bypass dampers are
recommended on both airstreams.
The most common air-to-air energy recovery technologies include total-energy
wheels, heat pipes, fixed-plate heat exchangers, and runaround coils. Each of these
technologies is outlined in the following sections. For more information on these
and other types of air-to-air energy recovery devices, refer to ASHRAE Handbook—HVAC Systems and Equipment, Chapter 26 (ASHRAE 2016c).
Total-Energy Recovery Devices
Fixed-plate exchangers with moisture-permeable membranes and total-energy
wheels transfer both sensible heat and moisture. Wheels (or rotary energy exchangers) are a common choice for DOAS units. They are based on water vapor and sensible heat being absorbed by the desiccant-coated wheel from one airstream and
then transferred to the other airstream (Figure 4.5). In summer conditions, water
vapor and heat are transferred from the outdoor air to the cooler, drier exhaust air.
In winter conditions, water vapor and heat are transferred from the warmer, wetter
exhaust air to the incoming outdoor air. Total-energy wheels can transfer up to 90%
of the heat and humidity difference between two airstreams, depending on the
wheel’s rotation speed and other factors (Harriman et al. 2001).
Sensible Energy Recovery
Sensible-only energy recovery devices include rotary wheels, fixed-plate heat
exchangers, heat pipes, and coil runaround loops. These devices will only transfer
Figure 4.5 Total-energy wheels.
Harriman et al. (2001)
sensible heat, and their effectiveness is based on the dry-bulb temperature difference between exhaust and outdoor air. In some cases, evaporative cooling devices
are located upstream of the exhaust side of a sensible energy recovery device.
When it is warm and dry outdoors, these devices can take advantage of the lower
wet-bulb temperature of the exhaust air to cool this exhaust air by evaporating
water. This will reduce the dry-bulb temperature of the exhaust as it approaches the
wet-bulb temperature, thereby increasing the temperature difference between the
exhaust air and outdoor air, and subsequently increasing the sensible heat transferred.
Heat Pipes. Heat pipes can be used to preheat or precool outdoor air entering a
DOAS unit. They can also be used to reheat dehumidified air after leaving a cooling coil. An example is shown in Figure 4.6.
Figure 4.6 Heat pipe operating principle.
Harriman et al. (2001)
Heat pipes are sealed tubes containing a refrigerant. The lower end of the pipe is
placed in the warm airstream. The upper end is placed in a colder airstream. The
liquid inside the bottom of the tube boils as it absorbs heat. This cools the warm air
passing over the bottom of the tube. The resulting warm refrigerant vapor drifts
upward to the top of the tube where it condenses, releasing heat to the cold air passing over the top of the tube.
For DOAS applications, heat pipes are usually designed to transfer between 45%
and 70% of the temperature difference between two airstreams (Harriman et al.
2001). Like a cooling coil, the transfer effectiveness of the heat pipe is influenced
by the number of rows of pipes in the array, the fin spacing and the velocity of air
as it passes through both sides of the array. More heat is transferred when
there are more rows of pipes in the array,
the fin spacing is closer together,
air velocity through the array is slower, or
the heat pipe is inclined to promote liquid returning to the “warm” end.
Plate-Type Heat Exchangers. In a plate exchanger, the hot and cold airstreams
are separated by thin plates. Air flows through the device in an X or a Z pattern. The
colder airstream absorbs heat from the warmer airstream on the other side of the
plates (Figure 4.7). The plates are typically made from aluminum and provide sensible heat recovery only (no moisture recovery). Several manufacturers also offer
plates made from membrane materials that allow water vapor to be transferred
between the two airstreams.
These plate-type heat exchangers are perhaps the simplest and least expensive
heat exchangers used in DOAS units. They are static devices without moving parts,
other than optional bypass dampers. Their function is typically to preheat or precool outdoor air entering a DOAS unit. Although sensible only, plate heat exchang-
Figure 4.7 Plate-type heat exchanger in a packaged system.
Harriman et al. (2001)
ers are also often used effectively to reheat cold, dehumidified air using either
warm exhaust air or warm outdoor air upstream of dehumidifying coil.
Most configurations of plate-type exchangers will transfer between 60% and
65% of the sensible heat difference between two airstreams (Harriman et al. 2001).
These rates are usually achieved using face velocities of 300 to 600 fpm (1.5 to
3.0 m/s).
Coil Runaround Loops. Coil loops are typically used when there is a need to
prevent any cross-contamination between outdoor air and exhaust air, or to add
energy recovery to existing systems. Typical applications include hospitals or laboratories. These devices transfer sensible heat only (no moisture) and typically operate at 35% to 65% sensible effectiveness (NREL 2003, ASHRAE 2016c). The coil
runaround loops are slightly different from the other heat exchangers in the sense
that they include two heat exchangers, typically water-to-air heat exchangers
(coils). One coil is located in the exhaust side airstream and the second is located in
the outdoor airstream; they are connected via piping and a small pump. Both of
these heat exchangers have to be optimized for best performance. A three-way mixing valve can be used to adjust the flow resulting in changing the heat exchangers
effectiveness; it is typically used for frost prevention of the exhaust-side coil.
Section of ANSI/ASHRAE/IES Standard 90.1-2013 establishes a minimum requirement for the effectiveness of exhaust air energy recovery. However,
the definition of effectiveness in the 2013 version of Standard 90.1 differs from the
definition established in ASHRAE Standard 84, Method of Testing Air-to-Air Heat/
Energy Exchangers (ASHRAE 2013b) and the accompanying AHRI Standard
1060, Performance Rating of Air-to-Air Exchangers for Energy Recovery Ventilation Equipment (AHRI 2013). This was corrected in the 2016 version of ANSI/
ASHRAE/IES Standard 90.1, which changed the language to use the term enthalpy
recovery ratio (ERR) rather than the term effectiveness. This enthalpy recovery
ratio is defined as
ERR = (h1 - h2)/(h1 - h3)
where hx corresponds to the enthalpy of the respective airstreams as described by
Figure 4.8 (adapted from Mumma 2014). This definition is a simple representation
of the fraction of total available energy transferred to (or from) the outdoor airstream. Using this measure, ANSI/ASHRAE/IES Standard 90.1 (ASHRAE 2016b)
requires that the exhaust air energy recovery device has an ERR of 50% or greater.
In comparison, the total rated effectiveness t, as defined by ASHRAE Standard
84 (ASHRAE 2013b) and AHRI Standard 1060 (AHRI 2013), is different. The
rated effectiveness increases as the exhaust airflow decreases:
t = (mOA/mmin) × (h1 – h2)/(h1 – h3)
Figure 4.8 Energy recovery example.
Mumma (2014)
mOA = outdoor air mass flow rate
mmin = smaller of the outdoor and exhaust air mass flow rates
= defined as in Equation 1
Physically, this equation describes an increase in relative energy transfer effectiveness. It can be misleading, because it only characterizes how good the device is
at recovering heat from/to the given size heat sink/source mEA, but does not represent the actual amount of energy recovered. Therefore, 50% rated effectiveness, as
defined by AHRI Standard 1060 (AHRI 2013) is not equal to 50% ERR (as defined
by ANSI/ASHRAE/IES 90.1 [2016]) at unbalanced flow conditions (when mEA <
mOA, for example).
The most useful measure to the HVAC designer is ERR as defined by ANSI/
ASHRAE/IES Standard 90.1 (2016b). ERR accounts for any air imbalance between
the outdoor and exhaust airstreams as well as the effect of purge air. Manufacturers
often provide software programs to engineers to calculate ERR for a specific set of
application conditions.
The enthalpy recovery ratio of air-to-air energy recovery devices is negatively
affected by unbalanced outdoor and exhaust airflows. This is because unbalanced
airflows result in less of the total available energy being transferred. Figure 4.9
shows the effects of unbalanced airflows on both rated effectiveness and ERR.
As indicated by the dashed line in Figure 4.9, when the exhaust airflow equals
half of the outdoor airflow, the system recovers less than 70% of the energy that it
would recover if the flows were equal. In addition, ERR drops below 50%, which is
the minimum required by ANSI/ASHRAE/IES Standard 90.1, Section
(2016b) for exhaust air energy recovery.
Figure 4.9 Impact of unbalanced flow on energy recovery effectiveness and
Adapted from Mumma (2014)
Space Pressurization and Energy Recovery
Most HVAC systems rely on the outdoor airflow to maintain a desired level of
building pressurization, the magnitude of which may change with season and/or
geographic location. Pressurizing a building during humid weather helps to prevent
air and moisture from leaking into the building envelope and/or occupied spaces.
A DOAS unit can be used as an effective method of providing building pressure.
However, because some of the air leaks out of the building because of this pressurization (note that ANSI/ASHRAE/IES Standard 90.1 (2016b) specifies minimum
envelope tightness), the exhaust airflow returning to the DOAS unit will be less
than the intake airflow. Figure 4.9 shows the effect of unbalanced airstreams on
both energy recovery effectiveness and enthalpy recovery ratio. The HVAC
designer should take care to avoid a large imbalance of outdoor to exhaust airflow
that might affect the performance of the unit. For example, frosting on an air-to-air
energy recovery device will occur sooner if there is less exhaust air than outdoor
air. For more information refer to the ASHRAE Journal article “DOAS and Building Pressurization” (Mumma 2010).
Figure 4.10 Frosted air-to-air heat exchangers: (a) plate heat exchanger and
(b) wheel heat exchanger.
DencoHappel GmbH; SEMCO, LLC
Frost Prevention
For DOAS units that contain some type of air-to-air energy recovery device,
frosting may occur on the device during extreme cold weather conditions, especially when the occupied space uses an active humidification system. As heat transfers from the warm exhaust airstream to the cold outdoor airstream, the dry-bulb
temperature of the exhaust air decreases. Eventually, the air passing through the
exhaust side of the device may cool to a point where it reaches a saturated condition
and moisture will begin to condense onto the exhaust-side surface of the energy
recovery device. If the surface temperature of the device is colder than 32°F (0°C),
this condensed moisture will begin to freeze.
As ice starts to form it expands, which may damage the heat exchanger. It also
blocks the flow of air, reducing the heat recovery benefit of the device. Because
enthalpy exchangers transfer moisture from the exhaust airstream to the drier outdoor airstream, frosting occurs at much lower outdoor air temperatures in enthalpy
exchangers than in sensible heat exchangers. How frequently this frosting might
occur depends on the type of energy recovery device, the prevalence of cold
weather, and the condition of the air entering the exhaust side of the device (e.g., if
the space is humidified or has a high humidity generation).
The various types of energy recovery devices each have their own methods of
frost prevention. For example, a total-energy wheel may modulate a supply-side
bypass damper to reduce the amount of cold outdoor air that passes through the
wheel, thereby reducing the heat transferred and preventing the condition of the air
passing through the exhaust side of the device from reaching a saturated condition.
Or, a coil runaround loop may use a three-way mixing valve to divert warm fluid
leaving the exhaust-side coil to mix with the cold fluid leaving the supply-side coil
Table 4.1 Common Frost Prevention Strategies for
Air-to-Air Energy Recovery Devices
Frost Prevention Strategies
Coil runaround loop
Three-way mixing valve
Outdoor air or exhaust air preheat
Heat pipe
Face and bypass dampers
Outdoor air or exhaust air preheat
Fixed-plate heat
Supply-side frost damper (may reduce outdoor airflow)
Outdoor air or exhaust air preheat
Total-energy wheel
Wheel speed modulation (only partially effective)
Supply-side bypass damper
Outdoor air or exhaust air preheat
such that the resulting mixture enters the exhaust-side coil at a temperature above
32°F (0°C). For very cold climates, or for buildings with higher indoor humidity
levels during cold weather, a common strategy is to install a heating coil upstream
of the energy recovery device (either the outdoor airstream or exhaust airstream).
Manufacturers of commercially available air-to-air energy recovery devices routinely incorporate frost prevention strategies or mechanisms appropriate to their
equipment and its targeted application. The performance of several frost control
strategies is discussed in ASHRAE research project RP-543 (Phillips et al. 1989a;
Phillips et al. 1989b).
Table 4.1 highlights a selection of typical frost prevention strategies for air-to-air
energy recovery devices. Frost prevention strategies that reduce the amount of outdoor air should be avoided as this will affect the contaminant levels within the
occupied space. Any bypass strategy should incorporate a downstream heating coil
to prevent freeze stat trips or cold supply air temperatures.
In some climates, the outdoor airflow rates required by ANSI/ASHRAE Standard 62.1 (ASHRAE 2016a) can lead to very low indoor relative humidity in the
wintertime. In applications where humidification is used, DOAS units can typically
accommodate a number of humidifier options. The more common humidifier types
used with DOAS include the following:
• Integral Steam Absorption Coils. These are typically factory-installed devices
located between the cooling coil and supply fan. Steam is generated using a gasor electric-driven steam generator and introduced to the airstream through dis-
Tips and Traps
Many packaged rooftop units have fiber-faced insulation inside the casing
instead of a second sheet of metal to cover that insulation. It is not recommended to add humidifiers to such units. Moisture produced by a humidifier can saturate fibrous insulation. Be sure to specify double-walled
enclosures for DOAS units if humidifiers are to be used, sufficiently insulated to maintain an inside wall surface temperature above the airstream
dew-point temperature during winter design conditions.
persion tubes. The temperature of the airstream is typically raised slightly
because of the high temperature of the steam dispersion tubes.
• Duct-Mounted Steam Absorption Coils. These are typically installed in a
straight section of ductwork adjacent to the outlet of the DOAS unit. Similar to
integral coils, the steam is often generated locally using a gas- or electric-driven
steam generator, but can also be generated at a central plant. The steam is introduced to the airstream using duct-mounted dispersion tubes. The temperature of
the airstream is typically raised slightly because of the high temperature of the
steam dispersion tubes.
• Evaporative Humidifiers. These are typically installed integral to the DOAS
unit and are fed from either a pressurized water-atomizing or compressed airfogging device located adjacent to the unit (Figure 4.11). Foggers produce many
small droplets of water vapor, providing high capacity in a short distance with
less risk of fallout, but atomizers are usually simpler and less expensive. Evaporative humidifiers will substantially reduce the temperature of the airstream as
the airstream energy (and not a steam generator) is used to evaporate the liquid
DOAS units can be used to meet indoor air quality needs for most applications
by removing particulate and/or gaseous contaminants from outdoor air before introducing it to occupied spaces. DOAS units have an advantage over central all-air
systems: rather than potentially spreading contaminants to all spaces through the
use of recirculated return air, they are able to expel contaminants directly to the outside and introduce only clean air to occupied spaces. Figure 4.12 shows some filter
Particulate Contaminant Removal
DOAS has proven to be an economical approach to particulate contaminant
removal from the outdoor air. Compared to all-air systems, DOAS can provide
Figure 4.11 Compressed air fogger.
Harriman et al. (2001)
son between DOAS and VAV filter efficiencies (Mumma 2009), indicating that the
DOAS unit consistently results in lower five-year total costs. In each case, the filter
efficiency for the DOAS will be higher than the VAV system although the filter
change frequency is reduced. This is one specific example intended to highlight relative comparisons. More information can be found in the work by Mumma (2009).
Gaseous Contaminant Removal
Outdoor air may also contain gaseous contaminants at unacceptable concentrations; for example, in an urban or industrial environment with high levels of smog
or similar gaseous contaminants that can be harmful to occupants or sensitive mate-
Figure 4.12 Particulate filter options.
Mumma (2014)
rials contained in the building. If so, it requires treatment by gaseous contaminant
removal equipment (different from particulate filtration equipment) before being
supplied to occupied zones. Gaseous contaminants should be removed by the
DOAS unit before any mixing with return air, to minimize the total amount of air
that needs to be treated. For further information on selection and application refer
to ASHRAE Handbook—HVAC Applications, Chapter 46—Control of Gaseous
Indoor Air Contaminants (ASHRAE 2015a).
All DOAS units should have a prefilter installed at the outdoor air inlet.
Additionally, if the DOAS unit includes an air-to-air energy recovery device, a
prefilter should be placed upstream of the exhaust side of this device. These filters
not only prevent energy recovery devices from becoming clogged with larger dust
and debris (Figure 4.13), but they also extend the life of the more expensive highefficiency filters. A typical prefilter used in DOAS units is a MERV 8 (30%–35%
dust spot efficiency).
Table 4-2 Optimal Solutions for VAV and DOAS Filters
Giving Equal Performance
Fan 5-Year
Operating Cost
Based on Filter
Face Area, Between 5-Year
First Cost
ft2 (m2)
61.5 (5.7)
9.3 (0.86)
33.4 (3.1)
8.8 (0.82)
30.5 (2.8)
8.9 (0.83)
Figure 4.13 Enthalpy wheels unprotected by filters.
Tips and Traps
When a filter is placed immediately downstream of either a coil or a
humidifier, it can become saturated with moisture. Placing filters upstream
of coils or humidifiers avoids this problem, and also protects equipment
from accumulations of dirt and dust, further reducing the potential for
microbial growth.
Many components can be used to configure a DOAS unit. Different climate and
application types dictate how the DOAS is configured. For example, cooling/dehumidification coils in hot, humid climates must be designed to remove significantly
more moisture from the outdoor air than units designed for hot, dry climates. And
heating coils and frost prevention strategies may be needed for colder climates but
not required for hot climates.
One common component of DOAS units is usually an air-to-air energy recovery
device. These devices can greatly reduce both energy consumption and moisture
load on dehumidification coils by transferring sensible and latent heat between the
exhaust air path and the outdoor air path. They also may allow heating and cooling
equipment to be downsized, saving money on first costs. DOAS units can be configured to include one or more types of air-to-air energy recovery; for example, one
device to recover energy from the exhaust air (such as a wheel) and another to provide reheat after moisture removal by the cooling/dehumidification coil (such as a
plate heat exchanger).
Space pressurization is a major concern for most commercial buildings. To
address this, HVAC engineers often provide slightly more outdoor air than exhaust
air. Mumma (2010) suggests that the actual amount of outdoor air needed to pressurize a building can be as much as 50% more than the exhaust air. Larger amounts
of outdoor air will significantly reduce the effectiveness of the air-to-air energy
recovery device. The HVAC designer should design the DOAS unit to correct for
any reduced energy recovery effectiveness or consider a supplemental makeup air
unit solely to control building pressure.
Figure 5.1 Mission Control Center at NASA’s Johnson Space Center.
NASA (2009)
Although building controls are (thankfully) not as complex as those required
to land a spacecraft, they are an important element of any HVAC design. As
building technologies advance, the importance of well-designed control
sequences increases accordingly. Giving adequate time and thought to the control
and integration of building systems can affect energy consumption, indoor air
quality and occupant comfort, directly affecting the success of a project.
Providing sufficient details on the sizing, configuration, and components of
DOAS in construction documents is essential to a successful DOAS installation. Concise and detailed descriptions of how the DOAS components are
intended to work together are also essential to the success of the system. Without adequate controls guidance, even a perfect DOAS installation will be
unable to meet its goals.
It is therefore important that the design engineer describe how the DOAS
will work in sufficient detail that a controls contractor can program each component without trying to guess the designer’s intentions. The controls contractor typically does not have the detailed information about the intent and design
of the system, so without specific guidance, they are forced to either repeat
what they have done for other projects, guess, or ignore.
Detailed sequences with schematic diagrams are particularly helpful tools for
conveying this information to contractors and future operators. Although schematic diagrams may not provide any information that is not already provided in
the floor plans, they present an overview of the system without the complication of construction details, as demonstrated in Figure 5.2. The objective of this
chapter is to provide the reader with an overview of topics that an engineer
needs to consider when writing control sequences and schematic diagrams for
One of the main advantages of DOASs is that they condition outdoor air separately from recirculated air. This enables accurate tracking of outdoor airflow,
and can improve control of humidity levels in occupied zones.
Local HVAC equipment is typically selected based on required cooling
capacity at peak outdoor dry-bulb conditions. This satisfies the sensible component of the design load, but will only partially satisfy the latent load, because
the peak dew-point temperature will likely occur on a different day. Because
the mode of control for local HVAC equipment is typically a thermostat, as the
sensible-cooling load decreases in the space, so does the latent capacity of the
cooling coil.
In climates requiring humidity control, this can result in elevated indoor
humidity at part-load conditions. Designing the DOAS unit to address all latent
loads (from the outdoor air and those created in the space) provides an effective
solution to this problem. Local HVAC equipment is then responsible only for
maintaining dry-bulb temperature in the space.
The most common way to activate a DOAS unit is based on the building
occupancy schedule, programmed by the building automation system (BAS).
The fan in the DOAS unit is energized to bring in the required outdoor air. Outdoor air temperature and humidity sensors are used to calculate the outdoor air
dew-point temperature, and cooling, dehumidification, humidification and heat-
Henneman Engineering Inc.
Figure 5.2 Schematic diagram at left gives a quick overview of system function compared to plan view at right.
ing components are modulated to ensure that the conditioned air does not
exceed the desired leaving air dew point.
There are four general modes of operation for the DOAS unit that are based
on outdoor air conditions: dehumidification and cooling, sensible cooling, heating, and ventilation only (Figure 5.3 and Table 5.1).
Different control strategies are used depending on the specific components of
the unit, and these approaches are discussed in the following sections.
Figure 5.3 DOAS unit control modes (outdoor air control).
Murphy (2012a)
Table 5.1 DOAS Unit Control Modes
Control Mode
Outdoor Conditions
and Cooling
Outdoor air dew point > dehumidification set point
Sensible Cooling
Outdoor air dew point  dehumidification set point
Outdoor air dry-bulb temperature > cooling set point
Ventilation Only
Outdoor air dew point  dehumidification set point
Heating set point outdoor air dry-bulb temperature 
cooling set point
Outdoor air dew point  dehumidification set point
Outdoor air dry-bulb temperature < heating set point
Although several methods can be used to dehumidify the outdoor air, the most
common is to cool the air down to the dew-point temperature required to meet the
desired humidity level in the spaces (see Chapter 2 for how to calculate this temperature). This is typically done with a cooling coil, often with the assistance of an
air-to-air energy recovery device. Other approaches to dehumidification include
desiccant wheels or liquid-desiccant dehumidifiers.
When leaving cooling coil temperature is used for humidity control and the coil
has full modulating capabilities, the control strategy is straightforward: simply control the discharge air temperature from the cooling coil to the required dew point of
the conditioned outdoor air. In cases where the cooling coil has limited control
capabilities (e.g., one or two stages or a high percent minimum cooling capacity),
the temperature leaving the cooling coil should on a time average be kept at or
below the set point required to maintain the desired space humidity levels
(ASHRAE Standard 62.1 User’s Manual, Example 5-C footnote (ASHRAE
Control strategies associated with desiccant dehumidification are similar to those
used for other applications, and are detailed in ASHRAE Handbook—HVAC Systems and Equipment, Chapter 24 (ASHRAE 2016c). Depending on the type of desiccant and system configuration, these processes may result in warm and dry air,
such that some postcooling of the air may be required before it is introduced into
the space.
Resetting Dehumidification Capacity
When the latent load in the space is lower than at design conditions, the need for
dehumidification of the outdoor air is also lessened. In this case, the dew point for
the conditioned outdoor air could be reset upward to reduce dehumidification
energy. A humidity sensor could be installed in each space (or in several representative spaces) with a BAS polling these sensors to determine the critical space—
that is, the space with the highest humidity. Based on a signal from the BAS, the
DOAS unit then reduces its dehumidification capacity, raising the leaving air dew
point just enough to still maintain the humidity level in the critical space at the
desired upper limit. By responding to actual humidity conditions, the system maintains the humidity at or below the desired upper limit in all spaces while minimizing dehumidification energy use.
Humidity Control During Unoccupied Hours
During long unoccupied periods such as weekends, holidays, and vacation periods, a common control strategy is to simply turn off the supply and exhaust fans in
the DOAS unit and close the outdoor and exhaust air dampers. In humid climates,
moisture may continue to infiltrate the building and some building processes that
often are scheduled for these unoccupied periods such as carpet cleaning and floor
waxing can significantly increase the indoor humidity levels. This can result in toohigh humidity levels that may lead to condensation and subsequent moisturerelated problems.
When the DOAS unit is configured to deliver conditioned air directly to spaces, a
recirculating damper can be added to provide a means to address high humidity
during unoccupied periods. The recirculating damper will allow the DOAS unit to
condition recirculated air to maintain desired humidity conditions in the space (Figure 5.4). The exhaust fan and total-energy wheel (if installed) will be off, the outdoor and exhaust air dampers closed, and the recirculating damper open. The
supply fan and cooling coil will be activated during this mode. The airflow rate can
be reduced in this unoccupied mode, reducing fan power consumption.
Because of potentially low sensible-cooling loads, a source of reheat in the
DOAS unit or local spaces may be necessary to avoid too-low temperatures in the
building. Also, to avoid airflow imbalances between occupied and unoccupied
Figure 5.4 After-hours humidity control.
CA = conditioned air RA = recirculated air
Adapted from Murphy (2008)
modes, the HVAC engineer should size the recirculating damper such that the air
pressure drop through the return air path is similar to the air pressure drop through
the outdoor air damper and energy wheel.
The dry-bulb temperature at which air leaves the DOAS unit is typically a result
of the dehumidification process. Generally, it is most energy efficient and cost
effective to deliver the conditioned outdoor air as close to the temperature that
results from conditioning (dehumidifying in most cases) as possible, as long as it is
acceptable for comfort in the space and consistent with the capabilities of the local
Although controlling the conditioned outdoor air temperature of a DOAS is not
significantly different from a traditional system, determining whether to reheat or
not may be somewhat different for a DOAS. Although the primary functions of a
DOAS unit are outdoor air delivery and dehumidification, it can also assist in
addressing cooling loads.
The amount of air delivered to the spaces from the DOAS is typically less than
the minimum setting of a variable-air-volume (VAV) terminal in a mixed-air system, because the DOAS provides only outdoor air. In a few cases, even this relatively small amount of cool air can be too much for the zone. The following
sections discuss situations when some spaces may become too cool, and suggest
possible strategies to address this.
Methods to Avoid Too-Low Space Temperatures
Demand-Controlled Ventilation. This strategy dynamically resets the amount
of outdoor air delivered to a zone based on varying occupant levels in that zone
(Figure 5.5). This is typically done using pressure-independent dampers (such as
VAV terminals), one for each zone, and a variable-frequency drive (VFD) on the
supply fan of the DOAS unit. In addition a time-of-day (TOD) schedule, occupancy
sensor and/or carbon dioxide sensor is used in each zone to vary the outdoor airflow
delivered to that zone. Not only will this approach help avoid too-low space temperatures, but it provides significant energy savings (cooling, dehumidification,
heating, and fan energy).
Activate Heat in Local HVAC Unit. Depending on where the conditioned outdoor air is introduced into the space, a useful approach to avoid too-low space temperatures is to activate the heating coil in the local HVAC unit (Figure 5.6). This is
a common strategy that can be used in systems with local fan-coil units with heat,
water-source heat pumps, baseboard heating, or radiant heating where only a few
local units need heating and the other units can remain in cooling and take advantage of the energy savings from delivering the cool conditioned outdoor air.
Reheat Dehumidified Air at DOAS Unit. Using outdoor air to address a portion of the indoor sensible-cooling load by supplying air at cool temperatures often
enables downsizing of local HVAC units. However, in cases such as the following,
Figure 5.5 Demand-controlled ventilation.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
RA = recirculated air
Murphy (2008)
when the air leaving the DOAS unit may provide too much cooling to some spaces,
reheat will be needed to temper the dehumidified outdoor air before introducing it
to the spaces:
• To Avoid Low Space Temperatures at Part-Load Conditions. As a zone’s sensible-cooling load decreases (because of milder outdoor air conditions, reduced
solar heat gain, and/or internal loads) the cold air supplied by the DOAS may
provide more sensible cooling than the zone requires. When this happens, the
local HVAC unit will add the necessary heat to the space. This might be the most
efficient approach if only a few zones are in heating mode. When many of the
local HVAC units are in heating mode, it may be more efficient to reheat the
dehumidified outdoor air using recovered heat (an air-to-air energy recovery
device or a hot-gas reheat coil) than to, for example, activate the boiler.
• Where Zone Sensible-Cooling Loads Are Highly Variable. In spaces such as
hotels or dormitories, the sensible-cooling loads can vary significantly from
room to room. This can potentially result in too-low space temperatures in those
Figure 5.6 Activating heat in local units.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
RA = recirculated air
Murphy (2008)
areas that have little to no sensible-cooling load. In cases such as this, where the
benefit of delivering cold air occurs less frequently, it may be warranted to
deliver conditioned outdoor air closer to space temperature.
• When Very Low Conditioned Outdoor Air Dew Point Is Required. For applications that have very high latent loads or require lower than normal space dew
points, the outdoor air will need to be dehumidified to a very low dew point. In
this case, the temperature leaving the cooling coil may be lower than the engineer
feels comfortable with introducing into the space. As an example, the dew-point
calculation in Chapter 2 requires a conditioned outdoor air dew point of 47°F
(8.3°C). Even though systems can be designed to deliver air this cold, the HVAC
engineer might feel more comfortable discharging air at a more conventional
temperature of 55°F (12.8°C), requiring reheat. Reheating this dehumidified air
to a dry-bulb temperature closer to space temperature at all times is generally not
• To Avoid Condensation in Ceiling Plenum. If outdoor air is introduced directly
into a ceiling plenum space near the intake of local HVAC units (see Figure 3.6),
it should be supplied at a dry-bulb temperature above the expected dew-point
temperature of the air within the plenum. If cold air is supplied into the ceiling
plenum, it could cool surfaces (e.g., structural beams, electrical conduits). At
night, when the DOAS unit is off, wind or operating exhaust fans may cause
humid outdoor air to leak into the plenum, which may lead to condensation on
the cooled surfaces.
Depending on climate and internal loads and requirements, these conditions may
warrant that the conditioned outdoor air is reheated for zones with no heating capabilities. This reheat might be added using heating coils mounted for each zone in
the DOAS ductwork, or by reheating the dehumidified outdoor air before it leaves
the DOAS unit.
The following are some DOAS unit control strategies to avoid too-low space
temperatures at part-load conditions:
• Reset DOAS Leaving Air Temperature Based on Outdoor Air Temperature.
A possible control approach is to activate the DOAS unit reheat coil (or air-to-air
recovery device) based on outdoor air temperature (Figure 5.7). For example, if
the outdoor air temperature drops below a set-point limit of 55°F [13°C], the hotgas reheat coil or wrap around heat pipe (see Chapter 2 for more details) can be
activated to increase the dry-bulb temperature of the conditioned air, preventing
too-low temperatures in space with no heating capabilities. This limit is typically
adjusted after a few months of “trial-and-error” operation. This approach might
appeal to building operators as it is simple and similar to the conditioned outdoor
air temperature reset strategy used in many traditional VAV systems. If dehumidification is still required, this reset should not affect the leaving-air dew point.
That is, the cooling coil would still be controlled to dehumidify the outdoor air to
a dew point low enough to achieve the desired space humidity levels.
Example Control Sequence for Outdoor Temperature Reset:
The building automation system shall continuously monitor the outdoor drybulb temperature.
• When the outdoor dry-bulb temperature is warmer than 55°F (13°C)
(adjustable [adj.]), the DOAS leaving air dry-bulb temperature (DBTCA) set
point shall be 52°F (11°C) (adj.).
• When the outdoor dry-bulb temperature is between 45°F (7°C) (adj.) and
55°F (13°C) (adj.), the DOAS DBTCA set point shall be reset proportionally
(see chart) between 52°F (11°C) (adj.) and 67°F (19°C) (adj.). If DBTOA >
52°F (11°C) (adj) the cooling coil leaving dew point shall remain 52°F
Figure 5.7 (a) Typical DOAS configuration with (b) supply air temperature
reset based on current outdoor air temperature.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
Murphy (2008)
RA = recirculated air
(11°C) (adj.) to continue to dehumidify; only the reheat coil will be modulated to raise the DOAS DBTCA.
• When the outdoor dry-bulb temperature is colder than 45°F (7°C) (adj.),
the DOAS DBTCA set point shall be 67°F (19°C) (adj.).
• Activate Reheat Based on Coldest Zones. In this scenario, the building automation system (BAS) monitors the terminal equipment in each zone served by
the DOAS unit, and determines if local heat has been activated in any zone (or if
the temperature in any zone not equipped with local heat has dropped below its
heating set point. Based on a signal from the BAS, the DOAS unit can then modulate its reheat capacity, resetting the leaving air dry-bulb temperature upward
just enough to avoid or minimize local heat and prevent too-low space temperatures in most (or all) of the zones (Figure 5.8).
This strategy delivers the conditioned outdoor air at a temperature that offsets
much of the zone sensible-cooling loads without creating too-low zone tempera-
Figure 5.8 Reheat activated by coldest zone.
OA = outdoor air
CA = conditioned outdoor air
EA = exhaust air
SA = conditioned supply air
Murphy (2008)
RA = recirculated air
tures while reducing the need for the zone equipment to operate in heating mode.
Of course, reducing the use of the local heating is only a benefit if the DOAS unit
uses recovered energy for reheat (wrap around coil, an air-to-air energy recovery
device or hot-gas reheat). If the DOAS uses gas or electricity for reheat, it would
likely be more efficient to allow the zone equipment to operate in heating mode.
Example Control Sequence for Zone Monitoring Reset:
The building automation system shall continuously monitor the dry-bulb temperature in all zones served by the DOAS unit. Exclude any zones that are in
• When local heating has been activated in at least two (adj.) zones, or if the
space temperature is colder than its heating set point –1°F (0.6°C) (adj.) in
at least two (adj.) zones that are not equipped with local heat, the DOAS
leaving air dry-bulb temperature (DBTCA) set point shall be reset upward in
increments of 1°F (0.6°C) at a frequency of 10 minutes, until local heating
has been deactivated in all but one (adj.) zone and the space temperature in
all but one (adj.) zone is warmer than its heating set point +0°F (0°C) (adj.),
or until the DBTCA set point is reset to its maximum setting of 70°F (21°C)
• When the space temperature in all but one (adj.) zone is warmer than its
heating set point +1°F (0.6°C) (adj.), the DOAS DBTCA set point shall be
reset downward in increments of 1°F (0.6°) at a frequency of 10 minutes,
until the space temperatures in all but one zone (adj.) are colder than their
heating set point +0.5°F (0.3°C) (adj.), or until the DBTCA set point is reset
to its dehumidification setting of 52°F (11°C) (adj.).
• The cooling coil leaving temperature shall remain at 52°F (11°C) (adj.) as
required for dehumidification when the outdoor air temperature is above
52°F (11°C) (adj.).
Total-Energy Recovery Control
Many proponents explain the benefits of exhaust air energy recovery by focusing
on the hottest and coldest days of the year. The more extreme the outdoor conditions, the greater the energy savings and the more the heating and cooling equipment can be downsized. However, during less severe outdoor conditions, improper
operation of the energy recovery device (see Figure 5.9 for a sample unit configuration with wheel as this is the most common device for total-energy recovery) can
actually increase overall system energy use (Murphy 2012). Therefore, proper control of the device is critical for maximizing the energy-saving potential while avoiding (or minimizing) energy waste.
Total-energy wheels can be controlled in two ways: (1) enthalpy control, where
temperature and humidity are measured for both the outdoor and exhaust airstreams, or (2) temperature-only control. If enthalpy control is selected, ensuring
the accuracy of humidity sensors is essential, including the monitoring of their
accuracy over time. The relative difference between the sensors’ readings is more
important than the absolute accuracy of the sensors. The same argument applies to
temperature sensors, but typically these sensors are more accurate and less susceptible to drifting over time than humidity sensors.
There are typically five distinct modes of control for total-energy wheels, as seen
in Figure 5.10 and Figure 5.11. These modes are defined by outdoor air (OA)
enthalpy/temperature conditions and desired conditioned air (CA) set points:
1. Full recovery (cooling)—OA conditions exceed the enthalpy/temperature of the
exhaust air (EA).
2. No recovery (cooling)—OA conditions are lower than the enthalpy/temperature
of the EA, but warmer than (or near) the CA set-point temperature.
3. Partial recovery (heating)—OA conditions are lower than the enthalpy/temperature of EA, and colder than the conditioned air set-point temperature. However,
Figure 5.9 DOAS unit configured with a total-energy wheel and bypass dampers.
OA = outdoor air
CA = conditioned outdoor air
OA' = outdoor air pretreated by wheel
Mumma et al. (2013)
EA = exhaust air
Figure 5-10 Total-energy wheel control modes.
Adapted from Quinnell (2016)
Figure 5-11 Total-energy wheel control modes on the psychrometric chart
(region A is applicable to enthalpy control).
Sustainable Engineering Group, LLC
the OA is warm enough that the total-energy wheel can provide all the heat necessary, without requiring supplemental heating.
4. Full recovery (heating)—OA conditions are cooler than the CA set-point temperature, and cold enough that even with the total-energy wheel operating at full
capacity, some supplemental heating is also required.
5. Frost prevention (heating)—OA conditions are cold enough that frost may be an
Region 1 (and Region A, if Enthalpy Control)—Full Recovery (Cooling). In
this area, the total-energy wheel should be running at 100% capacity to reduce the
temperature and/or enthalpy of the incoming outdoor air as much as possible. This
applies when the outdoor air exceeds the enthalpy (Region A in Figure 5.11) or
temperature (Region 1 in Figure 5.11) of the exhaust air.
If temperature-only control (i.e., dry-bulb control) is chosen, adjustments can be
made to obtain some of the benefits of running the wheel when outdoor air conditions fall into Region A (hOA > hEA, but DBTOA < DBTEA). One dry-bulb control
strategy is to start the wheel when the outdoor air dry-bulb temperature DBTOA is
still cooler than the exhaust air dry-bulb temperature DBTEA. This will result in
cooling energy savings during those hours in Region A (the total-energy wheel will
reduce the enthalpy of the air before it enters the cooling coil). However, this will
increase heating energy use during those in Region 2. But in many climates there
are relatively few hours when this occurs (when the outdoor conditions are in the
lower right portion of Region 2) so an optimized dry-bulb temperature for starting
the wheel can provide a good balance (the increased heating energy in Region 2 is
very small compared to the reduced cooling energy associated with operating the
wheel in Region A).
The optimal dry-bulb temperature to begin running the total-energy wheel varies
depending on climate (typical range is 69°F to 75°F [20.6°C to 24°C] and needs to
be optimized for each climate [Taylor and Cheng 2010]). In humid climates, this
optimal temperature will likely be closer to the estimated wet-bulb temperature of
the exhaust air, while in drier climates it will likely be closer to the estimated drybulb temperature of the exhaust air.
Region 2—No Recovery (Cooling). For a total-energy wheel, when the
enthalpy of the outdoor air drops below the enthalpy of the exhaust air but is still
warmer than the conditioned air set-point temperature (hOA < hEA and DBTOA >
DBTCA [Figure 5.11, Region 2]), the wheel provides no cooling benefit. In fact,
unless it is turned off, the wheel will actually increase the load on the cooling coil
by increasing the dry-bulb temperature and/or the humidity ratio of the outdoor airstream.
At the example conditions depicted in Figure 5.12, the enthalpy of the outdoor
air (hOA = 24.3 Btu/lb [38.5 kJ/kg]) is less than the enthalpy of the exhaust air (hEA
= 28.2 Btu/lb [47.7 kJ/kg]). If the total-energy wheel continues to operate at these
conditions, it increases the enthalpy of the air leaving the wheel (OA') to 27.0 Btu/
lb [44.9 kJ/kg], which increases the load on the cooling coil (Figure 5.12).
However, if the wheel is turned off when hOA < hEA and DBTOA > DBTCA (Figure 5.11, Region 2), the enthalpy of the air entering the cooling coil is lower, avoiding this increase in coil load. For a 10,000 cfm (4700 L/s) DOAS unit, operating the
wheel at this condition increases the cooling coil load from 14 tons (50 kW) with
the wheel off, to 25 tons (88 kW) with the wheel on—an 80% increase.
In this configuration, consider adding bypass dampers (Figure 5.9) on one or
both sides of the wheel. Opening these dampers when the wheel is turned off can
reduce the air-side pressure drop and minimize fan energy use if VFD control of
fans is used to control outdoor airflow. Also note that, in this operating mode, the
wheel is typically cycled on for a few minutes each hour to help keep it clean.
Region 3—Partial Recovery (Heating). When it is cool outside (Figure 5.11,
Region 3), some means of controlling the capacity of the total-energy wheel may be
needed to avoid overheating and possibly overhumidifying the entering outdoor air.
At the example conditions depicted in Figure 5.13, the dry-bulb temperature of
the outdoor air (45°F [7°C]) is colder than the desired temperature of the conditioned outdoor air, which is 50°F (10°C). Therefore, the total-energy wheel could
Figure 5.12 Total-energy wheel control on a mild, rainy day.
Murphy (2012b)
Tips and Traps
Some designers choose to control a total-energy wheel based on the drybulb temperature of the two airstreams, rather than enthalpy. Although this
avoids the cost and maintenance required to install humidity sensors, it may
also reduce energy savings during the cooling season.
If the total-energy wheel is turned off whenever the dry-bulb temperature of
the outdoor air drops below the temperature of the exhaust air (DBTOA <
DBTEA), the wheel will be off for many hours when it could have been used to
reduce cooling energy use. During those hours when DBTOA < DBTEA, but
the enthalpy of the outdoor air is still higher than the enthalpy of the exhaust
air (hOA > hEA), the wheel could be operating to reduce the enthalpy of the air
entering the cooling coil (Region A in Figure 5.11). Mumma (2011) illustrates
that many hours of potential useful heat recovery can be gained by using
enthalpy control versus using 75°F (24°C) dry-bulb control.
However, if using temperature-only control, setting a wheel shutoff drybulb set point equal to the exhaust air temperature is never optimal, and far
fewer hours of savings will be lost if the dry-bulb set point is lower than the
exhaust temperature. The typical range for optimal dry-bulb wheel shutoff settings is between 69°F and 75°F [20.6 to 24°C] (lower for humid climates and
higher for dry climates).
When choosing between dry-bulb and enthalpy control, the designer should
consider that humidity sensors are typically only accurate to ±5% rh, so rather
than achieving the anticipated energy savings, large inaccuracies in humidity
readings may result in increased energy consumption (Taylor and Cheng
2010; Mumma 2011). The accuracy of humidity sensors can degrade over
time, which may decrease the benefits of enthalpy control. In addition,
enthalpy control can prove complicated for operators. Therefore, if enthalpy
control is chosen, it should be carefully executed.
be turned on to transfer sensible heat from the warmer exhaust air to preheat the
entering outdoor airstream. In this example, if the wheel operates at full heating
capacity, the air leaves the wheel (OA') at 66°F (19°C), which is warmer than
Because a DOAS is accompanied by local HVAC equipment that provides heating or cooling for each zone, whether this overheating of the outdoor air actually
results in wasted energy depends on whether the zones currently require heating or
cooling. If a zone served by this DOAS unit requires heating at this example condition, then the overheated outdoor air may be beneficial because it reduces the need
of local equipment to add heat to the zone. However, if a zone requires cooling, the
overheated outdoor air may cause the local equipment to use additional recooling
For buildings that are dominated by internal cooling loads (thus requiring cooling almost year-round), it has been proposed that the wheel should remain off
during cool weather (Mumma 2005b). In this case, the cooler air supplied by the
DOAS unit—unheated at 45°F (7°C) for the example depicted in Figure 5.13—offsets more of the zone cooling load. For an application dominated by internal cooling loads, this strategy likely reduces overall system energy use. Even though a
typical DOAS cannot provide 100% economizer cooling capacity like a traditional
VAV system, allowing the system to deliver cooler air at such conditions extends
its ability to provide some amount of free cooling.
Regarding humidity, in this example if the total-energy wheel operates at full
capacity, it transfers water vapor from the more humid exhaust air into the entering
outdoor airstream. Outdoor air then leaves the wheel (OA') at a 52°F (11°C) dew
point, which is higher than the 50°F (10°C) set point. The result is that the cooling
coil may need to activate to dehumidify the air (Figure 5.13). Therefore, unnecessarily operating the total-energy wheel at full heating capacity may require recooling and/or operating the dehumidification equipment, both of which are
unnecessary uses of energy.
Figure 5.13 Total-energy wheel control on a cool, dry day.
Murphy (2012b)
Reducing the capacity of the wheel can prevent both overheating and overhumidifying. Modulating an exhaust-side bypass damper reduces the amount of air
passing through the wheel, which decreases the amount of energy recovered. In the
example depicted in Figure 5.13, reducing airflow through the exhaust side of the
wheel results in less heat transferred to the outdoor airstream, and air leaves the
supply-side of the wheel at the desired 50°F (10°C) dry-bulb temperature, rather
than being overheated. Modulating wheel capacity also avoids overhumidifying the
outdoor air. In this example, air leaves the wheel at a 45°F (7°C) dew point, which
is below the 50°F (10°C) set point, so dehumidification is not needed.
Slowing the rotational speed of the wheel could provide an alternative means of
reducing capacity. Modulating an exhaust-side bypass damper is preferred, however, because it provides a wider range of capacity control and has a more linear
unloading characteristic, which results in simpler and more stable control (Murphy
and Bradley 2009). Another option with fixed-speed heat recovery is to start and
stop the wheel such that the time-averaged conditioned outdoor temperature is
equal to the desired conditioned outdoor air set point (Jeong and Mumma 2007).
This also requires a sequence to avoid that the heating and cooling coil is activated
when the temperature gets too high or too low.
Region 4—Full Recovery (Heating). In this region, the outdoor air is cold and
all the heat recovered from the exhaust air will reduce the heating load on the building. This happens when the recovery device reaches 100% recovery capacity with
no air bypassing and the speed of the wheel is 100%. In this mode, supplemental
heat may be required to heat the OA to the desired dry-bulb temperature set point of
the conditioned outdoor air (DBTCA).
Region 5—Frost Prevention (Heating). In this region, there is a chance that
frosting might occur on the exhaust side of the total-energy wheel. As an estimate
of whether frost buildup is a concern for a specific set of outdoor air conditions,
draw a line on the psychrometric chart between the outdoor air conditions and the
exhaust air conditions (this assumes the same sensible and latent recovery efficiency). If this line passes through the saturation (100% rh) curve, frosting is a concern. Manufacturers’ selection software can also be used to determine if and when
frosting might occur.
An alternative way to identify frost buildup is to monitor the pressure drop
across the energy recovery device. If this pressure drop begins to increase in very
cold conditions, it is likely caused by a frosting event.
To obtain the full benefits of the total-energy recovery device, frost prevention
cycles should not be initiated prematurely. During a frost prevention cycle, considerable heat that could have been recovered is lost, and the heating equipment has to
make up the difference. Typically, in unhumidified office buildings, frosting will
rarely occur on enthalpy recovery devices, even in extremely cold climates. Please
refer to the Frost Control section for specifics.
Sensible-Only Heat Exchangers
Figure 5.14 divides the psychrometric chart into sensible heat exchanger control
Region 1—Full Recovery (Cooling). Region 1 is defined by the area on the psychrometric chart for which DBTOA > DBTEA. In this region, full recovery is
enabled to lessen the mechanical cooling load as much as possible. Any bypass
dampers should be closed to allow the sensible heat exchanger to operate at full
capacity, for maximum recovery of cooling energy. The cooling coil is modulated
to satisfy the conditioned air (CA) dry-bulb target.
Region 2—No Recovery (Cooling). When the outdoor air temperature is lower
than the exhaust air temperature, but warmer than the desired conditioned air temperature (DBTCA < DBTOA < DBTEA), energy recovery should be disabled to avoid
transferring unwanted heat to the incoming airstream. Bypass dampers should be
opened to divert all exhaust air (and/or outdoor air) around the sensible heat
exchanger. The cooling coil is used to meet the desired conditioned air dry-bulb target.
Region 3—Partial Recovery (Heating). In this scenario, the outdoor air drybulb temperature is cooler than the desired conditioned air temperature (DBTOA <
DBTCA) but still warm enough that the heating load can be entirely satisfied by sensible heat recovery. When outdoor air conditions fall in this region, the exhaust-side
Figure 5.14 Psychrometric chart showing modes of interest for sensible heat
exchanger control.
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bypass dampers and/or the wheel speed should be modulated to control the capacity
of the sensible heat exchanger. This is done to avoid overheating the conditioned
outdoor air. Supplemental heating is not required because the heating load is less
than the heat exchanger capacity.
If there is no means to control the rate of recovery, the warm part of Region 3
should be combined with Region 2 and the cooler part should be combined with
Region 4. Exactly how Region 3 is split into Regions 2 and 4 will depend on how
cold the conditioned outdoor air can become before it creates comfort issues,
whether the building is cooling dominant at these temperatures, and whether heating is available in the DOAS unit. An option with fixed-speed heat recovery is to
start and stop the wheel such that the time averaged conditioned outdoor temperature is equal to the desired conditioned outdoor air set point (Jeong and Mumma
2007). This also requires a sequence to avoid activating the heating or cooling coil
when the temperature gets too high or too low.
Region 4—Full Recovery (Heating). Control in this region will be the same as
for the enthalpy recovery system. The outdoor air is cold and all the heat recovered
from the exhaust air will reduce the heating load on the building. This happens
when the recovery device reaches 100% recovery capacity with no air bypassing
and the speed of the wheel is 100%. In this mode, supplemental heat may be
required to heat the outdoor air to the desired dry-bulb temperature set point of the
conditioned outdoor air (DBTCA).
Region 5— Frost Prevention (Heating). Compared to the same exhaust air
conditions, frosting on the exhaust air side of the air-to-air energy recovery wheel
will occur at higher temperatures with a sensible heat recovery only wheel than
with a total-energy (enthalpy) wheel. However, the exhaust air conditions will normally be drier because of no latent humidity recovery (unless active humidification
is taking place). Manufacturers’ selection software can be used to determine if and
when frosting might occur.
An alternative way to identify frost buildup is to monitor the pressure drop
across the energy recovery device. If this pressure drop begins to increase in very
cold conditions, it is likely caused by a frosting event.
To obtain the full benefits of the air-to air energy recovery device, frost prevention cycles should not be initiated prematurely. During a frost prevention cycle,
considerable heat that could have been recovered is lost, and the heating equipment
has to make up the difference.
Total-Energy Recovery
When it is very cold outside, an exhaust air energy recovery device is subject to
frost buildup. Because a total-energy recovery device transfers sensible heat and
water vapor from the exhaust air to the cooler, drier outdoor airstream, the exhaust
air is cooled and dehumidified. If the condition of the exhaust air passing through
the energy recovery device reaches saturation, moisture will begin to condense on
its surface. If the surface temperature of the device is below 32°F (0°C), this condensed moisture may begin to form frost on the exhaust side. This reduces the
amount of energy recovered, blocks the airflow through the device, and may result
in structural damage.
The outdoor temperature at which frost begins to form depends on the effectiveness of the wheel, the temperature and humidity of the exhaust airstream, and the
outdoor and exhaust airflows (Murphy and Bradley 2009; Mumma 2001). Manufacturers’ selection software can be used to determine when frosting might occur.
For a total-energy wheel, one of two control strategies is typically used to prevent frosting:
• Reduce the Capacity of the Wheel. Modulating a supply-side bypass damper
decreases the amount of heat transferred, thereby raising the surface temperature
of the device to prevent frost from forming. This approach is often used in climates and applications where frost formation is expected to be a rare occurrence.
In many cases, the bypass dampers are already incorporated into the equipment
to reduce fan energy when the wheel is turned off.
• Preheat the Outdoor (or Exhaust) Air Before it Enters the Wheel. Raising
the temperature of the air entering either the supply or exhaust side of the wheel
prevents the exhaust air from reaching a condition at which frost might begin to
form. This approach is used in climates and applications where frost formation is
expected to be more common (Murphy 2012). Although this approach requires
the installation of a small preheat coil, it allows the wheel to continue operating
at full energy recovery capacity even during the coldest times of the year.
Sensible-Only Heat Exchangers
When the outdoor air is cold enough to produce frost on the heat exchanger (the
frost threshold; Region 5 of Figure 5.14) control measures must be taken to prevent
frosting. For fixed-plate heat exchangers, a frost avoidance damper can be closed to
prevent frost from forming on the exhaust side of the heat exchanger. Using a frost
avoidance damper will allow heat exchangers to operate at subfreezing outdoor
temperatures. If colder conditions are expected, a preheat coil should be used for
frost prevention (on either supply side or exhaust side).
For runaround loops, a three-way valve is typically used for frost control by mixing the warmer water returning from the exhaust-side coil with the colder water
returning from the supply-side coil. This will increase the water temperature entering the exhaust-side coil, keeping it warmer to preventing frosting.
For heat pipes, a supply-side bypass damper can be used to decrease the amount
of heat transferred and prevent frost from forming on the exhaust side.
In any case, heat recovery effectiveness will be reduced when the frost sequence
is enabled.
Many DOAS units are constant-volume systems that do not adjust the outdoor
airflow based on variations in occupancy. These systems are straightforward to
control, requiring only the overall building occupancy schedule. The DOAS unit
does not typically have to operate during morning warm-up or cooldown periods,
or when the heating/cooling systems cycle on to maintain temperature during unoccupied periods (unless there is a need for dehumidification).
Reducing the amount of outdoor airflow when spaces are at low occupancy or
unoccupied can save a great deal of energy (Crowther and Ma 2016). This strategy
is commonly called demand-controlled ventilation or dynamic reset of the outdoor
air, because outdoor airflow is adjusted in real time based on the actual occupancy.
These control strategies typically rely on occupancy sensors (OCC), time-of-day
(TOD) schedules, people counters, and/or carbon dioxide (CO2) sensors to adjust
the outdoor airflow (Figure 5.15). ANSI/ASHRAE Standard 62.1 (2016a) allows
this type of control as long as individual spaces are always provided with the minimum required amount of outdoor air for the current population.
To enable the outdoor airflow to vary with occupancy, the DOAS supply fan and
(in most cases) exhaust fan must be capable of reducing their airflows. Typically,
this is accomplished by using a VFD motor. In addition, dampers should be
Figure 5.15 Demand-controlled ventilation with a DOAS.
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installed in each area served by the varying outdoor airflow so that the distribution
of the air can be modulated to the correct amounts as occupancy varies.
As outdoor air is varied, the exhaust air may also have to be modulated to avoid
negative building pressures. The low limit for the exhaust airflow will typically be
determined by code exhaust requirements (for restrooms, laboratories, etc.). The
low limit for the outdoor airflow will typically be determined by code ventilation
requirements, makeup air needed for code-required exhaust, or the minimum outdoor airflow needed to maintain the desired building pressure. If the exhaust is
physically separated into two sections (code-required and general exhaust), an airflow measurement device may be warranted on the code-required exhaust side to
ensure that spaces with code-required exhaust maintain their minimum exhaust airflows.
Varying the outdoor airflow can produce significant savings in fan, cooling, and
heating energy, but also requires a more expensive and complicated distribution
system. Payback will depend on the amount of time the building is occupied, the
magnitude of outdoor air reduction, the efficiency of heat recovery (if installed),
and other project-specific factors.
Average building pressure is determined by the amount of outdoor air brought
into the building, the amount of air exhausted from the building, and the tightness
of the envelope. Envelope tightness should not be overlooked, because it can
greatly affect the ability to establish and maintain desired building pressure (for a
detailed treatment of building pressurization considerations, please see Harriman et
al. [2001]).
Additional factors such as wind and the stack (chimney) effect can significantly
alter building pressure, creating local pockets of positive or negative pressures.
These forces can be so powerful that it is unreasonable to expect an air-handling
system to overcome them in all situations, but it is the engineers’ responsibility to
create an indoor environment that (on average) is typically either slightly positive
when the outdoor air is humid or neutral when it is cold.
A slightly positive or neutral average building pressure can be achieved using
one of two distinct approaches. One method is to measure the building pressure and
control outdoor (intake) and/or exhaust airflows to achieve the desired average
building pressure. The other method involves measuring (or estimating) outdoor
and exhaust airflows, and maintaining a set difference between them to create the
desired building pressure.
When using the building pressure sensor method, there are two main challenges.
The first is to find good locations for the indoor and outdoor pressure sensors. The
outdoor air sensor should be selected and located to minimize the effects of wind
(i.e., avoid exterior walls). The indoor sensor should be installed in a location that is
representative of the average building pressure. A good indoor location can be difficult to find, particularly if the building has more than one story (i.e., susceptible to
the chimney effect). In addition, the sensor needs to be in a representative space
(not ceiling plenums or pressurized spaces), away from exterior doors and other
openings, and away from diffusers etc. that may blow on the sensor.
Another challenge facing building pressure control is accurate pressure measurement. The building pressure set point will be low (compared to duct pressures) so
the differential pressure sensor has to be sensitive to very low pressures. A suggested range for differential building pressure sensors is ±0.1 in. of water (25 Pa).
For comparison, a slightly positive average building pressure set point is typically
in the range of 0.002 to 0.050 in. of water (0.5 to 12.5 Pa), depending on the tightness of the envelope.
The alternative method of controlling building pressure is to measure and adjust
the outdoor and exhaust airflows. This means exhausting less air than is being
brought in if positive pressure is desired, or maintaining equal airflows to achieve
neutral pressure. In these cases, it is important to include all exhaust and intake airflows, including intermittent flows, or those that are not routed through the DOAS
unit. For constant-airflow DOAS, this can be achieved during the balancing of the
system, but it requires that the testing, adjusting, and balancing contractor has been
provided the ability to accurately measure exhaust and outdoor airflows.
In systems capable of varying outdoor airflow, it is important to have accurate
measurements of both the exhaust and outdoor air intake, or desired building pressures will not be achieved. Measurement can be challenging, because airflow monitoring devices typically require some amount of straight duct to be accurate. The
designer should take this into consideration early in the design process. In addition,
some airflow measurement devices can be affected by dirt and dust buildup. This
can be particularly problematic on the exhaust side if there is no filter upstream of
the airflow measurement device. If the exhaust flow is measured too low, the
exhaust fan will speed up and create negative building pressure.
It might seem tempting to simply offset the exhaust air and outdoor air fan
speeds to eliminate the cost associated with airflow measurement devices. However, because the two fans operate along different system curves, this can lead to
unintended consequences such as negative building pressures, in certain situations.
Therefore, careful planning, with thorough testing of all situations and fan speeds,
is needed if a fan speed offset strategy is to be used for building pressurization.
Regardless of the choice of building pressurization method, the amount of
exhaust and outdoor air intake cannot be decreased below code requirements. There
will also be an upper limit for how much outdoor air the DOAS equipment is capable of delivering. The building pressurization system must work within these
DOAS uses the same type of sensors and control devices as any traditional airhandling system, including temperature and humidity sensors, airflow measurement devices, actuators, freeze stats etc. All sensors and actuators need to be cali-
Tips and Traps
• Be careful to avoid kinks in pressure tubes extending from the differential
building pressure sensor (both outdoor and indoor).
• Use filters ahead of any airflow measurement devices.
• Design for the entire range of operation when planning a building pressurization system, not just maximum airflows.
• Work with architects and engineers to design a tight envelope—it is much
easier to pressurize an airtight building, and it will save energy as well.
• If fan speed offset is chosen as a strategy to provide building pressurization, make sure to design and test for the entire range of airflow situations. A fixed fan speed offset is not likely to succeed over a broad range
of flows.
• Do not use a duct pressure sensor to measure the building pressure. A
whole-building pressure sensor has to be sensitive to very low pressures.
• Use a slow response or timed average sensor reading for controlling the
building pressure to avoid unnecessary ramping of the exhaust fan when
someone opens or closes a door or otherwise affects the building pressure
reading for a short period of time.
brated in the field and inspected to make sure that they are connected to the right
BAS input or output.
Sensors should be checked and recalibrated after a certain amount of time. CO2
sensors, humidity sensors, and airflow measurement devices tend to drift the most
over time. A CO2 sensor should typically be recalibrated at least every 6 months,
and humidity sensors every year. Airflow measurement devices should also be
checked at least once a year, particularly if they are responsible for pressurizing the
building. They can produce incorrect readings if they are dirty, and may need to be
cleaned on regular intervals. If there are no filters located upstream of these
devices, they should be checked and cleaned more frequently.
In some DOAS control strategies, differential temperatures, humidities, or airflows are used as part of the control strategy. In these cases, it is important that the
differential measurement is accurate. For example, two airflow devices measuring
the exhaust and outdoor airflow might be within acceptable calibration limits, but if
the exhaust airflow is reading on the low side of the acceptable range and the outdoor airflow on the high side, the building might end up less positively pressurized
than designed (or even neutral or negative).
Enthalpy control strategies rely on accurate humidity sensor readings. Some
authors have argued that humidity sensors tend to be unreliable (Taylor and Cheng
2010) and therefore it is more practical (and potentially more efficient) to control
Tips and Traps
based on dry-bulb temperature only. Others have argued that the impact of this sensor inaccuracy on actual energy use is small (Mumma 2011). As humidity sensing
technology improves, however, these arguments may become less applicable.
Always make sure that the outdoor air dampers close completely when
the DOAS is turned off, as this can cause significant maintenance issues as
well increased energy consumption. Also check actuator operation at regular intervals (e.g., two to four times each year, or whenever filters are
The DOAS should be as easily accessible as any other system serving the building. For larger buildings and campuses, this means that the DOAS should be available on a central, Internet-accessible interface. For smaller systems, the DOAS unit
may come with a stand-alone controller, but higher-end systems often come with a
wireless interface.
Easy access to system data makes it possible to track performance at frequent
intervals, making the fine-tuning of schedules and early detection of issues possible. Setting up trends on appropriate data points also enables the users (or any troubleshooter) to identify issues that need attention. Chapter 7 discusses this subject in
more detail.
Building automation systems require that the engineer define how the system is
to be controlled. These sequences are then programmed into the system by the
building automation system installer. Writing clear, concise, yet comprehensive
sequences of operations is key to the energy and comfort performance of the
HVAC system, yet it requires effort and careful thought. It is important to have a
solid understanding of how controls work, the limitations of the specific building
automation hardware specified, and the HVAC system design. Techniques for writing successful control sequences are discussed in ASHRAE Guideline 13 (2015b).
A DOAS can come in many configurations, from very simple to quite complex,
to meet goals for different climates and applications. Because of these variations,
there are multiple methods and sequences that can be used to control a DOAS. This
chapter addresses the most common challenges that are faced by the engineer while
designing DOAS controls.
This chapter has provided suggestions regarding the control of outdoor air dehumidification as well as controlling the conditioned outdoor air temperature delivered to the space. Methods for maximizing air-to-air energy recovery savings are
also discussed. Other topics that typically need to be addressed with the operation
of a DOAS, such as frost prevention, building pressurization, and airflow control
are presented. Finally, general discussions of the instrumentation, central control
system, and sequences of operation have also been provided.
Figure 6.1 Well-designed, poorly constructed.
Getty Images
All of the time and cost that has been invested in designing a project can go to
waste if it is not constructed correctly. Keeping in mind how the final product
should function will help ensure that the right steps are being taken to get there.
Double-checking design documents (or in the case shown in Figure 6.1, instructions) before completing each phase of construction can also prevent costly mistakes from occurring.
This chapter provides an overview of construction phase considerations related
to DOAS that may prove useful to design engineers. The submittal review process
reveals how the contractor plans to implement the HVAC design. This provides an
opportunity for the design engineer to offer feedback on equipment selections,
helping to ensure that the systems meet the design intent and are on the path to
operating successfully.
During the construction phase, the use of installation checklists and system performance tests help to confirm that the DOAS units and HVAC system will perform as intended. These efforts are typically organized by a commissioning
provider, with the design engineer contributing by describing the intent of the
design and conveying expectations for operation.
Documentation is crucial to the ongoing successful operation of any HVAC system. The engineer plays a key role in helping to define documentation requirements
in the design specifications, and verifying that the contractor has provided this documentation. The design engineer can extend their impact on the project by following through with these activities in the construction phase.
The HVAC engineer continues to make important contributions to a project
during the construction phase, even after the DOAS unit has been selected and the
HVAC design is complete. Some of the major construction considerations related
to DOAS are discussed here. Issues related to other types of HVAC systems are
discussed only when they intersect with DOAS-specific considerations.
Submittal Review
The submittal review process offers a design engineer the opportunity to understand what equipment and components the contractor intends to use to make their
design a reality. The design engineer typically reviews the submittal documents to
ensure their compliance with the design documents. Any deviations are noted and
communicated back to the contractors.
Items of importance to DOAS units that should be given careful consideration
during the submittal review include the following:
Heating and Cooling Coils and Desiccant Dehumidifier
• Ensure that air velocity, water velocity, air pressure drop and water pressure drop
fall within specified limits (these can vary considerably from standard airhandling unit coil parameters).
• Confirm that both the latent and sensible capacities meet design requirements
with 100% outdoor air.
• Verify that performance data has been corrected for the type of antifreeze solution specified.
• For DX cooling coils, confirm that they have sufficient capacity and modulation
capability to achieve the required humidity removal at all operating conditions.
• Confirm that there is sufficient clearance at the electrical panel.
• Review the location of access doors to ensure that each major component can be
easily serviced and/or removed for cleaning.
• Confirm that the conditioned outdoor air dew point is at (or below) the indoor air
dew point during peak outdoor dew-point design conditions.
Condensate Traps
• Verify that cooling coil traps follow the manufacturer’s guidance for allowing
condensate to drain completely out of the drain pan (Figure 6.2).
• Confirm that the manufacturer’s required trap depth can be accommodated by the
equipment pad or roof curb.
• If a preexisting trap does not meet specified requirements, a custom trap should
be made.
• Confirm that the fan and motor are capable of delivering the required maximum
airflow at the specified external and submitted highest internal pressure loss (i.e.
dirty filter, wet coil etc.).
• Verify that the type and efficiency of the fans are as required.
Air-to-Air Energy Recovery Devices
• Confirm that the correct types of recovery devices have been submitted.
• Confirm that the effectiveness of each device meets the required effectiveness.
• The pressure drops across the air-to-air recovery devices must be as required or
• Confirm that filters and racks meet the performance criteria specified in the
design documents.
• Verify that the pressure drop across filters does not exceed design limits at the
conditions specified in the design documents (e.g., clean, midlife, dirty).
• Make sure that filters are located where they can be easily accessed for inspection and replacement. Review the manufacturer’s required clearances and verify
that the planned installation provides adequate clearance.
Bypass Dampers
• Bypass dampers must be sized large enough to avoid causing an excessive pressure drop.
• Damper type must be selected to allow for range of control specified in the control sequences.
• Verify that controlled devices match the building automation system (BAS) submittal. For example,
• Does the DOAS unit have factory-fitted or field-supplied controllers (or
• Are all BAS-required control points available from the DOAS controller?
• Are there duct smoke detectors on supply and/or return connections?
• Are motorized dampers provided with the DOAS unit, and will they operate
via signals from the BAS?
• If a factory-fitted controller is provided with the DOAS unit, confirm that control
points in the BAS submittal match those in the DOAS submittal.
• Ensure that factory controllers share the same protocol as the BAS system (e.g.,
BACNet®, LONWorks®).
• Verify that the DOAS will operate to dry (or heat) any incoming air provided to
conditioned spaces, whenever exhaust fans are operating to remove conditioned
air from the building.
• Verify that during unoccupied mode, the DOAS will either stop operation or
will recirculate and dry the indoor air rather than bringing in humid or cold
outdoor air.
Flow Diagrams
• Ensure that comprehensive flow diagrams state the air volume, air dry-bulb temperature and humidity ratio after each component in the system, and that separate
flow diagrams are provided for at least three outdoor air entering conditions:
peak dew point, peak dry bulb, and winter design.
• Ensure that the capacity required of each component in the system is defined on
all three flow diagrams, when operating the stated outdoor air entering conditions.
• Verify that the sum of the building’s exhaust air flows is equaled or exceeded by
the sum of the intake air flows from systems that provide conditioned outdoor air
or makeup air.
• Verify that all required alarms are communicated from the DOAS unit to the
BAS. DOAS units are often located on roofs or other locations that are difficult
to access, so visual inspection may occur less frequently than with other equipment. Additionally, malfunction of the unit may not produce as noticeable an
impact on the indoor environment as, for example, failure of heating/cooling
units would. It is therefore particularly important that the DOAS unit is equipped
with alarms if components such as cooling stages, fans, or energy recovery
devices should fail. Failure of these components will cause a lack of ventilation
or reduced dehumidification ability, resulting in reduced indoor air quality.
Even if the contractors ideally install everything according to the construction
documents, the reality is that the construction documents are not always correct
and/or field adjustments are necessary for a variety of reason. These changes can
affect the DOAS installation and, while there are numerous possible impacts, we
have discussed a few common items for a successful completion of a DOAS project. The installation checklists (see a sample in Appendix A) address many more
items that the contractors need to complete correctly.
Exhaust, outdoor air intake, vents and roof equipment (including the DOAS unit)
tend to be moved, turned, or relocated during construction. This can change the distance between outdoor air intake and possible sources of pollution (e.g., loading
docks, idling buses and trucks, etc.) or strong odors (e.g., sanitary vents, kitchen
exhausts etc.). It is important to make sure that the final location of the outdoor air
intake is such that recirculation of exhaust air is minimized and a sufficient distance
from sources of pollution and strong odors is maintained.
Another possible challenge associated with bringing exhaust back to the DOAS
unit is ductwork routing, particularly in the case of retrofits. If external ducting is
used, both the exhaust and supply ducts must be insulated and sealed to avoid ambient energy losses and condensation issues.
The roof curb or mounting pad must be high enough off the roof or floor to allow a
drain trap as detailed in Figure 6.2. It should be verified that the height of the roof
curb or mounting pad can accommodate the required drain trap assembly height at the
location of the drain for the selected DOAS unit design pressure at the drain pan.
Installation Checklist. Construction checklists can, when correctly implemented, significantly reduce the number of issues that arise late in the building process and/or cause poor building performance. Checklists can help minimize
contractor rework, avoid suboptimal solutions created by last-minute corrections,
and reduce conflicts. The effective use of checklists can therefore benefit subcontractors, general contractors, owners, architects, engineers, and (ultimately) occupants.
In the design phase, the engineer may want to include a sample checklist in the
specifications to introduce the concept to the contractors. Items on the checklist
may apply to any or all phases of the DOAS construction process, such as delivery,
storage, installation, and/or start-up. A sample DOAS checklist is provided in
Appendix A.
Start-Up Testing
Like all building equipment, DOAS units should be started up and tested to confirm that all components function as designed. Start-up testing should be completed
by contractors or factory representatives on all units throughout the facility, and
verified by a commissioning provider. Start-up testing includes making sure that all
Figure 6.2 Example of condensate drain trap for a draw-through fan.
Tips and Traps
Harriman et al. (2001)
Packaged DOAS rooftop units are often simply placed on top of the supporting roof curb without a full air and water seal between unit and curb. The
designer and contractor must ensure that this connection is designed and
installed to be weathertight and airtight and high enough off the roof to
accommodate the condensate trap.
the main components such as fans, coils, filters, dampers, and energy recovery
devices are functional. It also includes confirming that control components such as
actuators, sensors, and alarms are correctly wired, calibrated, and accurate.
Test and Balance Verification
Testing, adjusting, and balancing (TAB) (Figure 6.3) is required for any system
that provides or requires air or water flow, and a DOAS is no exception. Because of
Figure 6.3 Testing and balancing.
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this, it is essential that the DOAS and its associated ductwork provide a balancer
with good options (i.e., straight and accessible main ducts) to measure outdoor and
exhaust airflows. This is important whether or not the unit has airflow measurement
devices. If the TAB cannot be successfully accomplished, possible impacts include
the following:
• Not enough outdoor air supplied, which can negatively affect IAQ
• Too much outdoor air supplied, which may increase energy use
• Unbalanced outdoor (intake) and exhaust airflows that cause overly positive or
negative building pressure, which may negatively affects IAQ, increase energy
use, and/or make entry doors stand open or difficult to open
The conditioned outdoor air also has to be correctly distributed to the spaces. For
constant-volume DOAS, the balancing dampers have to be adjusted such that the
airflows to all zones is as required with ideally at least one balancing damper 100%
open. With constant-airflow systems, it is important that the pressure at the air terminations do not vary over time, such as when the outdoor air is ducted to the
intake of a local unit (e.g., a heat pump) with a fan that is set to cycle on/off with
temperature. Varying pressure at the outlet can significantly disturb the distribution
of the conditioned outdoor air in a constant-volume air system. If variable pressure
on the terminations cannot be avoided, pressure-independent dampers and airflow
measurements devices (such as variable-air-volume [VAV] terminals) might be
For VAV systems (such as with occupancy or demand control), the supply side is
typically controlled well with terminal units. The exhaust might be somewhat more
challenging than normal and could require controlled dampers and flow devices to
keep, for example, restroom exhaust air from falling below acceptable levels as the
airflow varies.
One unique issue with DOAS is that the airflows to a zone can be considerably
smaller than with systems that provide cooling and/or heating. The ducts, diffusers,
and airflows should be designed such that it is practical for a TAB contractor to
measure and balance the DOAS air distribution for any zone with low airflow.
The engineer should review the TAB report to verify that the outdoor airflow is
within acceptable limits, and this should also be verified by commissioning provider.
System Performance Testing
In addition to confirming that the components of the DOAS work properly, it is
important to test whether they are able to meet design requirements. For example,
the cooling coil may be working, but if the air temperature leaving the cooling coil
is 70˚F (21°C), a dehumidification strategy that requires 50°F (10°C) off the coil
will not be achieved. It is particularly important to test design-level functionality in
units that do not have sensors or connect to a central system, and units that are
physically difficult to access.
Making sure that the DOAS is able to meet its design requirements is referred to
as system performance testing (SPT [Figure 6.4]). Contractors should confirm
through testing that every unit and system is able to meet its stated requirements. It
is also beneficial to involve a commissioning provider, who can define test procedures and verify that testing has been performed. While contractors are experts at
understanding how and what to install, they may be less familiar with control
sequences, particularly if they are vaguely worded (e.g., “The heating coil, mixed
air dampers, and the cooling coil shall be controlled in sequence to maintain the
discharge air set-point temperature.”).
DOAS units are particularly important to test, as mentioned above, because the
effects of their malfunction can be difficult to detect directly. Although it is prefer-
Figure 6.4 System performance testing. An engineer and building operator
conduct a system performance test.
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able to test the DOAS under actual conditions, this may not always be possible, so
many expected conditions must be simulated or created by overrides (such as CO2
readings, humidity levels, occupancy, etc.) It is important to test DOAS functionality as thoroughly as possible before occupancy to avoid problems, but waiting for
all test conditions to occur naturally is typically not practical or beneficial. Ambient
conditions can be simulated to some extent, but might only give an indication of
actual performance. Therefore the DOAS should be retested as soon as conditions
are appropriate (i.e., seasonal testing) and verified by the commissioning provider.
Demand-controlled ventilation can be difficult to accomplish if the controls contractor and engineer are not familiar with these sequences. For example, the
restroom exhaust must not be allowed to drop below a code-required minimum
value, often restricting how low the DOAS outdoor airflow can be reduced, and
introducing the need for a sequence to control how much exhaust must come from
the restrooms versus from the remainder of the building. The engineer also needs to
address what to do when the outdoor airflow is shut off in enough zones that the
resulting exhaust airflow drops below the minimum required by code. Communicating this to the controls contractor and commissioning provider in an unambiguous manner, and making it easy to test, can be challenging.
Operator training (Figure 6.5) is essential for ensuring optimal performance of
any system. Although facility operators are likely to be familiar with the individual
components of a DOAS unit, they may be less familiar with some of its functions
(e.g., energy recovery). It can be very beneficial for the design engineer to explain
design intent for the system to the building operator, including how it functions.
Although the DOAS unit will usually operate automatically, a solid understanding
of how it works and interacts with the other HVAC components in the building,
may help avoid potential pitfalls, such as accidentally making detrimental changes
to set points or sequences.
Closeout Documentation
As with other systems, DOAS record documents (TAB report, record drawings,
control sequences, etc.) should be included in the operation and maintenance
(O&M) manual. Because DOAS may be an unfamiliar system to some operators, it
is particularly important that its functionality is well documented, especially
Figure 6.5 Operators and users training session.
Sustainable Engineering Group, LLC
because this type of information is difficult to extract from construction documents
alone. Operators can then refer to these documents before making any system
adjustments, and can also use them to train new operators if needed.
If the specifications require a facilities requirements and operation and maintenance plan (for example, as needed for LEED v4 projects [USGBC 2013]), at a
minimum the following information should be included in this plan:
DOAS equipment runtime schedules
Set points for DOAS equipment
Outdoor air requirements
Any variations in schedules or set points for different seasons, days of the week,
or times of day
• Systems narrative describing the mechanical systems and equipment
• Preventive maintenance plan for the mechanical equipment
• Commissioning program narrative that includes periodic commissioning requirements, ongoing commissioning tasks, and continuous tasks for critical facilities
The majority of a DOAS design engineer’s responsibilities occur during the
design phase of the project. However, an engineer should remain involved throughout the construction phase as well, to ensure that the intent of the DOAS design is
achieved. This includes reviewing submittals, communicating with contractors
during the installation process, and reviewing TAB, start-up, and other testing
The engineer may also want to be involved with operator training, particularly if
the owner’s facility personnel are unfamiliar with DOAS operation. Although a
DOAS unit may look similar to a traditional AHU, the purpose and method of operation is fundamentally different.
Figure 7.1 Filter replacement for rooftop DOAS unit.
Sustainable Engineering Group, LLC
Prevention is better than cure. All of the time and effort that goes into designing a building reaches fulfillment when it begins operating. Providing building
staff with important system documentation will help ensure that the HVAC sys119
tem is operated correctly. In addition, regular maintenance of DOAS components
such as coils and filters (Figure 7.1) will improve performance and help prevent
expensive repairs.
Transferring knowledge of building HVAC systems to facilities staff by staying
involved during early building operation is an important aspect of the design engineer’s role. Instruction regarding the intent and function of the system is particularly important to deliver in the case of less common systems such as DOAS.
Although most of the design engineer’s work has been completed before building
operation, providing guidance during this phase can profoundly impact the longterm success of the project.
This chapter provides an overview of operational considerations related to
DOAS. It is intended to guide HVAC engineers regarding which documents to
include during the design phase, as well as how best to assist owners once the
DOAS is operational. It is essential to the long-term success of the project that
engineers identify the needs of the maintenance staff and tailor their DOAS design
to match staff skill levels and existing operation and maintenance (O&M) methods.
It is also crucial to a project’s success that all documentation provided to the
owner lasts as long as the system itself, whether submitted on paper or electronically. Ready access to these documents will maximize the chances that the HVAC
system will operate optimally over its lifetime, and that successful operation will be
maintained through any staff changes.
All building documentation should be provided in a format that lasts as long as
the building itself. Although paper copies have the potential to last that long, they
seldom do, and often end up misplaced or destroyed. Electronic files are easy to
retrieve and copy, but can also be lost. In addition, electronic files can become difficult to access after a few years because of rapidly changing software. Because
each format has some drawbacks, it is recommended that both paper and electronic
documents are provided to the owner. Combined, they are likely to last the longest.
The following documentation should be provided to the owner of a DOAS:
Operation and Maintenance Manual
• All submittal information, testing, adjusting, and balancing (TAB) reports, manufacturer’s operating instructions for major equipment, etc.
Systems Manual
• Describing how the systems work, the components included in each system, and
how the components will work together. Also summarizes maintenance requirements.
As-Built Documentation
• Construction documents updated to reflect how the system was actually installed.
Training Plan for New Facilities Personnel
• Include recorded training sessions.
It is recommended that electronic files are made available on the same computer
or server as the building automation system so that anyone using the system has
easy access to design details as necessary. Copies and backups of building documentation should be maintained for redundancy. Cloud-based storage provides a
ready solution for both easy access and reliable backups.
Although operators are likely familiar with the individual components of a
DOAS unit, its function may be less well understood. There is therefore a risk for
DOAS to be operated incorrectly. To aid operators’ understanding, the engineer
should consider providing the following:
• A description of the purpose of the DOAS in the control sequence, with schematic drawings of the DOAS
• A DOAS training document for operators, which should also be accessible to
future operators
Recommended Control Points
The following control points can be useful for DOAS operation and troubleshooting. They are listed by component, and should be viewed as initial suggestions, not necessarily a complete or exhaustive list:
• General
• Entering outdoor air dry-bulb temperature
• Entering outdoor air humidity
• Fan(s)
• Supply fan status and speed
• Exhaust fan status and speed, if equipped
• Dampers
• Position of dampers
• Cooling
• Leaving coil dry-bulb temperature
• Position of chilled-water coil valve actuator or modulation (or staging) of
• Fault status of each compressor
• Heating
• Leaving coil dry-bulb temperature
• Position of gas valve, hot-water coil valve actuator, or modulation (of staging) of electric heater
• Filter(s)
• Differential pressure across filters
• Airflow measurement device(s)
• Total airflow at each sensor
• Energy recovery device
• Entering supply-side dry-bulb temperature (and humidity, if enthalpy recovery)
• Leaving supply-side dry-bulb temperature (and humidity, if enthalpy recovery)
• Entering exhaust-side dry-bulb temperature (and humidity, if enthalpy recovery)
• Leaving exhaust-side dry-bulb temperature
• Control points for any actuators/speed control associated with the energy
recovery device
• Wraparound heat exchanger (if used)
• Leaving upstream-side dry-bulb temperature
• Leaving downstream-side dry-bulb temperature
In addition to an annual physical examination by a qualified technician, DOAS
units should be monitored regularly to confirm their continued successful performance. Operational performance checks should be carried out on a regular basis,
such as during the change of seasons when other preventative maintenance items
are reviewed. A general checklist of items that should be monitored regularly
during DOAS operation is given in Appendix B. It should be viewed as a starting
point and not an exhaustive list. Some of the major DOAS functions that warrant
regular monitoring are as follows:
• Ventilation. Verify that the unit provides the correct amount of outdoor air as
defined in the construction documents.
• Dehumidification. Verify that the leaving air humidity ratio (or dew point) is as
specified, and that space humidity levels fall in the acceptable range.
• Air-to-Air Energy Recovery. Verify that outdoor and exhaust airflows are as
specified, and that the effectiveness of the recovery device meets design/installation expectations. Confirm that the bypass function of the energy recovery device
is performing as intended, if installed.
Air Quality Maintenance
To ensure the DOAS continues to deliver clean outdoor air to the building, the
following items should be reviewed monthly:
• Inspect and change filters on a regular interval (monthly, or as needed to maintain desired collection efficiency). Make sure filters fit tightly and do not bypass
air, as this will cause downstream components to become dirty.
• Confirm that the following components are clean:
• Cooling coils and drain pans should be inspected to make sure there is no
visible biological growth or fouling.
• Air-to-air energy recovery devices on both the exhaust side and supply side
should be inspected to make sure all openings are clear and free of obstructions.
• Heating coils should be unobstructed.
• Fans should not have accumulated dust.
• Control components should not be covered or plugged by dust.
• Grease bearings as necessary.
During its first year of operation, a university dormitory in Wisconsin received
complaints about humidity levels in its first floor resident suite. This was unexpected, as the university had invested in a DOAS specifically to help control
humidity levels. The maintenance staff initially tried running the fan-coils continuously in the hopes of reducing humidity, despite the fact that the fan-coils had been
designed to run intermittently.
Measurements revealed that humidity levels in the resident suite were swinging
between 70% and 95% over periods of about 30 minutes. This period corresponded
to times when the local fan-coil was activated and deactivated (Figure 7.2). Further
investigation showed that the building was very negatively pressurized. While there
were some discussion of what could cause this negative pressure (laundry exhaust
fans, etc.), a look at the DOAS serving the space revealed that the exhaust fan was
running continuously at 100%, independent of what the supply fan was doing.
The supply and exhaust fans had both been set up with airflow measurement
devices, and the exhaust fan was controlled to track the supply fan (with an airflow
offset to keep the spaces slightly positive). However, the exhaust fan airflow could
not meet its airflow set point, so it ran at 100% at all times. An investigation of the
fan revealed that the airflow measurement device was plugged by dust (Figure 7.3).
Cleaning this sensor got the exhaust fan back under control, and both the negative
pressure and humidity issues were resolved.
The DOAS unit had a 30% filter upstream of the airflow measurement device
that may have been partially bypassed, allowing dust to collect on the sensor. After
identifying the issue, regular cleaning of the exhaust airflow measurement device
prevented future issues.
Design engineers represent the primary source for information on how building
HVAC systems are intended to operate, and they are therefore the best candidates
for conveying it to facilities staff. Multiple sources of information should be available to operators, including both documents and training sessions (recorded for
future reference). Engineers should also be accessible for initial troubleshooting
and responding to questions regarding system intent.
Although building occupancy occurs long after the design phase is completed, it
is the period that will determine whether or not the project is a success. To achieve
Figure 7.2 Trends showing high humidity levels in dormitory residential suite.
Sustainable Engineering Group, LLC
Figure 7.3 Airflow measurement device (a) before and (b) after cleaning.
J&H Controls (Brian Abler)
smooth operation, it is important for facilities staff to become familiar with the
intent and function of building systems, and develop a clear understanding of how
these systems should be operated.
Model number
Filter rack size
Rated outdoor airflow/total static pressure
Number of supply fans
Supply fan motor speed/power
Exhaust airflow/total static pressure
Number of exhaust fans
Exhaust fan motor speed/power
Energy recovery section type
(wheel, flat-plate heat exchanger, heat pipe, etc.)
Refrigeration type
The unit is physically undamaged.
Unit is free of water damage.
Manufacturer's wiring diagram and nameplate are visible and
match unit configuration.
Protective coverings over duct and pipe openings are in place and
Fans are secure and undamaged.
Dampers are undamaged and move freely.
Coils and heat exchangers are undamaged and fins are straight.
Energy recovery wheels are undamaged.
Unit is provided with variable-frequency drives for supply and exhaust
There is enough room to install the condensate trap as designed.
Installer has read the manufacturer’s installation/start-up manual.
Unit is visibly tagged with DOAS model number.
Manufacturer's unit nameplate and wiring diagram are readable.
Unit is mounted and anchored [add specific requirements potentially
including curb/concrete pad height, connection type] per Detail X/XXX
and/or manufacturer’s installation manual, and/or Specification section
Unit is sealed to the roof curb.
Unit is level.
Interior surfaces of unit are undamaged and clean.
Filters are in place and mounting rack is undamaged.
Service and maintenance clearances are in accordance with manufacturer's
requirements [on page X of the installation manual and/or submittal].
Coverings over duct and pipe openings are secure and not breached.
Screws and handles for access panels are in place and undamaged and
latches operate properly.
Access doors are undamaged, mounted square and open and close freely.
Ducts are firmly connected to the unit.
Supply and exhaust ducts do not obstruct access and maintenance doors/
Duct interiors are clean.
Duct insulation is firmly attached up to duct/unit flange.
All dampers, actuators and sensors are accessible with access panels per
Specification section XX.XXXX.
All dampers close tightly and stroke fully and easily.
Sound attenuators are provided [include specific location] in accordance
with Drawing XXX and/or Specification section XX.XXXX.
All electrical connections are tight.
Unit is electrically grounded.
Conduit allows for clearance around unit for maintenance and service.
Disconnect switch is attached to or nearby unit in a visible location.
Piping does not obstruct access and maintenance clearances and allows
for unit removal.
All piping is installed in accordance with Specification section XX.XXXX,
Part 2.XX and/or Part3.XX [include any specific or unique requirements].
All piping is supported with/by [include specific requirements] per Specification section XX.XXXX.
All valves, pressure/temperature ports, and other piping components are
installed per Detail X/XXX and Specification section XX.XXXX, Part
2.XX and/or Part 3.XX.
Pressure gages and thermometers are installed per Specification section
XX Part 2.XX and Detail X/XXX.
Piping fittings and components are installed with extensions and clearances in hangers for insulation per Specification section XX.XXXX and/or
Detail X/XXX.
Valves are tagged.
Piping is clean (i.e., no gravel, sand, or debris inside piping).
Piping is insulated per Specification section XX Part 2.XX.
Clearances for access panels are maintained.
Dielectric fittings are installed to isolate dissimilar pipe materials.
Condensate drain is installed per Detail X/XXX.
Condensate drain piping is pitched and installed per manufacturer’s installation manual [page XX].
Condensate drain piping is marked with [specific marking requirements]
per Specification section XX.XXXX.
Condensate drain piping is insulated per Specification section XX Part
Piping does not obstruct access and maintenance clearances and allows
for unit removal.
All piping is installed in accordance with Specification section XX.XXXX,
Part 2.XX and/or Part 3.XX [include any specific or unique requirements].
All valves and other piping components are installed per Detail X/XXX
and Specification section XX.XXXX Part 2.XX and/or Part 3.XX.
Valves are tagged.
Interior of unit has been cleaned and is free of dust and debris.
Clean filters are in place and are secure in housing, and are the proper
MERV rating as specified.
Fans rotate per the direction arrows shown on the unit.
Unit operates without excessive noise or vibration.
Refrigerant circuits are fully charged with refrigerant and oil.
Fans and motors are lubricated and aligned.
Fan belts have the recommended tension, are in good condition, and
show no signs of fracture [if applicable, delete if direct drive fan].
Protective shrouds for fans and belts are in place and secure.
Condensate flows freely and discharges at an approved location.
Unit and connections are free of apparent large air leaks.
Manufacturer’s checklist has been completed.
Start-up is performed by the manufacturer’s approved representative [if
Filters and coils are clean.
Fan RPMs are within manufacturer’s rating.
Supply Fan motor BHP [kW] does not exceed nameplate HP [kW].
Exhaust Fan motor BHP [kW] does not exceed nameplate HP [kW].
Outdoor airflow (cfm [L/s]) is [insert {cfm (L/s)} per manufacturer’s submittal and scheduled value in Drawing XXX.
Exhaust airflow (cfm [L/s]) is [insert {cfm (L/s)} per manufacturer’s
submittal and scheduled value in Drawing XXX.
Water flow (GPM [L/s]) is [insert {cfm (L/s)} per manufacturer’s submittal
and scheduled value in Drawing XXX.
MCA of unit is less than scheduled when all compressors and fans are
All actuators, sensors, and control devices are installed in the location
required by the construction documents.
The installed actuators, sensors, and control devices match the requirements in the construction document for each location.
All actuators, sensors, and control devices are accessible for service,
adjustment, calibration and repair.
All dampers are in the position indicated by the actuator stroke position
Damper linkages are adjusted with no free play.
All damper actuators in critical applications (such as outdoor air and
exhaust air dampers) are spring return (not fail-last-position) for freeze
Control sensors are field calibrated per Specification section XX.XXXX.
Control actuators and sensors are labeled [include specific requirements]
per Specification section XX.XXXX.
Control wiring is grounded at [location in submittal detail].
Power is provided to each controller, and status light(s) are lit.
Wires and pneumatic tubing is labeled at each end are per Specification
section XX.XXXX.
Control relays and transducers are labeled per Specification section
Controllers are labeled per Specification section XX.XXXX.
Wiring is neat and easily traceable.
Controls cabinets are clean and free from installation debris.
Controls cabinets have controls drawings in them.
Wiring terminals have insulation stripped to appropriate length, no chance
of electrical shorting or pinching of insulation.
Filters are clean.
Components are clean. If not, check the following:
Filters are in place upstream of components, are clean and not damaged.
The filters fit tightly into the filter rack without allowing excessive bypass
air. If the filters do not cover the entire cross section, adjustments must
be made. Filters could be incorrect sizes, blank-offs or gasketing are
missing, or the filter racks need repairs or adjustments.
The upstream filters have adequate filtration efficiency.
The fans rotate in the correct direction. Belts are in good shape (if present).
There is no standing water in the unit during cooling season. If there is, check
the following:
The filters are clean.
The condensate trap is present and has adequate height to pull out condensate.
The amount of negative pressure in the DOAS cooling coil section is as
designed and not overly negative at full airflow.
The temperature sensors are accurate and calibrated. This might be easiest to
check when the unit is under stable conditions with no coil or component
adding any heating or cooling.
All airflow measurement devices are clean and calibrated.
All pressure sensors are clean and calibrated.
All humidity sensors have been calibrated within the last year.
All CO2 sensors have been calibrated within the last 6 months.
Any air-to-air energy recovery device performs close to the designed
The control sequences match what was installed (or there is a known reason
for the change). Use forms developed by the commissioning provider.
The schedules match the occupancy of the building. Because this is a DOAS,
the unit does not typically need to provide outdoor air when the building is
The cooling coil delivers sufficient cooling/dehumidifying as expected.
All compressor stages are operational and not in fault (if applicable).
The heating coil delivers sufficient heating as expected.
All dampers fully open and close tightly.
The DOAS unit does not have excessive air leakage.
The supply air temperature is stable and as expected.
The static pressure is as expected (if applicable).
The freeze stat is functional (check with ice on freeze stat if possible).
AHRI. 2013. AHRI Standard 1060. Standard for performance rating of air-to-air
exchangers for energy recovery ventilation equipment. Arlington, VA: AirConditioning, Heating, and Refrigeration Institute.
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outdoor air system units. Arlington, VA: Air-Conditioning, Heating, and
Refrigeration Institute.
Ahmed, R., and J. Appelhoff. 2013. Frost-protection measures in energy recuperation with multiple counterflow heat exchangers. REHVA Journal: October.
ASHRAE. 2013a. ASHRAE handbook—Fundamentals. Atlanta: ASHRAE.
ASHRAE. 2013b. ASHRAE Standard 84. Method of testing air-to-air heat/
energy exchangers. Atlanta: ASHRAE
ASHRAE. 2014. Standard 90.1-2013 user’s manual. Atlanta: ASHRAE.
ASHRAE. 2015a. ASHRAE handbook—HVAC applications. Atlanta: ASHRAE
ASHRAE. 2015b. Guideline 13. Specifying building automation systems.
Atlanta: ASHRAE.
ASHRAE. 2016a. ANSI/ASHRAE Standard 62.1-2016. Ventilation for acceptable indoor air quality. Atlanta: ASHRAE.
ASHRAE. 2016b. ANSI/ASHRAE/IES Standard 90.1-2016. Energy standard
for buildings except low-rise residential buildings. Atlanta: ASHRAE.
ASHRAE. 2016c ASHRAE handbook—HVAC systems and equipment. Atlanta:
ASHRAE. 2016d. Standard 62.1 user's manual. Atlanta: ASHRAE.
Crocker, S., and P. Smith. 2013. Service clinic: Servicing desiccant system
enthalpy wheels. ContractingBusiness.com. http://contractingbusiness.com/
Crowther, H., and Y. Ma. 2016. Design considerations for dedicated OA aystems. ASHRAE Journal: March.
Fisk, W.J. 2000. Health and productivity gains from better indoor environments
and their relationship with building energy efficiency. Annual Review of Energy
and the Environment 25:537.
Harriman, L.G., D. Plager, and D. Kosar. 1997. Dehumidification and cooling
loads from ventilation air. ASHRAE Journal: November.
Harriman, L.G., J. Lstiburek, and R. Kittler. 2000. Improving humidity control for
commercial buildings. ASHRAE Journal: November.
Harriman, L.G., G. Brundrett, and R. Kittler. 2001. Humidity control design guide
for commercial and institutional buildings. Atlanta: ASHRAE.
Harriman, L.G., and J.W. Lstiburek. 2009. The ASHRAE guide for buildings in hot
and humid climates. Atlanta: ASHRAE.
ICC. 2012. International energy conservation code. Washington, D.C.: International Code Council.
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Jeong J., and S. Mumma. 2007. Binary enthalpy wheel humidification control in
dedicated outdoor air systems. ASHRAE Transactions 113(2).
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Standard 62-1989. ASHRAE Journal: March.
Kumar, S., and W.J. Fisk. 2002. IEQ and the impact on employee sick leave.
ASHRAE Journal: July.
Mumma, S. 2001. Dedicated outdoor air-dual wheel system control requirements.
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Other Titles from ASHRAE
The ASHRAE Guide for Buildings in Hot and
Humid Climates provides a summary of building science, moisture management, and techniques for reducing energy consumption in hot
and humid climates, all based on real-world
field experience as well as on recent ASHRAE
Humidity Control Design Guide for Commercial and Institutional Buildings provides the
HVAC designer with more than 500 pages of
complete coverage of humidity control from
basic principles to real-world design advice, and
is organized in a logical, easy-to-follow layout.
Procedures for Commercial Building Energy
Audits is a full-color guide that contains upto-date application and operational information for energy audits. The second edition
provides information on what to expect from
an audit, defines three levels of audit effort,
and includes more than 25 customizable audit
guideline forms.
ASHRAE Design Guide for Tall, Supertall, and
Megatall Building Systems is a unique reference for all specialists and owners using and
designing systems for buildings taller than 300
ft [91m] with a broadened scope and updated
content that reflects current standards and
industry practices.
Complete, Up-to-Date DOAS Guidance
Guided by the information in this book, HVAC system designers will be
able to optimally incorporate DOASs into their projects. Architectural
designers, building developers and owners, maintenance professionals,
students, teachers, and researchers may also find the contents useful.
Featuring practical checklists, full-color graphics and psychrometric
charts, and common tips and traps for designers, ASHRAE Design
Guide for Dedicated Outdoor Air Systems is an indispensable guide
for the working HVAC professional with interest in DOASs.
ISBN 978-1-939200-71-6 (ppbk)
ISBN 978-1-939200-72-3 (PDF)
9 781939 200716
Product code: 90304
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Atlanta, GA 30329-2305
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ASHRAE_DOAS_Cover Spread.indd 1
Dedicated Outdoor Air Systems
Experienced DOAS designers will find this guide helpful in dealing
with deviations from the norm, while HVAC designers without DOAS
experience will find a complete guide to implementing a DOAS. The
guide can be read front to back or in parts, depending on the needs
of the designer.
Dedicated outdoor air systems (DOASs) provide HVAC designers
with opportunities for advantages in simplicity, efficiency, and economy.
This book represents the most complete and up-to-date guidance on
the design, installation, and operation and management of DOASs in
nonresidential applications.
Dedicated Outdoor
Air Systems
operation and maintenance
5/18/2017 10:59:44 AM