NJ GREEN BUILDING MANUAL
NEW COMMERCIAL UPDATED 1-MAY -11
Properly Sized HVAC Equipment
What is Properly Sized HVAC Equipment?
Properly sized (or right-sized) HVAC
equipment provides the necessary amount of
heating and cooling for a building. Heating,
ventilating, air-conditioning, and controls
(HVAC) systems account for nearly 40% of the
energy used in commercial buildings in the
United States. 1
Oversized equipment results in higher
installation and capital costs, reduced operating
efficiency, and potential negative impacts on
comfort. In humid climates, “short-cycling” of
air conditioning equipment can mean poor
humidity control – stopping and starting the
equipment from excessive cycling wears on
equipment. Oversized systems use more fan
power for their blowers and impact peak
demand in summer in hot climates.2
Figure 1 – Basic HVAC System (Source: Center
for Biosecurity. Copyright © 2009 UPMC. All
rights reserved. http://www.upmcbiosecurity.org/)
Oversizing can occur for a variety of reasons.
Contractors may use standard rules of thumb
based on their experience, for example a set amount of air conditioning based on square
footage, without adequately analyzing unique situations, 3 and often customers may
simply request oversized units with a bigger is better view without understanding the
calculations that go into sizing equipment. 4
How to Incorporate Properly Sized HVAC Equipment
Right-sizing equipment is the result of proper planning in new buildings – performing the
appropriate calculations to determine a reasonably sized system rather than working off
1
Whole Building Design Guide. http://www.wbdg.org/resources/env_hvac_integration.php (accessed May
25, 2010).
2
US DOE. Right-Size Heating and Cooling Equipment.
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/31318.pdf (accessed May 25,
2010).
3
Andrews, Steve. 2003. HVAC Equipment Sizing Calcs. Steve Andrews. Home Builder. August 2003
4
US DOE. Right-Size Heating and Cooling Equipment.
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/31318.pdf (accessed May 25,
2010).
NJ GREEN BUILDING MANUAL
NEW COMMERCIAL UPDATED 1-MAY -11
of assumptions, and incorporating the HVAC system into the overall design of the
building from the beginning.
Utilizing researched standards for annual extreme temperatures is important, as
overestimating will lead to oversizing. 5 The American Society of Heating, Refrigerating
and Air-Conditioning Engineers (ASHRAE) handbook of fundamentals provides
information about standards. 6 Programmable thermostats can maintain constant
temperatures rather than forcing equipment to rapidly cool down or warm up interiors.
This affects sizing and energy usage. 7 Variable speed air handlers slowly build up the
speed of blowers on hot humid days to save energy and improve dehumidification. 8Air
leakage and heat loss from ducts reduces efficiency. Duct leakage means a loss of
conditioned air and depressurization of a building. Placing ducts in the insulated building
envelope reduces the energy it takes to condition air to reach a desired temperature
Further, shading devices should also be considered – it can be assumed, for example, that
tenants will pull down blinds during hot, sunny days. External shading should also be
factored in, such as overhangs or adjacent buildings. 9
When planning for HVAC equipment, there are several additional considerations that will
help avoid oversizing equipment. For example:
o Accept HVAC safety factors and pick-up load allowance outlined by ASHRAE as
an upper limit
o Apply safety factors to a baseline that was created using reasonable assumptions.
o Use computerized analysis tools, such as DOE 2.1E, to reduce uncertainty and
reduce oversizing.10
Benefits
o Contributes to energy, emissions and cost savings
o Provides improved occupant comfort and indoor environmental quality
5
US DOE. Right-Size Heating and Cooling Equipment.
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/31318.pdf (accessed May 25,
2010).
6
Andrews, Steve. 2003. HVAC Equipment Sizing Calcs. Steve Andrews. Home Builder. August 2003
7
US DOE. Right-Size Heating and Cooling Equipment.
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/31318.pdf (accessed May 25,
2010).
8
US DOE. Right-Size Heating and Cooling Equipment.
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/31318.pdf (accessed May 25,
2010).
9
Andrews, Steve. 2003. HVAC Equipment Sizing Calcs. Steve Andrews. Home Builder. August 2003
10
WBDG. High-Performance HVAC. http://www.wbdg.org/resources/hvac.php (accessed January 2,
2011).
NJ GREEN BUILDING MANUAL
NEW COMMERCIAL UPDATED 1-MAY -11
Costs
Right-sizing HVAC equipment should not add cost to a project. Sound planning and
analysis about the building’s heating and cooling needs and the incorporation of the
strategies noted above should result in a properly sized HVAC system that serves the
needs of the building. Costs may be associated with purchasing analysis software and/or
conducting analysis.
Resources
US DOE Computer Program Listing for a range of HVAC measurements and
calculations
http://apps1.eere.energy.gov/buildings/tools_directory/subjects.cfm/pagename=subjects/p
agename_menu=materials_components/pagename_submenu=hvac_systems
The National Institute of Building Sciences Whole Building Design Guide maintains a
listing of Energy Analysis Tools
http://www.wbdg.org/resources/energyanalysis.php?r=hvac
The National Institute of Building Sciences Whole Building Design Guide maintains a
listing of relevant codes
http://www.wbdg.org/resources/hvac.php#rcas
The US Department of Energy’s Labs for the 21 st Century Program provides a guide on
right-sizing HVAC systems for energy intensive laboratory facilities
www.labs21century.gov/pdf/bp_rightsizing_508.pdf
Toolbase Services – Right-sizing Heating and Cooling Equipment
http://www.toolbase.org/Design-Construction-Guides/HVAC/right-size-hvac-equipment
Whole Building Design Guide - High-Performance HVAC
http://www.wbdg.org/resources/hvac.php