Building HVAC Requirements – Air Distribution System Ducts, Plenums, Fans,and Filters
Table 4-10: Return Duct Sizing for Single Return Duct Systems
System Nominal Minimum Return Duct Minimum Total Return Filter Grille
Cooling Capacity (Ton) Diameter (inch) Gross Area
(Inch2)
1.5 16 500
2.0 18 600
2.5 20 800
From Table 150-B of the Energy Standards
Table 4-11: Return Duct Sizing for Multiple Return Duct Systems
Two Returns
System Nominal Return Duct 1 Return Duct 2 Minimum Total Return
Cooling Capacity Minimum Diameter Minimum Diameter Filter Grille Gross
(Ton) (inch) (inch) Area (inch2.)
1.5 12 10 500
2.0 14 12 600
2.5 14 14 800
3.0 16 14 900
3.5 16 16 1000
4.0 18 18 1200
5.0 20 20 1500
From Table 150-C of the Energy Standards
4.2.1.1 Zonally Controlled Central Forced-Air Cooling Systems
The primary purpose of zoning ducted air conditioners, heat pumps, and furnaces is
to improve comfort. Increased comfort is attained by having the capacity of the
HVAC system (cooling or heating delivered) follow the shift in load as it changes
across the house. For example, it is common for two-story homes to be too hot on
the second floor in summer and winter. Zoning has the capability of diverting more of
the HVAC capacity to the area with the increased load. Another common example is
a home with a significant area of west-facing and east-facing windows. In the
summer, the east rooms overheat in the morning, and the west rooms overheat in
the afternoon.
Providing the most agreeable temperature to all the zones is comfortable, but it
carries with it the possibility of increased energy consumption. Since the most
common home is single-zoned and has only one thermostat placed near the center
of the house, temperatures in the rooms distant from that thermostat will vary,
sometimes significantly. If zoning is added, the more distant rooms may be
conditioned to a more comfortable temperature. This increased conditioning requires
more energy. When designed correctly, zoning allows only the zones that need
conditioning to be conditioned, thus potentially saving energy.
It is common for single-speed zonally controlled central forced-air cooling systems to
produce lower total system airflow through the returns when fewer than all zones are
,Building HVAC Requirements – Air Distribution System Ducts, Plenums, Fans,and Filters
calling for conditioning. The reduced airflow lowers the sensible efficiency of single-
stage heating or cooling equipment. Two primary causes of lower airflow in multiple
zone dampered systems are:
1. Restriction of some system supply ducts by closing zoning dampers in zones
that do not need additional cooling, while other zones do need cooling.
2. Recirculation of already-cooled air from the supply plenum directly back to the
return plenum without first delivering the cooled air to the conditioned space
by use of a bypass duct.
To prevent the lower efficiency that results from reduced system airflow or from
recirculated bypass duct airflow, single-speed compressor zonally controlled central
cooling systems must demonstrate they simultaneously meet mandatory fan efficacy
and airflow requirements in all zonal control modes, which is possible only with a
superior duct system design that does not restrict the system total airflow when
fewer than all zones are calling for conditioning, and does not use a bypass duct.
Section 150.1(c)13 prohibits use of bypass ducts prescriptively, but bypass ducts
may be used if the efficiency penalty due to the reduced airflow through the return
grille is modeled as described in Section 4.4.1.19 below.
Multispeed or variable-speed compressor-type zonally controlled cooling systems
are not required to verify mandatory fan efficacy and airflow requirements in all zonal
control modes; however, these systems must be HERS-verified to confirm they meet
the mandatory fan efficacy and airflow requirements with the compressor on high
speed and all zones calling for cooling.
4.2.1.2 Zonally Controlled Cooling Systems – Airflow and Fan Efficacy
Requirements
Recent studies have shown that zonally controlled cooling systems with or without
bypass dampers (multiple zones served by a single air handler with motorized zone
dampers), usually do not meet the airflow and fan efficacy (AF/FE) requirements
when fewer than all zones are calling. The energy penalty that results from this is
greater than the benefit of having zonal control; therefore, zonal control is no longer
simply assumed to be a “better-than-minimum” condition, and there are special
compliance requirements for these systems.
Zonal control accomplished by using multiple single-zone systems is not subject to
the requirements specified in Energy Standards Section 150.0(m)13C.
Two-speed and variable-speed compressors are considered multi-speed. Multispeed
compressors allow the system capacity to vary to more closely match reduced
cooling loads when fewer than all zones call for cooling. Therefore, an exception to
Section 150.0(m)13C gives multispeed compressor systems special consideration
when used in zoned systems and these systems are not required to verify
performance in all zonal control modes. Instead, the airflow and fan efficacy testing
is required to be performed only at the highest speed when all zones call for cooling.
Zoned systems with single-speed compressors must be tested and pass in all
operating modes.
,Building HVAC Requirements – Air Distribution System Ducts, Plenums, Fans,and Filters
An exception to Section 150.0(m)13C allows single-speed compressor systems to
comply with HERS verification of the mandatory AF/FE requirements only at the
highest fan speed when all zones call for cooling. The exception applies provided the
system also uses the performance compliance approach and complies with HERS
verification of the requirements for AF/FE in all zonal control modes specified by the
software user input for minimum airflow rate when fewer than all zones call for
cooling. Single-speed compressor systems, with or without bypass dampers, are
less likely to meet the mandatory AF/FE requirements in Section 150.0(m)13C with
fewer than all zones calling for cooling. Therefore, the performance compliance
software calculates a penalty for the reduced airflow (specified by the user) during
operation when fewer than all zones call for cooling. Other energy features for the
building must offset this penalty for reduced airflow when fewer than all zones call for
cooling. In the performance compliance software, if the system is modeled as a
zoned system with a single-speed compressor, the minimum allowable airflow drops
to 150 CFM/ton. But because the standard house is assumed to have an airflow of
350 CFM/ton, there is a penalty imposed on the compliance calculation unless the
designer specifies a value of 350 or higher. Entering a value between 150 and 350
can lessen the penalty resulting from the minimum allowed value of 150 CFM/ton.
It is extremely important that the energy consultant model airflow and fan efficacy
values that are reasonable and can be verified by a HERS Rater; otherwise, the
system will fail HERS verification, and the compliance calculations will have to be
revised to specify user input equivalent to the actual values that could pass HERS
verification. Energy consultants should coordinate with the HVAC designer before
registering the certificate of compliance.
Bypass dampers may be installed only if the certificate of compliance specifically
states that the system was modeled as having a bypass damper.
Example:
1. A home is to be built with a heat pump connected to a zoned system (two
zones) with a single-speed compressor and bypass ducts. From experience,
the HVAC contractor knows that it will not be possible to meet the 350 CFM/ton
requirement, but 275 CFM/ton is likely.
2. The energy consultant models the system in the proposed house with 275
CFM/ton and 0.45 W/CFM (value for a gas furnace). Because the standard
house assumes 350 CFM/ton, there is an energy penalty that must be made up
by including other better-than-standard features in the performance compliance
input, but the penalty is not as large as it would be at a value of 150 CFM/ton.
3. 275 CFM/ton must be tested in all control modes.
4. The home is built, and the system is verified by a rater and passes at 287
CFM/ton with one zone calling, 298 CFM/ton with the other zone calling, and
372 CFM/ton with both zones calling. The system is also measured with all
zones calling to confirm it meets or exceeds 350 CFM/ton.
, Building HVAC Requirements – Air Distribution System Ducts, Plenums, Fans,and Filters
5. If this same home was to be built with a multispeed compressor, it would be
tested only with all zones calling, but the target airflow would be no less than
the mandatory 350 CFM/ton. Compliance credit can be achieved by modeling
airflows greater than the mandatory CFM/ton and/or fan efficacies less than the
mandatory watts/CFM.
Table 4-12: Single-Zone Ducted Central Forced-Air Cooling Systems
Single-Zone Ducted Cooling Systems (Single Zone Off a Single Air Handler)
Compressor Mandatory Requirements for Airflow Performance Performance
Type and Fan Efficacy Compliance Compliance
Option Option
Proposed System Modeled Airflow
Defaults and Fan Efficacy
Single-Speed, Airflow: Airflow: Airflow:
Multispeed, or ≥ 350 CFM/ton if not a small duct high
Variable-Speed: 350 CFM/ton ≥350 CFM/ton
velocity type (non-SDHV) (non-SDHV)
≥ 250 CFM/ton if is a small duct high 250 CFM/ton ≥250 CFM/ton
Testing velocity (SDHV) type (SDHV) (SDHV)
Performed on Fan Efficacy:
Highest Speed and/or
only ≤ 0.45 W/CFM for gas furnaces (GF)
Fan Efficacy: Fan Efficacy:
≤ 0.58 W/CFM for air handlers that
are not gas furnaces (non-GF) 0.45 W/CFM ≤0.45 W/CFM
(GF) (GF)
≤ 0.62 W/CFM for SDHV type
0.58 W/CFM ≤0.58 W/CFM
Exception: Airflow and Fan Efficacy
(non-GF) (non GF)
HERS verification not required if return
system meets Tables 150.0-B or C. 0.62 W/CFM ≤0.62 W/CFM
However, HERS verification that return (SDHV) (SDHV)
duct installation meets Tables 150.0-B or
C is required
Table 4-13: Zonally Controlled Central Forced-Air Cooling Systems
Zoned Ducted Cooling Systems (Multiple Zones off a Single Air Handler)
Compressor Mandatory Requirements for Airflow Performance oned Ducted Cooling
Type and Fan Efficacy 1 Compliance 2 Systems (Multiple
Zones off a Single
Proposed System
Air Handler)
Defaults 3
Modeled Airflow and
Fan Efficacy
Airflow: Airflow: Airflow:
Single
≥ 350 CFM/ton (non-SDHV) 150 CFM/ton ≥ 150 CFM/ton
Speed
and/or
Fan Efficacy:
Fan Efficacy:
≤ 0.45 W/CFM (GF)
0.45 W/CFM Fan Efficacy:
≤ 0.58 W/CFM (non-GF) (GF)
Table 4-10: Return Duct Sizing for Single Return Duct Systems
System Nominal Minimum Return Duct Minimum Total Return Filter Grille
Cooling Capacity (Ton) Diameter (inch) Gross Area
(Inch2)
1.5 16 500
2.0 18 600
2.5 20 800
From Table 150-B of the Energy Standards
Table 4-11: Return Duct Sizing for Multiple Return Duct Systems
Two Returns
System Nominal Return Duct 1 Return Duct 2 Minimum Total Return
Cooling Capacity Minimum Diameter Minimum Diameter Filter Grille Gross
(Ton) (inch) (inch) Area (inch2.)
1.5 12 10 500
2.0 14 12 600
2.5 14 14 800
3.0 16 14 900
3.5 16 16 1000
4.0 18 18 1200
5.0 20 20 1500
From Table 150-C of the Energy Standards
4.2.1.1 Zonally Controlled Central Forced-Air Cooling Systems
The primary purpose of zoning ducted air conditioners, heat pumps, and furnaces is
to improve comfort. Increased comfort is attained by having the capacity of the
HVAC system (cooling or heating delivered) follow the shift in load as it changes
across the house. For example, it is common for two-story homes to be too hot on
the second floor in summer and winter. Zoning has the capability of diverting more of
the HVAC capacity to the area with the increased load. Another common example is
a home with a significant area of west-facing and east-facing windows. In the
summer, the east rooms overheat in the morning, and the west rooms overheat in
the afternoon.
Providing the most agreeable temperature to all the zones is comfortable, but it
carries with it the possibility of increased energy consumption. Since the most
common home is single-zoned and has only one thermostat placed near the center
of the house, temperatures in the rooms distant from that thermostat will vary,
sometimes significantly. If zoning is added, the more distant rooms may be
conditioned to a more comfortable temperature. This increased conditioning requires
more energy. When designed correctly, zoning allows only the zones that need
conditioning to be conditioned, thus potentially saving energy.
It is common for single-speed zonally controlled central forced-air cooling systems to
produce lower total system airflow through the returns when fewer than all zones are
,Building HVAC Requirements – Air Distribution System Ducts, Plenums, Fans,and Filters
calling for conditioning. The reduced airflow lowers the sensible efficiency of single-
stage heating or cooling equipment. Two primary causes of lower airflow in multiple
zone dampered systems are:
1. Restriction of some system supply ducts by closing zoning dampers in zones
that do not need additional cooling, while other zones do need cooling.
2. Recirculation of already-cooled air from the supply plenum directly back to the
return plenum without first delivering the cooled air to the conditioned space
by use of a bypass duct.
To prevent the lower efficiency that results from reduced system airflow or from
recirculated bypass duct airflow, single-speed compressor zonally controlled central
cooling systems must demonstrate they simultaneously meet mandatory fan efficacy
and airflow requirements in all zonal control modes, which is possible only with a
superior duct system design that does not restrict the system total airflow when
fewer than all zones are calling for conditioning, and does not use a bypass duct.
Section 150.1(c)13 prohibits use of bypass ducts prescriptively, but bypass ducts
may be used if the efficiency penalty due to the reduced airflow through the return
grille is modeled as described in Section 4.4.1.19 below.
Multispeed or variable-speed compressor-type zonally controlled cooling systems
are not required to verify mandatory fan efficacy and airflow requirements in all zonal
control modes; however, these systems must be HERS-verified to confirm they meet
the mandatory fan efficacy and airflow requirements with the compressor on high
speed and all zones calling for cooling.
4.2.1.2 Zonally Controlled Cooling Systems – Airflow and Fan Efficacy
Requirements
Recent studies have shown that zonally controlled cooling systems with or without
bypass dampers (multiple zones served by a single air handler with motorized zone
dampers), usually do not meet the airflow and fan efficacy (AF/FE) requirements
when fewer than all zones are calling. The energy penalty that results from this is
greater than the benefit of having zonal control; therefore, zonal control is no longer
simply assumed to be a “better-than-minimum” condition, and there are special
compliance requirements for these systems.
Zonal control accomplished by using multiple single-zone systems is not subject to
the requirements specified in Energy Standards Section 150.0(m)13C.
Two-speed and variable-speed compressors are considered multi-speed. Multispeed
compressors allow the system capacity to vary to more closely match reduced
cooling loads when fewer than all zones call for cooling. Therefore, an exception to
Section 150.0(m)13C gives multispeed compressor systems special consideration
when used in zoned systems and these systems are not required to verify
performance in all zonal control modes. Instead, the airflow and fan efficacy testing
is required to be performed only at the highest speed when all zones call for cooling.
Zoned systems with single-speed compressors must be tested and pass in all
operating modes.
,Building HVAC Requirements – Air Distribution System Ducts, Plenums, Fans,and Filters
An exception to Section 150.0(m)13C allows single-speed compressor systems to
comply with HERS verification of the mandatory AF/FE requirements only at the
highest fan speed when all zones call for cooling. The exception applies provided the
system also uses the performance compliance approach and complies with HERS
verification of the requirements for AF/FE in all zonal control modes specified by the
software user input for minimum airflow rate when fewer than all zones call for
cooling. Single-speed compressor systems, with or without bypass dampers, are
less likely to meet the mandatory AF/FE requirements in Section 150.0(m)13C with
fewer than all zones calling for cooling. Therefore, the performance compliance
software calculates a penalty for the reduced airflow (specified by the user) during
operation when fewer than all zones call for cooling. Other energy features for the
building must offset this penalty for reduced airflow when fewer than all zones call for
cooling. In the performance compliance software, if the system is modeled as a
zoned system with a single-speed compressor, the minimum allowable airflow drops
to 150 CFM/ton. But because the standard house is assumed to have an airflow of
350 CFM/ton, there is a penalty imposed on the compliance calculation unless the
designer specifies a value of 350 or higher. Entering a value between 150 and 350
can lessen the penalty resulting from the minimum allowed value of 150 CFM/ton.
It is extremely important that the energy consultant model airflow and fan efficacy
values that are reasonable and can be verified by a HERS Rater; otherwise, the
system will fail HERS verification, and the compliance calculations will have to be
revised to specify user input equivalent to the actual values that could pass HERS
verification. Energy consultants should coordinate with the HVAC designer before
registering the certificate of compliance.
Bypass dampers may be installed only if the certificate of compliance specifically
states that the system was modeled as having a bypass damper.
Example:
1. A home is to be built with a heat pump connected to a zoned system (two
zones) with a single-speed compressor and bypass ducts. From experience,
the HVAC contractor knows that it will not be possible to meet the 350 CFM/ton
requirement, but 275 CFM/ton is likely.
2. The energy consultant models the system in the proposed house with 275
CFM/ton and 0.45 W/CFM (value for a gas furnace). Because the standard
house assumes 350 CFM/ton, there is an energy penalty that must be made up
by including other better-than-standard features in the performance compliance
input, but the penalty is not as large as it would be at a value of 150 CFM/ton.
3. 275 CFM/ton must be tested in all control modes.
4. The home is built, and the system is verified by a rater and passes at 287
CFM/ton with one zone calling, 298 CFM/ton with the other zone calling, and
372 CFM/ton with both zones calling. The system is also measured with all
zones calling to confirm it meets or exceeds 350 CFM/ton.
, Building HVAC Requirements – Air Distribution System Ducts, Plenums, Fans,and Filters
5. If this same home was to be built with a multispeed compressor, it would be
tested only with all zones calling, but the target airflow would be no less than
the mandatory 350 CFM/ton. Compliance credit can be achieved by modeling
airflows greater than the mandatory CFM/ton and/or fan efficacies less than the
mandatory watts/CFM.
Table 4-12: Single-Zone Ducted Central Forced-Air Cooling Systems
Single-Zone Ducted Cooling Systems (Single Zone Off a Single Air Handler)
Compressor Mandatory Requirements for Airflow Performance Performance
Type and Fan Efficacy Compliance Compliance
Option Option
Proposed System Modeled Airflow
Defaults and Fan Efficacy
Single-Speed, Airflow: Airflow: Airflow:
Multispeed, or ≥ 350 CFM/ton if not a small duct high
Variable-Speed: 350 CFM/ton ≥350 CFM/ton
velocity type (non-SDHV) (non-SDHV)
≥ 250 CFM/ton if is a small duct high 250 CFM/ton ≥250 CFM/ton
Testing velocity (SDHV) type (SDHV) (SDHV)
Performed on Fan Efficacy:
Highest Speed and/or
only ≤ 0.45 W/CFM for gas furnaces (GF)
Fan Efficacy: Fan Efficacy:
≤ 0.58 W/CFM for air handlers that
are not gas furnaces (non-GF) 0.45 W/CFM ≤0.45 W/CFM
(GF) (GF)
≤ 0.62 W/CFM for SDHV type
0.58 W/CFM ≤0.58 W/CFM
Exception: Airflow and Fan Efficacy
(non-GF) (non GF)
HERS verification not required if return
system meets Tables 150.0-B or C. 0.62 W/CFM ≤0.62 W/CFM
However, HERS verification that return (SDHV) (SDHV)
duct installation meets Tables 150.0-B or
C is required
Table 4-13: Zonally Controlled Central Forced-Air Cooling Systems
Zoned Ducted Cooling Systems (Multiple Zones off a Single Air Handler)
Compressor Mandatory Requirements for Airflow Performance oned Ducted Cooling
Type and Fan Efficacy 1 Compliance 2 Systems (Multiple
Zones off a Single
Proposed System
Air Handler)
Defaults 3
Modeled Airflow and
Fan Efficacy
Airflow: Airflow: Airflow:
Single
≥ 350 CFM/ton (non-SDHV) 150 CFM/ton ≥ 150 CFM/ton
Speed
and/or
Fan Efficacy:
Fan Efficacy:
≤ 0.45 W/CFM (GF)
0.45 W/CFM Fan Efficacy:
≤ 0.58 W/CFM (non-GF) (GF)
