User Guide

Zoning System Design Manual
70-2321—06 6
Size the Pressure Relief Damper
Install between supply and return.
All zoning systems should be installed with a supply-to-return pressure relief damper. A Constant
Pressure Regulating Damper (CPRD) is used to bypass excess airflow from the supply duct into the
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return duct.
Supply duct systems should be sized to handle the total airflow
when all zones are open, and use a CPRD bypass capable of
handling the excess air when only the smallest zone is calling.
Excess air should be bypassed into the return air duct.
Determine the size of the bypass as follows:
CFM System
- CFM Smallest Zone
= CFM Bypass
For example, on a 4-ton system ....................................... 1600 CFM System
with the smallest zone designed to handle ................ - 400 CFM Smallest Zone
the bypass must be able to handle ................................. = 1200 CFM of airflow in bypass
When few zones are calling, static pressure in the system increases, and air is bypassed from supply to return. This
ensures that the throw of air at the register is correct whether one zone is calling or all zones are calling.
This method of duct design is also a great way to control excess humidity in the home, especially in southern cli-
mates. Bypassing air from the supply duct into the return duct accelerates the dehumidification ability of the equip-
ment, creating greater comfort for the homeowner.
Alternative Pressure Relief Methods
If pressure relief dampers are not allowed in your state/region, or if the space is too confined to install a bypass
back to the return, use one of the below methods for reliving the pressure-
1 If the system has 23 zones that are similar in size, you may be able to get by without any pressure relief damper.
If the equipment has a high-speed/low-speed fan terminal (typically labeled DS, BK, ODD, DHUM, or DEHUM)
wire this to the DS/BK on the zone panel. This lowers fan speed when a single zone is calling for cooling (or heat
with heat pump systems). Then test each zone individually to verify pressure is not too high.
2 If there is no space for bypass ductwork, the zone dampers can be set to allow some air to leak. By setting the
larger zone dampers to a minimum position, excess air pressure can be relieved even when only one zone is call-
ing. Minimal leakage to zones that are not calling for heat or cooling does not typically cause noticeable tem-
perature swings, but the homeowner should be advised that air will be coming from these registers even when
that zone is not calling.
3 Instead of running the pressure relief damper from the supply to the return ducting, you might be able to run the
duct from supply to a “alternate pressure relief area” in some houses. This would be an area of the house in which
overheating or cooling o the space is not a concern; for example a hallway, false ceiling, or basement.
Table 1. CPRD Sizes.
Round CPRD
Model Number Bypass
CPRD8 600 CFM
CPRD10 1000 CFM
CPRD12 1400 CFM
CPRD14 1700 CFM
Use duct collars (not supplied) when connecting
a CPRD round bypass to rectangular ducting.
PRESSURE RELIEF DAMPERS AND DATS