User's Manual

Zoning System Design Manual
70-2321—03 6
Size the Bypass Damper
Install between supply and return.
All zoning systems should be installed with a supply-to-return bypass damper. A Static Pressure
Regulating Damper (SPRD) is used to bypass excess airflow from the supply duct into the return duct.
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Supply duct systems should be sized to handle
the total airflow when all zones are open, and use
a SPRD bypass capable of handling the excess air
when only the smallest zone is calling. Excess air
should be bypassed into the return air duct. If the
space is too confined to install a bypass back to the
return, excess air can also be dumped into a hall-
way, false ceiling, or basement.
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 by-pass 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
climates. Bypassing air from the supply duct into the return duct accelerates the dehumidification ability of the
equipment, creating greater comfort for the homeowner.
Table 1. SPRD Dimensions.
Round SPRD Rectangular SPRD
Model Number By-Pass Model Number By-Pass
SPRD7 300 CFM SPRD12X8 1000 CFM
SPRD8 400 CFM SPRD12X10 1200 CFM
SPRD9 600 CFM SPRD12X12 1400 CFM
SPRD10 750 CFM SPRD20X8 1600 CFM
SPRD12 1200 CFM SPRD20X10 2000 CFM
SPRD14 1800 CFM SPRD20X12 3000 CFM
SPRD16 2400 CFM
ByPass damPers and dats