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Bypass Dampers And Dats - Honeywell TrueZONE HZ311, HZ322, HZ432 Manual

Zoning system design manual
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Zoning System Design Manual

ByPass damPers and dats

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.
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 ..............................
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
Model Number By-Pass
300 CFM
400 CFM
600 CFM
750 CFM
1200 CFM SPRD20X10
1800 CFM SPRD20X12
2400 CFM
1600 CFM System

Rectangular SPRD
Model Number By-Pass
1000 CFM
1200 CFM
1400 CFM
1600 CFM
2000 CFM
3000 CFM

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