White Rodgers CAZ-2 Zone Plus Control Panel Design Guide

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ZONING

DESIGN

Zoning any forced air system is easy once you know a few of the ba-

sic rules. The main consideration is to maintain a constant amount of 

air flow (CFM) through the HVAC Unit. This needs to occur when only 

one zone is open and if the zones are of varying size when the small-

est zone is open. The other consideration is not to oversize a duct 

system too much in order to maintain adequate velocity and airflow 

when all zones are open and may be calling for conditioning on those 

design temperature days.

The design of the duct system for today’s zoning is an important 

factor to a comfortable and efficient zoning system. The number of 

zones, along with their size, often determine the best type of design.     

There are scenarios for zoning. The first, which is typically on new 

installations where the duct work can be designed for zoning would 

be to oversize the ducts for each zone in order to get more air to the 

zone when it may be the only one calling. The scenario for all others 

would be to use a by-pass damper to relieve the excess air pressure 

in the duct system when a minority number of zones are calling. 

The reason for each zone duct being the same size is that any zone 

could be the only zone calling and therefore that zone must handle 

the CFM of the HVAC Unit. When the duct is sized for 2/3 of the total 

CFM the smaller size does restrict the airflow and forces the air at 

a higher pressure and velocity, however it does not increase the air 

typically over that static pressure rating of the blower motor, usually 

0.5”W.C. This also keeps the air velocity from being noticeably noisy.  

Below is a quick guide to determine the minimum equivalent size of a 

zone duct for each size HVAC Unit.

Systems over 5 Tons typically are commercial and would use a by-

pass damper to relieve the excess air pressure when the majority of 

zones shut down.

In retrofit and systems with 4 zones or more, over sizing the ducts is 

not practical. In these instances a by-pass damper is used to relieve 

the excess air back into the return air duct or dump the air into a 

central area of the building, such as a hallway, where often there is 

a common return. In this instance try to locate the by-pass air as far 

away from the return air intake as possible.

In new installations where ducts are being added it is recommended 

to size each zone duct the same and to size the duct for approximate-

ly 2/3 of the total HVAC System CFM. This is practical on systems 

with 2 or 3 zones and when all zones are approximately equal in size. 

This is NOT practical in an installation where 80% of conditioned area 

is one zone and 20% is the other zone. 

The key to a good zoning system is to deliver the conditioned air to 

the calling zone as fast and quietly as possible in order to satisfy the 

demand. Whatever air cannot be directed into the zone must then by 

by-passed. This develops the formula for calculating the size of the 

by-pass damper.  

otal CFM  -  Smallest Zone CFM  = By-Pass CFM

Once the amount of by-pass air is known it is just common sense to 

size a duct adequately to handle the amount of air. White-Rodgers 

has both round and rectangular/square by-pass damper sizes.