Reference Guide

The Braeburn Zoning Reference Guide 6 Part No. RG-0508-001
Understanding Bypass System Ductwork
In many zoning installations it is necessary to install a bypass damper. A bypass damper will
help the HVAC equipment maintain rated CFM past the heat exchangers and blower motor. If
a system does not have a bypass, and only a few dampers are open, excess supply plenum
pressure may be created. The result can be system limit trip-outs, excess noise, coil icing, or
other problems. The bypass damper is installed in the bypass system duct.
What is a Bypass System Duct?
It is a connection between the supply side and the return side air plenum. The connection
uses a bypass damper to open when needed. The bypass damper can either be electric, that
opens a damper with an electric motor when needed, or barometric (think barometer) that
opens up a weighted door when the system pressure increases.
Braeburn recommends using one of three methods to control excess supply plenum pressure.
The best method will be determined based on the speci c requirements of the application
such as cost, type of duct system used, actual duct design, and the ability to install a bypass
duct in tight equipment locations.
Three common methods are:
Eliminate the bypass by oversizing each branch of the duct system so that each 1.
branch is capable of  owing the system CFM.
return air
ducts
return air
plenum
bypass
duct
coil
supply air
sensor
zone dampers
electric or
barometric
bypass damper
filtration
system
furnace