Reference Guide

Part No. RG-0508-001 7 The Braeburn Zoning Reference Guide
Install a barometric bypass damper in the bypass duct from the supply plenum to 2.
the return air plenum. Size the bypass duct by using a formula that subtracts the
smallest zone CFM from the total system CFM to determine the bypass CFM. As
an example with a total system CFM of 1200 and a smallest zone CFM of 700,
the bypass needs to be 1200-700=500 CFM.
Install an electric bypass damper in the bypass duct from the supply plenum to 3.
the return air plenum. Sizing an electric bypass damper using the same sizing
formula as the barometric bypass damper.
Eliminate the Bypass by Oversizing Each Branch
This option is usually used on two zone systems that are about the same size and HVAC
load. It can be acceptable to oversize the ductwork in each zone so that when only one
of the two zones is open, all the air produced by the indoor blower is applied to the open
zone. Many installers have been successful by sizing each zone to handle about 75% of the
system CFM. This duct design has advantages and disadvantages. One advantage is that
no bypass duct or damper is required; and the system’s operating temperature and pressure
should be normal. One disadvantage is lower duct pressure when both zones are calling.
This means that the system’s ability to throw the air to the outside walls may be diminished.
Carefully calculating the face velocity in FPM at the diffusers can help.
Install a Barometric Bypass
This low cost option uses a short duct between the
supply plenum and the return air plenum. The Braeburn
barometric bypass is installed in this duct. The purpose
of the bypass duct is to allow the excess pressure from
the supply plenum to escape into the return air plenum.
Excess pressure is routed to the return air plenum by
adjusting the bypass damper. The bypass damper is
adjusted so that when all zones are open the bypass
damper is lightly closed. As supply dampers close and pressure builds in the supply air
plenum, the pressure pushes the bypass damper open. A simple, adjustable weight is used
to properly position the damper.
Adjusting a barometric bypass damper is simple and requires no instruments.
Install the bypass damper with the arrow pointed from the supply side to the 1.
return side.
Ensure the damper blade pivot shaft is horizontal to the ground and the large part 2.
of the damper blade is down.