21
AIRFLOW
1
2
4
3
1. Damper Shaft
2. Lock Nut
3. Lever Arm
4. Counter Weight
BAROMETRIC BYPASS
DAMPER
BYPASS DAMPERS
Bypass dampers are used to provide constant air delivery
through the air handling unit. This is done by bypassing excess
air from the supply duct back to the return duct. As a zone is
satisfied, its zone damper closes. When this happens, the
bypass damper opens just enough to bypass the excess air.
This will control static pressure and noise at the diffusers.
Zonex Systems offers two types of bypass dampers, Barome-
tric and electronic. Each is available in round or rectangular
configuration. Barometric bypass dampers are limited to
systems of 5 tons. Electronic bypass dampers can be used on
any size system. For residential HVAC systems with variable
speed blowers, the barometric or electric bypass dampers can
be used. NOTE: When using the electronic bypass (STBP/ST-
CDBP), see the Bypass Dampers - Electronic Section, Pages
23-26; or contact Technical Support at 800-228-2966.
The barometric bypass damper is for systems
5 tons or under.
It utilizes a weighted damper blade to maintain constant duct
pressure. This allows for easy installation without the need for
electrical power or wiring. The round barometric damper can
be installed in any position. The RBB rectangular damper must
be installed with horizontal air flow only.
SIZING: When only the smallest zone is calling, the maximum
amount of excess supply air flow through the bypass damper.
To determine the proper size bypass damper to use, do the
following steps:
Step 1: Calculate bypass air volume as
follows:
A) Calculate total air volume at 400
CFM per ton.
B) Calculate air volume of smallest
zone in CFM.
C) Calculate bypass air volume by
subtracting the smallest zone air
volume from the total. (A - B = C)
If bypassing more than 2000 CFM, use the electronic bypass.
Example: You have a 4 ton system. Your smallest zone will use
500 CFM. The total CFM is 1600 CFM (400*4). Your bypass
CFM is 1100 (1600-500). From the table, you determine that a
12” bypass damper is needed.
Do not use the barometric bypass in any system over 5 tons,
for systems over 5 tons, or to bypass more than 2000 CFM,
use the electronic bypass.
Step 2: Select damper from sizing table. Once you have calcu-
lated the bypass air volume from Step 1, use the BAROMET-
RIC BYPASS SELECTION TABLE. From the table, select the
bypass damper with CFM rating equal to or greater than the
value calculated in Step 1. For rectangular barometric dampers,
use a ductulator to convert from round to rectangular.
Diameter
CFM
9
”
650
10
”
800
12
”
1200
14
”
1600
16
”
2000
BAROMETRIC BYPASS
SELECTION TABLE
BYPASS DAMPERS – BAROMETRIC
BYPASS DAMPERS
RECTANGULAR & ROUND
BAROMETRIC BYPASS
