17
RECTANGULAR & ROUND
BAROMETRIC BYPASS
BYPASS DAMPERS
BYPASS DAMPERS – BAROMETRIC
AIRFLOW
1
2
4
3
1. Damper Shaft
2. Lock Nut
3. Lever Arm
4. Counter Weight
BAROMETRIC BYPASS
DAMPER
ZONE DAMPERS
SIZING ZONE DAMPERS
DAMPER INSTALLATION NOTES
Diameter
CFM
9”
650
10”
800
12”
1200
14”
1600
16”
2000
BAROMETRIC BYPASS
SELECTION TABLE
BYPASS DAMPERS
If the ductwork already exists, simply size the damper to fit
the ductwork. For new systems or retrofit jobs:
a) Determine CFM from heat gain or loss calculations.
b) Select damper size by using a duct sizing table or calculator.
c) Select a Zonex Systems damper to fit the duct size selected
for that zone.
1. Do not exceed 700 FPM in a register/diffuser branch duct.
2. If a damper is installed within 3 feet of register/diffuser,install
sound attenuating flex duct between damper and outlet.
3. Zone dampers should be preceded by 2’-4’ of straight pipe
where possible.
4. In attic installations and high humidity areas, the Zonex
Systems damper should be insulated along with the duct
work. The hat section on the damper is delivered with insula-
tion between the hat section and pipe. Therefore, insulation
should be applied to the round pipe and be butted against
the hat section, (do not insulate the motor terminal board).
The motor generates enough heat so no condensation will
develop on the hat section.
5. Remember to allow a 16” gap in the duct for Heavy Duty
rectangular CD dampers.
6. Low and medium pressure rectangular dampers slide into a
3” wide cutout in the ductwork.
7. Install TR round dampers to the motor in the 9 to 3 o’clock
position. Do not install damper so the motor is in the 4 to 8
o’clock position.
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 electric bypass
(STBP/STCDBP), see the Bypass Dampers - Electronic
Section, Pages 19-22; or contact Technical Support.
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)
BAROMETRIC 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.
If bypassing more than 2000 CFM, use a 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
calculated the bypass air volume from Step 1, use the
