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Installation (continued)
7. Install the dampers. Power close, spring open
dampers are preferred. Refer to the instructions
provided with the damper.
18 AWG thermostat wire minimum recommended.
Bypass Damper Sizing
When fewer than the maximum number of zones are
calling for heating or cooling, an excess volume of air is
delivered, and because of the excess air, an excess
amount of static pressure is produced as well. Zone
systems often require a bypass duct to relieve this
pressure. A properly sized barometric bypass damper
must be installed in the bypass duct which is run
between the supply and return air duct systems (see
Figure 4). The barometric damper and the bypass duct
must be sized to accommodate the excess static
pressure from the supply duct.
The bypass tap in the return air duct must be at least 6
ft. from the furnace /air handler to ensure that the hot or
cold air coming off of the plenum has time to mix with
the return air before it passes through the air handler
again. The provided discharge air sensor (field installed)
protects the equipment from overheating or coil
freeze-up by interrupting the HVAC equipment.
The bypass damper and duct should be sized to handle
the excess pressure created when the smallest zone is
operating alone (worst case). To size the bypass
damper, subtract the total air volume capacity of the
smallest zone from the total air volume of the system.
ZONE 2
ZONE 2
THERMOSTAT
ZONE
DAMPER
ZONE
DAMPER
ZONE 3
THERMOSTAT
ZONE 3
ZONE 1
ZONE 1
THERMOSTAT
ZONE
DAMPER
ZONE
DAMPER
ZONE 4
THERMOSTAT
ZONE 4
FIGURE 4 – BYPASS DAMPER INSTALLATION
Page 9
8. Re-install the cover on the base.
Example:
Total System air volume: 2000 cfm
Air volume of smallest zone: 600 cfm
Bypass requirement: 2000 - 600 = 1400 cfm
In this example, the bypass duct should be sized to
handle the 1400 cfm excess pressure created when
only the smallest zone has a demand. For bypass
damper air volume capacities, see Table 6.
Round
8 in. dia. – 400 cfm
10 in. dia. – 750 cfm
12 in. dia. – 1200 cfm
14 in. dia. – 1800 cfm
16 in. dia. – 2400 cfm
Refer to the parts and supplies catalog for information
on available barometric bypass dampers.
IMPORTANT: Bypass duct should be mounted as far away
from the heating cooling unit as possible to allow for a
better mix of the bypass air with the return air.
BYPASS
DAMPER
SUPPLY
HEATING/
COOLING
UNIT
AIR
DISCHARGE
AIR SENSOR
TRANSFORMER
FOR
PANEL, DAMPERS
AND THERMOSTATS
TABLE 6
Rectangular
12 x 8 – 1000 cfm
12 x 10 – 1200 cfm
12 x 12 – 1400 cfm
20 x 8 – 1600 cfm
20 x 10 – 2000 cfm
20 x 12 – 3000 cfm
BYPASS DUCT
RETURN
AIR
6 ft. (1.8 m)
CONTROL
PANEL
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