user manual

Page 5
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 will
prevent any damage to 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.
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 3.
Refer to the parts and supplies catalog for information
on available barometric bypass dampers.
TABLE 3
Round Rectangular
8 in. dia. – 400 cfm 12 x 8 – 1000 cfm
10 in. dia. – 750 cfm 12 x 10 – 1200 cfm
12 in. dia. – 1200 cfm 12 x 12 – 1400 cfm
14 in. dia. – 1800 cfm 20 x 8 – 1600 cfm
16 in. dia. – 2400 cfm 20 x 10 – 2000 cfm
20 x 12 – 3000 cfm
FIGURE 4 – BYPASS DAMPER INSTALLATION
TRANSFORMER
FOR
DAMPERS,
PANEL AND
THERMOSTATS
ZONE
DAMPER
ZONE 1
ZONE 2
ZONE 2
THERMOSTAT
ZONE 1
THERMOSTAT
HEATING/
COOLING
UNIT
ZONE
CONTROL
PANEL
SUPPLY
AIR
RETURN
AIR
ZONE
DAMPER
DISCHARGE
AIR SENSOR
BYPASS DUCT
BYPASS
DAMPER
6 ft. (1.8 m)