Lennox International Inc Zone Control Panel Bypass Damper Sizing, Example, Round Rectangular

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.

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

Refer to the parts and supplies catalog for information on available barometric bypass dampers.

ZONE 1	ZONE 1	BYPASS
THERMOSTAT
		DAMPER
				BYPASS DUCT
ZONE
DAMPER
		SUPPLY	HEATING/	RETURN
			COOLING	AIR
		AIR	UNIT
ZONE			DISCHARGE
			AIR SENSOR
DAMPER
				6 ft. (1.8 m)
		TRANSFORMER		ZONE
		FOR		CONTROL
		DAMPERS,		PANEL
ZONE 2		PANEL AND
	ZONE 2	THERMOSTATS
THERMOSTAT

FIGURE 4 – BYPASS DAMPER INSTALLATION

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