Barometric Bypass Damper

A question that comes up frequently when quoting zone control systems is, why and when do I need a bypass damper?  As the individual zone dampers close, the system static pressure will tend to rise.  In order to maintain constant airflow through the HVAC system, a barometric damper can be used to bypass some of the discharge air back to the system.  The bypass damper should be installed in such a way as to connect the supply air duct to the return duct.  Damper adjustment is done by moving the weight up and down the arm.  If only two zones are required, size each zone to handle about 75% of the total CFM.  This method does not require a bypass damper.  If one zone requires substantially more CFM than the other, it is recommended that the larger zone damper be set to bleed about 20% of its air.  If more than two zones are required, install a bypass damper.  It should be set to bypass just enough air to maintain a quiet, draft free system.  Barometric bypass dampers can be used on systems with static pressure ratings up to 0.75”W.C.  Bypass dampers are sized based on the cooling tonnage.

Condensing Unit Damper Size
2.5 tons or less 8” bypass
3 tons 10” bypass
4 tons 12” bypass
5 tons 12” bypass
7.5 tons 14” bypass
10 tons 14” bypass

For systems larger than 10 tons, subtract the CFM of the smallest zone from the total system CFM, then size the bypass damper to handle 100% of this difference at 2,500FPM.

Updated: July 17, 2008 — 3:29 am

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  1. Need assistance in designing an Aprilaire Systems with (3) 8870 zone thermostats.

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