Bypass Dampers

Bypass dampers for zoned forced-air systems, from Resideo.

Zoning a single-stage system creates a pressure problem. When only one small zone calls, the blower is still pushing full airflow into a fraction of the duct, and static pressure rises sharply — the symptoms are register noise, whistling and, on a furnace, high-limit trips from insufficient flow across the heat exchanger.

A bypass damper relieves that by routing surplus air from the supply plenum back to the return. Barometric versions open automatically against a weighted or spring-loaded plate as pressure rises; motorized versions are driven by the zone panel.

Bypass has a real cost worth understanding. Recirculating supply air back to the return raises supply temperature in heating and lowers it in cooling, which can push equipment toward its limits. Modern variable-speed systems often avoid bypass entirely by modulating the blower instead, and on that equipment a bypass damper can make things worse rather than better.

Size the bypass to the smallest zone, not to the total system.

Popular part numbers: Honeywell CPRD8, Honeywell CPRD10, Honeywell MARD14.

Frequently asked questions

Barometric or modulating?

A barometric damper such as the CPRD opens on its own as static pressure rises. A modulating damper such as the MARD is driven by the zone panel.

What size bypass damper do I need?

Size it with the zone dampers, based on the airflow the smallest zone cannot take. Round dampers are sold by duct diameter.

Are replacement regulators available?

Yes. CPR regulators fit CPRD dampers. See also zone dampers and zone panels.