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.
