A bypass damper is an automatic control damper that diverts excess supply air back to the return ductwork, maintaining constant airflow through the system while reducing heating or cooling when demand decreases. These dampers enable single-speed fans to modulate capacity without cycling on-off, improving comfort and reducing energy consumption.
Operation and Control
Bypass dampers open in response to increasing static pressure in the supply duct, typically set at 0.5 to 1.5 inches of water column. As zones satisfy demand, reduced heating or cooling load causes pressure rise, triggering damper opening. Dampers may be spring-driven, powered actuators, or pressure-sensitive designs. Modulating operation maintains constant return air temperature and reduces thermal cycling.
Applications and Benefits
Bypass dampers are standard on constant volume VAV systems and supplemental heating installations. They prevent short-cycling of furnaces or heat pumps during partial-load conditions. Systems typically include barometric relieve dampers set 0.25 inches higher than bypass setpoint to vent excess pressure. Proper installation requires adequate return ductwork capacity to handle full system airflow when bypass opens completely.