Bypass dampers modulate on a signal from a duct static pressure sensor to bypass air from the supply duct back into the return air duct. If the duct static pressure exceeds the adjustable setpoint, then the damper opens to bypass more air. If the static pressure drops below the setpoint, the damper closes to bypass less air.
Size the bypass dampers for the remaining airflow volume when all zones are at minimum airflow. Use the following equation to help size bypass dampers for the correct airflow:
Bypass Airflow (cfm) = Unit Airflow (cfm) – All Zones Minimum Airflow (cfm).
To size the damper, select a damper based on calculated bypass cfm and a maximum velocity between 1,750 and 2,250 FPM. When determining the bypass duct size, make sure to consider any transition fittings and associated pressure drops. See Table 1 and Table 2 for bypass damper selection.
Use a single bypass damper and round duct for the bypass. Use a rectangular damper if there are space limitations or total airflow requires it.