This system only supports a variable speed return air fan.
Speed Tracking: The return fan runs whenever the associated supply fan is on. Whenever both the return and supply fan are proven on the exhaust air damper opens, the damper closes otherwise. An active differential airflow setpoint is determined by scaling the designed speed differential (S-R-SPD-DIFF) by the ratio of the minimum outdoor airflow setpoint, as calculated based on ASHRAE 62.1 or California Title 24 requirements, to the designed outdoor airflow (V-AREA-ALL-ALL). Control the return fan to maintain the return fan speed equal to supply fan speed less the differential.
Direct Building Pressure: This option adds an input for building static pressure. Both the exhaust air damper and a return fan differential pressure setpoint are reset to maintain the building static pressure setpoint (network input BLDGP-SP). This scheme maintains slight positive pressure in the building compared to the exterior static pressure.