A factory-built return air plenum is also available for the 4100 series systems but is ordered as a separate accessory. This plenum incorporates a filter rack and tray for placement of a heating/cooling coil which must get set in the return ductwork when interfaced to a heat pump. The return air plenum connects directly to the 4100 series systems for either a right-to-left or left-to-right airflow pattern.
