Forced-air systems also have return ducts that connect to the HVAC equipment through an air-collection box known as a return plenum. As freshly conditioned air is blown out of the room supply ducts, ?used? air from the rooms is sucked into the return ducts. The air from the rooms collects in the return plenum and is drawn back into the HVAC equipment through the return inlet for another round of heating or cooling. Air filters often are located in the return plenum.