HVAC Plenum

A plenum is a box that connects the ductwork to the AHU (air handling unit).

An HVAC system typically has two plenums: a supply plenum and a return plenum. The supply plenum takes the hot/cold air from the AHU and distributes it through the building via the ductwork. The return plenum takes the room air and sends it to the AHU for treatment.