(Source: Building America Solution Center)
A plenum in heating, ventilation, and air conditioning (HVAC) systems is a box that connects to the HVAC system. It has a critical function and that is to bring in, distribute, and remove air.
Each HVAC system typically uses two plenum boxes: a supply plenum and a return plenum.
A supply plenum works by taking in the new cool or warm air from the heater or air conditioning system and, in turn, distribute it throughout the building using the HVAC’s ductwork.
A return plenum takes in that used air and, using the ductwork, sends it back to the core HVAC system. That air is recycled as a fresh batch later.