A HVAC AC plenum provides separate space for air circulation. In a furnace system, the plenum is a compartment located between the furnace and the ductwork that distributes the heated air to the various rooms of a building. Furnace Plenum Parts are typically made of sheet metal and connected to the furnace with a large metal collar that is sealed to the furnace with mastic to prevent ... https://www.partshnc.com/furnace-parts/plenum