A plenum is a box made of sheet metal that connects an HVAC systems air handler to the ductwork. It is a part of a building that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/conditioned or return airflows, usually at greater than atmospheric pressure. Plenums are usually situated on either side of the fan or heat exchanger and have the important function of bringing in, distributing, and removing air. There are two types of plenums in an HVAC system: the supply plenum and the return plenum. The supply plenum box is connected to the supply unit of the furnace, air conditioner, or heat pump. Once the air is heated or cooled, it goes into the plenum box and is then moved into the ducts and through the home. The return plenum box takes air from inside the home and sends it to the air conditioner or furnace to be heated or cooled. Plenums are essential in improving airflow in a home or buildings HVAC system, reducing humidity levels, and creating a more comfortable environment.