A return plenum is a component of a heating, ventilation, and air conditioning (HVAC) system that is used to collect and distribute the return air from a building or a room. The return plenum is typically located near the furnace or air handler, and it is connected to the return ducts, which carry the return air back to the furnace or air handler. The return plenum helps to evenly distribute the return air throughout the system, and it also helps to ensure that the return air is properly mixed with the supply air to maintain a consistent temperature and air quality.

In some HVAC systems, the return plenum is integrated with the plenum box, which is a component that is used to distribute the conditioned air from the furnace or air handler to the registers or vents in the building or room. The plenum box and return plenum are typically located near each other and are connected by a series of ducts, which allow the air to flow freely between the two components. In other systems, the return plenum may be a separate component that is located near the furnace or air handler.

Return plenums are commonly used in commercial and industrial buildings, but they can also be used in residential buildings. They are typically made of sheet metal or other durable materials, and they are designed to withstand the high temperatures and pressures that are present in an HVAC system.