A plenum box is a component of a heating, ventilation, and air conditioning (HVAC) system that is used to distribute conditioned air throughout a building or a room. The plenum box is typically located near the furnace or air handler, and it serves as a transition point between the supply and return ducts.

The plenum box is typically connected to the supply ducts, which carry the conditioned air from the furnace or air handler to the registers or vents in the building or room. The return ducts, which carry the return air back to the furnace or air handler, are also connected to the plenum box. The plenum box helps to evenly distribute the supply air throughout the building or room, 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.

Plenum boxes 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.