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

In some HVAC systems, the supply plenum is integrated with the plenum box, which is a component that is used to collect and distribute the return air from the building or room back to the furnace or air handler. The plenum box and supply 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 supply plenum may be a separate component that is located near the furnace or air handler.

Supply 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.