Above Ceiling Return Air Plenum

Return air is then directed into the grille and travels to each crac unit.

Above ceiling return air plenum.

Typically spaces between the structural floor or a dropped ceiling are considered plenum. Normally the space above the ceiling and the deck above is used to return air from the conditioned spaces below the ceiling to the air handling equipment. Dropped ceilings or suspended ceilings are often installed so that the space between them and the floor above or the roof deck can be used as a return air plenum. A commercial building with a plenum airspace.

With return air plenums there is less control over where the air actually travels. When both the supply and return ducts are constructed in this manner it is possible to insulate the ducts and the dropped ceiling so that the upper airspace is not heated or cooled increasing energy efficiency. Vertical section through a commercial building without a plenum airspace. This is most common in commercial buildings schools hospitals or libraries.

This would render the insulation thermally ineffective as the air space above the ceiling would be at the same temperature as the room below. The space providing air is called a supply plenum and the space providing air return is referred to as a return plenum. In thermal applications the area above the insulation should not act as a return air plenum. A plenum space is the part of a building that enables air circulation by providing pathways for heated conditioned and return airflows at a higher pressure than normal.