Above Ceiling Return Air Plenum

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

Above ceiling return air plenum.

Ducted return air plenum this method requires the installation of ductwork that runs from each computer room air conditioning crac unit up into the space above the suspended ceiling and to each return air grille. 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. When installing a return air plenum system it is absolutely essential that the system is constructed airtight without any leaks above the ceiling space. Typically spaces between the structural floor or a dropped ceiling are considered plenum.

The space providing air is called a supply plenum and the space providing air return is referred to as a return plenum. Thermal insulation in return plenum areas would best be added at the roof and side walls. 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. A commercial building with a plenum airspace.

With return air plenums there is less control over where the air actually travels. Vertical section through a commercial building without a plenum airspace. This would render the insulation thermally ineffective as the air space above the ceiling would be at the same temperature as the room below. This is most common in commercial buildings schools hospitals or libraries.