Above Ceiling Return Air Plenum

If there is a common plenum above the ceiling then return air can be pulled from rooms through open return air grilles in the ceilings and back to the mechanical equipment room without the cost of rigid return air ductwork.

Above ceiling return air plenum.

Return air is then directed into the grille and travels to each crac unit. Any large space designated for any type of air flow or routing of systems can be called a plenum. Above the insulation should not act as a return air plenum. Shafts can be smaller as the complete open section of the shaft and ceiling are accessible for the return airflow.

The air conditioning system supplies heated or cooled conditioned air through ducts in the above ceiling or attic space and this air comes into the room through a device called a register vent or diffuser. This would render the insulation thermally ineffective as the air space above the ceiling would be at the same temperature as the room below. However some drop ceiling designs create a tight seal that does not allow for airflow and therefore may not be considered a plenum air handling space. Sonobatts insulation can also.

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

They also reduce the amount of energy used and are self balancing so they remove the need for balancing labor. The term plenum could be a large section of ductwork or the upper space above a ceiling.