What is a Plenum?

Definition of Plenum in Construction
A plenum is a space designated for air flow or can describe a space devoted to mechanical, electrical or sprinkler lines.

The term plenum, could be a large section of ductwork, or the upper space above a ceiling. Any large space designated for any type of air flow or routing of systems, can be called a plenum. The space providing air is called a supply plenum and the space providing air return is referred to as a return 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. If this is an area used as a return, it is normally referenced as a return plenum . Fire protection within a return plenum is very important to the integrity of the entire structure, and special non combustible material must be used within the return plenum spaces. Non combustible wiring the use of steel and metal tubing and piping, which is fire resistant, must be used within the ceiling spaces. In addition, the type of insulation used above this type of ceiling is also specified as non combustible. Plenums normally have fire and smoke dampers to stop the transfer of heat and smoke through the plenum system. In some cases, plenums are under the floor and are used to move air from one location to another. In other cases the use of a pipe plenum can provide the ability to encase multiple mechanical lines within a common space.