return-air plenum
Quick Summary
A return-air plenum is a space within a building that is kept under negative pressure to facilitate the return of air from occupied spaces to the ventilation system. This space is typically located between the ceiling and the structural deck above.1
In commercial buildings, this negative pressure is created by an opening from the return side of a rooftop unit into the plenum. Air from the occupied space enters the plenum through grilles or openings in the ceiling. This return air can also pass over light fixtures, allowing for heat recovery in winter.1
Plenum ventilation systems, in general, apply motive force at the inlets, driving air through rooms that then act as plenums, preventing cold drafts.2
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