Plenum ventilation in buildings

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Plenums are air compartments or chambers that are sometimes found in buildings, either above suspended ceilings, in the gap between the ceiling and the floor slab, or below raised floors in the gap between the raised floor and the floor slab. They form part of the ventilation system for the building.

They may be supply or return plenum, and may serve either an entire building, or a specific zone in a building. Supply plenum supply ventilation air to the occupied space, whilst return air is generally extracted through ductwork. Conversely, in return plenum the air is generally ducted to the space, whilst the return air is extracted through the plenum.

Plenum can be combined with thermal mass to help moderate peak conditions, or to allow night time purging. See thermal mass for more information.

Underfloor air distribution (UFAD) uses the underfloor plenum beneath a raised floor to provide ventilation air through floor diffusers directly to the occupied zone. The air spreads across the floor forming a reservoir of fresh, cool air. Any sources of heat (such as people or computers) generate a thermal plume lifting contaminated air to high level where it can be removed from the space. See Underfloor air distribution for more information.

Plenum can give good flexibility for the layout of buildings. However, they can provide a space within which fire can spread, particularly because of the air supply they offer, and so they may be required to include fire and smoke detection, and potentially combustible material (such as cables) may be controlled. They can also create a pathway for the transmission of sound between separate spaces.

The inclusion of dampers and acoustic attenuators can be beneficial, but this results in pressure losses. Good air-tightness is required to the plenum construction to optimise performance.

It is important to ensure that spaces above suspended ceilings and below raised floors do not inadvertently become plenum as a result of the disconnection of, or damage to ductwork.

NB the phrase ‘open plenum’ can be used to describe buildings in which there is no suspended ceiling, but instead the floor slab and services are exposed giving an ‘industrial’ look.

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