By Robert Trotter, CBO, CFM
In the built environment a plenum is a separate space for air circulation for HVAC systems, typically provided in the space between the structural floor/ceiling and suspended-grid acoustical tile ceiling. Incorporating a plenum can have its design advantages but corridors serving adjoining areas of health care occupancies are prohibited from being used for a portion of an air supply, air return, or exhaust air plenum.
Using building cavities as return air plenums can draw them below atmospheric pressure if not properly designed, causing outdoor air to be drawn into the building fabric. In humid climates, this can result in condensation of moisture from outdoor air within architectural cavities, consequently resulting in mold and mildew growth. Also, under certain circumstances, plenum configurations can have an adverse effect on life/fire safety. Clearly, plenum returns should not be used where codes prohibit them, such as health care occupancies, where individual space pressures must be controlled. These provisions are prescribed in the NFPA 101®, Life Safety Code® and NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems.
