Plenum space

Not to be confused with Plenum chamber.
Vertical section through a commercial building using HVAC plenum instead of airspace plenum. When both the supply and return ducts are constructed in this manner, it is possible to insulate the ducts and the dropped ceiling so that the upper airspace is not heated or cooled, increasing energy efficiency. This is a fully enclosed plenum
A commercial building with a plenum airspace.
A non fully insulated HVAC plenum created by accident can go unnoticed and become a fire hazard, due to cabling installed under the assumption that this will always be a fully enclosed plenum. the fire risk come from that the non fire resistance material of the building might expose to the fire.

A plenum space is a part of a building that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/conditioned or return airflows, usually at greater than atmospheric pressure. Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum; 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.[1][2]

  1. ^ Thomas, Timothy. "What is plenum? - Definition from WhatIs.com, TechTarget, September 21, 2005. Retrieved July 25, 2017.
  2. ^ National Fire Protection Association (2017). "Article 100 Definitions". NFPA 70 National Electrical Code. 1 Batterymarch Park, Quincy, Massachusetts 02169: NFPA. Retrieved October 9, 2023. Plenum: a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.{{cite book}}: CS1 maint: location (link)