Return System 2 : Connected ducts, air passages, or plenums and fittings through which air from the space or spaces to be ventilated are returned back to the circulating equipment
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}