A plenum is an enclosed portion of the building structure that is designed to allow air movement, and thereby serves as part of an air distribution system. Return, exhaust, and relief air plenums shall be limited to uninhabited crawl spaces, areas above a ceiling and areas below the floor, attic space and mechanical equipment rooms. Ceiling and floor plenums may be used for supply air if they are accessible for cleaning. Plenums shall be limited to one fire area. Separations between different supply plenums shall be made of sheet metal and sealed tight. Fuel-fired appliances shall not be installed within a plenum.