9.33.6.13.Return-Air System (See Note A-9.33.6.13.)

(1) The return-air system shall be designed to handle the entire air supply. (1.1) Except as provided in Sentences (2) and (3), return ducts shall be constructed of material having a surface flame- spread rating of not more than 150. (2) Where any part of a return duct will be exposed to radiation from the furnace heat exchanger or other radiating part within the furnace, such part of a return duct directly above or within 600 mm of the outside furnace casing shall be noncombustible. (3) Return ducts serving solid-fuel-burning furnaces shall be constructed of noncombustible material. (4) Combustible return ducts shall be lined with noncombustible material (a) below floor registers, (b) at the bottom of vertical ducts, and (c) under furnaces having a bottom return. (5) Spaces between studs or joists used as return ducts shall be separated from the unused portions of such spaces by tight-fitting metal stops or wood blocking. (6) A vertical return duct shall have openings to return air on not more than one floor. (6.1) A public corridor shall comply with Sentences 6.3.2.7.(4) and (5). (7) The return-air system shall be designed so that the negative pressure from the circulating fan cannot (a) affect the furnace combustion air supply, nor (b) draw combustion products from joints or openings in the furnace or flue pipe. (7.1) Return-air from a dwelling unit shall not be recirculated to any other dwelling unit.