603.6.1.1 Materials.
MC § 603.6.1.1
All air duct materials shall be suitable for continuous exposure to the temperature and humidity conditions of the environmental air in the air duct. Air ducts shall be constructed of any of the following materials: 1. Iron, steel, aluminum, copper, concrete, masonry or clay tile.
2.Class 0 or Class 1 rigid or flexible air ducts tested in accordance with UL 181 and installed in conformance with the conditions of the listing. Exceptions: 1. Class 0 or Class 1 rigid or flexible air duct shall not be used as a vertical air duct that is more than two stories in height.
3.Where the temperature of the conveyed air does not exceed 125°F (51.7°C) in normal service, negative pressure exhaust or return air ducts shall be permitted to be constructed of gypsum board having a maximum flame spread index/rating of 25 without evidence of continued progressive combustion and a maximum smoke-developed index/rating of 50. Air ducts formed by gypsum boards shall have a surface temperature maintained above the airstream dew-point temperature, and shall not be used in air-handling systems utilizing evaporative coolers. Exception: The maximum conveyed air temperature of 125°F (52°C) shall not apply to gypsum board material used for post-fire smoke purge.
