603.6.1 Air Ducts

Air ducts shall be permitted to be rigid or flexible and shall be constructed of materials that are reinforced and sealed to satisfy the requirements for the use of the air duct system, such as the supply air system, the return or exhaust air system, and the variable volume/pressure air system.

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.
2. Class 0 or Class 1 rigid or flexible air ducts shall not be used for air ducts containing air at temperatures in excess of 250°F (121°C).
3. Where the temperature of the conveyed air does not exceed 125°F (52°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 air stream 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.

The materials, thickness, construction, and installation of ducts shall provide structural strength and durability in conformance with recognized good practice. Air ducts shall be considered to be in compliance with this requirement where constructed and installed in accordance with the New York City Building Code. Where no standard exists for the construction of air ducts, they shall be constructed to withstand both the positive and negative pressures of the system.