Maximum smoke developed classification and exposed surface flame-spread rating of the outer covering of ducts, plenums and other parts of air duct systems used within an assembly of combustible construction, in buildings described in Sentence 1.3.3.2.(1) of Division A.

This also applies to:

• the fire safety characteristics of heating, ventilating and air-conditioning systems, as stated in Article 6.2.1.2., and
• buildings to which Part 9 applies [see Sentence 1.3.3.3.(1) of Division A for application of Part 9], with respect to:
  • mechanical ventilation systems other than self-contained systems serving single dwelling units, as stated in Sentence 9.32.1.1.(2),
  • the ventilation of storage garages for more than 5 motor vehicles in residential occupancies, as stated in Sentence 9.32.1.1.(3),
  • mechanical ventilation systems used for the ventilation of rooms and spaces in dwelling units during the non-heating season, where natural ventilation is not provided in accordance with Article 9.32.2.2., as stated in Clause 9.32.2.1.(1)(b),
  • heating and air-conditioning systems other than those serving individual dwelling units, as stated in Sentence 9.33.1.1.(2), and
  • air duct distribution systems in which the rated heat input exceeds 120 kW, as stated in Sentence 9.33.6.1.(2).

To exempt the outer coverings of ducts, plenums and other parts of air duct systems from the requirements of Sentence 3.6.5.4.(2), which would otherwise impose more stringent limits with respect to flame-spread rating and smoke-developed classification, and permit higher limits to the flame-spread rating and smoke-developed classification, on the basis that the higher limits do not significantly add to the risk of fire and smoke spread within an assembly permitted to be of combustible construction.

This is to limit the probability that the outer coverings will have inappropriately high flame-spread and smoke-development properties, which could lead to the development of an inappropriate amount of smoke and the spread of fire across the exposed surfaces of the material, which could lead to the spread of fire and smoke to other parts of the building by means of the air duct systems, which could lead to harm to persons.