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3.6.5.8.(4)
OS1
ObjectiveOS1 Fire Safety
Attribution
Application
Application 1: 
Combustibility of material lining combustible return ducts :
  • below floor registers,
  • at the bottom of vertical ducts, and
  • under furnaces having a bottom return.
This applies to buildings described in Sentence 1.3.3.2.(1) of Division A.
Application 2: 
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).
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Intent
Intent 1: 
To limit the probability that combustible debris accumulating in certain horizontal locations in combustible return ducts will not be isolated or separated from the combustible surface of the duct by a duct lining of noncombustible material, which could lead to a fire involving the debris igniting the duct and then spreading to other parts of the building, which could lead to harm to persons.
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