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9.32.3.4.(2)
ApplicationIntent
OH1OS3
ObjectiveOH1 Indoor Conditions
Attribution
[F50,F51,F81-OH1.1] [F51,F81-OH1.2]
Application
Application 1: 
Tempering of outdoor air where the actual normal operating exhaust capacity of the principal ventilation fan exceeds the maximum permitted outdoor airflow for the lesser of:
  • a mixed air temperature of 15°C, or
  • the minimum acceptable return air temperature specified by the furnace manufacturer.
This applies to outdoor air introduced to self-contained heating-season mechanical ventilation systems coupled to forced air heating systems serving single dwelling units with 5 or fewer bedrooms that are supplied with electrical power and a heating system, in buildings to which:
  • Part 9 applies [see Sentence 1.3.3.3.(1) of Division A for application of Part 9], or
  • Part 6 applies [see Sentence 1.3.3.2.(1) of Division A for application of Part 6 and Sentence 6.2.2.1.(3)].
Exceptions: 
except as stated in Sentence 9.32.3.1.(1), which applies to heating-season mechanical ventilation systems designed to comply with CAN/CSA-F326-M.
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Intent
Intent 1: 
To limit the probability that the flow of outdoor air through the furnace, which is needed to balance the actual normal operating exhaust capacity of the principal ventilation fan, will exceed the amount that can be tempered by the available return airflow to a temperature that is safe for the furnace heat exchanger.
This is to limit the probability of:
  • thermal shock to heat exchangers, or
  • the condensation of combustion gases or water vapour.
This is to limit the probablity of cracking or premature failure of heat exchangers, which could lead to:
  • the leakage of combustion gases into living space, or
  • an inoperative furnace.
This is to limit the probability of:
  • negative effects on the air quality of indoor spaces, and
  • the inadequate thermal comfort of persons.
This is to limit the probability of harm to persons.
Intent 2: 
[Clause (b)] Where the normal operating exhaust capacity of the principal ventilation fan exceeds the maximum permitted outdoor airflow, and the system does not incorporate means for tempering outdoor air, to direct Code users to Clause 9.32.3.1.(1)(a) or Article 9.32.3.5.
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