Activation of fans in self-contained heating-season mechanical ventilation systems that are not 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: 

[Clause (a)] To limit the probability of the operation of principal ventilation fans when outdoor air supply fans are not operating, which could lead to excessive depressurization, which could lead to:

• the spillage of combustion products from fuel-burning appliances that are susceptible to spillage, or
• the inadequate replacement of indoor air with outdoor air.

This is to limit the probability of the inadequate control of:

• airborne pollutants,
• oxygen and other components necessary for breathable air,
• relative humidity, which could lead to condensation, which could lead to the generation of pollutants from biological growth or from materials that become unstable on wetting, or
• the entry of combustion products into living space.

This is to limit the probability of negative effects on the air quality of indoor spaces, which could lead to harm to persons.

[Clause (b)] To limit the probability of the operation of supply fans when principal ventilation fans are not operating, which could lead to excessive pressurization, which could lead to condensation within assemblies, which could lead to the generation of pollutants from biological growth or from materials that become unstable on wetting.

This is to limit the probability of negative effects on the air quality of indoor spaces, which could lead to harm to persons.