Intent 1:
To limit the probability of:
- the entry of cold air into buildings (backdraft) through
exhaust outlets that are subjected to positive wind pressure when
fans are not operating, which could lead to the excessive cooling
of exhaust ducts, which could lead to the condensation of water vapour
on the exterior surfaces of such ducts, which could lead to the generation
of pollutants from biological growth, or
- the entry of vermin into buildings, which could lead
to the introduction of disease organisms.
This is to limit the probability of
negative effects on the air quality of indoor spaces, which could
lead to harm to persons.
Intent 2:
To exempt exhaust outlets serving heat recovery
ventilators from the requirement to install backdraft dampers, on
the basis that the internal resistance of the heat recovery ventilator
core provides sufficient resistance to backdraft.