To limit the probability of an inability to regulate the supply of outdoor air to the dwelling unit, which could lead to excessive negative pressure (where supply fans alone are affected), which could lead to the spillage of carbon monoxide gas from fuel-burning appliances that are susceptible to spillage, which could lead to the acute poisoning or asphyxiation of persons.
To limit the probability of an inability to regulate the supply of outdoor air to the dwelling unit during cold weather, which could lead to thermal shock to the heat exchanger, which could lead to premature failure or cracking of the heat exchanger, which could lead to the leakage of carbon monoxide gas into living spaces, which could lead to the acute poisoning or asphyxiation of persons.