To limit the probability that flow-regulating dampers will create too small an opening, 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 that flow-regulating dampers will create too large an opening during cold weather, which could lead to thermal shock to heat exchangers, which could lead to the premature failure or cracking of heat exchangers, 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.