To limit the probability of excessive negative pressure, 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 expand the application of Subsection 9.33.4. to ducts in residential occupancies where the size of a duct cannot be determined using Table 9.32.3.11.A. or 9.32.3.11.B.