To limit the probability of connecting return ducts to rooms on multiple floor levels with different stack pressures, which could lead to an inadequate flow of return air from upper-level rooms, which could lead to an inability to regulate the flow of warm air to rooms, which could lead to an inability to maintain the minimum indoor air temperatures [see Sentence 9.33.3.1.(1)], which could lead to condensation, which could lead to deterioration, which could lead to compromised structural integrity, which could lead to harm to persons.