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When a vent is less than 10 feet high, the vent should be at least as large as the flue collar outlet.b) Vents for draft hoods or flue collars 12 inches in diameter or less should NOT be reduced more than one pipe size.
With a 1.25 -inch diameter pipe, sloping at 1/4-inch per foot, the pipe 5 feet away or more from the trap will be flooded by water just coming over the crown weir of the trap. This is why vents must be within 5 feet of a trap to be effective. Any fixture within 5 feet of the stack should not need a separate vent. Since many drain and waste pipes are larger than 1.25 -inch diameter, there is a safety factor built into this.