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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.
A 3 or greater sized floor drain with p-trap doesn't need a vent providing,the fixture drain or(the pipe leading horisontally to the p-trap)is, as or longer,than 18.A p-traâ¦p lesser than 3 must be vented.