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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.
Plumbing codes strictly prescribe where vent pipes can connect to the stack and how far they should travel. In most cases a wet section of pipe -- the part that carries wastewater -- cannot be used as a vent, even if it is usually dry.