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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.
tom of all houses that are 24 to 32 inches tall. Front vents should be as long as a house is wide, side vents 6 inches. tall by 1/2-inch wide. Houses 36 inches tall or more should have vents 10 to 12 inches from the bottom.