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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.
Buy the right gas pipes and fittings for the job. Most domestic gas lines use 1/2-inch (1.27 cm) through 1 1/2 inch (3.81 cm) black pipe, while large commercial projects sometimes use larger fittings up to 8 inch. You should buy a total pipe length of 6 to 12 inches (15.24 to 30.48 cm) longer than you need to account for pipe overlap and waste.