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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.
Weâll discuss the easiest to install, standard 8â³ x 16â³ vents. 1  First mark off the space where you want the soffit vents to go, spacing them so they fit between joists or rafters. 2  Cut the hole slightly smaller than the vent itself with a circular saw or a saber saw. Donât forget to wear eye protection.