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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.
However, as soon as a larger common vent size is required, such as 14 inches (area 154 square inches), the vent connector rise must be increased to 3 feet to avoid draft hood spillage.This requirement does NOT apply when the connector rise is originally over 5 feet and consequently self-venting.