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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.
Dryer vent pipes properly called vent ducts, come in a variety of materials. Most dryer duct is round and 4 inches in diameter. Some are flexible and others are rigid. Since all types are sold for use with dryers, you would naturally assume that all are suitable for this application. Unfortunately, that is not the case. Vinyl duct, and in some situations, all flexible duct, is not allowed by building codes.