Bathroom fan duct size matters because the fan’s rated CFM assumes suitable installation conditions. A small, long, or restrictive duct can reduce airflow and increase noise.
Many older fans use 3-inch or 4-inch ducts. Many modern higher-CFM fans may call for 4-inch or 6-inch ducts. Always check the specific product manual before buying.
Air moves more easily through a wider, smoother, shorter duct. Tight bends, too many elbows, crushed flexible duct, or a blocked exterior cap can all reduce performance.
Bathroom exhaust should vent outdoors through an approved wall or roof cap. It should not dump moist air into an attic or enclosed cavity.
Bottom line: before buying a fan, confirm duct diameter, duct length, number of elbows, and where the duct terminates. Good ducting helps the fan perform closer to its rated CFM.
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