How to Install Exhaust Fan Ducts
- Use smooth metal duct material.
- Install short, straight runs.
- Make a positive connection to the outside.
Ducts and fittings must be smooth, 30 gauge metal or thicker. The diameter of the duct should match the outlet connection on the fan housing. However, always use at least 4 inch duct. If the fan has a 3 inch duct collar, add an adapter to accept a 4 inch duct.
Keep runs as short and straight as possible. When turns are needed, two 45º angle fittings create less resistance to air flow than one 90º. Avoid 180º turns and S-shaped runs.
Support the duct every 18 inches with galvanized wire, metal strapping or other approved duct support. Slope the duct so that any condensation will drain either to the outside or to the fan. (A puddle in the duct could support mold or bacteria.)
Secure each joint with 2 sheet metal screws. Then seal it with duct mastic or other approved material.
Never dump exhaust air into an attic, garage, basement, or crawlspace. If the end of the duct simply hangs in the general vicinity of a vent, some moist air will stay inside. Instead, install a fitting at the end of the duct to make a positive connection to the outside. These fittings can be purchased from sheet metal suppliers, or you can make one from metal flashing.
If converting an existing vent, be sure there is still enough net free area for adequate ventilation.
In cold climates, insulate the duct to reduce condensation.
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