Plumbing and heating contractor Richard Trethewey quiets a whistling heating duct. (See below for a shopping list and tools.)

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Shopping List for How to Install a ForcedAir Bypass Damper:
Bypass damper, used to quiet a noisy duct
Start collars, for attaching bypass damper to existing duct

Tools for How to Install a ForcedAir Bypass Damper:
Drill/driver equipped with a sheet metal duct cutter

Steps:
1. Mark the location of a start collar on both the return and supplyside duct.
2. Cut holes in each duct using a sheet metal duct cutter attached to a drill/driver.
3. Screw a start collar to the returnside and supplyside ducts.
4. Install elbows to each end of a bypass damper.
5. Attach the bypass damper between the two start collars, effectively connecting the return duct to the supply duct.

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How to Install a ForcedAir Bypass Damper | This Old House
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