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In this video, This Old House plumbing and heating contractor Richard Trethewey quiets a whistling heating duct.


1. Mark the location of a start collar on both the return- and supply-side 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 return-side and supply-side 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.

Shopping list:

  • Bypass damper, used to quiet a noisy duct
  • Start collars, for attaching bypass damper to existing duct