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Upgrading a bathroom fan and I need room to convert a 3" duct to a 4" flapper on the fan (about 7" of length overall). The ductwork is rigid sheet metal and ran directly to the old fan. The duct is above a drywall ceiling that I'd really like to avoid cutting into more if possible.

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    Are you trying to shorten the 3" duct in the ceiling by 7"? You might be able to do it with a dremel with a abrasive wheel. An automotive cut off wheel the kind used for exhaust pipes may work too. Post a photo showing how much room you have to work with and you'll get better answers. Jun 9, 2021 at 21:36
  • As a pro I would just stick my plasma cutter inside the pipe and cut it off then put out smokers. A dremel tool with a few cutoff disks may be possible to cut the pipe, or a nibbler tool but that is tight I don’t think my smallest nibbler would fit inside the pipe and cutting in 7” and then trying to cut from inside the pipe with shear metal sheers and in a circle both methods sound like a trip to the ER for stitches. It may be best to take more ceiling down or use a dremel with cutoff disks. I was slow to type and see goose says the same, 3” pipe no cutting from inside possible with sheers
    – Ed Beal
    Jun 9, 2021 at 21:42
  • Where does the duct terminate. Is it possible to shorten the duct on that end and pull the duct towards the termination the appropriate distance?
    – Kris
    Jun 9, 2021 at 22:12

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