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I am installing a range hood and due to a ceiling joist in the way, I have to offset the ducting going through the ceiling by about 3" from the center of the hood. I have to do this within about 30" of height from the vent hood to the ceiling. The easiest thing to do would be to just use semi rigid ducting however from what I've read this is considered less than ideal because of potential for grease to build up on the ridges. Though I think if I stretched it out 100% it would minimize the ridges. I don't have enough room to put elbows in and that would offset it too much anyway.

The options I can think of are...

  1. Just use semi-rigid ducting
  2. Use rigid ducting for the first few feet (where maybe grease buildup will be the worst), then go to semi-rigid through the ceiling and back to rigid in the attic.
  3. ??? I have no idea.

Any suggestions?

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  • Stay with rigid whereever possible - semirigid induces a lot more turbulance making for noisier operation.
    – Criggie
    May 31 '21 at 3:01
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30 inches of height? 3 inches of offset? You need to learn how to use multi-section rigid duct elbows, which can be rotated to be considerably less than the 90 degrees you buy them at. You have plenty of room to make a nice little offset using a pair of elbows rotated to whatever matching degree gets you just the offset you need. You can literally rotate them all the way to straight, so 5, 10, 15 or 27.3 degrees can be easily achieved.

Image from directflues UK - no affiliation

Each section of the elbow can be rotated with respect to the others. Image from directflues UK - no affiliation, but a good illustration (inclusive of the possibility that you could to the offset with one, if it happened to work out that way.)

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  • I was about to post an answer to this question and then see that you already posted the same exact idea. :-)
    – Michael Karas
    May 30 '21 at 20:24
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If your range hoods use a rectangular duct such as a 3.25 X 12" or 3.25 X 14"and not a round duct, I would go to a duct fabricator or HVAC shop and have them make a duct exactly as what you need. Explain to them what the duct is for so they can make a duct to suit the application.

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