I am moving ducting three studs down, so I need to extend an insulated 7” flex pipe run that bridges a 7” elbow with a 7” floor register duct.

7” flex is hard to find locally but I have plenty of 8” insulated ducting left over from another project. Would it be okay to bridge that run between the elbow and the register with a 8” flexible duct?

1 Answer 1


You can do it but it will make a larger dead air space I won’t say I haven’t done something like that because I had extra of the larger size and ran out of the smaller.

  • thanks! Alternative is to make a 30 min trip to supply house and get 25 ft of 7” for $42. I’m only running 6ft and I do have room between joists. Would it affect airflow to that single register enough to warrant the effort of getting right size?
    – David
    Sep 16, 2020 at 16:36
  • 2
    I agree and have no concerns about extra airspace. It's trivial for that situation.
    – isherwood
    Sep 16, 2020 at 18:09
  • 1
    Exactly the case I have run into , No problems only dead air space in the duct it takes a second longer to start heating Or cooling so it will be exactly the same flow or if it could be measured slightly more because of reduced friction but I doubt there will be enough to measure it.
    – Ed Beal
    Sep 16, 2020 at 18:37

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