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I'm renovating my basement and the big round 7" ducts that cross under the ceiling joists are in the way. I'd like to convert them to 3.25" by 12" flat rectangular ducts, so that the ceiling can be higher.

Is this likely to pass permit inspection?

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    Make sure to properly anchor the new ducting, I have never had an inspector even check the flow calculations there is no round or square requirement. – Ed Beal Dec 5 '17 at 16:04
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7" round: 38.5 square inches
3-1/4 x 12: 39 square inches

You have a little more area in the rectangular duct, so you should have no problem.

  • Thanks @isherwood. Math teachers always told me to 'show my work'... – Aloysius Defenestrate Dec 6 '17 at 3:53

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