We are replacing a branch of the main HVAC distribution ducts in a 70's home that uses forced air from a gas heater. The HVAC distribution duct is rectangular 10 inches x 16 inches - which supports branching off to five individual room duct lines that use standard 3.25 inches x 10 inches.

I am having difficulty finding any of the rectangular ducts that are similar in size: the ones I find are all 3.25" x 10". Are there other approaches / formats for the 10" x 16" rectangular ducts now? We do need approximately 100 square inches of surface area in one branch that feeds three of the 3.25 x 10". So that could be something like a 10" x 10" or maybe a 8" x 12".. But I was unable to find any of those dimensions. Should I look for round ducts instead?

We are going to go from 5 three of the 3.25x10 individual duct lines into one larger one (3 x 3.25in x10in) - and then redistributing to the three

The portion being replaced needs to support 3 lines - so about [3.25in x 10in] x 3 or ~100 square inches

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    Quite possible they where made on site. Quite a few furnace installs they just brought sheet metal and cut and bent what they needed for larger ducts sizes, only used pre-made for the small stuff.
    – crip659
    Commented Apr 3, 2022 at 21:59
  • If you are going to change the size, get someone who knows what they are doing. It is not ONLY cross-sectional area, but also aspect ratio. If you are going from 10X16 to 8X(something) then that something is going to need to be bigger than 20. If you go with just the cross-sectional area then you can choke the flow to the 3 branches, and change the loading on your indoor blower, and increase your home energy use for the same conditioning. Commented Apr 4, 2022 at 13:14

1 Answer 1


When I was still doing HVAC work the sheet metal shops all stocked 8" high ducts in sizes 8 x 8 through 8" x 24", all the other sizes had to be made. So, any size duct other than 8" high had to be fabricated. If you can't find the duct sizes needed to complete your upgrade I would find an HVAC company that can fabricate duct work and have them make a transition piece to go from the 10" duct to an 8" duct. Buying a standard size duct would be much cheaper. A 10" x 16" duct is equal to an 8" x 21" duct which is not a standard size so I would buy the next larger "stocked" size which would probably be 8" x 24". By the way, a fab shop can make anything you would like if you have the money. One last point, a 3.25 x 10" duct is the same as a 6" round duct which is much cheaper than the 3.25 x 10" rectangular duct and much easier find and install. Hope this helps.

  • It could also be made from the insulated duct board - fiberglass insulation board with a foil coating on one side. Cuts easily with a knife, then tape it together with foil tape. I'd agree that round duct of the same cross section would be easier, though.
    – FreeMan
    Commented Apr 4, 2022 at 12:08

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