We have a return air duct that is 18 inches in diameter that had to be soffited. Not a big deal except it is taking up closet space in what is otherwise a very small closet for a kids bedroom.

Can this 18" duct be "adapted" into 4 of those 8" oval ducts that fit inside a 2x4 wall and then tied into the main return air line to the furnace? How would I figure out if it would provide the same airflow/cfm requirements as the single 18" duct?

2 Answers 2


At the first level you need to compare cross sectional area of the 18" round duct with the cross sectional area of the four 8" oval ducts. There is a secondary effect of using the multiple smaller ducts because it exposes the air flow to more inside surface area which increases the pressure drop some but that can be compensated for by making the replacement duct area be larger than the original.

The cross sectional area of an 18" diameter duct is:

enter image description here

I do not know the shape of the 8" oval duct that you were considering but assuming that it is a standard ellipse shape like this:

enter image description here

We can see that the A dimension would be half of the 8" size and the B dimension would be half of the standard 2x4 wall stud width of 3.5"

enter image description here

If using four of the oval ducts the total area would be:

21.99 * 4 = 87.96 sq inches.

So to compare 254.45 from the large duct to the 87.96 from the four smaller ones you can see that only about a third of the required area is provided. So the answer would be no, this is not a suitable replacement.

Of course if the 8" oval duct has a slightly different shape this could affect the calculation some but I'd still say that it wouldn't be enough.

Here is a graphical comparison:

enter image description here

Update for Obround Ducting

The obround ducting available from one source is 3" by 8" in size (assuming the 8" specified by the original poster).

enter image description here

The cross sectional area of a duct this size:

enter image description here

enter image description here

So for the 3x8 size being considered it would take (254.45 / 22.06) = 11.53 ducts to be equivalent to the one 18" round duct!!!

  • 1
    Sounds like I would need 12 of those to provide that air flow! Crazy!
    – esac
    Oct 20, 2013 at 0:28
  • @esac - Seems crazy but then again an 18" pipe is a huge airway.
    – Michael Karas
    Oct 20, 2013 at 4:51
  • Oval ducts are not ovals, they are obround like this.
    – Tester101
    Oct 21, 2013 at 10:36

Michael Karas' answer correctly shows the huge cross-sectional area discrepancy between 4 small oval ducts and the one 18" round duct, so the ovals you had considered would not be an equivalent replacement.

However, I would think another alternative that you may find more attractive for this application would be to make yourself a custom rectangular return duct out of duct board.

I'm not an expert on this product or its applications, but it's something that I have seen used, and I think the build-to-suit aspect may be good for you.

You could choose the height/width combination that would best suit your application, as long as you maintain the ~250 sq.in. cross sectional area of the 18" round duct you are replacing. Maybe a 10" x 25" rectangle would work better for you.

bare duct board

duct board assembly

finished ducts

  • Duct board has a lot of downsides, including low quality installs (leakage) and potential unseen rodent damage/intrusion. I can't see a reason to choose it over tin. Jan 4, 2016 at 6:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.