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Community and Q&A

Oval ducts for ERV

raul4817 | Posted in General Questions on

So I have committed to installing a fully ducted erv.  I’m going to be closing my 2nd floor subfloor with my plumbing install and inspection completed now.

I planned on running a 4″ round for the fresh air supply to sit on the wall just below my central return for my fujitsu. I have plenty of space for the 4″ round and can easily fur out the wall to accommodate the diameter, however I am having a tough time locating an appropriate stack head for the round duct.  So I planned on transitioning to 5″ oval just below the floor penetration. 
This seemed to be alot of transitions so I’ve looked at some 6″ or 7″ oval that I could run continuously from basement .   

Is oval duct really a just a poor choice? I have read somewhere that rectangular is better than oval and round being the best.  

The erv will have two fresh air supplys feed from the basement, most likely off 6″ round.  The 1st somewhere centrally located on the the 1st floor near a return and the 2nd upstairs also centrally located just under the return for the ducted minisplit.


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  1. Expert Member
    Akos | | #1

    There is nothing wrong oval provided you size it correctly. 6" oval is about the same as 5" round but some of the oval fittings are VERY lossy, make sure you use the correct equivalent length, they are not the same as round fittings.

    You should be able to push about 100cfm through a 6" oval duct with minimal fittings on boost, but I would keep the velocity less than that on normal operation.

    1. raul4817 | | #2

      Thanks Akos.

      I only need around 50 cfms for this run. I am having some trouble calculating but here is the relevant info. I planned on using a Panasonic intelli balance 100. The total oval duct length would be around 25ft to the basement, then will be converted to 6" round at that point.

      I would need
      1X 6" oval to 6" round 90 adapter.
      1X 6" oval flat 90
      2x 6" oval 90

      not sure what the equivalent feet is for these fittings is.

      also will have 8' of round 6" pipe in the basement with perhaps
      2x 6" round 90's

      I purchased some spiral pipe recently at a local sheet metal manufacturer and would be willing to bet they stock some oval spiral. The 90's I purchased for my round spiral seem to have long sweeps so If the stock oval fittings I will most likely be purchasing from them, If not the local Menards stocks some 6" oval. I can however get 7" oval from them and would be willing to upsize if needed.


      1. Expert Member
        Akos | | #3

        You'll have to go through Manual D, I think the appendix has the EL for all those fittings. At 50 CFM you velocity is bellow 400FPM, so even the lossy fittings shouldn't add too much pressure drop.

        I would probably loose the 6" to oval 90 though, that is a pretty high EL, if possible go with a 6" 90 followed by a round to oval adapter.

        Spiral is the way to go, they are significantly lower loss.

        A 6" main trunk is plenty large for 100CFM, you should have no issues there.

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