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oval vs. rectangular ducts

| Posted in Energy, Heating & Insulation on April 25, 2003 12:11pm

I’m getting bids from HVAC contractors. One guy insists that using the 10″x31/2″ oval ducts will not be sufficient especially for moving cold air to the second floor rooms. He wants to use rectangular ducts. He explained that the cross section simply isn’t large enough. Other contractors insist that it is equal to rectangular.

Do any of you pros have some useful info for me? The one guy said he has seen brand new homes where they had to install window ac units in the upper floor windows due to these ducts.

Thanks for any help.

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  1. User avater
    BillHartmann | Apr 25, 2003 04:35pm | #1

    "He wants to use rectangular ducts. He explained that the cross section simply isn't large enough. Other contractors insist that it is equal to rectangular."

    I don't know what they are talking about of course the 10 x 3 1/2 oval has much more cross section than a 3 1/2 by 3 1/2 rectangular. Is that what you where compare it to. Or was it a 30 by 30 rectangular.

    And are you talking about 500 sq ft on the 3nd floor or 5,000 sq ft?

    Also I see you talking about ducts to move air to the 2nd floor, but no meantion of ducts to return the air.

    I am not an HVAC pro, but there is simply not enough information for anyone to answer your question.

    1. robbird | Apr 26, 2003 04:44pm | #3

      Ok. I'm not exactly sure what the rectangular size is ,but got the impression that it is about the same size only rectangular. The main floor is 1900 sf. and the second floor is about 1500. Cold returns were said to balance the system. I guess the question I had was more general, has anyone ever heard of these ovals not flowing as well as rectangulars, all things being equal.

      1. robbird | Apr 26, 2003 04:48pm | #4

        Thanks. I'll make sure they can explain the calculations.

  2. NPitz | Apr 26, 2003 04:13pm | #2

    The answer is it depends. Oval duct sizes are not quite the same as rectangular and round sizes. 3-1/2 x 10, if I remember correctly, is what they call 7" oval. This would seem to imply that it is the equivalent of a 7" round duct, but in fact it is more like the equivalent of 6.5" round, if there were such a thing. There is also what is called 8" oval which, is more like the equivalent of a 7.5" round. The bottom line is that what is important is that the person installing it is calculating it correctly for what they need in terms of capacity. Ask the installer how they go about figuring the duct size. If the guy understands concepts like friction, cross-section, capacity, etc., then they probably know what they're doing. I don't think you can make a blanket statement that oval duct won't do a proper job.

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