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  1. #1

    Question

    I'm getting ready to have my a/c replaced. I read somewhere that one type of duct (round or square/rectangular) flows more efficiently than the other, but I can't remember which one. Can anyone point me in the right direction?

  2. #2
    Join Date
    Feb 2005
    Posts
    40
    I don't think you are going to find any difference from round to rectangle. As long as you know the externanl static pressure (in H20) that you have left to work with for ducting design/sizing. With a ductulator, this remaining external static pressure value along with velocity and or cubic feet per minute values will give you both round and rectangle ducting sizes equivalencies. In other words the friction losses will be the same. This would leave you with the choice, of preference. Which is simpler, easier, and less costly, both in material cost and labor cost. Which would be simpler to install in the area that you are working with. Round duct requires less work installing than rectangle. Rectangle requires special tools to install it. The simpliest, easiest,and costliest is flexduct. In the last 20 years the manufactures of flex have made big strides in producing a good product. If you are looking at residential, for me, I would use flex. If commerical, building codes will probaly dictate metal ducting. In some cases I have been albe to use metal duct trunk lines and flexduct drops for ease and accessiblity.

  3. #3
    Join Date
    Feb 2005
    Location
    Toms River, NJ
    Posts
    419
    To answer your question, round duct is more efficient than square duct.

    [Edited by smadave on 02-22-2005 at 10:06 PM]

  4. #4
    Join Date
    May 2001
    Location
    Bennington, Vermont U.S.A.
    Posts
    13,864
    Air loses more velocity at elbows and fittings that anywhere else in the duct system. So you want to assure that the elbow has a means of "turning" the air in the direction that your want it to go. This being true then a round elbow will turn the air easier than a regular square 90 with no turning vanes.

    But don't make the mistake that alot of people do by thinking that a 8" round duct will carry the same amount of air as an 8 X 8" duct. Do the math.

  5. #5
    Join Date
    Apr 2004
    Location
    where you at?
    Posts
    3,359
    ASHRAE and/or SMACNA tables are a good source for Pressure Drop data...assuming that is what "duct efficiency" implies.

    If this is a residential installation, at the typical flow intensities involved (i.e. << 1000fpm) either duct profile will work fine, as the pressure drop differentials are low as compared to the dump-losses at the duct termination and the static P available by the fan.
    And, rectangular ducts offer a potential space saver - or not.

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