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  1. #1
    Join Date
    Sep 2012
    Posts
    37

    Is this return big enough?

    I have a 2.5 ton AC on an 80,000 BTU gas furnace. The existing (single) return consists of a 20 x 20 filter grille and approximately 20 feet of 14" round pipe with two 90 degree elbows. This connects to a return plenum under the furnace.

    I know I should increase the filter size to either 20 x 24 or 20 x 25 (the 20" width can't be increased, but I can accommodate some extra height). Is the 14" pipe large enough for this installation?

    Thanks!

    D.

  2. #2
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
    Posts
    7,635
    nope
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  3. #3
    Join Date
    Sep 2007
    Location
    Boise, ID
    Posts
    4,259
    The pressure drop of the return duct (assuming it's galvanized sheet metal) is only 0.036, and that's on the high side. Not knowing the plenum configuration (= system affect), type of filter, grill etc. it's hard to answer your question. Off the cuff, the 14" dia. duct is ok as pressure loss in duct is minimal.
    If a nation expects to be ignorant and free, in a state of civilization, it expects what will never be. (Thomas Jefferson 1816)

  4. #4
    Join Date
    Mar 2011
    Location
    Galveston Texas
    Posts
    530
    If it was me neither would be big enough. A 20x20 grill is only large enough for a 2 ton unit, and the 14 in duct if it is galvanized would only handle ~700 CFM so that's less then 2 ton at 400cfm/ton. The 20X25 R/A would be just big enough for a 2.5 ton unit, and a minimum of a 16" duct is required (and that's still a bit light but only by ~20cfm) if galvanized and an 18 if flex (again it's still a bit light by 25cfm). These are for a bare minimum 1000cfm of airflow.

  5. #5
    Join Date
    Sep 2007
    Location
    Boise, ID
    Posts
    4,259
    20' of galvanized duct at 1000 cfm: .09 p.d./ 100 ft. =.09*20/100=0.018 loss. That is nothing. It's not "how much air a duct can handle", the only thing the fan cares about is how much static pressure it needs to overcome. I agree the filter is small, based on velocity, but all that does is reduce filter effectiveness of the filter and add to total system static pressure losses, well, it also increases NC (noise criteria).
    If a nation expects to be ignorant and free, in a state of civilization, it expects what will never be. (Thomas Jefferson 1816)

  6. #6
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    16 inch

  7. #7
    Join Date
    Sep 2012
    Posts
    37
    This broad range of answers is an interesting illustration of both the value and challenge of getting multiple professional opinions on most anything. It would appear that there is no absolute answer, though I suspect most of the responders would disagree.

    D.

  8. #8
    Join Date
    Sep 2007
    Location
    Boise, ID
    Posts
    4,259
    When I am selecting a fan motor for a 20K cfm air handler I need to know what the total system pressure is. I need to find which branch has the highest static pressure loss, both supply and return. Knowing what my loss is the only way I can ensure I select the proper horsepower of motor to overcome the total system static pressure. This is why manufactures have fan charts. Heck, try asking a vendor to give you a selection on a 2000 cfm roof mounted exhaust fan. The first question they will have is "what's your static pressure".
    If a nation expects to be ignorant and free, in a state of civilization, it expects what will never be. (Thomas Jefferson 1816)

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