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  1. #1
    Join Date
    Jun 2010
    Location
    Charlotte, NC
    Posts
    155

    How do I calculate return duct size?

    I already learned from reading other posts my return grills are too small.

    How do I calculate if the return ducts are too small, too?

    I have a three ton downflow furnace a/c. I have two return ducts, hard metal round, one 14 inch and one 12 inch. How many square inches do I need to get the right airflow for 3 tons?

    Both returns are above the ceiling, with lengths of 12 and 8 feet, two ninety bends, (one up, one down).

    I want to find out if I need to tell whichever HVAC company I hire to replace my AC to just replace the return grills, or add another vent. I have not yet found anyone I trust to tell me what to do.

  2. #2
    Join Date
    Feb 2010
    Location
    The midwest.
    Posts
    663
    14" equals 925 cfm @.08
    12" equals 600 cfm " "
    Total cfm 1,525
    Needed for three ton: 1,200 cfm (min).
    Now, what size are the grilles?

  3. #3
    Join Date
    Jun 2010
    Location
    Charlotte, NC
    Posts
    155

    grill size

    Thanks, Todd for the answer on the duct size.

    Grills are 14 by 14 and 12 by 14. I am pretty sure they are too small.

    But if the ducts are right sized, I can just have the grills replaced.

  4. #4
    Join Date
    Feb 2010
    Location
    The midwest.
    Posts
    663
    Quote Originally Posted by Maikerum View Post
    Thanks, Todd for the answer on the duct size.

    Grills are 14 by 14 and 12 by 14. I am pretty sure they are too small.

    But if the ducts are right sized, I can just have the grills replaced.
    Hopefully they can be made larger. (2 cfm per sq. in.) 364 x 2= 728 cfm
    Less than 2 ton of airflow.

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by Todd S 2 View Post
    14" equals 925 cfm @.08
    12" equals 600 cfm " "
    Total cfm 1,525
    Needed for three ton: 1,200 cfm (min).
    Now, what size are the grilles?
    That's fine if the friction rate is actually .08,which would be a guess without knowing.

    However,chances are they are okay for three tons of air.

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by Maikerum View Post
    Thanks, Todd for the answer on the duct size.

    Grills are 14 by 14 and 12 by 14. I am pretty sure they are too small.

    But if the ducts are right sized, I can just have the grills replaced.
    How are they connected to the ducts?

    Aren't they noisey?

  7. #7
    Join Date
    Oct 2006
    Location
    The Beach
    Posts
    641
    Your return ducts are just right but the return grills are way to small. You have only 364 sq. inches. You need close to 600 sq. inches. We like to have 200/ton. This gets up down to around .05.
    Blue Fox

  8. #8
    Join Date
    Jun 2010
    Location
    Charlotte, NC
    Posts
    155

    Thanks everyone

    Thanks very much. I am glad my ducts are ok. Especially since I plan to install a smaller unit as a replacement, 2 or 2.5 ton. (I have significantly upped my insulation.) But still waiting on j-calcs.

    I really want to replace my return grilles with a more standard size. Two 16 by 20's give me 640 sq. in., which should be plenty. I can't think of a downside to being oversized a little. Does not look like it will be too easy to do but should be doable. Just got to find an hvac guy who likes to bend metal and cut drywall.

    They are in a hallway ceiling, and surprisingly, have not been too noisy. Not over the rumble of the blower motor, anyway.

  9. #9
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,159
    Not to burst your bubble, it's not the duct size but the length and how well installed the flex is that will determine if these 2 "hits are big enough. Is it pulled pretty tight or is it a loose solinky? Big differece in air flow and pressure drop. You need to take a monometer ( pressure meter) to see what the pressure readings are in the supply and return plenums to get an idea if the return is "choked"
    Was this a concern because you weren't getting air in one or more of the outlets?

  10. #10
    Join Date
    Jun 2010
    Location
    Charlotte, NC
    Posts
    155
    Thanks, genduct for the input, but I think my bubble is still safe. It may have been in a side thread so you may have missed it, but I mentioned my returns are hard metal duct, not flex. I also gave an approximate length and # of bends.

    I never had any problem with the system until my 21 year old ac gave up last week, but will be glad to enlarge my grills and hope to get slightly performance from it.

  11. #11
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Actually it was in the first post of this thead;

    I have two return ducts, hard metal round

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,787
    Are they just return grilles, or are they return air filter grilles?
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  13. #13
    Join Date
    Jun 2010
    Location
    Charlotte, NC
    Posts
    155
    They are return air filter grills. I use 1 inch pleated filters in them, and they have always seemed small to me, and irritating because they are odd size anyway.

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