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Thread: Return size ??

  1. #1
    Join Date
    Jul 2006
    Location
    b'ham , al
    Posts
    23
    how many square inches should i have for a 3.5 ton heat pump.
    I have 2 grills on same wall(corner), one is 20x20 the other 16x20, but they share the same box(metal) , as it goes into through the floor though the cut out is 14x21, which would be 294 sq in. now once it goes through the floor there is a large 22x22 that leads into thee AH, my question is the 14 x 21 BOOT in the floor....is that enough???how much do i need ???
    THX again ..

  2. #2
    Join Date
    Dec 2005
    Location
    California/Nevada
    Posts
    3,624
    according to my calculations , it's right on the nose.

    18" round return duct(for system size) , translates to a 14 x 20 square return register.



  3. #3
    Join Date
    Jul 2006
    Posts
    1,091
    wolfstrike,
    We use a .05 friction loss for return air ducts and 300FPM grille sizing. It work really well for heat pumps (Rheem use to recommend it)

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Ductolator can only be used to size the ducts ,AFTER determing the FR(friction Rate) of the actual duct system,using Manual D,from http://www.acca.org


    Using .05 or .1 is just a Rule of Thumb.

  5. #5
    Join Date
    Jul 2006
    Posts
    1,091
    Yeap... rule of thumb... but on the other hand! Ha! (Ya get it... on the other hand!)

    dash, you are right.

    I've found that most of our applications (the style of homes built in our area) that rule of thumb falls right in line. There are exceptions. But I have never found a case where we had too much return air.

  6. #6
    Join Date
    Jul 2005
    Posts
    717
    [QUOTE]Originally posted by wolfstrike
    [B]according to my calculations , it's right on the nose.

    18" round return duct(for system size) , translates to a 14 x 20 square return register.-----------------
    ----------------------------------------
    WRONG.
    18 " diam = 14 x20 in rectangular ducted area, but a 14 x20 REGISTER or GRILLE will have a definite percentage of LOSS depending on its design.
    If the register that is installed has a 20 % design loss,then a register having an minimum area of 336 sq inches, or a 14 x 24 would have be installed.




  7. #7
    Join Date
    Jul 2006
    Location
    b'ham , al
    Posts
    23
    THX for the replies guys. like i said , i got GRILLES....LOL
    plenty of filter area. so is the cut out good to go ??or do i need to add another return?

  8. #8
    Join Date
    Jul 2006
    Posts
    1,091
    toddgotfish,
    you have enough.

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