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  1. #1
    Join Date
    Nov 2007
    Posts
    15

    Return Duct Size / CFM?

    After posting a prior question about return grille filters, I ended up opening a can of worms on how my duct is set up. Below is my info and then the questions.
    Goodman 3 ton, SSZ16 with a AEPF426016

    Main level return is 8x14 at the bottom of the wall in the dead center of the house in an open foyer. Grille size is 16x20 (with 4" of depth)but the duct opening is now cut to about 7.5x18. There is also seven more feet of duct running up the wall before it's boxed off.

    Second, 14x8 return runs down from the upstairs hall close to the ceiling. About a 22ft run with two 90s. 16x20 grille (same depth) and about 13x18 duct opening.

    Both lead to a 10x20 main duct.
    I've been seeing the basic recommendation of 144" per ton.
    1) If each duct is 112" then how could I even come close to 432"? Assuming no filters until the 21x23 at the handler, I know that this is an unrestricted 224", but it still seems low. I was considering adding more returns, but will it make a difference in terms of air flow?

    Also, I have two, 8x14 supply lines. One to each floor.

    Thanks,

    Mike

  2. #2
    Join Date
    Jun 2008
    Location
    South,Tx
    Posts
    331
    Get a Air Duck Calculator , and start doing math.........
    Matt 7:12 The Golden Rule
    "Do for others what you would like them to do for you. This summary of all is taught in the law and the prophets.

  3. #3
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by Mike71 View Post
    After posting a prior question about return grille filters, I ended up opening a can of worms on how my duct is set up. Below is my info and then the questions.
    Goodman 3 ton, SSZ16 with a AEPF426016

    Main level return is 8x14 at the bottom of the wall in the dead center of the house in an open foyer. Grille size is 16x20 (with 4" of depth)but the duct opening is now cut to about 7.5x18. There is also seven more feet of duct running up the wall before it's boxed off.

    Second, 14x8 return runs down from the upstairs hall close to the ceiling. About a 22ft run with two 90s. 16x20 grille (same depth) and about 13x18 duct opening.

    Both lead to a 10x20 main duct.
    I've been seeing the basic recommendation of 144" per ton.
    1) If each duct is 112" then how could I even come close to 432"? Assuming no filters until the 21x23 at the handler, I know that this is an unrestricted 224", but it still seems low. I was considering adding more returns, but will it make a difference in terms of air flow?

    Also, I have two, 8x14 supply lines. One to each floor.

    Thanks,

    Mike
    Would you be able to find out (or estimate) your return air speed in feet/min. A local pro once gave me a table relating to single round return ducts, with recommended sizes. Also my Manual D book gives recommendations for max air speed, pretty much same answer. Looking at air speed is only one factor of duct design, but it is an important one.

    Hope this helps -- Pstu

  4. #4
    Join Date
    Nov 2007
    Posts
    15
    Quote Originally Posted by pstu View Post
    Would you be able to find out (or estimate) your return air speed in feet/min. A local pro once gave me a table relating to single round return ducts, with recommended sizes. Also my Manual D book gives recommendations for max air speed, pretty much same answer. Looking at air speed is only one factor of duct design, but it is an important one.

    Hope this helps -- Pstu
    I have the dip switches set so that it should be running at 1200cfm, which I've read is the maximum for the 10x20 and 600cfm for the 8x14.

    Thanks for your help!

  5. #5
    Join Date
    Jun 2009
    Location
    massachusetts
    Posts
    22

    Mike71

    You can never have to much Return Air, but you can have to little, adding another Return will hurt nothing

  6. #6
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    If you run 1200 cfm thru that 10*20 it will be moving 864 feet/min. Unless I have made an arithmetic mistake. Assuming you have rigid ducts, page 3-6 of ACCA Manual D recommends a maximum 700 feet/min.

    600 cfm thru that 8*14 duct will be moving 771 feet/min. I am not qualified to say what will happen when you exceed the Manual D maximum. Maybe little or nothing.

    If you want to follow the recommended velocity for returns, it is 600 fpm for trunk ducts and 400 fpm for branch ducts. As a previous poster has already said, I believe there is no downside to having a very generous return. That might encourage you to enlarge or add to the present system.

    Hope this helps -- Pstu

  7. #7
    Join Date
    Jun 2009
    Location
    massachusetts
    Posts
    22

    pstu

    Think of it this way, your blower motor is pushing the supply air and pulling the return air, if you have to little return air on a furnace you will trip the high limit because your not moving enough air across the heat exchanger. If you have to little return air on an air handler you will end up with an iced up coil, once again this is because your not moving enough air across the coil. The blower motor is only going to push and pull the amount your dip switches are set for low, med, or high, if you have an over sized return trunk it is not going to hurt anythning

  8. #8
    Join Date
    Feb 2004
    Location
    5280
    Posts
    466
    getting all your info from a ductulator can lead you down a bad path. I would start with ESP to determine if changes are needed and if its needed in the SA or RA.
    "We'll have to outwit the fiend with our superior intelligence." Yukon Cornelius

    Some people are like Slinkies---not good for anything, but still bring a smile to your face when you push them down the stairs!

  9. #9
    Join Date
    Jun 2009
    Location
    massachusetts
    Posts
    22
    I would measure the rooms, windows, doors,. Do your insulation and window type, etc. and do a manual j and get your BTUs. Size your cond, go .5 ton bigger on the evap coil or Air handler. Find your CFM and use the right size pipe or flex.(supply houses have charts) and put it together, simple. Some people have just measured rooms and x 1.08 to get the CFM, it may work but I honestly have never done it that way.

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by stenly View Post
    I would measure the rooms, windows, doors,. Do your insulation and window type, etc. and do a manual j and get your BTUs. Size your cond, go .5 ton bigger on the evap coil or Air handler. Find your CFM and use the right size pipe or flex.(supply houses have charts) and put it together, simple. Some people have just measured rooms and x 1.08 to get the CFM, it may work but I honestly have never done it that way.
    Manual D from www.acca.org is
    te industry and ANSI Standard to size the ducts. There's a bit more to it then a chart that the supply house has.

  11. #11
    Join Date
    Jun 2009
    Location
    massachusetts
    Posts
    22
    The chart that I am talking about is the one that will tell you how many CFM that a particular pipe size is equal to, thats all. Absolutely Manual D has a load of info that any person that is installing Duct work should own and read.

  12. #12
    Join Date
    Aug 2003
    Posts
    906
    Quote Originally Posted by Mike71 View Post
    After posting a prior question about return grille filters, I ended up opening a can of worms on how my duct is set up. Below is my info and then the questions.
    Goodman 3 ton, SSZ16 with a AEPF426016

    Main level return is 8x14 at the bottom of the wall in the dead center of the house in an open foyer. Grille size is 16x20 (with 4" of depth)but the duct opening is now cut to about 7.5x18. There is also seven more feet of duct running up the wall before it's boxed off.

    Second, 14x8 return runs down from the upstairs hall close to the ceiling. About a 22ft run with two 90s. 16x20 grille (same depth) and about 13x18 duct opening.

    Both lead to a 10x20 main duct.
    I've been seeing the basic recommendation of 144" per ton.
    1) If each duct is 112" then how could I even come close to 432"? Assuming no filters until the 21x23 at the handler, I know that this is an unrestricted 224", but it still seems low. I was considering adding more returns, but will it make a difference in terms of air flow?

    Also, I have two, 8x14 supply lines. One to each floor.

    Thanks,

    Mike
    The 144 in2/ton figure is for filter size, not duct size. It corresponds to an air speed of 400 fpm at the filter. In general ducts can handle faster air speeds. The end goal is a reasonable static pressure at the air handler. That depends on the shape and length of the ductwork vs air speed (Manual D etc, etc, amen), and also the air speed at the filter (and obviously the filter material itself). If you had two 16x20 filters (one at each grille and none at the air handler) that would give 320 in2 each or 640 in2 total, which is ample area (1200 cfm/640 in2 x 144 in2/ft2= 270 fpm). You mention a 21x23 = 483 in2 at the air handler and that yields 358 fpm, still good.

    So, the filter area sounds fine (assuming that you have filters either at the grilles or the air handler and not both, the filters are not HEPA, are changed regularly, yada yada). If you need anything, one fix would be bigger pipes between the return grilles and the air handler. My suggestion would be to measure the static pressure at the air handler, and if it's too high, calculate how much bigger the upstairs return duct should be to bring it down to a reasonable value, instead of 8x14 (as the longer run is probably where the most pressure is lost). Or, if you keep the filter on the air handler, add another return at a location that will help balance airflow (most likely needed on the second floor, but that's just a guess as we don't know if the supply side on the second floor is similarly more restricted).
    -If you won't turn it on then nothing else matters.

  13. #13
    Join Date
    Aug 2003
    Posts
    906
    Quote Originally Posted by stenly View Post
    You can never have to much Return Air, but you can have to little, adding another Return will hurt nothing
    There is a possibility that it could change slightly the supply of air or circulation pattern in various rooms or floors. That is, assuming you care about having perfectly balanced airflow and precisely delivered air. I'll grant you most people won't lose sleep over that and will simply adjust supply registers, if they even do anything. *shrug*
    -If you won't turn it on then nothing else matters.

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