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Thread: Air flow?

  1. #1
    Join Date
    Sep 2005
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    Anyone know the difference in flow from a gas furnace that is rated for 3 tons vs. 4 tons? I have 3 tons and want to know if the upgrade is worth it, since I got no real airlow right now.

  2. #2
    Join Date
    May 2005
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    Seabeck, WA
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    1,870
    400 cfm / ton. On Trane variable speed units you have more options. 40 to 65 degree split.
    Jason
    Live for yourself and ask no one to live for you.

  3. #3
    Join Date
    Jul 2003
    Location
    Massachusetts
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    288
    The answer lies in your ductwork. If it isn't sized for a 4 ton, it will overheat the furnace and it will go off on high limit. Probable that your issue is not with the furnace. Have someone check it out for you.

  4. #4
    Join Date
    Sep 2005
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    Ductwork is 20x13 rectangle, working fine. The problem lies with the AC unit not moving air, it is a 3.5 ton ac on a 3 ton furnace, I just bought the house knowing this was a problem, but now I need to fix it with 100+ temps outside. Thanks for the help guys.

  5. #5
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    Jan 2004
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    PA
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    Might just be the evap coil is dirty.
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  6. #6
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    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Originally posted by socrates
    Ductwork is 20x13 rectangle, working fine. The problem lies with the AC unit not moving air, it is a 3.5 ton ac on a 3 ton furnace, I just bought the house knowing this was a problem, but now I need to fix it with 100+ temps outside. Thanks for the help guys.
    You need to look at the "Whole" duct system,and/or test the static,to know if the duct system is "fine".

    Yes a 20X13 ID duct sounds "fine",but what about the fittings ,turns,grilles ,branch duts,Etc.,etc..?

    Tell us more about the ducts,etc. and the model numbers involved.

    Check for dirty filter,blower or coil,have a Pro,take a look.When calling ask"can you test the airflow,of the system,and if so how?Many can't and that will waste their time and yours,as well as your money.

  7. #7
    Join Date
    Sep 2005
    Posts
    4
    Ductwork is 20x13 from the plenum (20x17) to the branch ducts which are all 6" insulated hard line. There are 9 of these branches. The furnace uses the cavity below the closet as a return plenum with a single 12x24 grille. Coil is clean, I cleaned it, filtering system is brand new and there are no major restrictions that I can see right now with the ductwork, the only thing is where the plenum goes into the main duct it is a hard 90 degree elbow instead of a rounded one.

    Model numbers:
    Furnace: Goodman GMS80703ANA
    Evap coil: Goodman CAUF048C2A
    Condenser: Goodman CLT48-1

    From what I have found the difference in the 3 ton blower and 4 ton blower is that the small one is 6" wide and the bigger is 10", for this reason I think I got the coil freeze.

    I am not looking for free advise here, I have called three contractors out and gotten three different suggestions as follows:

    One said put a huge booster blower in the ducting just after the plenum, one said I need a new blower assembly in the furnace, but that will not fit by my calculation and the last said I need freon, even though I just had it charged and the guy who charged it said it was good with the superheat calc.

    I do not want a huge booster pulling a vacume on the coil and causing it to not drain, nor do I want to have to put a new furnace in place of the new one that came with the house.

  8. #8
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    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    IMHO.

    A manual D calculation is what is needed to know for sure,but looks like;
    9 ,6" round or sq. branch ducts are not enough ,or too small for 4 tons.
    12X24 return grille is too small,1600cfm ,is 900 feet per min.,should be 400 fpm maximum.

    Is the 20X13 ,ID or OD,if metal ,insulated inside or wrapped?


    You said"Filtering system is brand new",if it's more restrictive than the old one ,it could be part of the problem.

  9. #9
    Join Date
    Sep 2005
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    4
    GI have asked all of the contractors for the manual d test, all of them just say get more airflow with a blower or new furnace, never even checked for a test. I am right on the border of US/Mex and most of the AC guys are from Mex and don't really go by the book here. 13x20 is the metal, it is wrapped externally. The air cleaner is rated to flow for the 4 ton unit, but yes it is more restrictive that the factory little fiber crap one.

    Guess I'm off to look for another contractor, thanks guys.

  10. #10
    Join Date
    Jan 2004
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    PA
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    Originally posted by socrates

    Model numbers:
    Furnace: Goodman GMS80703ANA
    Evap coil: Goodman CAUF048C2A
    Condenser: Goodman CLT48-1

    From what I have found the difference in the 3 ton blower and 4 ton blower is that the small one is 6" wide and the bigger is 10", for this reason I think I got the coil freeze.




    the blower wheel of the 703, is 10" x 6", and the blower wheel for the 704 is 10" x 8".

    the 703 uses a 1/3 hp motor, and the 704 uses a 1/2 hp.


    You need a load calc done on your house to see if the a/c is the right size, then determine if you need the a/c swapped out, or furnace.
    The 4 ton furnace mod # I think is gms80704bna


    Hard to keep a 4 ton a/c from freezing the coil with only 1200 cfm, at mild outdoor temps.

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  11. #11
    Join Date
    Jul 2003
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    Massachusetts
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    Originally posted by socrates
    it is a 3.5 ton ac on a 3 ton furnace

    definatlety a problem, however, you may not be able to upgrade the furnace without changing some ductwork also. You should find out which one is wrong. The furnace size or the condenser size? That is what I think everyone is trying to say. I mean what if you swap out the furnace and everything works great and the ac is working and you have great airflow - BUT, now it runs for 2 minutes at a time all day short-cycling and never gets the house really comfortable?
    "ok, everyone just calm down."

  12. #12
    Join Date
    Mar 2004
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    CHICAGO SUBURBS
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    am right on the border of US/Mex
    and most of the AC guys are from Mex and don't really go by the book here. .........................doing jobs americans don't want to do, no doubt.[satire of coarse]
    Some people swear by me and some at me

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