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  1. #1
    Join Date
    Mar 2006
    Posts
    134
    I have a 3 ton AC and a 100K furnace
    My main duct is 12 inches x 18 inches
    I live in NJ

    Are my ducts over-sized, under-sized or correctly sized

  2. #2
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,122
    Originally posted by gsubrec
    I have a 3 ton AC and a 100K furnace
    This should be you first concern not the ducts, that is a big furnace for 3-tons of air.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  3. #3
    Join Date
    Feb 2005
    Posts
    249
    Have a Pro check the ESP(resistance of the duct system to air flow),and use the mfrs. fan data chart,to determine the air flow.

    A manual D ,from http://www.acca.org would be needed to determine if the room by room duct/grille sizes are correct.If they are not you will experience temperature differences from room to room.

  4. #4
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    those ducts are sized perfectly. 100k with a 3 ton drive is common as well. i see no issues.
    FILL OUT YOUR PROFILE!!

  5. #5
    Join Date
    Mar 2006
    Posts
    134
    In case, I need a bigger furnace / heatpump / ac,

    given my duct size of 12" x 18", how high can I go in Furnace ( without redoing my ductwork ), 120K BTU??

    given my duct size of 12" x 18", how high can I go in AC ( without redoing my ductwork ), 5 Tons??

  6. #6
    Join Date
    Feb 2005
    Posts
    249
    No one can answer that without a lot more info.,tinner saying thay are sized perfectly ,is just messing with you.

  7. #7
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,789
    Since we don't know the length of your duct, nor how many els, and transitions, we can't tell you if its big enough for 3 tons, let alone for a bigger a/c.
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  8. #8
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    We know a few limits

    Manual D is about a lot more than duct velocity, but that is one rule in there. I believe with hard duct you are requested to keep air speed below 900 fpm and required to keep it below 1000 fpm. If you get airflow above 1500 cfm thru your duct which is 1.50 sqft in cross section area, then you are surely violating one rule in Manual D.

    Of course you will need to do the full Manual D calculations in order to know whether the other rules are followed. Unless I am mistaken, one of the main goals of Manual D is to predict an acceptable ESP (External Static Pressure) with the design airflow. Having a tech measure ESP directly will tell you some key facts.

    My furnace manual indicates the Trane TUD100R9V5K 100K furnace wants a minimum of nearly 1300 cfm airflow, that is the low setting and is rated at 58 degree heat rise. Medium airflow would want 1600 cfm and high would be 1850-1900 cfm. Your furnace may have similar airflow requirements, you can ask this question of your own brand. Using the classic 400 cfm/ton for air conditioning, this suggests that section of ductwork might be the limiting factor when AC gets above 3.5 ton capacity.

    Cross your fingers and verify these opinions with a tech before you believe them as reliable info. Do you have solid reasons for believing you might want to upsize either the furnace or AC?

    Hope this helps -- Pstu

  9. #9
    Join Date
    Feb 2005
    Posts
    249
    Manual D velocity limits are for noise ,not sizing.

  10. #10
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,122
    Originally posted by tinner73
    those ducts are sized perfectly. 100k with a 3 ton drive is common as well. i see no issues.
    I must have missed something were did he list his 100,000 btu furnace has a 3-ton drive?
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  11. #11
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    Originally posted by mrbillpro
    Originally posted by tinner73
    those ducts are sized perfectly. 100k with a 3 ton drive is common as well. i see no issues.
    I must have missed something were did he list his 100,000 btu furnace has a 3-ton drive?
    well Bill..around here a 100k furnace with a 3 ton drive is pretty standard...16" wide blah blah. now, you can get one with a 4-5 ton drive as well with a larger cabinet. so if he has a bigger drive he'll just have to slow down the blower. i've never seen a 100k furnace with a blower smaller than 3 ton. in fact i've never seen a furnace with a blower smaller than 3 ton.

    so if he has an 18x12 trunkline at .08 friction is valued at 1250 cfm at close to 900 fpm. so i think those are pretty good numbers for the trunkline. just use your ductulator.
    FILL OUT YOUR PROFILE!!

  12. #12
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,122
    Originally posted by tinner73


    well Bill..around here a 100k furnace with a 3 ton drive is pretty standard..
    See in Houston a 50,000 btu is the standard with a 3-ton drive I guess I did not read what city the HO was in sorry about that.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  13. #13
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    so...

    >>Manual D velocity limits are for noise ,not sizing.

    What are you saying here, please? What are the circumstances under which it makes sense to exceed Manual D velocity limits?


    Thanks in advance -- Pstu

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