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Thread: 1 CFM /sq ft?

  1. #1
    Join Date
    Apr 2006
    Posts
    35
    OK

    I have a contractor who tells me I need 1 CFM per sq ft. That means I need 1980 CFM or 2000 CFM (no model J done)so a 4 TON will not work since 4 TON is 1600 CFM. Is this guy full of it or not. I hear people with 3.5 TON units and 1900 Sq ft in other areas of the country.

    HELP!!

    Thanks
    Greg

  2. #2
    Join Date
    Apr 2001
    Location
    Arkansas
    Posts
    2,111
    If that guy said it is raining outside, go look!!!!
    You need a load calc run, not some rule of thumb.
    I've got a lot of 3-ton systems on that size house.
    "FIGHT CRIME: SHOOT BACK"

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,599
    Click on the red tab, and pay the 50 bucks and do your own load calc.

    Then you'll know what size you really need.
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  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,599
    Not knowing where you are in New Mexico, he could be right.
    Depending on your house.
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  5. #5
    Join Date
    Apr 2006
    Posts
    35
    I have the software. I calculates 2.5 tons. That is why I am confuse. I am worried I input some data wrong. I am rechecking my measurements now.

    Thanks
    Greg

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,599
    What is your outdoor design temp, and what is your indoor design temp.

    Manual J doesn't tell you the size of the unit per say, it tells you how many BTU's at your design conditions, then the contractor is suppose to select the size unit that will meet that.
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  7. #7
    Join Date
    Oct 2003
    Location
    Valdosta Ga
    Posts
    847
    We are not in yur area I do not know factors involved to make a this judement. Thats why you want to have a couple bids.
    Talk to the contractor have him show cost of what he is doing.
    Maybe another contractor will give you more like a pad for the unit to set on , larger duct, sizes, easy access, a oudoor disconnect sw, digital tstat, emergency cut offs
    Sometimes you need alot of air exchange. If this contractor is the one you trust work it out.

  8. #8
    Join Date
    Feb 2004
    Location
    New Mexico
    Posts
    5,651
    You can hang yourself with rules of thumb. They just don't apply to every situation.

    I live in New Mexico and at 5200 ft of altitude there are factors that don't follow rules of thumb.

    Sun intensity

    27deg temperature swing

    Thinner air transfers temperature 4% per 1000' less than sea level.

    Light construction compared to moderate or heavy

    If a contractor bids a lot in a development that have similiar homes he probably can wing it.

    But like it was said pick up a copy of ACCA Manual J or go to HVAC Calc and they have an offer for a one time load.

    Tracers work both ways.

  9. #9
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
    Posts
    2,036
    "I have a contractor who tells me I need 1 CFM per sq ft."

    Is this a new rule of thumb for southern states or have I been out of school so long that someone dreamed up a new one?
    I personally still like the "how many fingers from across the street?" one.

  10. #10
    Join Date
    Apr 2001
    Location
    Arkansas
    Posts
    2,111
    Beleive or not....I had a contractor tell me that is the way he sizes the duct. If a room has 100sq ft it gets a 6in pipe.
    "FIGHT CRIME: SHOOT BACK"

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