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Thread: CFM Formula

  1. #1
    Join Date
    May 2005
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    2
    I'm trying to remember the formula for calculating the CFM of a indoor split system air handler. I know that you're supposed to take the amp draw of the unit, and the delta T between the supply and return, but what is the formula that you plug you info into to figure out how much CFM you are getting across the coil/heat strips? HELP ME PLEASE!

  2. #2

    CFM = Load / 1.1 x TD



    This is a sensible heat equation for airflow requirements.
    Is it the one you are asking about?

  3. #3
    Join Date
    Mar 2005
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    South/West of Quebec in the other part of Canada
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    2,331


    CFM = volts x amps x 3.414 / 1.08 x temperature rise

  4. #4
    Join Date
    Apr 2005
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    28
    Chillbilly;

    I your TD the same as delta Temp across the evap?

    What about the CFM for a gas furnace?
    thanks
    gn

  5. #5
    Join Date
    Mar 2005
    Location
    South/West of Quebec in the other part of Canada
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    CFM = btu output/ 1.08 x temperature rise

    P.S. your temp rise is not across the evap, it is across the heating elements or heat exchanger.

    P.P.S. volts x amps x 3.414 = btu output on electric furnace

    [Edited by key on 05-18-2005 at 06:46 AM]

  6. #6
    Join Date
    May 2005
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    2

    Smile

    That's the one. Thanks a million.

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    The accuracy of the measurements can make a large difference,using this method.

    A manometer and fan data chart,is a lot easier and more accurate,IMHO.

  8. #8
    Join Date
    Sep 2004
    Posts
    172
    I fooled with those calculations to determine CFM several years ago, I finally gave up and went and bought a good quality Pitot tube airflow tester, a good traverse on the main supply trunk will eliminate all the guess work.
    When you give a lesson in meanness to a critter or a person, don't be surprised if they learn their lesson.

  9. #9

    CFM testing problem

    In my refigeration book the cfm calc formula is CFM=BTUH/1.1XTD on my test they used BTUH input (Not Output) and at sea level. What is up with this.
    I have also seen 1.0505 and 1.08 as numbers in the formula. At any rate my answers were not the same as those given to chose from.

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