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  1. #1

    How to measure CFM?

    I am conditioning my crawl space and local code is to open a vent on the HVAC in the crawl space. I noticed that there was already a vent down there. The code is 1 cfm for each 50ft2 of floor area, so I need 18 cfm coming out if the vent. Does anyone know how I measure that?

  2. #2
    Join Date
    Feb 2008
    Location
    Western NC
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    2,504
    This is not a DIY site.

    Call your local contractor to come look at your crawlspace and determine the best solution to your situation
    I fully support the military and the War on Terrorism.


    If you don't know, then don't do. If you don't know and still do, then be prepared to pay someone else a lot to undo what you did and then do it right.

    If you do know, then do. But do it right. Otherwise, you may not be doing it long.

  3. #3
    Join Date
    Dec 2007
    Location
    East central Indiana
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    477
    That seems like a VERY bizzare code that would be nearly impossible to enforce.

    How many building inspectors do you know who are capable of doing CFM analysis???

  4. #4
    From talking with the city today, it is more like a recommendation that anything else.

  5. #5
    Join Date
    Feb 2008
    Location
    Western NC
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    2,504
    Quote Originally Posted by timchi29 View Post
    From talking with the city today, it is more like a recommendation that anything else.
    And from talking with us...its more of a recommendation that this type of work be performed by a professional that understands how to measure CFM, how to adjust CFM, and more importantly, knows whether or not this type of action needs to be done.
    I fully support the military and the War on Terrorism.


    If you don't know, then don't do. If you don't know and still do, then be prepared to pay someone else a lot to undo what you did and then do it right.

    If you do know, then do. But do it right. Otherwise, you may not be doing it long.

  6. #6
    Easier said than done. One contractor said that they need to replace the duct work then they can get an accurate # and then another said that they would have to rent the equipment, which will require a down payment. Amazing what HVAC contractors can think what they can get away with if you let them.

  7. #7
    Join Date
    Mar 2005
    Location
    Houston, Texas
    Posts
    11,847
    Quote Originally Posted by timchi29 View Post
    Amazing what HVAC contractors can think what they can get away with if you let them.
    You do know, you're speaking to HVAC contractors, don't you? Catch more flies with honey, than vinegar.

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,266
    Quote Originally Posted by timchi29 View Post
    I am conditioning my crawl space and local code is to open a vent on the HVAC in the crawl space. I noticed that there was already a vent down there. The code is 1 cfm for each 50ft2 of floor area, so I need 18 cfm coming out if the vent. Does anyone know how I measure that?
    Even good ducts leak more than that. Expect a 8-10% leakage on good ducts. 3 ton a/c has 1000 cfm of flow or 80 cfm of leakage. Thats more than enough. Plastic on the earth and seal the outside vents to keep the moisture out. Monitor the %RH and if %RH exceeds 60%RH, get a good dehu like Santa Fe Compact. For monitoring, check out the Accurite remote %RH meter from Wal-Mart or thermastor.com for about $35. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  9. #9
    Thanks Teddy. I am actually getting 4" of concrete installed today to seal from the earth.

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