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Old 11-03-2008, 06:23 PM   #1
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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?

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Old 11-03-2008, 09:07 PM   #2
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How to measure CFM?


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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?
Fasten a green garbage bag to a coat hanger, in the same manner a net is fastened to a fish net.
Then calculate the volume of the inflated garbage bag!
Then, with the bag deflated, hold the coat hanger over the vent opening! And turn on the blower!
Using a stop-watch, time how long it takes for the bag to inflate.
If the bag has a volume of 4 cubic feet and it takes 15 seconds to inflate, you divide 60 seconds by 15 seconds and multiply by the volume.
In this example, the result would be 16 cubic feet per minute!
I wish that I had invented this method, but I dint' .

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Old 11-05-2008, 07:28 AM   #3
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How to measure CFM?


Wish I could find the study, but a University compared this method to many high tech HVAC tools and concluded this was reasonably accurate.
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