# Thread: Maximum air flow thru 12 flex duct?

1. Since we're talking FPM and CFM, here's a traverse I took a few months ago with a pitot tube. Notice how there are positive airflows, negative airflows, and points where there is no air flow.

The point I'm trying to make here is that just taking one measurement of airflow and assuming everything is the same everywhere can lead you in the wrong direction.

2. Originally Posted by freonfreak
i think everyone is missing my point.the duct size is to small as per the duculator and recommended .05 column you should size ducts for but with 0.43 total static clearly the ducts are not to small soo what is going on is the duculator wrong or am i loosing my mind on this one!!
You keep saying your duct is too small to get .43 static but again you are not taking into consideration the length of the duct. A 14" return duct that is only 14 ft. long can run 1000 CFM at less than .05 friction. The velocity will be way up there but the static will not be high. You say you're getting 700 CFM which means you're velocity is around 890 fpm on your 12" supply duct. On my ductulator it shows about .19 friction at that velocity but that is the friction rate for 100 ft. of 12' duct. If you have 50 ft. then your friction rate is cut in half so it would be .095. The same goes for the return so its not that the ductulator is wrong but that it only shows the friction rate for 100 ft. of duct. You actually gave me the incentive to add that function to my online ductulator so you can adjust for the duct length. I just finished it tonight (I think).
http://www.oceanhvac.com/ductulator/

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Originally Posted by garyed
You keep saying your duct is too small to get .43 static but again you are not taking into consideration the length of the duct. A 14" return duct that is only 14 ft. long can run 1000 CFM at less than .05 friction. The velocity will be way up there but the static will not be high. You say you're getting 700 CFM which means you're velocity is around 890 fpm on your 12" supply duct. On my ductulator it shows about .19 friction at that velocity but that is the friction rate for 100 ft. of 12' duct. If you have 50 ft. then your friction rate is cut in half so it would be .095. The same goes for the return so its not that the ductulator is wrong but that it only shows the friction rate for 100 ft. of duct. You actually gave me the incentive to add that function to my online ductulator so you can adjust for the duct length. I just finished it tonight (I think).
http://www.oceanhvac.com/ductulator/
Thanks for updating that ductulator.. I was using that before..Interesting...

Ill play around with that new function and let you know...

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