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Old 08-17-2012, 08:36 PM   #16
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Fun with Ductwork


Use this for your calculations.
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Old 08-20-2012, 12:29 PM   #17
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Ive been real busy lately with my new job. Havent started it but all the preliminary requirements are insane. After looking at this chart, it seems to just move all the ducts up 1 size. My 7's go to 8's, 8's go to 9's and so forth. Whats the FPM on this type of setup? It doesn't say what the CFM is with a corresponding FPM. It says at an ESP of .5 for flex duct which i think is what most contractors try to achieve when installing a full unit? (maybe? closer to 1 without going over better?)

I just want to make sure this chart is accurate because if i calculate the new sizes with the old flow with flex duct it puts me WAY over CFM and i really want to make sure its accurate. 700 FPM is an average speed of airflow for ducts under 25 feet long yes?

I realize the contractor i get, if hes good, can do all this work and make it awesome but i dont want to rely on him 100%. I want to attempt to know what hes going to do before he gets here, tell him what I suggest and compare it to what he suggests. My house is old, getting ducts in different places is not easy and obviously if flow calculations have changed then i need to ensure myself that i am getting a good setup.

This chart, would it also be comparable to return air also? I always feel return air is somewhat faster FPM than supply air. My main return is a 16 inch, does that mean im only supplying my return with 1000CFM??? If so i need to go out and get a freaking 20 inch right now.
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Old 08-20-2012, 04:00 PM   #18
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What's the size of your furnace...tonnage, btuh? That determines more than any other factor what the cfm is on your unit.
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Old 08-20-2012, 04:12 PM   #19
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Your supply FPM will be in the 400s.
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Old 08-20-2012, 06:23 PM   #20
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75k i beleive, i had a picture of it here somewhere. i could always go back up and get one but i know it was 75k. 4ton.

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