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Vent Boots All Have a 6” Round Duct Attachment

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I am laying in new ducts for part of a home. After I determined what size ducts and vents were needed, I ran to the hardware store. I was surprised to find that all of the vent boots had a 6” round duct attachment. If a 4x12 vent would support 150CFM (assuming a 0.1 FR), why would the boot restrict to 6” (85CFM)? If you only needed 85 CFM for a vent, why not use a smaller size than 4x12”?
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There is more to it than that. FPM is a factor, I stopped doing this kind of work a few years ago, and can't seam to find my duct calc chart, So I can not tell you any specs on size. But when I did this, I kept velocity higher in some places than others. We slow the velocity down before the air registration to help keep sound levels down. So the beginning of the run is larger and after every take off, supply size reduces... Amazon sells a calculator like the one I always used.

Static pressure average was around .1"WC. 6" is a standard take off for a register unless you need a lot more than what one can provide. Can only suggest taking a class on this to understand or find a guy that can look at you case and size the duct correctly. The above from Amazon helps a lot to do this calc.

Good Luck.
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