Author Topic: Size a boot for flex duct  (Read 1501 times)

Offline wizedup

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Size a boot for flex duct
« on: July 14, 2015, 03:58:24 PM »
Want to increase 7" flex duct to 8", to the only vent in MBR / Vanity area. Now, not enough CFMs for the room(s) sizes; & has sloped, high ceiling - no attic.  Will put 8" collar at plenum.  May change the boot (a job in this heat). 

* What is standard boot size for 8" flex, or is there one?
* Do mfgs use outer box dimensions for "called" boot sizes, or the size opening needed in sheet rock?

* The old insulated boot is 6" x 12" (metal sticking thru sheet rock). Outer box dimensions are bigger.
The vent on it is 13.75" x 7.75".

Like to know ahead of time, required "frame" size for new boot (and if I can make it fit, where old vent is; how much trouble).

If new boot w/ 8" connection needs a different 2x4 frame size (& can't easily sawzall / scroll the old frame), like to get it ready to fasten in, before shutting down AC.

* Else, I could use the same boot & some reducer - for now.
* Any idea how much CFM loss, going from 8" flex to 7" at the boot?

Other option is leave the 7" and run a 2nd duct (maybe 6") & vent to same MBR wall.
But the 2nd run would take more work, be ~ 21 - 24 ft, have (3) 90's, or a 90 & couple 45"s (if use Els).
Either way, the old 7" flex needs replacing - w/ 7" or go to 8".