About 50 CFM. Provided the entire plate has been cut out.

 

After destroying the building structure,
one might get 270 CFM in a perfect world.

0.3 Sq Ft * 800 FPM = 240 CFM or
see attachment

 

Wood is a much rougher surface then sheet metal. So that chart does not apply.

 

160 cfm @ .08 according to the chart which is based on 100 feet.

 

So the total dP will be 1/10th as much for only 10 feet.

How do you determine 0.08" friction rate is applicable to the composite surfaces of the interior wall?

 

Is the "paper" back side of wall board smoother than metal?

 

Okay, from what I can gather, the original contractor did not get the full benefit of this return being he only used 6" flex duct. This return is in a master bedroom where there are 4 supply registers when you include the ones in the master bath and walk in closet.. I've been learning about what to do about my upstairs system which needs replaced now, but in the process, I realize the design problems with the downstairs system.. I now realize that my down stairs heat pump system is lacking on the return side by over 25%. I have a 3 ton heat pump and one 20" x 20" filter grille in the main return and this little wall cavity with only 6" flex leading back to the air handler return... So I'm trying to decide how to best correct the situation..

Thank you guys for taking the time to help those who ask
Sincerely: John

 

16" on center will only have 14.5" between boards...

 

If you assume the roughness factor for the wall cavity to be conservatively similar to flex duct then I get 125 cfm for a friction factor of 0.09". Thank you very much

 

3.5" x 14.5" = 50.7 square inch

8" Round area Equivalent that is stretched 10 feet out of 11 foot length = 210 CFM with dP of 0.04"

 

Thank you ALL for your input. This made for a pretty thorough discussion on the matter..
I think that changing everything from 6" flex to 8" flex is about all that would benefit..

Thanks Again.
Sincerely: John