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# CFM through wall cavities.

Member Posts: 2,214
How many CFM is a reasonable number to figure in a 2x4 wall cavity with studs 16" on center? Like wise 2x6?

Thanks

ramermechanical.com

• Member Posts: 805
what do you mean ??
Air infiltration ?
or,
Like a ducted return for forced air ?
• Member Posts: 2,214
A wall stud space return on a forced air system.

ramermechanical.com
• Member Posts: 805
this is my second disclaimer today,
I am not a contractor, nor engineer,
but I found this for ya,
www.comfort-calc.net/cfm_chart.html
says 3x14 = 150 cfm,
not sure how to apply length of run though,
others should chime in,
• Member Posts: 12,716
150 cfm would give you 8 feet per second in that space. Which might be OK for smooth duct -- but the inside of a stud space isn't smooth, and your head loss would be quite high, seems to me. But that's just a gut feeling.
Br. Jamie, osb

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
• Member Posts: 2,214
I'd be nervous figuring more than a 100 CFM but I really can't find any manual D data on the equivalent length and friction rates.

ramermechanical.com
• Member Posts: 2,144
edited March 2015
Well I guess that depends on your design return static. If using .05 I'd KindaSorta say 100CFM is a good #, all that rough surface and all. . At .08 its a little more. Can you line the wall cavity with custom made trunk to get the CFM up a bit. I saw something somewhere about using the wall cavity, now where the *ell is it?
• Member Posts: 2,214
My total ESP is .5 The friction rate of the duct system is .094 per/100' of effective length.

ramermechanical.com
• Member Posts: 805
how much derating should you really need to do for 2x4/2x6 and sheetrock, that's can't be all that rough, even if there's a wire or 2,
now if we're talking old rough sawn, lathe and plaster keys , , ,
then I can see quite a turbulent pathway.
• Member Posts: 2,144
OK ,I see your point, but,what cfm would you recommend?
• Member Posts: 316
Group 7, page 163.
• Member Posts: 2,214
Thanks

ramermechanical.com
• Member Posts: 316
Sure is wonderful to see someone who cares!
• Member Posts: 53
Manual D 3rd Edition, Appendix 3, Fitting Equivalent Lengths, Group 7 Panned Joists and Panned stud return air fittings. page 163, Reference Velocity = 700 FPM, Friction rate = 0.08 IWC per 100ft. 200 CFM maximum in stud space, 400 CFM maximum in Joist space.

It Doesn't say anything about a 2x6, I would Guess around 300 CFM, unless they are only talking about 2x6 stud space, But I don't think so.....

I have not opened that book in a while but is sitting right behind me where it belongs. I used to have it on a PDF can't find it now.

Contact me if you need some more help....
• Member Posts: 53
Found it, if you can open excel spreadsheets go to http://www.acca.org/standards/acca-speed-sheets/

Download the speed sheet for Manual D and open the G7 tab.
• Member Posts: 2,214

Found it, if you can open excel spreadsheets go to http://www.acca.org/standards/acca-speed-sheets/

Download the speed sheet for Manual D and open the G7 tab.

ramermechanical.com
• Member Posts: 2,144
OK ALL WELL AND GOOD, NOW!What does that give you for an UN-panned chase,Mr.Harvey? I'm lowerrrr down in the trenches than you!
• Member Posts: 2,214
Welll.... I really don't know how you would figure that one out. Unless the pressure drop was large enough to take a physical measurement.

ramermechanical.com
• Member Posts: 316
Since there is no roughness index for unpanned air spaces, it is safe to assume unpanned spaces have the same friction rate as does duct liner.
• Member Posts: 1
edited January 2016
You normally wouldn't want to use a 2 x 6 studs space for return air because no one would ever use an exterior wall for return air. Unless of course you don't mind having No insulation in that stud space or really hot or really cold return air temp on extreme dayson 2 x 4x16 studs space I usually account for 150-175 CFM at .05 static or 225-250 at .1 static. the legnth doesn't matter it can be 10' or 100'
• Member Posts: 2,214
I am quite aware that you don't use outside wall cavities for return air.

Would you care to elaborate a little on how you derived the calculations you posted above? Not sure I agree with them.

ramermechanical.com
• Member Posts: 2,538
More cfm @ at a higher static while nothing else changes???
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