CFM through wall cavities.

Harvey Ramer

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


Thanks

neilc

what do you mean ??
Air infiltration ?
or,
Like a ducted return for forced air ?

Harvey Ramer

A wall stud space return on a forced air system.

neilc

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,

Jamie Hall

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.

Harvey Ramer

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.

Techman

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?

Harvey Ramer

My total ESP is .5 The friction rate of the duct system is .094 per/100' of effective length.

neilc

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.

Techman

OK ,I see your point, but,what cfm would you recommend?

Spence

Group 7, page 163.

Harvey Ramer

Thanks

Spence

Sure is wonderful to see someone who cares!

NJ, Designer

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....

NJ, Designer

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.

Harvey Ramer

NJ, Designer said:

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.

Thanks! Very helpful!

Techman

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!

Harvey Ramer

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.

Spence

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.

ryanhowser

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'

Harvey Ramer

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.

Tinman

More cfm @ at a higher static while nothing else changes???

Rich_49

Welcome to HeatingHelp.com Doogie Howser , oh sorry , Ryan

Empire_2

40 cfm if your lucky......

aircooled81

I like the ductulator for quick refrences. if you want to engineer this to an exact science, there are books for that.
my quick math is, 0.5" static on the return, 4" stud (actual 3-1/2" stud) by 16" centers (subtract 1-1/2") would be 3-1/2x14-1/2 = to 50-3/4" free air space. plug that into ductulator, aprox 480cfm @ about 1500fpm. oh boy that does not sound realistic, but if your fan can pull that wall down that low, it might be a bit loud at the register!
for 2x6 wall cavity, about 950cfm @ 520 fpm.

anyone disagree with these numbers, for a loose observation?