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CFM through wall cavities.

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


Thanks
Ramer Mechanical
ramermechanical.com
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Comments

  • neilcneilc Posts: 722Member
    what do you mean ??
    Air infiltration ?
    or,
    Like a ducted return for forced air ?
  • Harvey RamerHarvey Ramer Posts: 2,200Member
    A wall stud space return on a forced air system.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • neilcneilc Posts: 722Member
    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 HallJamie Hall Posts: 11,195Member
    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.
    Jamie

    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
  • Harvey RamerHarvey Ramer Posts: 2,200Member
    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.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • TechmanTechman Posts: 2,144Member
    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?
  • Harvey RamerHarvey Ramer Posts: 2,200Member
    My total ESP is .5 The friction rate of the duct system is .094 per/100' of effective length.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • neilcneilc Posts: 722Member
    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.
  • TechmanTechman Posts: 2,144Member
    OK ,I see your point, but,what cfm would you recommend?
  • SpenceSpence Posts: 316Member
    Group 7, page 163.
  • Harvey RamerHarvey Ramer Posts: 2,200Member
    Thanks
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • SpenceSpence Posts: 316Member
    Sure is wonderful to see someone who cares!
  • NJ, DesignerNJ, Designer Posts: 53Member
    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, DesignerNJ, Designer Posts: 53Member
    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 RamerHarvey Ramer Posts: 2,200Member

    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!
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • TechmanTechman Posts: 2,144Member
    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 RamerHarvey Ramer Posts: 2,200Member
    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.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • SpenceSpence Posts: 316Member
    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.
  • ryanhowserryanhowser Posts: 1Member
    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'
  • Harvey RamerHarvey Ramer Posts: 2,200Member
    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.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Steve MinnichSteve Minnich Posts: 2,468Member
    More cfm @ at a higher static while nothing else changes???
    PHC News Columnist
    Minnich Hydronic System Design & Consultants
  • Rich_49Rich_49 Posts: 2,535Member
    Welcome to HeatingHelp.com Doogie Howser , oh sorry , Ryan
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Empire_2Empire_2 Posts: 2,343Member
    40 cfm if your lucky......
  • aircooled81aircooled81 Posts: 194Member
    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?
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