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Joist Bay Return

I've having a great AC system installed by some great professionals. I only have one concern, and thought I would like to post a question about it. I have an old house, and would not like to use an open joist bay for the main AC return. I would like to open the bay for the contractor's and have them install sealed ductwork in the joist bay rather than using the bay cavity. Is that the best way to do it? How much different would their return air calculation be with a sealed duct return rather than the joist bay....

I'm concerned dust, chemicals used for treating the old wood, and other unknowns, not to mention the likelihood of leaks, would impact the air quality and the efficiency of the return... Any/all comments welcome....


  • icesailor
    icesailor Member Posts: 7,265
    Return Ducts.

    I'm not an AC/WA expert of installer.

    I have never seen "The Old Dead Guy" installers do this in old systems unless they were hacks. The idea took hold. It is now common practice. In my travels today, the crap I have seen in these joist bay ducts are astounding.

    If you run a duct in the joist bay, you will decrease the size of the return duct.

    What I don't understand is that the AHJ/Code writers are making wet head installers insulate all heating and cooling pipes but Air Heads don't have to insulate returns. That doesn't make sense to me. But I don't do duct installs.
  • Three things to say

    I have three things to say about this topic:

    #1 - Have the installer run hard ducts in the bay.

    #2 - Make certain that the installer runs hard ducts in the bay

    #3 - It would be best for your piece of mind and health in most instances if the installer runs hard ducts in the bay.

    Although it has become common practice, you stand the risk of introducing additional contaminants to the system as you mentioned.

    Although there will probably be no long term serious health-related issues by going this route, the very fact you are questioning this means that in the months and years to come, you will be wondering..... Hmmmmmmm

    As far as the calculations go, adjustments will need to be made, but they should be minimal.

    Good luck!
  • Reynz_2
    Reynz_2 Member Posts: 98
    So, what you are saying is....

    Have them run hard ducts in the bay...:-)

    Thank you... I have probably gone overboard with air purification as I'm having both a Carrier Infinity Air Purifier and a REME HVAC cell installed. Air purification is important to me, and since I couldn't decide which one I preferred, I asked to have both... Since air quality was so important to me, I couldn't see how running a return in an open bay, especially "this old house" would be of any benefit. Thanks Eugene... Any comments or suggestions on using both the Carrier Infinity Purifier and the REME?
  • meplumber
    meplumber Member Posts: 678
    Question Eugene.

    I may be wrong, but I believe that the NFPA 90 prohibits the use of joist bay returns. I think it requires the use of ducted return.

    I will dig it out when I get to the shop.
  • Techman
    Techman Member Posts: 2,144
    edited September 2011
    Return duct

    As the Professor said -ductwork the return. But, keep in mind a 14" x 8" will carry about 500CFM of air, a 14x10 return will carry about 700CFM . One ton of AC is 400 CFM , 5tons is 2000 CFM. And a 14x 31/2  wall return is about 150 CFM .How many returns do you have? The return register has to have a bigger dimention than the duct , the register should have a " free air opening " measurement . If not, then the duct CFM drops.

    icesailor , we, insulate our returns when in an uncoditioned environment . I believe every mfg ( Carrier,York ,etc. ) says to insulate in unconditioned spaces, as does EVERY originazation -RSES , NATE , ACCA. So, we AirHeads  have some " not -so- gooders " in our ranks. I apologize for all of those hoopel's. But, a dope is a dope!
  • Reynz_2
    Reynz_2 Member Posts: 98
    edited September 2011

    I'm getting a 4 ton carrier, two stage with a variable speed blower..

    Well, I opened up the floor in the attic and made two bays available for return air. I'm hoping they will use the two bays and just run ductwork into the ceiling for a return... By the way, I have a supply and return in each bedroom, (total of six), one supply in the bathroom, one in kitchen, living room and dining room... The return in the common hallway upstairs will be the return for the four our so supply s downstairs... I hope two ducted returns in the hall will be enough to substitute for the open bay return that was originally suggested... By the way, I have a fantastic contractor and I know we will talk about this....
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,474
    Read this


     In addition,I'd be shocked if a manual J was done and the load was indeed 4 tons
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Thanks Robert!

    Thanks for posting that Robert.

    It's a shame that so many opt for the easy way out without thinking about the consequences.
  • Techman
    Techman Member Posts: 2,144
    edited September 2011
    Great link,

    thanks Robert! Hay, Professor,in that EV Article they mentioned the targeted return static pressure was .05"w.c.  As Carrier and other mfg's have taught ,the return static press is figured for .08 -.06" w.c.  depending. Is the .05 a new standard ? Or just their choice for sizing?As always,curious!

    With ANY of the above mentioned return static press's the return duct is LARGER then the supply trunk, every single time! We find so many under sized returns,but, we get to straighten them out.So, thanks! Having talked w/ many guy's who are mechanics/duct designers that install undersized returns I think the answer lays in the numbers. Carrier's( and others) basic residential duct sizing is usually 0.10" for the supply and .08" ( or .06 ) for the return .Now enters "fuzzy math ",many of the many guys I have spoken  to simply say , "well. .10 is 100% right! so .08 is 80% right! So, the return duct is supposed to be smaller! " I've convince only a few mechanics/duct designers of the proper sizing methods. One guy in particular installed hundred  and hundreds of systems for Sears,he also works for Brookhaven National Lab. After our "math" dicussion he went to several  Engineers @ BNL  and he was suprised and dimayed  to find out that .08 is not 80% . He instantly changed his return duct designing.
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