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  1. #1
    Join Date
    Jan 2008
    Posts
    196

    Air handler supply and return ducting...

    Here is what the techs will be dealing with next. The photo below shows the upstairs 12" supply on the far left, seperate ducted 8" MstBR return in the middle and 12" upstairs return on the right. You can barely see in the 12" return on the right that there is a 10" first floor ceiling return cut into it and pookied just above the sheetrock. Second photo shows the backside of these ducts, so right is left, left is right. Any ideas on the best way to tie the 8" and the 12" return together into the plenum? is as drawn functional? Comments?




    This photo shows the wall to the right of the last photo. These are all returns needing to be ducted into/with the previous 12" above, and on-top of the downflow AH. The first two 7" holes will be run down stud cavaties and become toe kick returns. The 8" hole further to the right, is from a ceiling return that will be ducted through a boxed soffit into the TJI's of the great room. I know this seems convuluted but this build and its many beams and perpendicular intersecting joist, have left no alternatives in a multi return design. This design was crunched by several techs and a structural engineer. It is what it is. Return capacity is slightly larger than supply according to the techs.

    The plan is to use a long correctly sized box to catch all the returns below and connect them to the return plenum below the 12" in the previous photo. Comments?



  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,721
    I'm still on my first "poy" of coffee.
    I think I know what you mean, and it should work ok. All 90* angles add restriction. On a return, if the ducts are tapped into the box too close together, it adds a lot of restriction. The air flow from one duct, cam interfere with teh air flow of the next one.
    Your return may seem sized for more capacity then the supply, but the return has the restriction of the air filter. The PD of the filter means teh return duct is usually larger then the supply to move the same amount of air, at teh same static pressure.
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  3. #3
    Join Date
    Jan 2008
    Posts
    196
    Thank you beenthere

    It was late last night when i posted the photos (major deal for me) and I was not entirely cohearant.

    Biggest concern seems to be the ability for all returns to be utilized and not have some branches simply get overpowered and end-up with high velocity and noise. This is admittedly entering a realm the techs have little experience in.

    The first photo has the seperate 8" return in the center. That duct is run straight from the back of the visible 8 inch 90 for near 24' into two gentle 45's and the MstBR ceiling. It involved an engineer letting us cut an 11x6 radiused hole (9" duct) in the center of main support beam. I worry about this "screaming". The 12" next to it (runs near 38' w/two 90's into the upstairs ceiling)has a 10" 12' long ceiling duct perpendicularly attatched just above the AH room sheetrock.

    With all three of these drawing from the same plenum and adding the two 7" stud returns plus the 8" return (10' long with 2 90's) from the right side, I see plenum design being a possible big deal.

    The techs are worried that the 8" will scream, so they were thinking of adding another elbow below the cieling and going into the side of a 14"x14" that encompases the 12" return. From the 14"x14" the plenum would 45 down to the oversized (2000cfm) 5" pleated filter box. The rightside square duct with the two 7" and one 8" return would be tapped into the "45 brake angle" of the plenum. The idea being that the AH would be more forced to draw on the side trunk/plenum instead of fighting the turbulence of a square box so to speak, if volumes were somewhat kept in mind while making tapers and transitions bringing things into the return plenum.

    Apparently we cannot install dampers on returns per code.

    I hope I am making sense and not confusing the heck out of this.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,721
    Only reason the 8" would be loud is if the other returns were more restrictive.
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