Page 1 of 3 123 LastLast
Results 1 to 13 of 32
  1. #1
    Join Date
    Jan 2008
    Posts
    196

    8x20 trunk off-set...

    This has got nothing to do with my system, but a curiosity as it could have been an option early on.

    Would you ever consider/allow a off-set into a trunk 5' or so out from the AH?

    If we would have made a 6" short side offset (drop down, go 3', then back up) into the 8x20 trunk, we could have avoided some structure re-work.

    It would have allowed the trunk to fit around a girder (headered off two joist bay's), and several take-offs could have been located between the "drop and raise" in the trunk, and run under the floor joist to registers.

    Seems a 6" offset over 3-4' is not that drastic. I am assuming in the real world it must be, considering the big time hassle we are going through to avoid doing it.

    Any comments about such a scenario?

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,737
    The offset is no problem. The takeoffs in the offset can be. The air can be turbulent enough that the air doesn't flow into the takeoffs.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    they probably couldn't make the offset.
    FILL OUT YOUR PROFILE!!

  4. #4
    Join Date
    Oct 2007
    Location
    Milwaukee,WI
    Posts
    1,068
    With the right length fitting, 6" really isn't anything at all . Seems like you probably wasted a lot of time instead of making 2 simple fittings. Plus you probably hacked out some of the support.
    ___________________________

    Chicago is an indian word for stinky!!!!!!
    -supertek65

  5. #5
    Join Date
    Jan 2008
    Posts
    196
    What would the proper methodoligy or analysis have been in-order to determine weather turbulence for flow would have been an issue in such an off-set...experience?

    tinner73...you are correct. While I tried to find someone with great metal working skills, I could not. The current techs rely on a metal shop for fabbed parts, which has proven to be a huge time waster as everything is then weeks out.

    The off-set trunk would have simplified the installation of three 6" branches. Currently the floor joist and some framing are being re-worked...maybe not the wisest decision in either time/money or system performance.

    Lash..."Hacked out something". Heaven forbid, not in my vocabulary. But I hear you!

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,737
    How hard of an offset it is, velocity in the duct.
    How close it is to the next offset.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  7. #7
    Join Date
    Jan 2008
    Posts
    196
    beenthere...the short-side offset would have needed to be a full 6 inches, so 2 inches shy of the 8 inch width of the 8x20 rec duct. The off-set needed to be made both down 6 inches and back up 6 inches, in about 36-37 inches. There was little room for error...like an inch and half at best.

    I heard one of the techs refer to this offset as a "sleigh", not sure if that is a correct term or thier own slang.

    The 8x20 is feeding (9) 6" branches from a 4 ton AH that has a split supply trunk (600cfm dedicated to the second story via a 12" round duct). So if I understand the cfm your asking about, I would say the 8x20 trunk by design is supposed to be for 900-1000cfm.

  8. #8
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    so... you had 36" to make 2 -6" offsets happen?



    Lash..........tell him how hard that would have been.
    FILL OUT YOUR PROFILE!!

  9. #9
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    so you needed a 20x8 drop and 20x8 raise with a 6in set both 18 inch long and you would be good to go

    offset would be sideways or kicking you over 6inches.

    this should not have been a big deal for any shop or true installer.

  10. #10
    Join Date
    Jan 2008
    Posts
    196
    Quote Originally Posted by tinknocker service tech View Post
    so you needed a 20x8 drop and 20x8 raise with a 6in set both 18 inch long and you would be good to go

    offset would be sideways or kicking you over 6inches.

    this should not have been a big deal for any shop or true installer.

    I would like to think not. Unfortunately, I was never able to find a tech that was both a competant sheetmetal worker and enthusiastic businessman with a good intelligent demeanor. Many wanted nothing to do with this job. This forum is full of passionate proffesionals, but I was not able to locate one.

    "beenthere" eluded to the fact that possibly there is some other issues with installing the "sleigh" into the 8x20 trunk and installing take-offs at that point. I never pushed this "sleigh" issue, letting the techs crunch it amongst themselves. But I see now that the tech's alternative is certainly more invasive to my structure and not all that clean of an install (more elbows). So I am re-looking at this wondering if it is worth revisting. Judging from what I am hearing it sounds like it may be. My concerns would be if I could be trying to change direction here into a design (the sleigh) that will functionally be worse.

    Thanks for everyones patience.

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,737
    Your looking at 900FPM for 900CFM.
    Thats a high enough velocity, that you'll have enough turbulance, that you won't want a takeoff with in 12" of the transition.
    Doesn't give much room to get 3-6" takeoffs on it.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  12. #12
    Join Date
    Jan 2008
    Posts
    196
    Thank you beenthere...I was going to bring this back up with the techs today.

    So any take-offs actually installed "between" the 8x20 transitions would be starved for air because of turbulence?

    Would it make any difference good or bad to increase the size of the take-offs, say to an 8" square to round?

    Maybe the techs were somewhat aware of this situation and is why they discounted it...giving them the benifit.

    Is "sleigh" a proper duct shape descriptive term?

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,737
    Going to 8" may or may not help.
    Over the internet it would be a bad guess either way.
    Sleigh, is a good term. Its a visualization.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

Page 1 of 3 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event