8x20 trunk off-set... - Page 2
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  1. #14
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    Jan 2008
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    The "over the internet diagnostic" is asking alot and subject to many variables, I know. But many of you have years of experience and a good understanding of the science at hand, which makes your tactical guesses better than what I am paying for, I hate to say.

    I went back to the install and studied it. The only other option where there is room for these needed take-offs, is to come directly off (downward)of the 8x20 trunk/plenum below the downfeed AH. Currently, the trunk design is such that the AH base plenum downfeeds directly into the 8x20 trunk. The trunk goes "right" to supply the downstairs where the "sleigh" transition would be located (if used). The direct opposite (left side of the AH base/plenum), has the 8x20 trunk immediatly transitioning into a short side 90 and going up through the sub-floor alongside the AH to feed the second story. Alongside the AH the 8x20 transitions into a 12" round to continue to the second story attic for 6 cieling supply registers.

    There is 11 inches of clearance below the 8x20 trunk under the AH. Is it concievable to use this room under the 8x20 for an extended plenum to be used for take-offs?

    There is a plumbing drain pipe located in this area currently, but I can have the plumbers re-route this.

    If so, what would be the correct way to integrate this extended plenum?

    I hope my description is making sense!

  2. #15
    Join Date
    Jan 2004
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    Lancaster PA
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    67,620
    Takeoffs in a plenum can have problems also.

    If takeoffs can be installed in the plenum area under the unit. Why can't they be installed in the 8x20 before the sleigh.
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  3. #16
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    Jan 2008
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    As soon as the 8x20 goes "right" from under the AH, with-in 3-5 inches there is a concrete pad that is near 30" wide. This pad supports a girder post and is with-in 3-4" of the bottom of the 8x20. The 8x20 just fits over the top of these pads and under the floor joist while it runs near 15'. The girder flanks one entire side of the 8x20. Three supply registers are on the "other" side of that girder, hence trying to get under it. As luck would have it, the floor joist (2x8's @ 12" o.c.) change direction on top of the girder, making the joist cavities a pain as well. That is what the techs are in the middle of currently, but it is not working well at all. There is 10-14 inches under the girders and they run every ten feet. The crawl is a very clean and dry concrete rat slab. But very tight and very rough. The new structure is being built on an existing 50's foundation as to be grandfathered.

    The "sleigh" would have actually been dropping down below the girder and between two of these pads, exposing enough of the side of the 8x20 for the possible take-offs.

    This install is difficult, I know. When I was interviewing techs I could see it on there face. They either wanted nothing to do with it or enough money for an extended vacation when it was done.

    The structure is of very high quality. I do not want the HVAC install to be less than the best it could be under the circumstances. The current techs are on a time and materials basis.

    Thanks again for everyones patience.

  4. #17
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    Jan 2008
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    beenthere...are you meaning "why not come off the bottom of the 8x20 trunk?", as in where the sleigh would be alocated or instead of the sleigh?

    Take-offs could be made to come off the bottom of the 8x20. They would just need to be made short or tight to the bottom of the trunk to allow a 6" elbow or maybe a take-off with an angle to it projecting out from under the trunk.

    Do you see this as a better alternative to several mentioned?

  5. #18
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    Lancaster PA
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    Bottom of 8x20 before the sleigh.
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  6. #19
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    The only area under the trunk aside from below the AH, would be where the "sleigh" would have been located (between cement pads). This area is about 5-6' from the AH.

    Is it possibly prefered (considering previous options) to locate (3) 6" take-offs on the bottom of the 8x20, all with-in about 36 inches?

  7. #20
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    Jan 2004
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    Within 36", Sure, done a lot.
    But not a good idea in the sleigh area.
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  8. #21
    Join Date
    Nov 2003
    Location
    northern mass
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    So let me wrap my head around this. Downflow AH into a crawlspace. The 2nd floor zone comes off the supply plunem in the crawlspace and goes back up through the floor again next to the unit and transitions to 12" round to continue up and do it's thing. Meanwhile, the other side of the supply plunem has an 8x20 supply trunk coming off of it at just the right spot as to avoid a concrete footing. And it is just below to joists, but just above the 'crete ? And the 15' of trunk runs next to the stick.

    And their problem is what exactly ? They can't come off of the top because the joists run the oppisite way ? The side is obviously blocked by the beam. It's a crawlspace with limited room.....so why won't they come off the bottom with top takeoffs ? I can't see lowering the duct down as to expose it's side mearly to put ATD's in. That makes no sence to me, the only thing I can guess at is maybe they don't know what a top takeoff is ?? When you say tech's looked at it, do you mean actual service techs or just refering to them as tech as meaning profesioals ?

  9. #22
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    Jan 2008
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    beenthere...I feel as though I have confused the situation the way I have presented it to you. I was proposing either the "sleigh" which would allow side take-offs, or just leaving the 8x20 straight/level (no sleigh section) and using bottom take-offs. The room available to do one or the other of those options would be in the same physical space of about 36 inches in trunk length between concrete pads. Sorry for the bad descriptions.

    brian in mass...you're interpretation of the systems physical aspects is correct. Yes on the joist, though they tried to use some convuluted elbow combinations that are not working cleanly (I want them taken out). The bottom of the trunk is about 10 inches from the concrete. For reasons I do not know, the techs did not want to come off the bottom of the 8x20, I believe for flow. Techs, as in actual service and install techs. Though not journey level in my eyes, but good attitudes and work ethic. My choices were slim, especially wanting to use metal duct and have it masticed, insulated and wrapped with a vapor barrier perfectly.

    The sleigh was my brainstorm (cant blame the techs) to allow side take-offs. The techs commented that it would be nice to run a 8" trunk feeding two of the three branches needing to be hooked up. Those two are near 15-17' away from the 8x20 trunk, the other register is only about 8-9' from the 8x20 trunk.

    I talked with one of the techs. He made a comment about a custom fabbed take-off into the lower corner of the 8x20 (about 3 inches are exposed below the girder), sort of encompassing both sides of the ducts corner and coming off at an angle helping flow. Large enough to supply three registers...crazy?

  10. #23
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    Nov 2003
    Location
    northern mass
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    Well duct work is a preasurized system. It is fine to come off of the bottom of the duct. And i'm glad to hear that they are both installers and service techs. I just still don't seem to understand why such a remedial task is of such debate and reflection. Is there some other issue not being mentioned ? Possibly access to areas of the floor that bottom mounts would cut-off ?

  11. #24
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    Mar 2005
    Location
    Suppy NC
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    now you seem to be coming a little clearer
    there is no reason to be comcerned about takeffs in the bottom or the duct. The air doesnt care top or bottom. To put this sliegh in will cause turbulants and give more problems with the takeoffs then going out the bottom ever will.
    been is rigt on about this. Te key is to reduce turbulants not increase them.

    i also am wondering why the 20x8 duct is not being reduced down after a certain amount of takeoffs to maintain static pressure and aid in pushing the air up the takeoffs. 20x8 straight through will kill your air flo in all your outlets IMO. Also if a transition can be put in before this spot you may be able to get enough roon to come off the side or the smaller trunk.


    not being there makes it real tough to see what is going on

    can you post some picturs of what you have already

  12. #25
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    Jan 2008
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    Quote Originally Posted by brian in mass View Post
    Is there some other issue not being mentioned ? Possibly access to areas of the floor that bottom mounts would cut-off ?
    Exactly, the laundry room plumbing, though I am near resigning to let it be landlocked.

  13. #26
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    Nov 2003
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    northern mass
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    Aha ! I see now, coming off the bottom will block some poor soul who has to go down there some day to work on the laundry room plumbing. But under the beam is under the beam........right ???

    And let me ask a real stupid question.....Has anyone given any thought to just offsetting the duct over, away from the stick, to accomodate coming off the side ?

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