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Thread: plenum length

  1. #1
    Join Date
    Mar 2008
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    4

    plenum length

    I am about to have my a/c replaced. The existing system doesn't work too well. On a 95 degree day the living area is 7 degrees hotter than the back of the house. My contractor wants to move the air handler (in the attic) from its central location to above the car port. This will make the supply plenum length about 70 feet long, and will require rebuilding about half of the supply and all of the return plenums. I am concerned that this great length will rob air to the bedrooms located at the end of the run. My contractor says this won't happen, but I have doubts. For $7+Grand, I want the job done right. I was thinking about just adding a damper to force more air to the living area. In this way I can keep the existing system and maybe save some $$.

    Could someone please advise me if my contractor's proposal will work o.k.? I have attached a .pdf file showing the a/c layout.
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  2. #2
    Join Date
    Mar 2005
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    Suppy NC
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    I would think it is a good idea to listen to te contractor. It seems he has done the proper calcs and is willing to stand behind it also. Dampers are not the cure all and in a lot of cases dont work like we want.


    either trust him or find you that will do what you want them to do and not care if your problem is resolved

  3. #3
    Join Date
    Mar 2008
    Location
    Houston, Texas
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    352

    long plenum?

    It looks like the plan is to go with trunklines. This idea is from the old school, but it is very effective if designed properly. The basic idea of a trunkline is that the whole trunk pressurizes and it backs up to the unit to allow the proper airflow @ each outlet. If it is done right, the last outlet doesn't just get whatever is leftover. It looks to me that your contractor has his stuff together.

  4. #4
    Join Date
    Mar 2008
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    looks like this contractor has done his homework, but when did running a trunkline become old school. what would have been another option using a typical ac system?

  5. #5

    More Information needed

    Is your contractor using a variable speed unit, also what type duct system is he installing?

  6. #6
    Join Date
    Nov 2003
    Location
    northern mass
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    Well, I don't know about old school. But I have changed the way I do Attic systems....well, in fact years ago.....but I run spiral or pipe instead of duct. Same thing though in princable I guess. But I run perimeter systems.
    I only try to be the best.....though i'm not......but in my efforts to be, I hope to learn and achieve more than most !!

  7. #7
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    If i'm reading your post correctly, the pdf file is the duct system you have now?

    From your post I don't see where the contractor has done any calculating or homework, maybe I'm missing something that these other folks are seeing.

    I would rather have the AH centered in the building although either way could work with the proper ducting.

    All electrical, refer. piping will have to be extended as well. The ducting needs to be sealed and well insulated to minmize heat gain/loss.
    I never let schooling interfere with my education... Mark Twain

  8. #8
    Join Date
    Mar 2008
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    ahh ok brian, gotcha. out of curiosity why do use spiral instead of square duct?

  9. #9
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    Mar 2004
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    Grottoes VA
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    5,856
    Quote Originally Posted by Skip 2 my lou View Post
    It looks like the plan is to go with trunklines. This idea is from the old school, but it is very effective if designed properly. The basic idea of a trunkline is that the whole trunk pressurizes and it backs up to the unit to allow the proper airflow @ each outlet. If it is done right, the last outlet doesn't just get whatever is leftover. It looks to me that your contractor has his stuff together.

    Old School?


    Thats is all we do, in fact I see very few systems here that are done any other way. The ones that are done differnt don't work and we replace them.

    I think he is talking about the flex monster that all the pros here hate.
    Karst means cave. So, I search for caves.

  10. #10
    Join Date
    Jan 2002
    Location
    East Grand Forks, MN
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    Could someone please advise me if my contractor's proposal will work o.k.? I have attached a .pdf file showing the a/c layout.
    I don't see your contractor's proposal anywhere?
    If the pdf is the proposal, it is not enough for me. I'm not going to guess!!!

  11. #11
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    Quote Originally Posted by badboyheel View Post
    ahh ok brian, gotcha. out of curiosity why do use spiral instead of square duct?
    Spiral/round pipe has less friction loss than rectangular metal.
    Fewer seams that have to be sealed.
    Less assembly time.
    Easier to hang.
    Easier to insulate.
    Costs less.
    Offsets and elbows require no special fabrication.
    Can use inexpensive premade reducers, instead of transitions, at points where the size has to be reduced to maintain the velocity.
    Less surface area for the same airflow, so less heat gain/loss in unconditioned spaces.
    Usually no space/headroom issues that would make rectangular duct desirable in an attic.
    If more government is the answer, then it's a really stupid question.

  12. #12
    Join Date
    Dec 2007
    Posts
    132
    Anttime round or spiral pipe can be used is a no brainer these guys are right on the money ,I've been installing spiral pipe in residetial applications for about ten years now ,coming from commercial background it only made sence to me ,Most residential contractors have never even seen it before,most of them still dont use it because of it requires skill to install they would rather drag a box of flex out and call themself sheetmetal workers and the real sad part is they think this the way it is supposed to be done when in fact that is the old school way , Back to the problem at hand ,there should be no problem at all running 70 feet to the end register in fact the end runs if properly installed should get the most air pressure.

  13. #13
    Join Date
    Nov 2003
    Location
    northern mass
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    411
    well, that about summed it up perfectly !
    I only try to be the best.....though i'm not......but in my efforts to be, I hope to learn and achieve more than most !!

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