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Thread: 3/4 supply.....1/2 section.....3/4 run....

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  1. #1

    Default 3/4 supply.....1/2 section.....3/4 run....

    Hello all!

    Just a quickie. I thought I saw this answer before somewhere, but now I can't seem to find it again.

    My question is one of fluid dynamics:

    I have a 1/2" (copper) supply coming into my bathroom that changed from 3/4", inside the wall about 4". Impossible to reach without destroying adjacent bath. It was feeding just a shower in this room, but now I want to feed a shower and bath so I need it to be 3/4" for another 15 ft or so....

    Can a 3/4 supply line have about a 1 foot section of 1/2" pipe in it without any noticeable drop in pressure or volume?

    Plumber buddy says "No sweat". Just put reducer coupling at accessible point and go back to 3/4" for remaining run....."Absolutely NO drop in performance"

    Of course I'm here for a second, third, and fourth opinion because this will be feeding a multiple head shower and I'd hate to regret it later.....

    Thanks a lot for your time!

    Nate

  2. #2
    Plumber Cass's Avatar
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    Default

    The simple answer is that no matter how hard you try you can't get the same volume through a 1/2" pipe as a 3/4" pipe if the pressure is equal.

  3. #3
    Master Plumber master plumber mark's Avatar
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    Talking pressure drop

    It depends on the incomming pressure.....

    if you have fairley high water pressure you probably
    wont notice the reduction....


    you will probably be all right and I would not destroy
    the other bathroom over it ......


    I lived in a house totally run on 1/2 copper
    comming in with 3/4 and immediately reduced to 1/2

    you never knew the difference....

    Last edited by master plumber mark; 02-27-2007 at 06:07 AM.

  4. #4

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    I realize, technically, the volume will suffer......but how much? Will I notice?

    Anyone ever had to do this for whatever reason? Results?

    I'm thinking it will increase pressure and slightly decrease volume. Correct?

    I'm not scared of formulas, if ya gottem'!

    Thanks!

    Nate

  5. #5
    DIY Senior Member TedL's Avatar
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    Other major variables you haven't mentioned:

    Static pressure
    Well or muni water
    Likelihood of simultaneous use of bath & shower

  6. #6

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    ~50 psi

    muni.

    hope to, but don't need to....

  7. #7
    Plumber jimbo's Avatar
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    Distance matters a lot. You are talking a short section of 1/2". I vote with others who say that if you have "normal" pressure available, the drop to the tub will be minimal.

  8. #8

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    I can't remember my thermodynamics from college, but it seems like you're creating a nozzle.

    I thought the pressure stays constant, but the velocity through the pinched section increases. Don't engines use this kind of arrangement to create thrust?

    I have no clue. Just crapping here.
    (important note: I'm not a pro)

  9. #9
    Master Plumber master plumber mark's Avatar
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    Talking do you have a choice anyway???

    If what you said is true, you
    would have to tear up another bathroom
    just to even get to 3/4 pipe.......


    just do it and see what happens...

    then worry about gutting the other bathroom.

    as far as flow charts and graphs goes ......

    Las Vegas odds are probably 85% good in your favor...

    so what more can you ask...






    my 9 year old is now thorwing up and sitting on toilet....

    a wonderful day had by all

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