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

  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!


  2. #2
    Plumber Cass's Avatar
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    Nov 2005


    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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    Feb 2005
    indianapolis indiana - land of the free, home of the brave....
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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


    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'!



  5. #5
    DIY Senior Member TedL's Avatar
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    Sep 2006
    NY Capital District


    Other major variables you haven't mentioned:

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

  6. #6


    ~50 psi


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

  7. #7
    Plumber jimbo's Avatar
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    Aug 2004
    San Diego


    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


    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

  10. #10


    Okay heres the setup:

    3/4 supply......~1 ft. of 1/2".......~15 ft. of 3/4"........tee 1/2" to thermo. shower valve and 1/2" to tub filler.

    This is just the hot, the cold supply comes further into the room so no issue there.

    Oh, BTW, don't think it matters but where the supply reduces inside the wall is actually a tee that also feeds 1/2" to a double sink.

  11. #11


    Referring to
    and assuming worst case that
    a) the 1/2" length is Type L copper (type M is thinner and thus larger in ID) and
    b) the flow rate is 5 GPM (very hot shower at very high volume),
    the pressure loss in 1 foot of 1/2" is less than 0.2 psi.
    Since a typical 3/4" metered town water supply presents at least 50 psi at 25GPM to a single family home,
    such a loss should not be noticeable.

    Since 1 foot of 1/2" type L has a volume of .012 gallons,
    the velocity of the water at 5GPM is
    (5/.012) = 417 feet per minute or about 7 feet per second, well beyond the 5 feet per second recommended as a working limit and likely quite noisy. Of course at 2.5 GPM, the velocity is 3.5 feet per second, still perhaps noisy but with no sidewall erosion dangers.

    Be safe,
    [In God we trust. All others, bring data ... ]

  12. #12


    So, it'll work relatively the same, but.....it'll be loud as all get out, huh?

    If I put split foam insulation around the 1/2" part and pack the stud cavity with fiberglass will the velocity noise still be noticable?

  13. #13
    Master Plumber master plumber mark's Avatar
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    Talking you wont notice any noise levels

    I really like those charts about the copper

    I cant figure them out, but they are very impressive.....

    you are not going hear a difference ,

    no way , no how.....

    especially while you are showering....

  14. #14


    Did I mention the shower will be running 7.5 GPM (three heads)? Any change in opinion?

  15. #15
    Homeowner geniescience's Avatar
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    Nov 2005
    humid summers hot, humid winters cold

    Default reducing noise. Measuring flow.

    good to know! Three.

    So, you want a lot of water, and as little pipe noise as possible. Very normal. I've never met a person who wanted the opposite.

    Friction losses are a linear sum of the distance. A short distance of narrower pipe is not a long distance. Everyone keeps posting the same info, worded differently. Your plumber friend is right, within reason. To go deeper, just FYI, it is irrelevant where the narrow piece is in the entire run; it is independent.

    So, a short distance doesn't add a lot of resistance to your flow.

    It is true that it is more like a nozzle -- and no-one can predict the noises it will produce when "stressed" to its maximum limit, even if one could be there and see all the shapes (bends in of the pipe). Any direct contact to studs or other structure will transmit noise through solid material and then into the air where you can then hear it. So, it you can slide a thin pad between the pipe and its strapping, that will help a lot.

    Musical instruments are complex geometry. Even the temperature of your hot water will change the noise the pipe produces. Personally, I think that the shape of the 3/4" pipe right after the 1/2" segment is what will determine a good part of the possible noise. Are you aware of how water produces microscopic vapor bubbles when it is pressurized and flowing fast over a microscopically rough surface like the inside of a copper pipe? This produces a hiss noise or lisping whistle. This noise gets amplified (slightly) at the 3/4" aperture acting like a horn spout does. When these bubbles come out of the 1/2" pipe, they won't get absorbed back into the main stream of water right away, but will at first expand a bit since the pressure has dropped. These two sudden changes will make some form of music or noise too, which then travels inside the water to an elbow or bend and then comes out into the air there. The length of the straight pipe determines at least one of the harmonics in the noise. That's the hypothesis, which you can verify by building a batch of similar setups and measuring noise as you change one variable at a time. If you were so inclined.

    If anyone has more to add about pipe noises, please feel free to contradict me; I am eager and willing to learn what ever you know.

    Now, Mr. Baum, if you really want to know whether you can fan out to three nozzles running 2.5 GpM each, and have acceptable plumbing performance elsewhere too, here is a way to test before installing. Build a pipe run equivalent (in number of elbows, and in total length, and in 3/4" copper) to the one you intend to install in your walls, and run it into the tub. Time how long it takes to fill the tub to e.g. 60 gallons, while turning on all the water (tub and shower, flushing toilet too). Note if you do not install the shower nozzles in this simulation as you are looking to know the total maximum amount of water you can get, you will not have the ultimate end flow, since a shower nozzle is a restriction, a friction, which reduces flow too. But I wouldn't sweat it. You can repeat this filling experiment with and without all the other variables.

    1. More length = more loss = less water.
    2. More bends = more loss = less water.
    3. Shape of bends is one factor in creating noise.
    4. Padding to prevent direct physical contact is important.

    Since noises can start to happen later, it is good to pad even if there is no noise.

    hope this helps.



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