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Thread: Help with a shower supply lines install

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  1. #1
    DIY Junior Member Dom Z's Avatar
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    Default Help with a shower supply lines install

    My wife and I are remodeling our bathroom.
    One part is to replace the fiberglass tube/shower surround with a 60X34 tile spa shower with 4 body sprays, a hand held shower and a rain shower head.
    The fixtures we are using are by Moen and require 3/4" supply lines.

    My problem is the cold water supply is 3/4" through the house but is T 'ed off to 1/2" at every fixture in the house, Kitchen, Bathroom sink, and Tub/shower.

    The hot water supply line is 1/2" from the furnace to every fixture.

    I think I can replace the T reducer with a 3/4" T for the shower cold supply.

    But, can I install a fitting to go from 1/2" up to 3/4". If I can will that cause me any other problems? Or do I have to install all new 3/4" pipe?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    A 3/4" pipe can flow nearly twice as much water as a 1/2" pipe. If you calculate the area of the pipe ppening (pi * r^2), you'll see why. Trying to flow more than about 6gpm in a 1/2" line will lead to erosion. So, it depends on what your specs are on the devices. Because the radius is squared, a little change results in a much bigger area, lower restrictions, and higher flow capacity. Just bumping the 1/2" pipe back up to 3/4" to make the connection won't help (much) getting more water there. High restrictions will decrease the flow, and therefore the performance of the shower.

    If the inlet piping to the WH is only 1/2", you may need to go way back to where it is 3/4".

    What is the max gpm of the shower fixtures you bought and what else to you think may be flowing at the same time?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Moderator & Master Plumber hj's Avatar
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    Default valve

    I have absolutely no problem with piping a 3/4" valve with 1/2" piping, knowing that the actual ports in the valve have a smaller area than the 1/2" pipe does. Whether you upsize from the tee or not, and whether it would make any appreciable difference, depends on how far it is from there to the valve.

  4. #4
    DIY Junior Member Dom Z's Avatar
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    So, it's ok to go from 1/2" up to 3/4" without a problem?

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    At the shower, if you turn everything on, how many gallons/minute does it use? If it is over 6gpm, I think you need it to be 3/4" all the way.

    Course, you won't be using all hot, so as HJ says, it probably won't be a problem. But, if it is trying to flow much more than that, you could have problems, especially if there are other hot users at the same time.
    Last edited by jadnashua; 07-31-2009 at 07:51 PM.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Default hot

    If "other hot users at the same time" cause a problem it would be because of something ahead of the tee, so it would not make any difference what size the pipe to the shower valve was. In fact the effect could be less with the smaller pipe because of a smaller demand.

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