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Thread: Water pressure and supply line question

  1. #1

    Default Water pressure and supply line question

    Hello! I'm adding a shower and the fixture recommends 3/4" supply lines. The city line comes in at 3/4" and then it is reduced before the water heater. If I change it to 3/4" all the way up to the bathroom, can I "T"-off to the existing 1/2" lines to the kitchen, 1st floor bathroom, laundry, etc. How would this effect the water pressure throughout the house?

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default pressure

    The dynamic pressure, which is what you are asking about, would depend on many more factors than just the size of the pipe. The distance from the largest water source, the amound of water flowing at the time, where it is flowing, etc., all determine what the pressure will be. When no water is being used, the pressure is equal everywhere in the house.

  3. #3

    Default

    Thanks HJ. So now I guess this becomes a volume question.
    With the shower fixture needing 3/4" for volume of water, do I need to run it before reductions to 1/2"?

    If I change it to 3/4" all the way up to the bathroom, can I "T"-off to the existing 1/2" lines to the kitchen, 1st floor bathroom, laundry, etc.

    The washing machines seems to start running when I take showers If
    I run 3/4" to the washing machine, will it make a difference or just leave it 1/2"?

    I want to be as effective as I can while I have access.
    Any help will be appreciated!!

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    3/4" pipe has 2.25x the volume of a 1/2" pipe. So, by running the 3/4" stuff to the high volume users, you've got much more available volume with the slightly larger lines. You'd be much less likely to notice a big drop in pressure with the larger line as long as you don't try to exceed the supply.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Default pipe

    You cannot use any more water than the valve or faucet will supply, regardless of how large the pipe feeding it is. Run 3/4" to the shower and branch off 1/2" as needed. The shower valve should compensate for any minor pressure variations. If there are major pressure variations, then the water supply source is inadequate.

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