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Thread: this size or that size

  1. #1
    DIY Senior Member DIY's Avatar
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    Default this size or that size

    A water main is being replaced,and in addition the fixtures that need supply are a tub/shower,toilet supply,bathroom and kitchen sinks,and in the near future a washer. The plumber says he can run the piping either of two ways: 1.) Run 3/4" piping up to and inside the house using a 3/4" reducing T to 1/2" going to each fixture. 2.) Or, run 3/4" piping up to the house then run into the house 1/2" piping. The plumber said option 1 might give you better water volume but reduced water pressure. Option 2 might increase the water pressure slightly going from 3/4" to 1/2" piping into the house. I simply want good water pressure and volume in the cases where fixtures are used simultaneously.

    Appreciate the help and suggestions!

  2. #2
    In the Trades Gary Swart's Avatar
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    Default

    The pressure will not change by reducing or enlarging the diameter of the pipe. What will change is the volume or flow. The pressure is predetermined by the pressure in the city water main. If that happens to be too high, it can be run through a pressure regulator valve and lowered, but if it is too low, you're stuck with it. One problem with using 3/4" inside the house is with hot water. The entire 3/4" pipe between the water heater and the hot water faucet has to be purged of cold water, and a 3/4" pipe has a lot more water in it than a 1/2" pipe. This wastes water and takes time. Of course, you could install a recirculating pump which requires a return line. That would give you instant hot water. For the fixtures you describe, I believe 1/2" inside the house would be quite enough.

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  4. #4
    Moderator & Master Plumber hj's Avatar
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    Default pipe

    Option #1 will give better volume AND better dynamic pressure because of reduce friction loss. Option #2 will give less volume AND lower dynamic pressure because of greater friction loss. Whether using 1/2" for the hot water system is advisable or not depends on the lengths of the lines.

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