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Thread: Is pipe/water pressue pressure limited to the smallest denominater ?

  1. #1
    DIY Senior Member chefwong's Avatar
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    Default Is pipe/water pressue pressure limited to the smallest denominater ?

    I'm got a tonnage of projects this year.
    One, which involved the deck.
    The pipe from inside the house runs along this path from the inside.

    From the inside, it's basically 3/4 off the main into a 2 way split. One split goes up and the other feeds the utilty room following the path and then out.
    If I run 3/4 from inside to out - with about 4 feet of so of 1/2" pipe off the 3/4 main -- nevertheless, will I see a increased water pressure on the outside pipe by doing this ?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Yes...the static pressure would stay the same, but the dynamic pressure is limited by the flow and the demand...as long as the flow matches the demand or has more capacity than you're trying to use, you won't notice. A section of 1/2" pipe that long will affect the dynamic pressure and volume.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    I am not following what you are describing. is the section of 1/2" between two pieces of 3/4" or at the end of the 3/4"? You will NOT notice any pressure increase, unless the existing design created enough friction loss to drop the pressure significantly. Your hose faucet design is probably what is causing any drop in pressure and therefore volume.
    Licensed residential and commercial plumber

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    DIY Senior Member chefwong's Avatar
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    Off the T on the main, I was going to just upsize off a fitting, and then branch back outside with 3/4.
    So between the two 3/4, there would be 1/2 - about 4 feet or so. I could just suck it up, and do more work and go 3/4 from the T.

    And yes, I'm using a fairly long hose, I think about 100 footer, 5/8 if memory serves. I know this plays somewhat of a role.
    Just wasn't sure if there was any Benefit by doing the run in 3/4 versus 1/2

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    Moderator & Master Plumber hj's Avatar
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    Your hose faucet's opening to the water line is about 5/16", so that is what will limit the amount of water you get from the hose.
    Licensed residential and commercial plumber

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    Forum Admin, Expert Plumber Terry's Avatar
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    will I see a increased water pressure on the outside pipe by doing this ?
    Pressure never increases

    You can have friction loss though. A larger pipe has less friction loss. Though a hosebib does just fine with 1/2"
    I do find that running 3/4" all the way keeps noise levels down because more water can pass through without increasing speed. But not much.
    Most hose bibs are run with 1/2"

  7. #7
    DIY Senior Member chefwong's Avatar
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    Interesting...keep it coming, for my *brainology*.
    Let's say off the main, 3/4 3 or 4 Way tap. Nice *city water pressure*. On a typical household with a run being 30-50 feet or or so, there is not ~increased pressure~ by going with 3/4 for the entire run. I've always thought that when the opportunity arose, like in this situation with the hose outlet, I would replace the 1/2 with 3/4.

    Aside from a really long run, when are are there benefits to going with the larger pipe

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    A bigger pipe does two things: has the ability to flow more water and second, decreases pressure losses from friction. So, it depends on how much you expect to demand from the pipe. There are flow tables available that will show you how much you can flow at various lengths at various pressures and known pressure losses. Long-term high demand from an insufficiently sized pipe can cause erosion failures of the pipe, especially after a fitting that may cause turbulence. A big contributor to turbulence and long-term failure is neglecting to ream the pipe back to full diameter after cutting with a typical tubing cutter - the wheel tends to narrow the end of the pipe - it must be restored to full diameter to avoid future problems.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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