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

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by chefwong, May 11, 2013.

  1. chefwong

    chefwong Member

    Messages:
    710
    Location:
    District of Columbia
    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 ?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,334
    Location:
    New England
    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.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,272
    Location:
    Cave Creek, Arizona
    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.
  4. chefwong

    chefwong Member

    Messages:
    710
    Location:
    District of Columbia
    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
  5. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,272
    Location:
    Cave Creek, Arizona
    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.
  6. Terry

    Terry Administrator Staff Member

    Messages:
    15,423
    Location:
    Bothell, Washington
    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. chefwong

    chefwong Member

    Messages:
    710
    Location:
    District of Columbia
    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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,334
    Location:
    New England
    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.
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