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Thread: water supply fixture units & pipe size

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    Default water supply fixture units & pipe size

    Hello,

    I am finishing my basement and have been planning to add a bar sink and a bathroom. Currently I have a one inch meter and a one inch pipe coming into the house. The plumbing inspector informed me that based on the number of fixtures that are in the house already, If i wanted to add any plumbing in the basement I would need to have the pipe running from the street into the house changed from 1" to 1 1/4". I did a little "googling" on water supply fixture units and found this table...

    http://www.metrokc.gov/health/plumbing/wsfu.htm

    If I am reading it right, it appears that an increase in WSFU can be obtained by increasing the pipe size within the house, downstream of the water meter. Off the top of my head that doesn't seem to make sense that connecting a 1 1/4" pipe to a 1" meter and supply would help things, but I'm looking for any other option other than trenching out the front yard.

  2. #2
    In the Trades Bob NH's Avatar
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    If you are in King County, I would go with the footnote that says:

    "All information is general in nature and is not intended to be used as a substitute for appropriate professional advice."

    Then I would get some "appropriate professional advice" with analysis that concludes that you don't have to increase the pipe size.

    You might start with a pressure measurement at your house to determine the pressure delivered by the utility.
    Last edited by Bob NH; 06-15-2006 at 10:40 PM.

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    In the Trades Gary Swart's Avatar
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    You can get a hell of a lot of water out of a 1" meter and 1 pipe, assuming the pipe isn't several hundred feet. That's the size I have and I can operate my sprinkler system and never even notice any change in volume or pressure inside. I only have about 30' between the meter and the house and the irrigation tees into the backflow preventer just a couple of feet from the meter. I'd think that unless you have an awful lot of fixtures, you should have plenty of pipe/meter size.

  4. #4
    Forum Admin, Expert Plumber Terry's Avatar
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    That table is pulled right out of the Code Book.

    It does make a difference when the pipe size is increased.
    You can argue with the inspector, but it won't make you right.

    Most homes in King County are going 1-1/4" on the water service.

  5. #5

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    Thanks for the comments. So before I make a fool of myself in front of the plumbing inspector I want to make sure I am interpreting the chart correctly. When it says "Meter and street services, inches" that is the size of the meter and the pipe coming into the house. The column "Building supply and branches, inches" is the size of the pipe after the meter. Then there's the "Maximum allowable length in feet". Is the length of pipe from the street to my house? Or is it the length of pipe after the meter to the first branch?

    Also, I am not in King county (I'm in a suburb of Chicago), but i was hoping that this type of calculation is done the same way in different parts of the country. Have you heard of instances where local codes deviate from this particular chart?

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    That's all folks! Gary Slusser's Avatar
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    In the Trades Bob NH's Avatar
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    Default It's Chicago - That Explains It!

    "Also, I am not in King county (I'm in a suburb of Chicago),. . . "

    That explains why the inspector is requiring you to upgrade the supply of an existing house. He is getting $500 for every one of the upgrades that his buddy installs.

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    Plumber jimbo's Avatar
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    Quote Originally Posted by richh

    Off the top of my head that doesn't seem to make sense that connecting a 1 1/4" pipe to a 1" meter and supply would help things,other option other than .
    Your initial reaction that you can only get X gpm from a 1" meter supplied by a 1" pipe is correct. However, the next step in the process is to consider the LOSSES through the system. Water in some ways can be compared to an electricl cirtuit. If you have certain amps running through a wire, there will be voltage drop that increase with the length of the wire.

    So it is with water: as water flows through a pipe, there is pressure loss due to friction etc. The loss is directly proportional to the length, and inversely proportional to the square of the diameter of the pipe. In a typical calculated situation ( sch. 40, 100 feet, 14 gpm) you would have about 4.5 PSI loss in 100 feet on a 1" pipe, and about 1.1 PSI loss on a 1" pipe. ALso, the water velocity would decrease from a very marginal 5.2 FPM to a very happy 2.9 FPM.

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    That's all folks! Gary Slusser's Avatar
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    Jimbo, that would feet/second no?
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