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Thread: Benefits from 3/4" Supply lines or a bad thing

  1. #1

    Default Benefits from 3/4" Supply lines or a bad thing

    Hi everyone,

    Great forum! This is my first post here, but searched here a few times searching for answers to my questions.

    We are building a new house, and are at the plumbing rough in stage. The question I have is basically, will I benefit from running 3/4" copper supply lines to all fixtures, then reducing to 1/2" at the connections or in my case, might it be a bad thing.

    We are on a well that gives us approx 10 gpm on a Constant Pressure system. However, there is a bit of silt coming through so we have two 'canister/bag' filters, a softener and then an 'Ultrafiltration' system (I know!!!).

    With all that said, if I use 3/4" copper supply lines to all the fixtures/bathrooms, will there be any added benefit, or could this be detrimental (ie more volume but sacrafice pressure)

    I have been getting differing opinions.

    Hopefully this is enough information

    Thanks
    Rob

  2. #2
    Plumbing Designer FloridaOrange's Avatar
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    For a given volume the pressure will go up when you reduce the pipe size. A "typical" house will have 3/4" hot and cold "trunk" lines with 1/2" branches to the bathrooms, kitchen, washing machine and hose bibbs. Having a 3/4" hot water line longer than you really need will mean having to clear out a larger volume of cooled down hot water to the end point before you get hot water.
    Matt
    Semi-professional plumbing designer
    Enjoying life in SW Florida

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    Master Plumber nhmaster's Avatar
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    I find that these days, there is no such thing as a typical house anymore. These questions can only be answered by doing the calculations based on SFU, distance, and pressure along with pipe material used. Anything else is a crap shoot.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The larger pipe would provide the flow, if you needed it (and you may not) without a pressure drop. Since you aren't paying for the water, except for the power to pump it, wasting some waiting for hot may not be an issue. The larger pipe will waste more water unless you recirculate. If it is, then plumb the house with a dedicated return line so you can (maybe later) add a recirculation system. If you are careful about the layout, you can get this to work without using a pump, but it's easier to add a pump. Lots of ways to control the pump - occupancy sensor, timer, demand switch, thermostat, and lots of companies that make systems.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5

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    Quote Originally Posted by nhmaster View Post
    These questions can only be answered by doing the calculations based on SFU, distance, and pressure along with pipe material used. Anything else is a crap shoot.
    Ahem, the correct technical term for 'crap shoot' is the waste pipe.

    Sorry I couldn't resist
    -rick

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    Master Plumber nhmaster's Avatar
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    When it's there, ya gotta take it

  7. #7
    Plumbing Contractor for 49 years johnjh2o1's Avatar
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    The old rule of thumb is don't put two fixtures on 1/2". There are exceptions to this, like showers with more then one head or large tubs with 3/4" tapings on the faucets. Most codes are based on this.

    John

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    Forum Admin, Expert Plumber Terry's Avatar
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    I believe that was only two fixtures on 1/2"
    If you have a kitchen sink 40 feet from anything else, it would take a long time to clear the line, and you are feeding it with a 3/8" supply.
    It would make no sense to run 3/4" to it.

    In the Seattle area, many homes are required to have a 1-1/4" water service.

    A plumber would take out his code book and add the values based on pressure and distance.


  9. #9
    Moderator & Master Plumber hj's Avatar
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    The thing that determines the water line size is the faucet it is connected to. Very few faucets will benefit from a 3/4" inch supply line, but there are no bad consequences, other than the cost, of doing it.

  10. #10
    In the Trades Gary Swart's Avatar
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    As pointed out already, a 3/4" pipe holds much more water than a 1/2" pipe, so the time and water wasted purging a 3/4" hot water line of cold water can be considerable if the length of pipe is very long at all. Even with 1/2" pipe, a recirculating pump is a great innovation as you have virtually instant hot water at any outlet and at any time.

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