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Thread: Hot water supply pipe size??

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    DIY Junior Member CP4343's Avatar
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    Default Hot water supply pipe size??

    I'm adding on to my home and I decided to run the addition on a tankless 4GPM,,, It came with 1/2" Input and out would it be in my best interest to stay 1/2" all the way or go to 3/4 after the water heater and split off with 1/2" at each faucet,,, the addition containes a master bath a second 3/4 bath a kitchen and Laundry room,,, I'm torn between thoughts Hop someone can help me,,,,

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    Journeyman/Inspector Inspektor Ludwig's Avatar
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    Yeah, you can only run maybe 3 fixtures at the most (2 lavs and a shower) on 1 ,1/2" line and that's on a good day. I don't know what plumbing code you use but in the UPC there are a few calculations that you need to do to figure out the proper pipe sizing (pressure, fixture units, length of run, etc.) It's too much for me to list here and every code has different calculations. If you want to try and size it yourself then you need to get the code section that deals with water supply pipe sizing and be prepared to do some math, or hire a plumber to come out for a consultation and have him calculate the sizing.

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    DIY Junior Member CP4343's Avatar
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    I was told I could do either but was not given a clear answer on what is best,,, If the water heater is 1/2" what good would it do to go to 3/4",,, Am I correct in thinking if the feed through the heater is only 1/2" the 3/4" after that would be pointless and would only slow the hot water from getting to the intedded destination by mixing with all that cold water in the line,, I think I saw a calculation that stated 3/4" has 2.25 times the water in the line as 1/2" would it take 2.25 times longer and take 2.25 times miore hot water before it gets to the faucit?

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    Journeyman/Inspector Inspektor Ludwig's Avatar
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    You can't run a master bath, 3/4 bath, a kitchen and a laundry on a single 1/2" line. The reason some tankless hot water systems bush down to 1/2 inch is so they can heat the water faster. Standard hot water tanks will bush down to 3/4" even though you may have a hot water supply demand that requires a 1". You won't have the volume of water that you need to run the fixtures correctly. If someone is taking a shower and someone is doing the laundry and someone is doing the dishes, you won't have enough water period let alone hot water. I'd make sure that the unit you're installing has enough capacity to handle most (if not all) of the fixtures being on at the same time. Even if that is a remote possiblity, the plumbing codes size piping for maximum water use potential.

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    Forum Admin, Expert Plumber Terry's Avatar
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    the plumbing codes size piping for maximum water use potential.


    They make small heaters for small jobs.

    Your home sounds bigger then that.
    You may need several heaters like the one you have, if you are to plumb more then two fixtures.

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    Retired prof. engr. gator37's Avatar
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    The plumbing code uses a predetermined diversity in its tabulation of "fixture unit vs gpm" chart if you are going to do the calculation. If I recall it also specifies that 1/2" is the minimum size connection.

    Increased pipe size gives you less pressure loss resulting in increased delivery. Thru the code method of calculation insures that the piping is sized adaquately if (you do it correctly). I think there is an example int the IPC and there used to be one for residendial in the SSC or IPC. As indicated and as the code teaches you will have to know some particulars for you installation like eq. length from meter to house, from house entry to furtherest fixture, fixture min. pressure, pressure at the water meter, etc.

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