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Thread: Hot water heater in/out line sizes

  1. #1

    Default Hot water heater in/out line sizes

    I need to replace an electric hot water heater. It is on a farm with a well. The water pressure is never very high. I need to gather all the materials then drive 3 hours to do the replacement. I am not sure of what I will find other than the existing lines are a mix of galv and copper of various sizes. I will use Pex as the crawl space is brutal (dirty and cramped). It has one bath, one kitchen sink and a wash machine.

    My question is what size of cold water inlet would be sufficient? Would 1/2 inch be large enough? I am planning on using 1/2 on the outlet side.


  2. #2
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    1/2" PEX would give you very unsatisfactory results. It has a small ID. If this were copper, I would use 3/4, and for PEX 3/4 would definitely be the minimum.

  3. #3
    In the Trades Bob NH's Avatar
    Join Date
    Oct 2005
    New Hampshire


    You could use 3/4" CPVC. It is a little larger than PEX in same size designation and is a lot easier to work than copper. The only problem is that you have to wait a while for the cement to achieve full strength.

  4. #4
    Plumber Cass's Avatar
    Join Date
    Nov 2005


    On a well, running PEX, I would go with 1", if possible for good flow, but a minimum of 3/4" like jimbo said.

  5. #5


    Thanks for the replies. Looks like I had better stick with at least 3/4 copper. I just measured some Pex and 3/4 is not really 3/4 is it. So what does the CST refer to anyway? I should probably try to go with 1", but I've not seen 1" Pex anywhere. Or 1" copper for that matter. Not sure that I want to dabble in CPVC right now.

  6. #6


    OK, as long as I am going to work on this system, I have another question. The house in question is my moms. If you are in the shower, and somebody uses cold water anywhere in the house, you get some pretty hot water in the shower. What would be a way to solve this problem? Thanks again.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    All new shower valves are required to have what is called a pressure balance valve. This basically throttles down the hot water if the cold water pressure drops, basically keeping the relative volumes the same. It is also called an anti-scald valve. Some of them also can be had with temperature control that automatically adjusts the volumes to keep the same temperature. It really helps as the hot water tank gets drained, but it won't work miracles.
    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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