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Thread: Hot Water Supply Sizing

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  1. #1

    Default Hot Water Supply Sizing

    i'm looking to supply a 20 gal hw tank (laars endurance boiler) with 3/4" line for 2 floor 2 full bath unit. then i guess i'll run 3/4" line out too. but recently was wondering if i shouldn't go with 1/2" instead, thinking tap and shower might warm up quicker.

    current connects to tank ar 1/2" but step up to 3/4" to enter the actual unit. i'm worried about adequate flow, etc. the hw part of this boiler has not been in use as of yet.

    any thoughts and advice much appreciated

  2. #2
    In the Trades Gary Swart's Avatar
    Join Date
    Sep 2004
    Yakima WA


    I think you have a valid concern about the time it takes to purge cold water from 3/4" lines, especially to the furthest fixture. I think 1/2" would be quite adaquate.

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
    Join Date
    Aug 2004
    Bothell, Washington
    Blog Entries

    Default Sizing water lines

    Most plumbers are running at most 2 plumbing fixtures on a 1/2" line.

    Tubs are rated at 4.0 fixture units, lavs at 1.0 for a combined total of 5.0
    If you check the charts, you will see that 1/2" lines don't handle much more, maybe one more 1.0 lav.
    Two bathroom groups, you should have 3/4" pipe.


  4. #4


    ok, thanks, i see this is a debated topic. i think i'll probably go with 3/4".


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