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How many tankless gas water heater will plumber need.

Please just the answer to my questions. Please no video of the bear telling me I am a idiot for wanting to sell or install tankless gas water heaters. There is a market for tankless gas water heaters and tank type gas water heaters

House will have seven (7) showers with 2.5 GPM shower heads. No tubs. Kitchen sink, 7 lava faucets, dishwasher, washing machine.

Well water 50 F degrees incoming 70 F temp rise to 120 F degrees.

I told the contractor to check how many GPM the well pump will produce.

How many 199,000 TGWH will be needed?

I say reduce all shower heads to 1.5 GPM and I would install two (2) 199,000 BTU TGWH and I should get about 9.5 GPM max at a 70 F degree rise.



  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    Yep. If you use 1.5 gpm shower heads, that should work. Are you planning on hooking these puppies up in parallel? If so, control may be an issue. Or you could split the system, making sure neither one is overloaded.

    if they insist on 2.5 gpm showers, they're going to need 3 units rather than 2.

    That, however, is a lot of BTUs. make sure the gas lines can handle it!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,546
    398k to bath, wash dishes, and do laundry. That’s barely enough.......

    Is there a chance everything will go at once?

  • hot_rod
    hot_rod Member Posts: 21,874
    that is the big question, how many load concurrently?

    1.5 gallons shower heads were mandated by the federal water act over 20 years ago
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    "that is the big question, how many load concurrently?" The old National Plumbing Code had a table for that -- which was pretty good. I have no idea where my copy has gotten to...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,546
    9.5 gpm covers only 7 showers with 1.5gpm heads......only :)

  • JohnNY
    JohnNY Member Posts: 3,226
    Are you buffering these with electric tanks? Add those BTUs too.
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Use a recirc pump and electric WH as buffer tank and i unit will be plenty with a 75 gallon buffer. Set to 140F on tank and use mixing valve.

    Recirc in large home should be code required. Will save 5 gallons each shower from a cold start and drop your overall demand by a lot.

    Might use a 2nd WH just to be safe for max flow rates. They will cascade. Only 1 needs Circ pump,

    AS a counterpoint, with well water, I’d surge you to use a 100 gallon 199k commercial condensing water heater like a AO Smith cyclone and be done with it. A little more equipment cost, but Less issues and can still use a recirculating pump. Can add an electric WH for extra buffer or mixing valve and store at 160-180F.
  • gennady
    gennady Member Posts: 839
    edited February 2019
    the question is what is probable maximum demand? what are the chances all these fixtures will work at the same time?