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instant domestic water heater

one of coworkers is doing est for a customer. has radiant floor heat with a rinnai gas water heater heating a radiant floor system.

I've seen some with baseboard and didn't work right, manuf of equipment said its a no-go. supposedly this one works, has primary secondary setup.

cust wants to put in an elec instant water heater since has solar pv system, up to 40kw on good day. Can this work well, seeing as how instant is running a much higher dt than a boiler and may not put out full btus if inlet water is 100 vs 65 degrees?


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    Has anyone bothered to figure out how much power an instant hot water heater draws? One vaguely suitable for a shower, for instance, will draw around 8 KW. A whole house unit might easily draw 20 to 40 KW. If your client's PV system has a battery, (I presume you mean his system produces 40 KWh on a good day, not 40 KW intantaneously? That would be a big array), it might work. Provided he has a big enough inverter and the cables (your looking at 30 amps/240 VAC).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,386
    Hi, This sounds like a good place to consider a heat pump water heater, with a good sized tank. :)

    Yours, Larry
  • hot_rod
    hot_rod Member Posts: 22,468
    What is the btu load of the system. Kw X 3.41 gets you to btu.
    A common 4500 W water heater element is 15,345 btu/ hr. That requires a 30A breaker.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream