Discussion in 'Tankless Water Heater Forum' started by SHAWNROOFERMAN, Sep 16, 2019.



    Sep 16, 2019
    Right now im using a tanked electric system. With limited space in that room I was hoping to go tankless, Here is the question There is currently 50 gallons being held in that tank. We have 2 wells on the property and I no between them both they carry at most 150 gallons we have ran the system dry with no water. I guess I want to no if that will make a difference ith a tankless system, I guess I have to consider there is 50 gallons of hot water in that tank, but that water in that tank has to be pushed out with new water from the system. so I guess I want to no if Im correct and the amount of water available isn't going to change? The 2nd question I have is I have a heat plate hooked up to a outside wood burner, When that burner is heated up that is obviously already hot water. Can the system detect if the water goes in at 110, compared to 60 or 70 degrees. 3rd as a electric unit what unit would you suggest to a 45 min shower girl?
  2. Terry

    Terry Administrator Staff Member

    Aug 17, 2004
    Bothell, Washington
    In Ohio I don't see a tankless electric doing the job for you. The wiring for a 50 gallon electric isn't even close to what is needed for a tankless install that's meant for anything more than washing you hands before dinner.

    A tankless will check the water temperature on the incoming and adjust to that. If you can keep the incoming at 110 degrees you wouldn't really need anything more. However, out of the ground in Winter you're looking at much colder water than that. You're most likely looking at 50 degrees.

    What you have now is 240 volts at 4500 watts.
    Heaters furnished with standard 240 volt AC, single phase non-simultaneous wiring, and 4500 watt upper and lower heating elements.

    Natural gas is how most of these are installed.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    Assuming a 45F worst-case incoming water temp from your wells in the dead of winter and a low-flow 2 gpm (=~1000lbs/hr) shower, 105F at the showerheatd you're looking at a 60F rise x 1000lbs/hr= 60,000 BTU/hr. Converting the units to watts that would be 60,000/3.412= 17,585 watts for just the shower.

    The very smallest that just covers the shower would be something like an EcoSmart ECO-18 or Rheem Rtex-18, and it would just barely cover it. If someone else uses hot water for any other purpose the person in the shower would get a jolt of tepid or cold water. That requires a pair of #8AWG wiring runs and a pair of 40A double-pole breakers. Do you have that much power to spare on the panel or service drop to the house?

    At about 18 kilowatts a 45 minute shower uses 13.5kwh. Even at Ohio's average 12.7 cents/kwh that adds up to well over a buck per shower.

    If there is at least 5' of vertical drain downstream of the shower you can buy some margin on that using a drainwater heat recovery heat exchanger, can cut the peak power draw in half or more, and cut the total power use/cost by the same fraction. The biggest one that fits is the "right" one, the marginal cost of the taller units is more than paid back in higher efficiency, and offers more margin for other uses, to help mitigate against the shower girl isn't screaming curses every time somebody else washes their hands during the 45 minute interval.

    Even though the heat exchanger could set you back the better part of a grand (or more) as a DIY this solution has a very decent return on investment in households that like to take long showers. For tub-bathers or 5 minute showerers, not so much.


    With any tankless there will be liming issues with the heat exchanger if your well's water hardness isn't pretty low. Installing it with isolation valves and ports for pumping white vinegar through it on a periodic basis is a good idea. (A half hour of de-liming solution pumping every year is good enough for most.)

    For higher flow applications there are bigger electric tankless units out there, but when you're in the 36kw range most homes don't have enough spare service capacity.
  5. WorthFlorida

    WorthFlorida The wife is still training me.

    Oct 28, 2009
    Orlando, Florida
    Do you mean provide 150 gallons per minute, per hour?
    Either system you cannot run your water system dry. Not sure as what your meaning is but a tankless system may be able to shut down with pressure below a predetermine level, but a 50 gallon tank with less than a full tank can expose the heating elements to air and burn out the elements.
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