40gal AO Smith Water-heater/electric/2 Occupants

The Tank 40gal AO Smith Water-heater/electric/2 Occupants

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    Last year I installed a TED5000 to track my whole house kwh consumption,
    and separately track my kwh usage for my Water Tank.

    My average kwh use with the water tank works out to 7/kwh/Day.

    The stand-by energy: hour/watt/80

    …….. how does this compare to a Tankless Water-Heater?

    Larry Weingarten

    Hello: Good for you on actually measuring energy use! If you don’t know what’s wrong, you can’t fix it.

    I can’t find a precise answer to your question, but can give a little food for thought. Tankless heaters, both gas and electric are often claimed to have no power usage when water isn’t being drawn. From experience, anything that has electronics does use power when it’s sitting idle. As an example, TVs use more power when turned off than when turned on, (unless you watch more than about four hours daily) So, with gas heaters, the power draw is likely only a few watts, though I have found no good data. Some electric heaters keep their heating chambers warm, so there will be less wait for hot water on next draw. This suggests some power draw for heating which depends on how warm the place is where the heater lives.

    So, there clearly is some power use in modern tankless heaters when there is no flow, but other questions are: How often does the unit need to be serviced and at what cost? ~~and~~ Is there an efficiency drop due to scale formation in the heater? Lastly, read the warranty carefully. These are not as straight forward as conventional tank-type heater warranties.

    Energy use is only a part of the life-cycle-cost equation 😎

    Yours, Larry


    Thanks elleent,

    Re: Tankless condensing electric water-heaters
    Re: Tankless condensing Ng water-heaters

    Location: SW Ontario/ Res. Family Home/ 2 Occupants/inflow water temp. 50F – 120F setting for heater.

    Where can I find the following information:

    .. ratings concerning electric consumption: * (RLA) and *(LRA)
    *(LRA) is a major concern/many startups in a 24h/day.

    …. ratings concerning NG/Condensing Tankless W-heater.
    *Start up and *running NG consumption? Thanks.


    If you are talking tankless electric, the only ones I am familiar with are the ones with resistance elements for heating. LRA (locked rotor amps) only applies to the starting of motor (compressor) loads. Heating elements would have only RLA (running load amps). Watts/voltage = current (amps) Tankless electric for homes typically range from 12 kW to 28 kW. At 240 volts this would yield 50 to 117 amps.



    — Thanks energyexpert, for confirming this data. (Truly energy hogs)

    Today, I also found info for under the sink condensing tankless w-heaters. (Also energy hogs)


    I would classify tankless electric as power hogs vs. energy hogs. Most people use kW and kWh interchangeable, but they are not; energy is kWh; power is energy consumed per unit time. Speed and distance are not the same thing. If you drive 60 mph for one hour you go 60 miles. If you drive 20 mph for 3 hours you go 60 miles. If you use energy at a rate of 18 kW for 15 minutes, you use the same energy as using 4.5 kW for one hour.
    Electric utilities frown on tankless electric WHs. My local utility devotes a whole website page to them. Normal distributions systems are designed for “normal” appliances and usage patterns. A 15 kva transformer will serve one house, a 25 will serve 2 houses, 3 houses might be served by a 37.5 kva. I have seen up to 9 houses served by a 50 kva transformer. My utility states that if the neighbor’s lights dim or other problems are reported and the utility discovers that you have a tankless electric WH, they will upgrade the system and send you the bill. That would certainly cut into any energy savings!
    My 4500 watt WH (105 gallons) was been rewired at the panel to 120 volts and rewired at the WH so thermostats and elements are in series. Calculated output is 563 watts. I operate it off peak only. Thermostats and elements should last forever. And I’m sure the utility loves me; a small load on all night long.


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