Electric hot water heater overheating then tripping

The Tank Electric hot water heater overheating then tripping

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  • #24052
    scott.f11
    Participant

    At first this seemed pretty cut and dry, bad thermostat. But it’s become harder to diagnose.

    It started with the upper thermostat tripping its high limit switch. Reset it, and it’s good for about 24 hours. The water would get really hot after a couple hours, but it would stay on. Adjusting the temperature on the thermostat didn’t seem to really help. Multimeter tested the elements and the lower thermostat, and got continuity instantly. Changed out the thermostat.

    Same problem happens, if not identical. I feel like the thermostat wasn’t the problem, although the 2 units give different continuity results. A plumber suggested I let it cool right down, so that when I reset it, it’ll have to heat for a while. Then, turn down each thermostat, one at a time, and listen for the relay to kick it off. I did this, after 10 minutes of running, and they both kicked off as I turned them down. Two hours later, the water was extra hot, tried again, only the bottom thermostat would click off as I turned it back.

    The hot water tank is 6 years old. It’s an AO Smith model ECT 66 200, and I’ve got photos of the old and new thermostat if that’ll help any. I’ve consulted an electrician and a plumber, and their phone support has them somewhat stumped.

    Does anyone have any thoughts on things to try?
    Thanks!

    #24053
    Larry Weingarten
    Participant

    Hello, Try this: Turn off power. Disconnect one wire from each element. Now, with your meter in ohms setting, (say ohms times 1000) measure continuity between each screw on the element and ground. Ground can be the hex base of the element itself. You should get no continuity at all… with both elements. If you get any, it’s a sign that there is a leak to ground, which essentially doesn’t let the element shut off, so it continues to heat slowly.

    Do let us know what you find! 😎

    Yours, Larry

    #24054
    scott.f11
    Participant

    Hi Larry,

    I just tried this, and had no continuity.

    Thanks
    Scott

    #24055
    Larry Weingarten
    Participant

    Hello, That’s strange! Older electric heaters are really quite simple. They have thermostats and elements. You replaced the upper thermostat and the elements check out. It’s time to toss out any assumptions and check; power supply, wiring diagram for the heater, proper insulation over the thermostats, and verify lower thermostat operation… or just replace that thermostat. Look for any evidence of moisture around thermostats or smell of burning.

    Maybe it’s just that I’m an old guy now, but I prefer analog meters as they can’t confuse me like digital units seem to. 😛 So, if you’re using a digital meter, and if nothing gets figured out, try an analog one.

    Yours, Larry

    #24059
    PG
    Participant

    factory wired top thermostat comes on first to heat if tank drained of hot water once that is satisfied then flips to lower thermostat – so if thermostat are against tank no space and door and insulation in place so it can sense temp of tank properly, one cause could be relief valve dripping or small drip at a tap and bottom element is on for a longer time i would check relief valve – also if you have a amp meter you could check if power is being used when element is off – i would have said element was shorting but you said there is no short but to heat that to trip is at 200 deg

    #24060
    PG
    Participant

    factory wired top thermostat comes on first to heat if tank drained of hot water once that is satisfied then flips to lower thermostat – so if thermostat are against tank no space and door and insulation in place so it can sense temp of tank properly, one cause could be relief valve dripping or small drip at a tap and bottom element is on for a longer time i would check relief valve – also if you have a amp meter you could check if power is being used when element is off – i would have said element was shorting but you said there is no short but to heat that to trip is at 200 deg

    #24097
    scott.f11
    Participant

    UPDATE:
    I stumbled upon a tech support line for AO Smith. It’s not on their warranty or parts site, just on their main site. I called them, and the guy walked me through testing almost every possible combination of wires and screws. His diagnosis was a bad top thermostat, the same part I had replaced. I went and got a new one, and a week later, no issues. Seems the part I ordered was broken/defective upon arrival.

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