We have a problem with an electric dual element heater with flip-flop thermostats (only upper element working). The plumber changed both thermostats once, the heater failed again in the same way, and now he's convinced that the lower heating element is gone. I hope he's right, and this is the solution, but reading how the system works, I began to wonder what happens when there's high hot water consumption? Can it happen that the upper thermostat never gets to its cut-off off point because cold water would come in the tank at a high rate, and, therefore, the lower thermostat will not ever switch on?

  • So test the resistance of the lower element - if O/C then duff.
    – Solar Mike
    Jul 13, 2022 at 8:19
  • Usually the design would have the cold water entering the tank at the bottom(the cold pipe goes all the way to the bottom) and pushes the hot water out from the top.
    – crip659
    Jul 13, 2022 at 10:58
  • Normally the lower elements kicks on first. But if you have high HW consumption, yeah, I guess that could happen, but double check your elements if bad, replace. An DMM with an amp clamp is useful. WITH THE POWER OFF, check the resistance of the elements, if very high resistance, the element is shot. Jul 13, 2022 at 14:50
  • Thank you, Solar Mike, crip659, George Anderson. The lower element gives highly unstable resistance readings: they change from very low to 16+, etc. - Unlike the upper element which gives stable readings at about 12.
    – KBRD
    Jul 13, 2022 at 23:10

2 Answers 2


it can happen, but this would require someone using large amounts of hot water all hours.

  • Thank you, Jasen, for your clarification!
    – KBRD
    Jul 13, 2022 at 23:11
  • Well, it looks like our plumber-bumbler got it right finally: the lower element appeared to be busted open near its root, and it looks like it was killing thermostats as well. Thank you very much, everybody!
    – KBRD
    Jul 17, 2022 at 3:18
  • 1
    @KBRD since nobody has that as an answer, please write it up and give yourself a check mark for it. Self-answers are perfectly acceptable here.
    – FreeMan
    Aug 12, 2022 at 14:10

I'm surprised the plumber changed both thermostats before checking the heating elements.

Yes. if there was a constant hot water use, the upper thermostat would never get up to heat and therefore never switch off and energize the lower element. If the lower element had failed, when the upper thermostat tried to switch it in, nothing would happen until the upper thermostat kicked the upper element back in and switched off the power to the lower one. Now, if the upper element was bad, the upper thermostat would not get up to heat to switch on the lower element and you'd have cold water. Most instruction booklets for water heaters recommend setting the upper thermostat a few degrees lower than the lower one.

  • Thank you, Jack! The plumber is, unfortunately, a bit jumpy in his conclusions and testing procedures. It's difficult to question a specialist . :-( Now I understand the scenarios better ...
    – KBRD
    Jul 13, 2022 at 23:18
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    I'm not surprised, changing the thermostats is much easier than changing the element (because you need to drain the tank to change the element, but the thermostats just unclip)
    – Jasen
    Jul 14, 2022 at 0:51
  • @Jasen Yes, but it takes 5 minutes to check the ohms on a heating element and confirm the problem.
    – JACK
    Jul 14, 2022 at 1:00
  • That's not a reliable test. busted elements can give all sorts of different readings on an ohm meter. if you also have an insulation tester then sure, but then you're probably not a plumber.
    – Jasen
    Jul 14, 2022 at 1:23
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    @Jasen It's worked for me for 50 years. 4500KW element at 240V is going to be around 12.8 ohms. You might get all sorts of readings but if you get close to 12, great chance of being good. and better than arbitrarily changing out two thermostats.
    – JACK
    Jul 14, 2022 at 12:26

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