120 v water heater installed with 240 v line

Discussion in 'Water Heater Forum, Tanks' started by Dawn, Aug 12, 2020.

  1. Dawn

    Dawn New Member

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    Aug 12, 2020
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    Florida
    Our contractor remodeled our bathroom and installed a new Rheem 20 gallon lowboy at that time. 18 months later we had to call a plumber because it stopped working. He called to tell us it was a 120v water heater installed on a 240v line and the thermostat had gone out. He said he was putting in a 240v element and thermostat. Will we have a problem if a 120v heater now has 240 v element and thermostat? Will this affect the circuits and invalidate warranty? Thanks!
     
  2. Reach4

    Reach4 Well-Known Member

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    Sep 25, 2013
    Location:
    IL
    This won't invalidate the warranty, I would not think, but the original installation would have already invalidated the manufacturer's warranty I would think. I am not a pro. Ask the original installer to pay for this fix.

    It should affect the circuits in a good way.
     
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  4. phog

    phog Active Member

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    Rochester NY
    When you install a 120V appliance on a 240V electric line, you are over-driving the heater element at a little less than 400% of its normal power. This is really hard on everything- the wires, the heater elements, the thermostat, etc. Depending on the element, it could trip the breaker on a typical 30A / 10ga water heater branch circuit.

    I wonder if your original contractor therefore put a bigger breaker in that is inappropriate for the wires. They clearly didn't know what they were doing in at least one major regard (wrong voltage); I'd check to see if they didn't cut another corner with putting a 40A breaker on a wire rated for 30A. That could be a fire hazard.

    You can DIY this, it says on the front of the beaker switch what the current rating is.

    Regarding putting the 240V-rated thermostat and heater element in, this is fine. It will fix the problem by lowering the power from 400% to 100% of the unit's rated level, which is exactly what you want. A different, equivalent fix might have been to just change the line in the electric box to make the circuit run at 120V. But course the plumber can't do the electric box since he's not an electrician.
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The circuit breaker protects the wiring, so it should be matched to that. That will also determine the wattage of the heating elements that the WH can accept safely. As long as the wiring can handle the wattage (and therefore the current which the circuit breaker can handle), it will be okay.

    Power = volts * amps. So, when they doubled the voltage, you're getting more power (watts). A heating element's resistance isn't quite linear, so as it heats up more than it was designed for, the power goes up more than that power formula would indicate. Because the elements are surrounded by water, that limits how much the temperature can rise.
     
  6. phog

    phog Active Member

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    At fixed resistance, watts = volts^2 / ohms, so when you double the voltage you get 4x the power. (But the temperature non-linearity of resistance means you get a little less than 4x in practice).
     
  7. Jeff H Young

    Jeff H Young In the Trades

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    Im not an electrition but dont see why a 240 volt circuit cant be easily hooked up to a 120 water heater if breaker is of capacity. perhaps not legal but should work fine.
    Also I agree that a plumber who is not also an electrition shouldnt do these modifications. But question whether a plumber can convert the aplliance by changing parts to become 240 volt, thats probebly beyond scope of a plumber . We can connect or repair a water heater but modify not sure
    This talk a bit technical but where the heck do these wild percentages come into play? A 120 volt uses 400% more amps than a 240? wont a 120 volt run off 15 or 20 amp breaker? So its possible breaker for 1 leg was undersized by 5 amp? Possible but not absolute. and possible that it was near cappacity and this one time a breaker popped. My guess is and just a guess orginal contractor simply put a water heater and pulled power from one leg .
     
  8. Reach4

    Reach4 Well-Known Member

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    Assume the resistance of the element is close enough to fixed. If you run 240 volts instead of 120, the current doubles. However power = (voltage * current). So while the current doubles, so does the voltage. So the power quadruples.

    (2x *2x)=4x
     
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  9. Jeff H Young

    Jeff H Young In the Trades

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    Got it ! your saying her 120 volt water heater was hooked up with 240 I figured it would have been just hooked up to 120 even though there was a 240 circuit. in other words one of the hot legs would have been capped off.
    Your warranty on water heater is one year parts so warranty is over but tank might be 6 year if tank leaks you might be able to get warranty shouldnt be an issue My water heaters have never even been inspected I called manufacture and made appointment to drop off at supplyier
     
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    A typical 240vac WH won't have a neutral wire, so there's no way to legally get 120vac. Yes, you could move the second current carrying lead to the neutral bus at the panel to get 120vac. It might work if you used the ground wire, but you are NOT supposed to do that - it's unsafe as the ground wire is not supposed to have current on it.
     
  11. Jeff H Young

    Jeff H Young In the Trades

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    nothing is right about running a 120 volt heater on 240. I dont think.
     
  12. Reach4

    Reach4 Well-Known Member

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    Location:
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    Sometimes there is a cable ("NM") with a black and a white used for 240 volts. The black goes to one hot on the 2-pole breaker and the white goes to the other.

    In that case, to start delivering 120 volts, you could move the white wire to the neutral bus in the breaker box, and try to remove the Sharpie ink or other marking on the white.

    When wiring a WH with 2-wire plus ground NM, I don't know what color the insulation on the two hots typically is.

    Yes, the person who put the 120 volt WH in, made a mistake.
     
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