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Thread: Runaway Thermostat?

  1. #1
    DIY Senior Member Briandl's Avatar
    Join Date
    Feb 2008

    Default Runaway Thermostat?

    Relief valve opened on my hot water heater. The water was very hot from the tap, so I turned the unit off and ran enough water through it to get cold water into it. I checked the valve to make sure it was working properly, and then tried to turn the unit back on, however it would not turn on. I reset the breaker on the thermostat and it came back on.

    So it seems to me I have a runaway thermostat.

    This is just a point of use Ariston 4 gallon water heater, just want to be sure before I go pick up a thermostat, because I suppose it could also be the element.


    (Also it's just a small point of use unit so it's off until I get it fixed)
    Last edited by Briandl; 11-25-2013 at 12:47 PM.

  2. #2
    DIY Junior Member Raspy's Avatar
    Join Date
    Dec 2013
    Northern Nevada


    The element only responds to the thermostat and cannot make the water heater "run away"

  3. #3
    DIY Junior Member WyrTwister's Avatar
    Join Date
    Feb 2014


    The tstat only switched one of the two hots ( 240 VAC ) I saw a WH that had an element shorted to ground on the un-switched hot . Therefore , part os the element was apparently running / heating 24/7 ?

    The big box stores carry a plumbers pack with upper and lower tstat and 2 elements . That might be the way to go ?

    However , if yours is a 4 gallon unit , is it 120 VAC ?

    God bless

  4. #4
    Jack of all trades DonL's Avatar
    Join Date
    Mar 2011
    Houston, TX


    I agree that a shorted element can cause the problem.

    If they short they can weld the contacts on the thermostat.

    Is that unit on 120V or 240V ?

    Good Luck.
    Theory only works perfect in a vacuum.

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