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Immersion heater fault

Hi any thoughts or insight into this fault much appreciated. So it's a 3kw immersion heater that's been causing the water to be scalding hot. It also blew a 15 amp rewritable fuse prior to me changing the stat earlier, anyway just had a text saying the water is still scalding and it's blown the fuse again. I'll go and have a thorough check tomorrow but I'm thinking the immerser may be on its last legs and needing completely replaced. Thanks for reading and look forward to reading any replies. 

  • Can only be the thermostat i would think. It should cut out unless its been set too high. It does sound like there is a problem with the element if it's blowing a fuse though.  Did you do an insulation resistance reading on it.    Gary 

  • I will do an insulation test tomorrow Thumbsup

  • The thermostat must be faulty. The modern stats have a functional set of contacts AND a safety manually resettable thermal safety trip to stop the water boiling. The element must be shorting to earth if the 15 Amp fuse wire is blowing. Presumably there is no R.C.D. in the circuit. I would renew the element and the thermostat. Also check the earthing while you are at it.


  • i just fitted a new stat today

  • Have you measured the hot water temperature? Scalding hot is a bit subjective. You wouldn't want to wash your mitts in 55°C water but that's the usual setting of your stat.

    When does the fuse blow? If it as soon as the immersion is switched on then the element must be  suspect. Otherwise . .

  • Fault from heater to earthed body of element partway along is the usual 'fuse blowing' failure mode. Perhaps if the stat is neutral switching instead of live (reverse polarity somewhere) then it would not go off even with stat.

    I suggest taking your big immersion heater spanner and a bucket with you this time....


  • I am slightly perplexed here. At least at the beginning, the fault current must be lower than the prospective current otherwise the water wouldn't even be tepid, never mind scalding.

    Still, I agree with Mike's suggestion. Good luck!

  • Just to update i got the plumber to fit a new immerser having found really low IR reading between terminals and earth on the old one , my new thermostat was also fried. anyway i thought all good when i left the job this afternoon but just recieved a text saying water still scalding and despite the boost on the economy7  controller having been been off for a few hours the customer noticed the meter spinning fast and isolated the immerser main switch ( old wylex board) . so the only other thing is the E7 controller that can be at fault ,but surely the stat  would cut out at 60 degrees even if the power is on constantly.... as ever all thoughts and advice appreciated.

  • None of that would overheat the water.

    Have you thought about taking a thermometer with you next time?

  • Doh! Is there a three day satellite delay on this forum?