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It does sound like 'normal' failure patterns for a thermostat immersed in water but seems like the intervals are a bit short. On the other hand most electric immersions that I come across these days are only a backup for a gas heating option so they're not heat cycling that regularly.
I wouldn't expect rust after 12-18 months. Could be internal failure not related to water but some sort of water ingress is also likely given it's immersed in a tank.
I'm scratching my head to think of some way the wiring could be a cause but can't come up with one.
Airhead-
Thanks again for the thoughts
Having just seen a picture of a charred out plug from an iron left on overnight, I suspect it is simply burning out. It’s set to 70*c (don’t ask) and although on a morning /afternoon timer, it’s probably just burning brightly for its short life… Not sure I have the inclination to fit a new tank with two elements, but just have some themostats on hand each winter I guess.
Chopsicle
Thanks for this info, v useful.
Have replaced the element a few years back - old one had no rusting - and the problem has continued at same frequency.
Thermostats replaced every 12-18 months, does seem to be during winter they fail with more frequent use. I’ll look at testing the failed one I’ve just taken out. The pattern is that once they trip, they’ll trip again in a week or so, then more and more frequently, until replaced.
CU has a certificate, and the person who installed cleared up a few other problems in the flat (1970’s build, legacy problems from previous) and this was flagged as an issue to test/address, but has continued since it was done, maybe 3 years ago.