Immersion heater switch hot & immersion heater not switching off (boiling the water!)

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My boiler needs replacing, therefore I switched on the immersion to use in the interim, although it didn't cut off once it reached temp and ended up boiling the water in the tank. When I turned the hot tap on, steam came out initially!
The immersion switch was also too hot to touch and had that burning bakelite 'fishy' smell.
I've clearly shut everything down and will start to investigate. My thought is that the heater is faulty and drawing too much through the switch.
Maybe I need to change the heater, thermostat and switch all at once?
Welcome your thoughts please.
Cheers.
 
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The heater is obviously OK but the thermostat is faulty. Easy enough to change.
 
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Ok thanks for the quick reply. What about the switch overheating?
Cheers.
 
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Is it a suitable 20A switch?

Check for loose wires
It is a 20amp switch, although very old. The wires were secure, although visible heat marks around the live input.

I'll replace the switch and thermostat and will monitor..

Thanks.
 
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That's a cause for concern (the cylinder boiling up)- there should be 2 thermostats on the immersion heater- the adjustable one that clicks on and off and a fixed temperature one that if activated has to be reset, intended to prevent exactly what you've experienced.
Have a look in the loft (or wherever the dhw header tank is), if it is thermosetting plastic (most are) make sure it hasn't deformed due to the heat.
If you have any placcie pipework in the dhw system you might want to check that- it doesn't cope well with boiling water.
Lastly, if you'll be running on the immersion for a while and it doesn't have a fixed resettable stat you should seriously consider fitting one (I'd be looking at a cylinder stat, a 20A relay and a momentary 240v switch)
 
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That's a cause for concern (the cylinder boiling up)- there should be 2 thermostats on the immersion heater- the adjustable one that clicks on and off and a fixed temperature one that if activated has to be reset, intended to prevent exactly what you've experienced.
Have a look in the loft (or wherever the dhw header tank is), if it is thermosetting plastic (most are) make sure it hasn't deformed due to the heat.
If you have any placcie pipework in the dhw system you might want to check that- it doesn't cope well with boiling water.
Lastly, if you'll be running on the immersion for a while and it doesn't have a fixed resettable stat you should seriously consider fitting one (I'd be looking at a cylinder stat, a 20A relay and a momentary 240v switch)
Thanks for the info. I've ordered a resettable 18" thermostat and a new switch. Are you referring to the one strapped to the outside also? Cheers
 

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