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Thanks. Yes I have an immersion heater. I have checked and it is switched off.

Thanks everyone for your contributions. Greatly appreciated. I will try some of your suggestions over the next couple of days. I may be back!!!!!

Yes, it would appear that the tank stat is working, clicking on and off, but if I leave everything at normal, the water still boils. It ignores the tank stat.

Hi Mandate, Thanks for contribution. The temperature has never been altered on the boiler (it's just under half way). The water suddenly started to get exccessively hot for no reason. Tank stat was set at 60C, which I understand is 'normal'. Tried it at 40C next day - still boiling water...

At the moment, the Actuator is unscrewed from it's position. Have just checked HW only. With tank stat on normally (60C) boiler flashes up. When turned right down, boiler goes off.

It used to be ok. No wiring tampered with, other than to fit the new tank stat - after it started to boil up.

Thanks for swift response. Yes, cylinder stat clicking at appropriate temperature (it's brand new). Tried the new stat on 40 and 60 but water still boils up whatever I do. Can't answer your third point at moment as I have switched everything off. Frightened that tank will blow it's top!!

I have a Potterton Suprima boiler. I have overheated water all the time, either when the CH and HW are on, or even when just the CH OR HW is on. Have repaced the strap-on tank thermostat. No luck. An amateur plumber I know has suggested I get a replacement Actuator MSV322 (he called it a 3 way...