MegaSack DRAW - This year's winner is user - rgwb
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Longshot. The 80c light is flashing/boiler stopped.
Turned off/back on and it works.
Leading upto this: Yesterday I turned the water stop tap off whilst away. Turned it back on and put the pressure much higher than normally set. Ran a bath and all well. 30mins later went to put the heating on and...thisfault.
Other points to note- water bar guage fine. Gas flow fine, no leaks.
Could it be too much water pressure flowing through the boiler?
(Will keep rechecking every ten mins).
Whats the make of the boiler?
Its a Glow worm Beta 30c. Their site just gives a descriptor/title not an explanation.
From your description of the fault and your observations, I would recommend you call an engineer.
Just out of curiosity. How do you know the gas flow is fine and there are no leaks? And if it wasn't, and there was. What correlation do you think this would have to the overheat fault. I can tel you. Non. If the boiler has reached overheat and tripped the sensor, it may need to be reset or replaced. Not familiar with this particular boiler. However, call a gas safe registered engineer.
Ours overheats when the pump seizes (water not going through system properly = too much heat build up in boiler = overheat).
I usually resolve that by taking the bleed cap off the pump and manually turning it with a flat-headed screwdriver to free it up.
Not sure if that is what is going on with yours but might be worth a shot if any of this looks familiar:
http://www.plumbteamltd.co.uk/heatingpump.html