Heating problem - Any boiler boffins out there?

[52classic]

Saulnier combi about 8 years old. Now on 'water only' setting with the temp set low. Pressure about 2 bar per instructions.

Suddenly noticed that the pump is running all the time, no matter what setting. Burner cuts in and out on demand but the pump runs always, unless the power is turned off at the switch or mains spur.

Also noticed that the stat is a bit tempramental. Sometimes the water is scalding and sometimes about the right temp for the dial setting.

Are these problems related? What are consequences of pump being on all the time?

What diagnosis/solution?

[Ricky_M]

Saulnier? Do you mean Saunier Duval?

Sounds like the two may be related. I'd have a guess at the hot water thermistor has failed and has triggered the high limit stat.

When the thermistor fails it doesn't sense the temperature of the hot water, that would explain why the water is scalding.

It will then carry on to heat the water to the point where a safety thermostat or high limit stat will kill the burner and run the pump to try and dissipate the heat.

It could also be that the boiler is continually calling for hot water when it is not supposed to, which means the heat has no where to go and will cause it to over heat

[52classic]

Thanks Ricky. It is indeed Saunier Duval.

What you say certainly makes sense so I'll get the manual out and spot the thermistors.