Overheat Thermostat Tripping on Bosch 240 Combi RSF?

Hi,

The overheat thermostat keeps tripping on our combi boiler. The pilot will fire, but when central heating is turned on it will fire for about 30 seconds and then cut out, with the overheat reset button having popped. Originally the pressure was a bit low but have upped it using the filling loop to 1-1.5 bar and still get the same problem.

Any ideas? 'Cos it's cold!!

Cheers

Chris