My 11 year old worcester 241 combi has just had a new pressure reservoir fitted. since then the boiler trips out the over heat button after 15mins. The engineer has taken off the bathroom rad stat to create a bypass but that has not solved the problem. He thinks the narrow bypass pipe in the boiler is blocked. What should he do now? He has mentioned having a room stat but that feels like a backward step to me.

magbow, May 2010

Check to see if gas valve is modulating down as the boiler is reaching temperature. You can do this using a multimeter at the terminals of the modulator on the gas valve (2 blue wires). You should see the DC voltage decrease as it gets hotter. Also check the thermistors resistance ( 3.2 kohms is about 70 degrees)

REG, December 2010

I have come across this problem many times on this boiler, it usually occurs if the appliance has been over pressurised, this can cause the heat exchanger passages to constrict , it can also be caused by magnetite sludge blocking the heat exchanger try a
chemical flush 1st.

copperpipeman, May 2010

15min just on heating ? is it on low burn , do u have pump over-run

tim, May 2010