new gas valve fitted but wont stay alight why?????

if when the pilot lights and after 30secs of holding the gas valve in the pilot goes straight out on releasing then you will have an overheat stat fitted in series with the thermocouple this needs a continuity test to determine if it as operated and needs replacing

combiking
March 2008

Your fan may not be slow running when there is no demand. This causes the pilot to suffocate after a while. A tell tale sign is the lack of any fan noise (can be very quiet) when there is no demand, and the top of the boiler being warm after a period of no demand.

Likely causes are a dry joint on the metal clad slow run dropper resistor, or a broken flow microswitch. Try operating the flow microswitch with your finger, it should have positive clicky operation like the DHW microswitch. Check for AC volts (careful, mains voltage) across the leads going to the fan when there is no demand.

c
March 2008

from the symptoms you describe it sounds like the thermocouple has either not been put in correctly or it has gone. replace with same type or universal one should sort it out.

gary
March 2008