This boiler is about two years old. I fitted it myself so Ravenheat don't want to talk to me. The boiler lights up but at the same instant that the flame lights the fan switches off. The flame stays on for a few seconds - I assume until the negative pressure is lost. It then goes through the cycle again. However if while the flame is lit I take one of the wires off the gas valve the fan comes back on and the flame stays on until it trips out when it has got up the temperature. Can anyone diagnose the problem from these symptoms?