I recently asked for help as my boiler was was cutting out after 15 mins. As advised I replaced the thermocouple and while I was at it the overheat cut off thermostat and the High and Lo thermostats. I thought everything was back to normal but now I've noticed that once up to temperature the main flame ignites then goes out in ten to fifteen secs. then repeats the cycle. Any ideas or is this normal.

george.morrison@blueyonder.co.uk, February 2009

I have an Apollo 80 and if the pilot-jet flame is not long enough to fully heat the thermocouple it shuts down the main burner when the water temperature thermostat shuts off the gas to the main burner. The main flame airflow "pulls" heat away from the thermocouple. If you have the drawing in the handbook it shows you where the adjustment is for the gas to the pilot jet. But check you haven't dislodged some crap into the pilot burner when you worked on the boiler, or knocked it slightly away from the thermocouple tip.

BryanL, February 2009