hi thanks for all the replies the boiler was actually having a high burning flame then a low burner so it was chanting at me now since he has been he did change the thermistor but said the old one was okay but now its nice and quiet and running okay

windowman, September 2015

I have had a problem like this with a mains gas fire - the flame would rise up and die down over a period of a couple of seconds.

The gas board came out and solved it immediately - they said it was "Water On the Service"

Water had entered the supply pipe in the street - pressure below and vacuum above caused the water to rise up the supply pipe - until the pressure differential could no longer support the column of water in the pipe - then it would all collapse - and the flame rise up. I could hear the water gurgling in the pipe that came from the ground and went into the gas meter.

They sucked the water out.

Your gas man will have changed a Thermostat - a thermistor is a heat sensitive electronic component.

I still doubt the honesty of your gas engineer.

Take Care..., September 2015