how to regulate gas?

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