The flame size should change based on the demand from the thermostat, yes. I do not know if you will see much change with the door open and going from max to min or not. The door being open may create a demand for more cooling. I would observe the flame height before opening the door and then check it by doing the knob twist to see if it gets bigger. The size of the flame corresponds to the BTU output of the burner. More BTU's makes more cold, it does not make sense, but thats the way it works.