Cheers Breesey, i take it you mean the heat to water exchanger? or dya mean gas to heat up in the main burner compartment?
I mean The heat exchanger!
You'll get kettling if the volume of the heat exchanger has been reduced buy scale because the smaller water quantity in it over heats and vapourises. You can get a similar effect if the pump is buggered and therefore not moving water through the heat exchanger quick enough.