I have a vented sytem which is overflowing into the header tank.The ball cock is submerged. I was also getting very noisy pipe banging when the system was on. Someone suggested turning down the boiler thermostat which I did and the banging stopped,unfortunately the tank is still overflowing.Do you think it could be the boiler thermostat or is it likely to be a blockage in a pipe.when you turn the boiler thermostat up and down it doesnt affect the flame. Any ideas please it is driving me scatty