My Father-in-Law has a gravity heating system which also works the DHW. On the boiler there is an option to work just the water or the water and heating together but not just the heating alone. When the heating is on the hot water cistern seems to 'over cook' and then hot water feeds back into the base of the header tank in the loft which then causes it to overflow. My first thoughts are that it could be the cistern has had its day or possibly the motorised valve is not working. I am a relative novice to the heating side of life and would appreciate any advice to enable us to narrow the problem down in order to change the 'correct' part.