Well WE have a boiling over imersion tank via loft vent!!! it filled the loft and upstairs with steam so shut it down. Its an old glow worm boiler with no separation between the hot water coming on and heating no elec, valves here. The fault like i say is heating on boils the hot water tank? maybe indirect pipework in tank U/S leaking into hot water tank? thats what ive been told so far. There is a valve on lower part of the imersion tank with D 1 2 on it is this the priority valve? pos u/s sorry bit new to this kinda work. any ideas .thanks