open vent Y-plan heating sysem, boiler goes to lock out as it overheats. I have bled the radiators three times and the boiler starts working but every next morning boiler goes to lock out again and there is no water in the sysem every time. what could be the problem? could it be pumping over into f&e cistern or a leak under the floorboards at ground floor? going to investigate tomorrow morning , any suggestions please , thanks .