My central heating is always boiling over. I have to keep the switch on the boiler to minimum, if the switch is set slightly over minimum the tank boils over. The radiators go cold but the boiler keeps going. Does anyone know why this is? Regards, Mark.
Boilers with low water content are prone to flash the water in the heat exchanger to steam if the water is not flowing fast enough to carry the heat away at a temperature below boiling. If the boiler flashes the water to steam, pumping stops, so the radiators get cold.
Investigate the pumping rate through the boiler to make it fast enough to carry the heat away at higher settings. This would be indicated by taking the water temperature entering the boiler and leaving the boiler. The difference in temperature should be less than 30F or 17C degrees.