Header exspansion tank on indirect tank constantly overflows I have put a new thermostat in , although I do wonder if this one may of packed up aswell as lastnight water in the top tank was red hot and today the water is cold , I have drained the cylinder totally and refilled as needed to move it. Mfi 30/180 Albion indirect cylinder. Its heated by an oilfired boiler. I have also replaced the ball valve . Can someone please advise what the problem might be and how to test for it etc please