I HAVE A COMBI BOILER 4 YEARS OLD. WHEN I SWITCH THE CENTRAL HEATING ON THE RADIATORS GET HOT, BUT THE PRESSURE GUAGE READS 3 BAR PLUS. WHEN I TURN THE HEATING OFF WITHIN MINUTES THE GUAGE READS ZERO.ON INSPECTING THE PIPE FROM THE PRESSURE RELEASE VALVE I HAVE NOTICED A VERY SLIGHT DRIP. ANY IDEAS WOULD BE WELCOME THANK YOU.
Take a look at the projects section and click through the central heating projects. There is a link in there to the Gas Mans site. He will email you answers to questions on boilers and central heating questions. You can also use the appointments section on the website for another month. DIYDOCTOR
The fault is more than likely to be the exapnsion vessel. it may have lost its charge or the diaphragm inside may have split. worst case will be you need a new vessel , best case it will just need recharging