The pressure gauge normally reads the c.h. system pressure approx 1 to 1.5 bar. The water is coming from your safety valve hopefully being vented outside in a safe manner due to the excess pressure more than 3.5bar. I think that you are unfortunate in as much as when you topped up it coincided with a failure of the flat plate heating exchanger which produces the domestic hot water. Isolate the boiler electrically then shut off mains water to the boiler and vent the system to 1 bar, shut off top-up jumper and disconnect ensure that no open ends are left on the boiler side,gently re-open main water to boiler if pressure gauge dramatically increases then I would conclude that the exchanger is leaking cold water into the heaing cct.
Guytawe, May 2010
