I have a combi boiler and whenever the heating is on the pressure guage rises to 3.0 bar pressure. As a result it then vents water out side the house and the pressure drops. The pressure in the system is then below 0.5 bar when the heating is off and I need to refill the system to build pressure again.
What could be causing this rise in pressure and how can it be fixed??
I think you need to get the expansion vessel checked, if it has lost it's air pressure charge your heating system water pressure will rise as the temperature of the water heats up causing it to expand, with no expansion in the pressure vessel possible it will cause the safety valve in the boiler to lift getting rid of the excess pressure (water). When the system cools dow it then shows a drop on your boilers gauge