I have had an issue where by the pressure on my Vokera Linea Combi boiler is topping out & then ejecting the water outside as it hits 3 bar.
I have re charged the vessel (twice), but i’m convinced its not the vessel thats the issue.
I have seperatley noticed that the boiler turns itself on for about a minute every hour, when this happens it knocks the pressure up a notch and doesn’t drop. So after a nights sleep…..where the pressure was sat @ around 1.5 bar its back close to 3 bar by the morning?
if you are positive you have repressurised the expansion correctly (no pressure in system and set to 0.7 bar) then from your description it sounds to me that the secondary heat exchanger is passing, it does happen and would explain the boiler coming on intermittently as the system is cooled down by the cold water, thats if the system doesnt have a pre heat turn on, there are a few questions i would need to ask but would answer what you have described hope that helps. have had many years experience with boilers of all makes and models