I have a Vaillant combi bolier (about 10 years old) with the pressure guage jumping to the max all the time. I have to use the pressure release tap to release the excess water once every week or even every two days!

I know the pressure represents the amount of water store inside the boiler, so is it a problem with my top-up tap that slowly letting excess water into the boiler or is it something else?

Thanks for any help or suggestions.