The domestic hot water system works fine but when the central heating
system comes into play the pressure in the system gradually builds up
until it reaches 3 bar which activates the pressure release valve.
After this, there is no pressure in the system and water needs to be
added to bring the pressure up to 1 bar. Then, the process repeats
itself. Is this a major problem with the central heating system or one
that I can easily repair? Any help or advice would be greatly
appreciated.