Sounds like leaking expansion tank to me,if topping up regulary.
With no pressure in system and heating cold, see if water comes out of expansion tank when shreider valve
core is pushed open.(it will need re pressuring) If excessive amount of water comes from tank, tank may need changing.Pressure safety valve should generally blow off at around 3bar.
The system pressure wants to be set around 1.25bar when system cold. Dont know exp.tank press.
spooney59
April 2013