Pressure constantly dropping and then the heating will not operate?

The pressure is set at approximately 1.5-2.0 bar. Over a period of 3-4 days the pressure drops to below 1.0 bar. At this stage the boiler unit does not operate and the heating does not function or kick in.
Once the inlet valve has been opened and the water pressure increased the unit again becomes operational.
This is a Vokera Compact 24 boiler. The heating engineer has been out twice and changed compnents in the unit. However I feel it may be a problem in the heating system itself. I am unsure of the comonent replacement as it is my grandmothers unit and she is 92 years old so does not understand the technicalities.