Vokera Linea 28 - Combi pressure Loss

Understand continual pressure loss is caused by leak in the central heating system. However despite four seperate visual inspections no evidence of a leak can be found. Cold pressure test 2.5 bar has failed to locate problem. Combi tested for one hour seperately under same pressure was satisfactory. Boiler installed January 1999. Lower floor area isolated and held normal pressure for 2-3 days but unfortunatley is now losing pressure. Suggestions please!! Assistance would be gratefully recieved.

have they checked inside the combustion chamber to see if the main heat exchanger is the point of leakage?? It wouldn't necessarily show if it is.

I suppose the press relase valve is not leaking?

common one on the vokera aav leaks and dries up no visable sign the water evaporates due to the heat,fit a new aav,check the prv,check all rad connections top and bottom,if all else fails fire in the leaksealer