Ariston 27 RFFI boiler - pressure drop?

I need to fill up the boiler every 2 days when it drops from 1.5 bar to zero. The pressure relieve valve has been replaced and no water loss is present from the pipe going outside. I have this week turned the system off and once the associated pipework around the boiler had gone cold I could see water dripping from underside of the boiler casing. I topped the water up to 1.5 bar and within a few minutes the water dripped out to zero pressure. What is the likely problem and will I need an engineer to fix it ?