vaillant system boiler loosing pressure

[LUNAT]

HI

wonder if anyone can help
vaillant system boiler keeps loosing pressure.looses about 1.5 bar in just two days.
1.it is unvented system
2.new boiler only used about once a week (empty property)
3.no water leak visible at the overflow

 

[nogoodatfaultfinding]

Solid or wooden downstairs?

 

[LUNAT]

HI

big house at least 22 rads pipes running into celing and into wall (done by someone else )