Ventilation: central or decentralized?

But only to a limited extent, because in the bedroom you also want fresh air... otherwise the whole undertaking is somehow pointless...

Exactly - but there is no device making noise in the bedroom. These (the fans) are, with proper planning, only placed in "non-living rooms." They extract the air. Fresh air comes into all other rooms through openings in the window frames.

Exactly - but there is no device making noise in the bedroom. These (the fans) are, with proper planning, only installed in "non-living rooms." They extract the air. Fresh air comes into all other rooms through openings in the window frames.

Then you have a system without heat recovery - hard to imagine for KFW 55.

With HRV you already have noise, but at night level 1/3 is no problem, during the day level 2 is also not noticeable.

I would install a central system on each floor.

Exactly - but there is no device making noise in the bedroom.

As already noted, this is then not a decentralized ventilation system but an exhaust system.

Welcome to the last century!

This technology is long outdated and offers hardly any advantages over manual ventilation. (But still, it is better than no forced ventilation at all) but I digress...

I would opt for central ventilation. For cost reasons, one could then consider going decentralized on the upper floor.