with / without central ventilation system experiences

Exactly, take it into account in the floor construction. With round ducts, you need even more. If a suspended ceiling is planned anyway, the pipes can of course also go in there. In any case, it must be considered in the clear room height of the shell construction.

Let me make a remark. If you want to consider it in the floor construction, the insulation is quite a hassle. If you do it yourself, it requires a lot of effort, especially if you want to do it properly (not just leaving a cavity next to the pipes or filling it with scrap material, you need bonded loose fill in there!). The absurd thing is that the specialist company probably often does not do it 100% properly but rather quickly just lays an insulation board over a cavity.

In the next house, I will probably have it poured into the intermediate ceiling instead.

One remark is allowed. If you want to consider it in the floor structure, the insulation is quite a bit of cutting. If you do it yourself, it's a lot of effort, especially if you want to do it properly (not just leave a cavity next to the pipes or fill it with offcuts, it needs to be filled with bound loose fill!).

Thanks for the remark. I hadn't thought about this beforehand and honestly don't know exactly how the specialist department carried it out here. Still, what exactly is the described loose fill supposed to achieve? Sound insulation?

On the one hand that, then of course moisture should not be able to collect in any cavity to then have the time (and amount) to really cause damage. And then it is probably also about the load distribution not being even. Theoretically, edge breakages can occur at the adjacent insulation or the ventilation pipe could get too much load. In case of doubt, the entire screed in one zone may then possibly settle. It is probably very unlikely, but it is well known that here in Germany we want everything to be double and triple safe. On the floor slab or towards the uninsulated basement you also have a thermal bridge, but I wouldn’t lay any ventilation there either.

...On the slab or to the uninsulated basement you also have a thermal bridge, but I wouldn’t install any ventilation there either.

We installed 75mm round ventilation pipes with 4cm expanded polystyrene insulation underneath (so almost directly) on the slab in the ground floor. No one saw a problem with this. We even had to install ventilation pipes in the (heavily insulated) facade. Also no problem so far.

Oh, interesting. And how much insulation is on it? Do you notice that the incoming air on the ground floor is somewhat cooler?

Hello, between the pipe and the floor covering there is again about 8cm of Styrofoam or about 13cm of 032 insulation wool. We have a very efficient controlled residential ventilation system regarding heat recovery, I have not been able to notice any difference in the air temperature at the valves so far (but I have not measured either).

Offtopic: If I ever have a lot of time again, I might do that, but at the moment I have long-term professional projects that I am involved in (like never before, as if there were no IT tomorrow :p). And privately, I will soon have to take care of the outdoor facilities with bike port, wooden deck, outdoor sauna, and the expansion of the utility room not to forget...