Concrete in the pipes of controlled residential ventilation

And does that then fit into your floor construction?

Yes, that fits. These are then special flat ducts that are installed in the screed.

Probably not in the screed but below it in the insulation.

Then the floor structure was previously too high. (Not technically too high, just too expensive.)

So, details would really interest me, purely out of curiosity. It sounds a bit too simple to me, and I can hardly believe that such a significant change in execution is just possible like that.

In any case, are the affected pipes being completely re-laid over their entire length? Are there crossings with other lines that are already laid on the raw ceiling?

Edit: Im Estrich wäre vermutlich nicht fachgerecht (Mindestüberdeckung und überhaupt).

The flat ducts must be on the raw concrete --> then insulation --> then screed

(so not IN the screed)

Sure, the flat ducts go under the screed, not IN the screed. There are 2 pipes that need to be newly installed, all others are fine. And according to the ventilation engineer, it fits exactly with the floor construction.