Floor construction in height with Vallox KWL pipe routing questionable

but the problem of the air distributors in the floor structure (missing overlap) is not solved by them either.

Well, the air distributors are not necessary with proper planning and flat ducts, so the problem resolves itself with the correct approach, but that's just an aside...

Would anyone happen to know which ventilation manufacturers offer relatively flat duct distributors or require less build-up in the system?

They are not necessary with flat sheet metal ducts. (In a typical single-family house)

Now we had our appointment. After the site manager, us as the client, the installer, and practically the entire staff of the ventilation company had to gather after working hours, there was intense pondering and additional phone calls were made to Vallox's company headquarters in Gießen. The fact is that Vallox themselves do not offer distribution boxes that are narrower. Alternatively, it would be possible to mount the distribution boxes on the wall, which would cost us living space and was categorically ruled out. Flat ducts (probably still available ) do exist, but the distribution boxes for them would be even taller. With a meticulous approach and the data sheets, the distribution box of 16 cm plus lower decoupling of 0.5 cm and hard insulation of 1 cm and liquid screed (with additional reinforcement and without underfloor heating loops) of 4 cm as the build-up was decided. With that, we have moved about 1.5 cm higher and were assured that there would definitely be no loss of heating in the floor, as they intend to lay particularly many heating loops alongside. Well, we are curious. Today or tomorrow the liquid screed insulation will arrive.

Um, and what about the cleaning of the distribution boxes?

1.5 cm can already decide whether the apartment door opens or not. The tiles will definitely crack in the area of the boxes. You will never be able to clean the pipes. No one has leveled whether the floor in the area of the boxes is not higher anyway. Bad compromise, but it's normal that as a layperson you can't do anything about it.

The boxes, like the pipes, must also be flushed through with a cleaning robot. However, the flap would no longer have been accessible in the wall construction either. Due to stability, metal mesh mats are now additionally used in the screed. Well, and as for the doors, they are not installed yet.

Alternatively, it would be possible to mount the distributors on the wall, but that would cost us living space and was categorically ruled out.

The problem was your planner, who should have explained the space requirements to you – or even better: banished the boxes to the technical room. But that won't allow the cable routing with round conduits anymore, right?

How about coming up with flat ducts from the technical room and laying them under the screed? And: especially do not use distributors that are two units high but only single-row ones!

Alternatively, you can also play 3D Tetris – do you have half-high partition walls in the bathroom, for example near the toilet? Maybe you can get the boxes in if you fully extend the wall there.