Interpretation of a central ventilation system

Intermediate ceiling - I think it's laziness, but you are the customer, he should provide hard facts...

Yeah, material price is one thing. The structural engineer has to, as he says, take out the slide rule again and that costs me quite a bit. Structural engineering in general was quite expensive, even though we have no "special features," see floor plan.
I will leave the on floor variant as it is now.

What I still don't understand, however, is the volume question in the upper floor. Why is the room volume, specifically the ceiling height, irrelevant for the design? According to Zehnder, there are different calculation methods or approaches for this. However, that is not a valid and (for me) comprehensible answer.

My GÜ also initially wanted to make the intermediate ceiling thicker. It would have cost 3k more; back then, it seemed cheaper to solve it with more insulation on the intermediate ceiling. The story height increase only cost 1900.
Couldn't have known that 9 months later the insulation prices would explode like that...
In the end, doing it myself for the installation didn’t pay off...

I also thought about doing it myself. But that means I have 1 hour of travel time per day of work and in the end I always have to work when it probably doesn’t fit my schedule to avoid delaying other trades. Besides, it’s suboptimal with 2 small children.

The room volume thing was the same for me. I even had the design planned by an MEP engineering firm. They also said it was irrelevant. For me it was only about 15 cm more, so I could somewhat understand that it doesn't make a serious difference, probably because it’s calculated generously anyway. For 3 m I might have probed more. What ceiling height do you have?

TGA Ing. would still be an option, thanks for the tip! Although I don't want to use a sledgehammer to crack a nut. Ceiling height on the upper floor (except corridor = 250) is about 4m at the highest point. I need to look for the section or ask the architect (based on the room volume). I intend to get a second offer from another manufacturer (Wolf, Maico, Hellios…?) anyway. I'm curious what they will say about the situation on the upper floor.

Just a brief note on our special requirements in case that's what you mean: as quiet as possible, especially in the bedrooms.

We don't have Zehnder but a combined unit from Tecalor, but I can also reassure you there. At night our system reduces the ventilation to the lowest level and is therefore barely noticeable in the bedroom. I would say I'm rather sensitive to noise and I can't hear it at night.

As already writes: the deeper you delve into the subject, the more uncertain you become. Flat channels are not easy to clean, higher pressure loss, flow velocity should be max. 1.5 m/s, no bends... I think I will follow the tip from after all and hire a TGA planner.