Controlled residential ventilation system - maximum duct lengths - consequences?

Yes, I have also discovered the program. Alternatively, I wanted to calculate with [Helios Easy Plan].

However, the question of the correct air volume still remains...

The 335 feel okay in general; calculating per person doesn't really make sense if you can't control the rooms individually or if an unusually large number of people live in the house. One circuit is fine up to 10m for simple rooms (study, children's room), but for living rooms, bedrooms, such circuit lengths are definitely too short. Nothing really gets through... if possible, I would still correct that; the conduits don't cost much.

The pipes are a significant cost driver for the system. Through the specialist trade 360 EUR per 20m, on the internet for 200 EUR.

What I don’t quite understand: Why does the manufacturer specify it like this?

The pipes or the source of supply?

The number of pipes to the valve

1. The long pipes would have been done with minimal exceedances.

There is a considerable difference between "Approved by the manufacturer" and "ideal for the average homeowner."

2. Flat duct had to be used because of the timber frame house with installation level

Maybe. I would have at least tried to fit a 90 mm round pipe, but of course, it is difficult to change such specifications afterwards. My list was not meant as a "criticism," but simply as a list of your problem areas.

3. These are also unavoidable with valves in the ceiling

That may be true, but they are disruptive to the airflow and should be avoided if possible.

The air distributor should be placed as close as possible to the device

Who says that? I can’t think of any reason for that...