Whom to entrust with the room-wise heating load calculation?

It was the former provider. As mentioned, the calculation can be used as a basis, but you have to question the values here and there. You also have ventilation losses without controlled residential ventilation, then significantly higher. As far as I know, the actual ventilation losses can only be calculated somewhat accurately if you know the n50 value from the blower door test. But the assumed 1.9/h in the calculation are quite high even without prior testing. For example, a KFW55 house must be below 1.5/h.

IMHO the BlowerDoor test shows how "airtight" your house is. So, all windows, doors, openings closed, controlled ventilation off... what escapes.

But this has nothing to do with the ventilation loss of the controlled ventilation (or window airing, or window rebate, or, or).

The controlled ventilation transports warmed air outside and brings cold air back in. This is then reheated using heat recovery. But some loss still occurs. In the room-specific heating load calculation, this varies from room to room. Depending on room temperature, air change rate, supply air temperature, etc.

This difference must be compensated by the heating system in addition to the transmission heat loss through the building components.

yes, in the end you end up calculating yourself sick based on the standard outside temperature, which here in the west last occurred 5 years ago, and come to the realization that the smallest heat pump is already oversized in everyday use (you basically get nothing under 6kw) and you will lay 5-10cm distance.

But then with proof.

It simply doesn't change the result in practical implementation.

And don't come to me with 10W pump power that you optimize away. That does not justify the effort.

You are mixing things up. It is not about the sizing of the heat pump. It is about the design of the underfloor heating. For me, the calculation took two evenings. More time is wasted on less useful things during house construction. But now I can show the [HB] that 30/25, for example, works well.

The sizing of the heat pump has to do with the energy demand of the house and the installation/planning of the underfloor heating with the heating load of the house and especially with the room-specific heating load?!

...of course, that has something to do with each other. But for the sizing, you don’t need a room-by-room heating load calculation. You can also calculate the heating load of the building differently with a few values during new construction.

The room-by-room heating load mainly serves for the correct sizing of the underfloor heating.

Sure, you can also estimate or simply say I install as close and as much as possible.

But it can also be done differently. If you now say for me a design with supply/return 35/30 is sufficient, then install what you can and you will manage with it. Although it has already been written... below 10 cm the effect becomes smaller and the installation more difficult depending on the pipe diameter.
If you want it lower, the calculation quickly reveals where it becomes tight and you know where you need to cover additional surfaces (wall heating, for example), or whether you perhaps (if not too late yet) consider additional insulation measures.
And above all, the room-by-room heating load calculation helps you when you have a somewhat stubborn heating professional.