Is the heating oversized?

3.6 kW at design temperature? We have the same dimensions but come up with 8.1 kW...

EDIT: So design temperature - calculated yourself? What output does your heater have?

Yes, I calculated it myself, I was picky at that point and made the calculation quite detailed. But the result also matches the measurements, the house has been standing for a while and I log temperatures and consumption etc. It is even slightly better, but not significantly. The heating has 5.8kW at B0W35. I didn’t want less because of the hot water, so that the storage charging for hot water doesn’t take too long. With a bit of tuning (hysteresis and compressor lockout) the device cycles absolutely reasonably, so when it’s warm, not too often, but also often enough that the cycles don’t get too long. Edit: Design temperature -14 degrees.

Ok, I'm just surprised that with our DIN calculation (modified for different indoor temperatures, controlled residential ventilation recorded and hallways also included), such a much higher value came out.

In our DIN calculation, 6 kW also came out - 3.8 kW is already impressive; but you were also lucky with the flow temperature from the hot source

I have taken all of that into account, no idea if [DIN], I adhered to the physics. But the dimensions alone don’t say anything. For example, we don’t have any bay windows etc., completely rectangular building shape. The roof is basically doubly insulated on top because of the controlled residential ventilation, once the upper floor ceiling and then completely between the rafters. In the children’s room/office no floor-to-ceiling windows (in the KZ there are higher ones with a fixed lower part, but still less window area), because I see the need for space there and I prefer to have my desk in front of a normal window at least. The basement floor is actually fully below ground level in terms of height. Maybe such details cause the difference?

Well, no idea – values from the thermal insulation calculation according to the Energy Saving Ordinance are used, where the components used have been included.

What Ht-value do you have? That is quite crucial...