Air-to-water heat pump: choice between Viessmann and Ochsner

The heat load calculation is still pending, but - roughly estimated: the new building according to the Energy Saving Ordinance 2016, a heating demand more towards 0.05kW/sqm, approx. 165sqm to be heated area: 0.05x165=8.25kW. The pump from Viessmann has 7.4kW. Why is it oversized? Just for understanding :)

But that's a bit of a crystal ball now... I would have speculated rather with 35 watts/sqm with my crystal ball.

With us, it is mathematically 44 watts/sqm, also not a KFW55 house but close. However, I cannot provide real values, the heat pump is still delayed. ;)

the new building according to the Energy Saving Ordinance 2016

The Energy Saving Ordinance 2016 no longer existed in 2020.

Semi-detached house, built solidly, not a KfW house (so approximately "KfW-60")

In 2023, at least KfW55 with tightened requirements applies. You also have a semi-detached house, which has a lower demand than a detached single-family house (thermal transmittance to the neighboring semi-detached house is great, there is no window that ruins your bill). Moreover, a heat pump is more likely to be undersized than oversized. And you have a controlled residential ventilation system. I’d bet my a.. that a 5 kW heat pump is completely sufficient for you. I would not let anyone install such a large heat pump. As far as I know, it cannot modulate below 4 kW, so at moderate temperatures like now, it runs basically at full power and doesn't know where to put the heat.