Calculate heating load to determine the size of the ground source heat pump

we will build a Kfw55 house. Solid 42.5cm exterior walls with a white bathtub basement. This is insulated. 2 full floors. House dimensions approx. 11.5 x 9.5 meters with an air space 3 x 3.5m. The basement should be partially heated, maybe completely. (we are currently disputing this with the general contractor because according to the building contract part should be heated and now this should not be possible because otherwise kfw55 would not be achieved). The heat pump was planned or rather estimated with a load of about 8kw and we have Vaillant VWF 88/4 in the contract. However, I read in the forum that some people manage with 6 KW for similar house characteristics. My general contractor says that probably we would have to upgrade to a 10KW heat pump if the whole basement needs to be heated. That would be about 291sqm area including the basement. Does anyone here have experience? A large part of the windows (total 24sqm) face south.

There my building is already an estimate. I don’t know your climate zone, but I have Kfw70 (on paper, actually better building technology), overall with a basement (real hot water, insulated) but a few square meters less. The unheated basement rooms, since they are within the insulated envelope, do not get colder than 15-16 degrees in winter (good as storage space – and I have no problem with condensing moisture). The additional demand for heating capacity for this should be limited. Our 5.8kW heat pump (brine!) has always had plenty of reserve; I have set the heating curve to minimum so it doesn’t get too warm, it just fits with a bit of fine-tuning so that the heat pump doesn’t short-cycle too much. I can’t imagine that a house with better insulation but a few more square meters would have a much larger heating load. The south-facing windows don’t matter, you need the capacity especially during cold winter nights. I think if you also take a brine heat pump and don’t undersize your probe, you shouldn’t need more than "one size bigger" than mine (unless you are in a particularly cold climate zone). It would definitely be best to do a proper calculation here rather than "guessing" by the general contractor.

We also have 11.5x9.5m with a basement, but a pitched roof - which makes no difference to the heating area.

We are almost KFW55, but have a poor Ht value of 0.29 - we did not overdo the insulation. However, we made a few modifications compared to the Energy Saving Ordinance calculation in detail. I might have it recalculated...

Everything is heated - including the hallways. And we come to 8.1kW heating load. Just as a reference value.

We have 190 sqm without basement, HT of 0.248. We heat with 5.3 kW - whether now 6 or 8 kW should not significantly affect the price or heating costs.

what would be a good HT value?

We also have 11.5x9.5m with a basement, but a gable roof - which makes no difference in the heating area.

We are almost KFW55, but have a poor Ht-value of 0.29 - we did not exaggerate with the insulation. Although we made a few modifications compared to the energy saving ordinance calculation in detail. Maybe I’ll have it recalculated...

Everything is heated - including the corridors. And we come to an 8.1kW heating load. Just as a comparison value.

I think you have a more complicated shape compared to my building (mine is purely rectangular). I would be curious about the actual required heating power, I am already wondering again why I have so much less with almost the same size. The few unheated square meters in the basement (corridors etc. are otherwise all heated) hardly make a difference, since they are within the insulated envelope. They are indirectly heated.