KFW 40, Ground-water heat pump worthwhile or is air-water better?

Take a look at the funding options for the brine heat pump. I'm not up to date on this, but the different subsidies used to close the cost gap quite a bit. The efficiency of the geothermal heat pump is independent of the weather. The annual performance factor is consistently maintained. I doubt that only €50 savings come out. If you build a 150 sqm house with KfW55, you have a calculated energy demand of 55 x 150 = 8,250 kWh p.a. The brine heat pump has an annual performance factor of 4 and the air-water heat pump an average of 2.8 (assumption that the maximum stated figure is not reached throughout the year). This results in a consumption for the brine heat pump of 8,250 / 4 = 2,062.5 and for the air-water heat pump a consumption of 8,250 / 2.8 = 2,946.429, so a difference of around 900 kWh; at €0.30/kWh that is €270 p.a.
What from my point of view also speaks for the brine heat pump are the cooling capacity and the significantly quieter operation.
My tendency – without having looked more closely at the actual expenses (costs minus subsidies) – would be the brine heat pump.

Many thanks for your assessment. I have already looked for funding opportunities as well, but for NRW I can't find anything significant, so with assumed additional costs of €12,000 (drilling, etc.) we have a payback period of 44 years. I keep coming back to the thought that the house has too good an energy rating and we don't have -10 degrees in winter here either.

I keep coming back to the thought that the house has too good an energy rating and we don’t have -10 degrees here in winter.

One thing is the payback period, another is what makes you feel comfortable. I know a few builders who chose drilling or opted against an air-to-air heat pump out of concern for problems with the outdoor unit. Certainly, this is not the norm, but it does happen, as you can occasionally read here. Our Daikin air conditioner, with which I sometimes also briefly heat upstairs, hums very lightly, and I don’t hear it indoors. There are no neighbors around outside, and even if there were, you wouldn’t hear it. It’s different with a house further up in the settlement, where I can actually hear the outdoor unit well when I walk by. Such a humming would be pure horror for me, which is why I didn’t do that, and even if such concerns are unjustified in over 90% of cases; still, it can be another decision factor. The “energy rating” or what you probably mean by it results only to a limited extent from the use of one heat source or another. As a user of a very well-insulated house, however, I would advise you to achieve a “good energy rating” precisely because then hardly any heating energy is needed at all; the house cools down far less than a less well-insulated one, and this already makes a positive difference at 10 degrees plus, so independently of Duisburg winter days. Initially, I had a strong wind noise here, which has now largely been resolved, but this was enough for me to have thought about selling. The neighbor with the humming heat pump was here recently; it doesn’t bother him; lucky guy. Actually, if I were you, I would worry far less about the type of house walls unless you want a very specific building material for concrete or necessary reasons. Otherwise, I would think much more about the many little things in the house that will really influence you directly later; the wall will be plastered or clad with wood and then it’s gone. The floor plan, window planning, type of heating (underfloor heating, wall heating, wood stove, etc.), floors, lighting, etc., all these things can make your house cozy but also beautiful, regardless of which stone you used to build it. Basically, I would not choose the heating just from the perspective of a payback period, but from my very personal comfort factor, whatever forms that for me.

Many thanks, Auraki!

I believe I may have expressed myself unclearly in one or two places ("good energy rating"). Of course, what I mean is the high energy efficiency class or the low final energy consumption of the house. Due to the good efficiency class, I consume so little energy that the costs are no longer in any relation to the savings. If money were no object, I would choose a brine-water heat pump. However, I prefer to invest the money in the garage or the new kitchen.

I don’t fully agree with you on the choice of masonry, as I want to be careful to avoid having to replace the ETICS panels in 30 years. But of course, you are right that other things will influence my daily life much more. I am already working on those topics, but both general contractors are flexible with the floor plans, so that won’t be a problem.

I don't completely agree with you on the choice of masonry, as I want to make sure that I don't have to renew the ETICS panels in 30 years.

There is certainly a different KO criterion for many decisions, but if you want to avoid the ETICS, whether everyone understands that or not, that's already clear. But you can also build with a thicker wall or use a different type of insulation. For example, we have 14cm of wood fiber insulation under the wooden facade, but I could have just as well built with a 50cm wall thickness; for us, that only resulted during construction from rather odd reasons with the general contractor. Do you want to build a very well-insulated house (with or without funding) or rather just meet the legally required standard? As I said, I would decide something like that out of my own accord, precisely with my parameters and backgrounds.

I keep coming back to the thought that the house has too good an energy rating and we don’t have -10 degrees in winter here.

Well then, the decision is almost made. I would place the utmost importance on noise levels when making the choice. Not just outside, but also inside.

I wonder whether a ground-source heat pump is really worth it? The costs for drilling amount to 12,000 - 13,000 euros. Basically, I ask myself whether I would ever get that money back by installing such a ground-source heat pump.

Instead of deep drilling, one could also consider a slinky collector. If the property is suitable for it and own work is planned, it could also be computationally worthwhile to opt for a ground-source heat pump. I have no personal experience with this, but in other forums on building technology there are many interesting posts and experience reports with instructions for implementation.