Gas condensing boiler, air-water heat pump, fuel cells - please advise

Since I don't like insulation boards on the house, the decision was quickly made. And I have to spend the 5,000€ anyway

Then I would have rather taken a controlled residential ventilation system with heat recovery. At least then I have a noticeable gain in comfort.

Then I would have rather chosen a controlled residential ventilation system with heat recovery. At least I have a noticeable gain in comfort.

We have that, but decentralized. However, I agree with you, in retrospect I would have invested more and gone for a central controlled residential ventilation system. For the second house, certainly then :-)

Solar thermal for hot water is a point of contention here. Whether it is worthwhile, no idea. The fact is, it already delivered well here in March, now in April I completely turn off the hot water on our heating system, a Junkers. It works. Still hot water only through solar. Whether it is worth it, no idea. We had no choice since we built without controlled residential ventilation. Karsten

Solar thermal energy for hot water is a matter of faith here. Whether it is worthwhile, I don't know. The fact is, it already delivered well here in March; now in April, I completely turned off the hot water on our heating system, a Junkers. It works. Still, hot water only via solar. Whether it is worthwhile, I don't know. We had no choice since it was built without controlled residential ventilation. Karsten

And there’s nothing to say against that. But here are people who first make completely baseless claims that denying the economic efficiency of such a solar system (for hot water) is complete nonsense. But when you come at them with a few numbers, it gets thin.

And purely economically speaking, it simply remains the case that such a system will not pay off. Whether you fill the bathtub twice a day or only every few weeks. Due to the high investment costs combined with the relatively low efficiency in winter, the already low savings potential (hot water heating demand), and the gas price, in my opinion, you cannot come to any other result.

By now, I am relatively indifferent: We pay 50€ per month for gas. That’s 600€ per year. I don’t yet know if it will be enough. If not, it will just be increased to 60-65€ per month. What else could you save on that would justify a four-digit investment?

From the initial desire for KFW 55/40 it became the Energy Saving Ordinance with gas (without solar) and controlled residential ventilation centrally with heat recovery and that’s that.

So the decision is often not just whether it makes sense or nonsense, but which nonsense brings more benefit.

Actually, that says it all.

And especially with an all-in-one GU, I would take the ST immediately if it's included in the package, because it usually doesn't cost extra since the catalog builders have accounted for it. On the other hand, if you leave out the ST and instead opt for more insulation or controlled residential ventilation or other substitute measures, you might even get credited 1000-1500 euros. But then you have to pay for the substitute measures.

Thank you very much for the numerous contributions. I have really started a discussion there. If I should decide on a solar system, is a domestic hot water tank of, say, 300l reasonable, or would it be better to have a combi tank that then holds significantly more capacity?