Newly built single-family house KfW55 standard - How to heat?

175l are heated super fast by the solar thermal system and the system stagnates. Otherwise, the water in the storage tank starts to boil and blows out. This causes the liquid in the ST system to boil, which is bad.

Are we talking about the same thing? I asked Dr. Google beforehand about a hot water storage tank and which one we need. The sites agreed. 175l is completely sufficient. Now you say it is too little because the water boils? Or are we not talking about the hot water storage tank?

You both are talking about hot water storage tanks. with (gas boiler +) solar thermal, you (presumably) with air-to-water heat pump (possibly with photovoltaics – but that is independent). Joedreck explains in the post you quoted why the hot water storage tank must be sufficiently sized for solar thermal. This is irrelevant when solar thermal is not used.

You are both talking about hot water storage tanks. with (gas boiler +) solar thermal you (presumably) with air-to-water heat pump (possibly with photovoltaics - but that is independent). Joedreck explains in the post you quoted why the hot water storage tank must be adequately sized for solar thermal. This is irrelevant if solar thermal is not used.

Ok. Understood, thank you.

For an air-to-water heat pump for a kfw55 house, 175 is also far too small. We have 300, 2 adults, 2 children - that usually is not enough for 24 hours. With gas it wouldn't matter, it just reloads. With an air-to-water heat pump, especially if there is also photovoltaics, you want to use the midday warmth and sun. A heat pump doesn't just charge the storage tank in 15 minutes.

Really? What is the water temperature set to at your place? Up to what temperature does the heat pump heat the water, and at what temperature does it start?

We have 180l and zero problems. On the weekend, there were even four adults and two children with us, everyone could always have a nice warm shower. Hot water runs normally 1-2 times a day. Water temperature is set to 48°.