Solar thermal domestic hot water additional heating support?

But isn’t it the case that with pure hot water heating by the sun, the storage tank is much smaller and there are also fewer square meters of solar panels? So, it’s about more than just a few valves, and whether it is then worthwhile? Karsten

Sure. The design of the heating system depends, among other things, on the usable area. How big the house is, how good the insulation is, how many reserves the new system has? No idea. With an 800-liter combined storage tank, it’s getting interesting. The 9.36 m² of its collectors (there have to be four) is a bit tight. With 800 liters, five collectors would be better.

With an 800-liter combined stratified storage tank, it slowly becomes interesting.

But only for the storage tank manufacturer, panel manufacturer, and the installer...

you can also save the space and money and the additional technology and simply operate the heating with it for several years...

Just like other extras in the house. Ventilation system, fireplace (with heating support), ETICS insulation, KNX, batteries (accumulators).

Oh, the other extras are not that expensive. Standard ventilation system tilt and turn with white handles on each element at no extra cost, but manually operated. Chimney, outside, designed as an open fire in the garden including grill, Landmann, 11.90 plus a few field stones, two 12-volt 70 ah batteries as emergency supply for the cars, brand Varta, 109.90 each plus deposit...Karsten

Yes, you already have an understanding of building very cheaply. But others gladly spend money on luxury. However, solar thermal is rarely worthwhile when an installer connects it. To operate heating support sensibly, you need a design for a very low flow temperature and a large collector area. You might be able to "save" during the transitional period. Personally, I would much rather invest in better insulation instead of these costs. Solar is only necessary if you are building with a gas boiler and the primary energy demand is too high (if I remember correctly).