Tank

The water tank (heat reservoir)

The water tank is a HSK500 (picture with German description) tank, which has a heat reservoir of 443 liters, and an internal heat exchanger for the domestic hot water (DHW) of 45 liters.

Why did I buy that one?

Hard to say...

as a non-commercial customer, it's not easy to buy a tank from the manufacturers directly, and not all mechanics are willing to search or call around for hours
so this was the best my heater mechanic was able to get
it fits perfectly into the room, which is very small
two people can transport it into the cellar (narrow stairs)
... and as the technical data was OK, I did accept it

There were others which had a bit more sophisticated pipe connections, better system of "layer loading", "hard foam" isolation, but also more expensive.

I also thought about buying one made of stainless steel, but I guess as I really fill it up to 95 degrees C, it will be full of lime after 20 years, so I will have to get a new one anyway. And they are a hell of a lot more expensive.

With the tank there came a 10 cm "soft foam" isolation that fulfills the minimum insulation requirements. But the cellar was a lot hotter than before (23 or 24°), and the tank cooled out in about 4 bad-weather days. Not bad, but it was clear that it could be better.

So after several months, I added another round of 16 cm aluminum coated glass wool, and now the tank stores enough heat for about 7 or 8 days when it is full loaded (temperature range is 95°C down to 40°C).

The cellar room now looks like this. It does not look very nice or professional, but the best I could do in an evening. I don't care, and it works as expected.

Mistakes I made, that others can possibly learn from

The bottom of the tank is not isolated at all. It stands on a metal frame (in form of a cylinder), and fortunately I put some foam in that cylinder. It could have been improved by using "hard foam" (PolyUrethane), or maybe put the whole reservoir on some wooden ground instead of concrete.
I could have put the tank in the room where my wife dries the laundry. As it still radiates heat, it would have improved the drying time, and reduced the neccessity of standard heating for that purpose. And the pipes could have been about 10 m shorter (each way!), allowing less heat loss in the pipes.