Thank you for the lively discussion. We were on a city trip to Hamburg, which is why I am only writing now. We will discuss it again with the energy consultant, but I think the [ST] is set. As mentioned, we live in an old building and are renovating it step by step; the [ST] does not take up space for a later photovoltaic system and can therefore be combined with it. For us, this is currently a good solution in combination with the gas heating, and all friends with old buildings and [ST] have reported only positive experiences even after several years of use.
Unexpectedly, our project has finally gained momentum. The cost estimate from the construction company is coming this week, we have a structural engineer, the plumber has now submitted his offer as mentioned, and through the structural engineer, we have come into contact with an energy consultant. After 5 months, finally some movement in the matter! If everything works out, it will start after Easter.