Well Euro, the fact that Brandenburg is not that far yet should not be our problem. Still stuck with cast iron radiators from GDR times and underfloor heating only heard of by word of mouth.
Just a question out of curiosity: What room temperatures can you roughly achieve with that, let's say, in a normal house built according to the existing Energy Saving Ordinance?
I can't imagine that a surface heating system, whether underfloor or wall heating, with a supply temperature of only 26 degrees, can make a house comfortably warm in the middle of winter.
Ok, for those who are okay with that, it may certainly make sense energetically. For me, it wouldn't be anything. 22-23°C would be too little personally. Pleasantly cozy for me means being able to lie on the couch in a T-shirt and shorts even in the deepest winter without shivering. 24-25° are my limit.
Then I can only recommend a sauna stove in the living room or save the extra cost of 4°C over 3-4 months entirely and head to the Caribbean. It's probably cheaper.