What to replace an oil heater from 1989 with in an old witch's house?

Hmm, I think 700 euros per year would be worth it for us :) Especially because I believe the difference will be smaller in the coming years. A new oil heating system would be around 20k according to the heating technician, I think a heat pump could be built somewhat cheaper.

Heat pump in a poorly insulated house with radiators does not work. You need easily 60°C flow temperature to get usable heat into the house with radiators. They were usually even designed for 70°C and above. This becomes extremely inefficient with the heat pump. Either a new oil burner or better gas if feasible. Pellets could also work, but in my opinion, it is almost never economically viable.

Nanana at -10AT I am currently at 44 degrees VLT

Hmm I think the heating system in our house was not really designed, they just installed a radiator the width of the window under each window. And at least the roof (in my eyes the most important?) is well insulated now. I would also be willing to implement further optimization measures, for example, larger radiators could be installed in some rooms without significant losses and partly the windows need to be replaced anyway.

Hmm I think the heating system in our house wasn’t really designed; they just installed a radiator under each window with the width of the window. And at least the roof (in my opinion the most important part?) is well insulated now. I would also be willing to implement further optimization measures, for example, larger radiators could be installed in some rooms without any real drawbacks, and some of the windows need to be replaced anyway.

Just for comparison with your unrenovated place. I have to renovate to KFW 55 standard in order to heat sensibly with a heat pump at all. Flow/return 40/35. And even that was only achievable with a proper radiator upgrade. Standard outdoor temperature around -7 degrees. You will generate brutal electricity costs in the unrenovated state. Sure, it's no problem to have 55 degrees flow temperature as well. But that costs extreme efficiency. It’s basically like heating water with an immersion heater.

Yeah, so does my calculation earlier hold true? Then the financial effort would actually be quite manageable despite lower efficiency?

I don’t have the illusion of building a high-efficiency structure here that energy consultants from all over the world would flock to admire my marvel. It’s more about choosing the least bad option among all inefficient solutions gas / oil / WP.