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

I am still not clear about what to make of the heat pump electricity tariffs.

The question is more what your network operator thinks about it. In many networks, a heat pump is only allowed behind a separate meter, then you have no choice. Otherwise, they tend to be rather unprofitable.

Hmm my first small heat pump simulation a few weeks ago:

Outdoor temperature -3°C
Flow temperature: boiler temperature 45°C (controlled via slope at 0.8)
Radiator in the living room, fully turned up. Other rooms not yet. Circulation pump unchanged.
Temperature in the living room: 20°C

The idea behind the instantaneous water heater was as follows:
I also want warm water in summer and in the transition period. With a heating element in the hygiene storage tank, I would then have to run the flow temperature higher than necessary and would lose the ability to cool in summer (even though this has never really been an issue before)?

Do I definitely need a descaling system with our hard water (21H)? So far, none has ever been installed and we have never been really bothered by the limescale.

Here is a small monitoring setup that I put together for my apartment:
[MEDIA=imgur]a/7eyF7qm[/MEDIA]

Unfortunately, I haven’t yet found any radiator valves where the opening can truly be controlled via API. With mine (tado), I can only set the target temperature. Once this is reached, the radiators close. This would be the opposite of what I want in summer during cooling operation.

Many heat pumps apparently always have all radiators turned up fully and regulate the temperatures centrally via the heating curve. I understand the approach that it is basically better to give less gas than to regulate the speed via the handbrake. However, in our house extension there are the kitchen and guest room, which are hardly heated so far, since there is a wood stove in the kitchen, there is basically less heating demand, and the guest room is above it.

The idea behind the instantaneous water heater was as follows: I also want warm water in summer and in the transition period. With a heating element in the hygiene storage tank, I would then have to run the supply temperature higher than necessary and would take away my ability to cool in summer (even though this has never really been an issue so far)?

I don't understand any of this. The heating element is usually not on at all, that's what the heat pump is for. And how do you want to cool in summer?

Many heat pumps apparently have all radiators

95% of all heat pumps don't have any radiators at all. And even if they do, the thermostats are turned up, but only with hydraulic balancing and regulated flow similar to underfloor heating.

Hmm maybe I also have a flaw in my thinking. But during the transition period, I maybe need 35 degrees flow temperature, so 35 degrees in the hygiene storage tank. That means the water would maybe come out of the faucet at 33 degrees? That is a bit cool for showering? In summer I don't have to heat for several months at all, but still want to bathe/shower warm. That's why warm up the whole hygiene storage tank? Hmm in my opinion, the house can be cooled in summer by circulating water that is cooler than room temperature through the radiators.

Hmm maybe I also have a thinking error. But during the transition period, I maybe need 35 degrees supply temperature, so 35 degrees in the hygiene storage tank. That means the water from the tap would maybe be 33 degrees?

You have 50 degrees in the storage tank. Supply temperature is for heating water, not for drinking water. With a heat exchanger, you get water hot enough to burn your skin. In summer, the storage tank is still heated. It's much cheaper via the heat pump, which converts one unit of electricity into four units of heat, than with the instant water heater (which only does 1:1). Storage losses are negligible.

Hmm in my opinion, the house can be cooled in summer by circulating water cooler than room temperature through the radiators.

Keep dreaming... Even with underfloor heating, maybe 2 degrees is possible in a well-insulated new building. Cooling effect via radiators will be zero. First, read up on how heat pumps work, different hot water storage tanks, and cooling options in houses. You are still a total novice. No offense.

Yes, I am definitely still at the beginning of my journey. That's why I am especially happy about posts from which I can learn. Hmm, there are some users here in the forum who say they also use their radiators for cooling, but cooling is not really a topic for us anyway. I think it might make sense to bring in an energy consultant for our project, who will hopefully also take care of all the funding applications. When we replaced the roof 2 years ago, one was briefly involved, but he wanted to wrap the whole house in plastic the whole time, and we didn't really see eye to eye.