new heat pump as a replacement for oil heating with existing radiators

Actually, I should receive 55% funding on the total amount (some people here in Thuringia think so, I live on the top 1st floor, slanted walls, and everything below is rented commercially), three separate entrances but DWW says I would not get that.

An energy consultant can tell you that reliably. Unfortunately, I am not very familiar with multi-family houses and commercial properties when it comes to funding conditions. But it has nothing to do with Thuringia; the KfW handles that uniformly throughout Germany. You will not get the 5% environmental bonus with R32 anyway. So, at most 50%.

That is in the counteroffer. But I can't interpret Dummy.

The estimated ~60,000 kWh annual consumption would mean that the oil heating had 100% efficiency. Since I am calculating with an old device, I rather assume 60-70%. The rest will have ended up in the chimney and possibly the basement. To me, all of this does not sound serious at all, and I strongly recommend putting the brakes on and starting over again.

Not 60,000 kWh, but 6,000 liters of oil consumption.

Why actually additional air indoor units if it got nicely warm with the radiators?

Because with the heat pump the flow temperature is only 45 to 50 degrees, and with oil I sometimes had almost 60 degrees.

Because with the heat pump the supply temperature is only 45 to 50 degrees, and with oil I sometimes had almost 60 degrees.

That is why only a fool switches from oil to a heat pump without further measures. The clever person replaces the radiators and manages with 40-45 degrees supply and does not wonder about high heating costs, which the fool will have. ;)

Not 60000 kWh, but 6,000 liters of oil consumption.

That's about the same. If you seriously calculate a COP of 3.5 in your old building, that is about 17,000 kWh of electricity. At 30 cents that leaves around €5,100 in heating costs. 6,000 liters of oil costs about €500 more. So you exchange high oil costs for high electricity costs.

Have I already mentioned that I have 8 kWp photovoltaics with a 5 kW storage system on the roof? But you are probably right, and I'll go searching again for another solution. However, I'm reluctant to replace more than 15 radiators. Some rooms simply don't have enough space for that. And the house isn't actually that old. Windows, insulation, everything is still excellent ... As I said, a prefabricated house with walls prefabricated in the factory.

Have I already mentioned that I have 8 kWp photovoltaics with a 5 kW storage system on the roof?

Nope, but that has nothing to do with the heating or hardly helps you at all in winter. With my 10 kWp system, I produced less than 1,000 kWh from November to the end of February. During that time, the heating runs at full power.

But I hesitate to replace more than 15 radiators. There's simply not enough space in some rooms. And the house isn't actually that old. Windows, insulation, everything still excellent...

1. You have to choose one of two evils. A few € one-time for radiators or high costs for the next 20 years... 2. Not enough space? I dare to doubt that. 33 cm radiators are only deeper; height and length are identical. 3. Everything still excellent? It may be that nothing is broken, but excellent in the sense of "insulates well" definitely not. Otherwise, you wouldn't need 6,000 liters of heating oil per year. 330 sqm in a new building don't need 5,500 € worth of oil. They only need 800 € worth of electricity. Just saying...

A different question. In my new building from 1990, I also had oil. With about 280 sqm in 2-3 apartments, I did not come anywhere near those 6000 liters of oil as far as I remember.