Old Heating Piping Experiences

but the specified total energy is always the same and the boiler has to provide it

In gas condensing boilers, the ratio of flow to return plays an important role for efficiency. Radiators with different surface areas (=surface that emits heat) do play a role here.

Because the new radiator with 40 degrees flow temperature delivers the same performance as the old cast radiator with 70 degrees flow temperature.

and that cannot be physically correct. The heat loss of the water in the radiator is converted into heat in the air. Where else should the energy go? It is clear that a cast radiator takes longer to heat up, but it retains the heat longer.

and that cannot be physically correct. The heat loss of the water in the radiator is converted into air heat. Where else should the energy go? It is clear that a cast iron radiator takes longer to heat up, but it retains the heat longer

That is why heat pumps with 28 degrees flow temperature and underfloor heating do not work, and the Earth is flat ;) Seriously. Compared to modern panel radiators, finned radiators have a smaller heat transfer surface. To achieve the same output, they must either be significantly larger or operate at higher temperatures. There is also additional physics involved. In the old heating system, the heating water must be heated to 70 degrees. However, it is much more effective to heat it to only 30-40 degrees but increase the flow rate.

and that cannot physically be correct. The heat loss of the water in the radiator is converted into air heat. Where else should the energy go? It is clear that a cast iron radiator takes longer to heat up, but it retains the heat longer

More surface area for emission = lower temperature = less consumption.