Control floor heating

In my experience, it is indeed possible to create significant and noticeable temperature differences between rooms, of course also with underfloor heating. In our current house, which is not very well insulated, we have 14-15°C in the bedroom and 25°C and 21°C in the adjacent rooms (bathroom, children's room).

If this were not possible, it would also mean a significant loss of quality of life for me, as I cannot sleep at all at 20°C.

It also does not make sense to me why there should be a difference in this matter between underfloor heating and regular radiators. The inertia is not an argument if the underfloor heating in the bedroom is ALWAYS set to 14°C (or "off").

The insulation between the rooms is certainly weaker than the insulation of the exterior wall, but for that the temperature differences are also about an order of magnitude smaller. However, the insulation is NOT worse by an order of magnitude when building, for example, with aerated concrete or Poroton or wooden studs + mineral wool. The heat transfer of the door naturally depends on the model of the room door, but a hollow chipboard probably does not insulate by an order of magnitude worse than a window of the same size in the exterior wall, just to reestablish this reference.

I do not believe that the flow temperature is raised when, for example, only one room is heated. The difference between flow and return temperature initially becomes smaller when only one floor is "flowed through". Of course, this depends on the type of control; I do not know if every heating system nowadays can regulate the volume flow by both the pump speed AND the flow temperature. Even if the flow temperature had to be raised slightly (which I do not believe), but the flow rate drops significantly (because fewer subordinate heating circuits are flowed through and the pressure loss increases or because the pump speed is throttled), then significantly less energy is still transported. This effect certainly compensates for the disadvantages that would arise from the heating device operating at a less favorable operating point (which would still have to be proven).

because I can’t sleep at all at 20 °C.

Many people like to sleep in a cool room, I find that interesting. (Leaving aside the fact that you certainly don't stay awake for months in summer. And people also sleep in more southern countries, in Spain even at midday ). People often perceive cool air as fresher, hence the desire to keep the bedroom cooler. However, what is relevant for good sleep is not the temperature but the air quality in terms of CO2 and humidity (pollutants excluded). And that brings us to controlled residential ventilation or open windows. But neither has anything to do with sleep temperature. Oh, and then there’s the duvet regulation. In new buildings, the summer duvet is sufficient all year round, which is not bad either. On the topic: Is the valve for regulating flow not sufficient according to the Energy Saving Ordinance, so that you can legally do without the control button on the wall? Assumption: This valve is always part of underfloor heating anyway; one valve per circuit, one circuit per room, at least.

So we have a MAXIMUM temperature difference of 2 °C... we can twist and turn however we want.

Relevant für den guten Schlaf ist aber nicht die Temperatur

Well, your explanation is all well and good, but I know better what is relevant for me. Of course, I can also sleep at 30 °C, but worse. I also have an air conditioner. The air is always fresh because the window in our room is always open for at least 2 hours before going to sleep, and our bedroom is also very large and high.

If that were not possible, it would also mean a significant loss of quality of life for me, because I can't sleep at all at 20 °C.

That's a matter of getting used to, see Bieber for details.

It should be clear that with an open window at an appropriate outside temperature, the room temperature can be lowered. However, your heating will try with all its power to bring all rooms within the thermal envelope to the same temperature, insulation from room to room can only partially prevent this.

Look at the insulation values of exterior walls, interior walls, doors and windows, for example, the U-value on the internet, calculate how the heat transfer is with your room geometries, and so on and so forth, that is "somewhat" more reliable than your feeling that the difference shouldn't be that big.

Otherwise, common sense should tell you that a room door with about 4 cm thickness is not used as an insulating exterior wall for no reason, which nowadays is about 10 times thicker... And at least with us, the glass elements of the interior doors are single-glazed and not triple-glazed like the windows to the outside...

since the window in our place is always open for at least 2 hours before going to bed

During the heating season, you are practically throwing money out of the window like that. You can do it, no question, it's up to you.