Is night setback sensible? - Experiences?

Does lowering the room temperature at night really make sense?

It depends on your "ruin". For old and uninsulated huts, it definitely makes sense!

I'll read along here. During the construction phase, we're sitting in a ruin, and I have to start turning up the heating because currently I'm as cold as a pig. Even though it's only September. Oh dear. I hope we get through the winter with our 3000 liters.

With underfloor heating, it doesn't make sense.

That depends on your "ruin." With old and uninsulated huts, it's definitely worth it!

1948 I don't think the walls are the problem, but that there are drafts and the heat quickly leaves the rooms. I urgently need to take care of that.

It depends on the use case. If you want to lower the temperature in all rooms equally for the night, a central system is the easiest and best way. If you have different rooms with different heating requirements during the day or night, you will quickly reach the limits of the heating system.

If I want to individually adjust each room’s temperature within a short time, then that is true to some extent. In reality, large temperature differences, especially in new buildings, are rather fantasy. Unless I have an 80-degree flow temperature, quickly raising or lowering the room temperature is difficult to achieve. A constant temperature in the house with a low flow temperature is hardly to be beaten in terms of comfort and coziness. And if the house has even been half modernized, it is usually also the most economical option.

The task of the heating system in the house is to compensate for the heat losses of the house at an indoor temperature X and an outdoor temperature Y. Therefore, the heating system only needs to supply as much heat as the house loses to the environment. If the house were perfectly insulated (U-value: 0.0) and hermetically sealed, the heating would not need to supply any heat to maintain, for example, 21 degrees inside the house.

The heat loss of the house is calculated from the insulation quality of the building envelope and the temperature difference between inside and outside. The topic of heat loss through ventilation would also be added, but we will leave that aside here.

By lowering the temperature at night, the indoor temperature is reduced, thereby reducing the temperature difference to the outside air and less heat is lost that the heating system needs to replace.

The statement that "heating up" a house consumes more energy than maintaining a certain temperature is not correct. The heat stored in the house is not lost; it is only stored there. Only the losses to the environment are decisive, and these decrease with the temperature difference.

In the case of underfloor heating, the energy savings are virtually zero because the heating system reacts so sluggishly that it simply "smooths out" switching off for a few hours. Of course, this presupposes a certain insulation standard.
With a heat pump, there is an additional factor, namely the strong increase in heating output in the morning, which causes less efficient operation than if it had run at a constant output.
This would only make sense from the perspective that, for example, heating with a heat pump during the day using cheaper photovoltaic electricity from the roof or that the significantly warmer outside air during the day makes the air heat pump correspondingly more efficient.

For all types of radiators with conventional combustion heating systems (oil, gas, pellets), a night setback is almost always worthwhile, especially if the building is poorly insulated.