Underfloor heating heating demand with at least 60 mm screed

Then I really don't understand it. I also cannot reproduce it in the U value calculator.

The losses in general are of course clear to me. It is also clear to me that with a higher temperature gradient the losses are higher as well.

A floor heating system running 24/7 (as it should be) will have a constant temperature anyway (under the same conditions). The only thing with a thicker screed is the time it takes for the screed to heat up.
And I could not reproduce the then higher surface temperature either.
The setup was from bottom to top

Ground 0 degrees
Concrete 2% 20cm
Bitumen sheet
10cm XPS
Foil
Screed as heating layer
Water temperature 35 degrees 10cm pipe spacing

When I changed the screed from 6cm and 4cm coverage to 60cm and 54cm coverage, nothing changed in the calculator regarding the surface temperature. Why should it, the heating continuously supplies energy and the losses of the component are the same since the other parameters remain the same.
Probably the screed is too good a heat conductor to change anything there.

Admittedly, if instead of screed a 1cm XPS board were placed ON TOP or a 20cm board, it would of course make a very big difference. With screed probably not, or the calculator really cannot represent it.

A correct thermodynamic calculation would also be anything but trivial. The U-value calculator will, for example, set horizontal losses to zero (I suppose), or not even call a parameter for that. Certainly adequate for household construction purposes.

And the fact that the heat flow downward is constant is true, but the thicker the screed, the more energy flows there (because the initial heating phase is longer until something reaches the floor). You absolutely need more energy.

Would you say, with a thick pan compared to a thin one (that they are the same size), that you can achieve identical frying results in both with the same absolute energy input?

No, of course not. More energy is required to heat up because more mass has to be brought from temperature x to temperature y.

However, this usually only happens once per heating period in a house.

By the way, I do the same with radiators at my place. Since the entire mass in the building is warm, it is much more comfortable. But that is, of course, independent of the actual topic.

Would you say that with a thick pan compared to a thin one (they being the same size), you can achieve identical frying results with the same absolute energy input in both?

The example is unimaginably bad for comparison, but my experience with pans is that once they reach temperature, they all need the same setting (power) to maintain it. When you consider that the thicker pan retains the temperature longer after being turned off, you have a pretty much identical energy consumption in absolute terms. For example, if I cook stuffed peppers in the pan, the heating phase hardly plays a role over 45 minutes.

The relevance of this topic is comparable to the question of whether a red house paint is better for the climate in the garden.

The relevance of this topic is comparable to the question of whether a red house paint is better for the climate in the garden.

Whereas the facade actually does have an influence. For example, my darker brick facade provides pleasant warmth in the evening on sunny days, a white facade does not.