Underfloor heating heating demand with at least 60 mm screed

And that is – in principle – not correct. That it probably isn’t significant in practice for the thickness differences discussed here is probably true.

You can see this better if you don’t distinguish between a few millimeters but simply take 1m instead of 60mm screed as a thought experiment.

But this argument absurdly derails the conversation – it’s like arguing that you can recognize the inefficiency of thin screed by how bad 1 or 2 mm of screed is. We should not cling to nonsensical theoretical (and practically nonsensical) thought experiments, but stay with the values relevant to the question.

As long as heat conduction towards the ground is not zero, the additional thickness will bring storage capacity and at the same time cost efficiency.
You need more energy to bring more screed to the same temperature than less screed. For identical material, mind you. But I think you meant that somehow differently?

For KFW55, the heating energy demand depends on the outer shell. The heating compensates for the heat energy loss of this shell (minus solar gains, other heat sources, and so on) by transporting heat energy into the house. The thickness of the screed is irrelevant for the required energy in this regard.
The only place where you find a tiny difference is if the insulation of the building from the lowest floor toward the ground (the energy of the other floors going downward is still within the insulated shell) is not so good, and the insulation under the screed is not so good either; then, at higher temperatures, the heat loss downward is higher. But you must not forget that the thicker screed can store the same amount of energy at a lower temperature, and that the more frequent reheating of the thinner screed actually happens RIGHT AT THE BOTTOM of the screed, exactly where most energy can potentially be lost.
But I think the difference for the overall system is practically negligible since there are both advantages and disadvantages. As I have already claimed, the main difference remains the cycling time.
My observation is that many heating systems are more efficient with long cycles than with short ones – at least with my brine pump I found when testing operating parameters that a large hysteresis – resulting in fewer cycles with longer runtime – causes measurably less energy consumption at the same room temperature (thanks to the inertia of the thick screed – the temperature does not fluctuate by 0.1 degrees, I log the temperature of several rooms). Besides, I am sure the compressor will thank me in the long run for having several hours of rest between starts instead of running 1-2 times every hour.
There is certainly some maximum cycle length at which efficiency (heat exchange with the ground, for example) decreases, but I have always been far from that. I had similar experiences in the previous apartment with a gas heating system, which was also more efficient at long cycles.

But the system never reaches a thermodynamic equilibrium, rather a steady state. A little bit of the stored heat also goes downward. For the whole system at constant temperature, IN = OUT.

Equilibrium only exists, of course, if it is exactly room temperature outside, and for that you have to heat for a long time!

but remain with the values relevant to the question.

Which is certainly true in the sense of the OP and therefore to these:

The differences in usual screed thicknesses will not decide anything here. Proper installation and good hydraulic balancing are the be-all and end-all!

We should not cling to nonsensical theoretical (and in practice nonsensical) thought experiments

Mea culpa. I like to reduce technical things to the physical basic principle. And how a steady state energy IN = OUT behaves when you change parameters is, strangely enough, always seemingly a non-trivial discussion – for underfloor heating unnecessary, admittedly.

Equilibrium of course only exists if it’s exactly room temperature outside, for that you also have to heat for a long time!

Exactly my humor :D

Merry Christmas to everyone!

So is a thermal floor slab good?

I also first thought it requires a lot of energy to heat the (thick) concrete slab. But the heating runs 24 hours anyway.

Hello Christian.
The only useful hint so far in your inquiry came from "Alex85"!
Everything else more or less missed the point.
It is correct that a cement-bound wet screed in residential construction must have a minimum thickness of 45mm. At least when the flexural tensile strength is 4N/mm2.
For heated screeds, this is the amount of coverage over the heating elements.
The usual diameter of the heating elements is 12mm or 15mm.
These are added mathematically to the 45mm, so that we easily reach 60mm screed installation thickness. You can see that the 60mm, which caused confusing statements in this round, is a standard value for heated screeds according to the norm (that is DIN 18560 Part 2), nothing special.
The density of the installation of the heating elements on the insulation, the diameter, and the flow temperature of the heating system ensure adequate heat output to the room air.
All other concerns raised in the responses in this context you can safely forget!
Regards: KlaRa

Why more energy input should be required for thicker screed is not clear to me and would need to be explained to me.

You don't really need to, because if the system is well insulated downwards, the energy goes into the screed and is released into the room air... but with thicker screed, the inertia increases, which overall leads to shorter operating times of the heat generator...