Underfloor heating heating demand with at least 60 mm screed

Less thick screed also means less storage area... ergo higher heating times here as well and higher energy demand to maintain temperatures.

No. The opposite is true. The heating times are shorter the less mass stores heat from the heating coils. Of course, it also cools down faster when the heating is turned off. The energy required to maintain the temperature of the surface is definitely not higher. Theoretically, you could turn off the heating earlier, since the screed supplies heat for a correspondingly long time at the right thickness. But you also have to start earlier to achieve identical conditions. So net gain is zero.

At least, because the insulation downward is good but not perfect.

Since I do not know the values in °C, it is of course possible that we are discussing something insignificant, as it ultimately concerns tenths of a degree. But the physical principle stands.

The most efficient underfloor heating has as good insulation as possible downwards and as fast heat transfer upward as possible (not to be confused with thermal conductivity). And that is even better with thin material, which is also a good heat conductor, than with thicker identical material. The more you go in this direction, the lower supply temperatures are sufficient.

No. The opposite is true. The heating times are shorter the less mass the heating pipes store heat in. Of course, it also cools down faster when the heating is turned off. The energy required to maintain the surface temperature is definitely not higher. Theoretically, you could stop heating earlier because the screed delivers heat for a corresponding time depending on its thickness. However, you have to start at least the same amount of time earlier for identical conditions. So, net gain is zero.

At least because the insulation downwards is good, but not perfect.

Since I don’t know the values in °C, it could of course be that we are arguing over nothing because, in the end, it’s about tenths of a degree. But the physical principle remains.

The most efficient underfloor heating has the best possible insulation downwards and the fastest possible heat conduction upwards (not to be confused with thermal conductivity). And this is even better with thin material that is also a good heat conductor than with thicker identical material. The more you go in this direction, the lower supply temperatures suffice.

I would claim you are thinking too much about temperature and too little about energy. For the energy supplied to the house, the thickness of the screed doesn’t matter; only that a thick screed is slower to respond. The supply temperature and efficiency do not depend on the thickness. KFW 55 is insulated so well that the heating system can afford to be slow. The only difference should be how much the heat generator cycles. More with thin screed, less with thick. Which is better now depends on the device. The difference between 1-2 cm more or less should be relatively irrelevant for heat pumps, I think.

In theory, one could turn off the heating earlier, since the screed continues to release heat accordingly long with the appropriate thickness.

Yes, I'm not just talking theoretically but practically... as soon as the system has reached equilibrium, the heating turns off and with thicker screed it also stays off longer than with thinner screed, since 5 tons of concrete can store more heat and release it longer than just 3 tons.

Since I don't know the values in °C, it might of course be that we are discussing something insignificant, because in the end it's about tenths of a degree.

That is probably the case.

Isn't there some standard that requires a minimum coverage of 40mm for the underfloor heating? Added to 20mm underfloor heating, this results in a typical screed thickness of 60mm.

The flow temperature and efficiency do not depend on the thickness.

And that is - in principle - not correct. That it is probably not significant in practice with the thickness differences discussed here is probably true.

You can see this better if you don’t distinguish between a few mm, but simply consider 1m instead of 60mm screed as a thought experiment. As long as the heat conduction towards the ground is not zero, the additional thickness will bring storage capacity and at the same time cost efficiency.

I would claim you think too much about temperature and too little about energy. For the energy supplied to the house, the thickness of the screed does not matter,

You need more energy to bring more screed to the same temperature than less screed. With identical material, mind you. But I think that’s what you meant differently?

The difference between 1-2 cm more or less should probably be relatively irrelevant for heat pumps.

That’s probably true.

once the system has reached equilibrium, the heating turns off and with thicker screed it also stays off longer.

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

The system never reaches a thermodynamic equilibrium

Purely physically and also mathematically no, but from a plant engineering perspective yes. Thus, the operating times of the heat generator are only higher during the first heating phase in autumn with a thicker screed. In the following winter, the system then only needs to reheat occasionally since the storage mass is higher.

With a thinner screed, the heat transfer is indeed more direct and faster, but the heat generator then has to run more frequently, which in turn leads to higher own losses and consequently increases maintenance effort.

Therefore, I stick to the opinion that a thicker screed is the better choice in a modern energy-efficient building.

But of course, we are all right here; it only depends on the perspective.