ᐅ Underfloor heating heat demand with at least 60 mm of screed

chand1986 schrieb:
And in principle, that is not correct. It is probably true that the practical differences in thickness discussed here are not significant.

You can understand this better if you don’t distinguish between a few millimeters but imagine 1 meter instead of 60mm (2.4 inches) of screed as a thought experiment.

However, this argument leads the discussion into the absurd – it’s like arguing that the inefficiency of thin screed can be seen by how bad 1 or 2mm (0.04 or 0.08 inches) of screed are. We should not hold onto pointless theoretical (and practically irrelevant) thought experiments, but rather focus on values relevant to the actual question.

chand1986 schrieb:
As long as the heat conduction towards the ground is not zero, the additional thickness will provide more heat storage capacity but also reduce efficiency.
You need more energy to bring more screed to the same temperature than less screed, assuming the same material. But I think you meant something different?

For KFW55 (energy standard), the heating energy demand is determined by the building envelope. The heating system compensates for heat losses through this envelope (minus solar gains, other heat sources, and so on) by transferring heat into the house. In this context, the thickness of the screed is irrelevant for the required energy.
The only place where you might find a tiny difference is if the insulation between the building and the ground at the lowest floor is not very good, and the insulation beneath the screed is also insufficient; then with higher temperatures, heat loss downwards will be higher. But you also must consider that thicker screed can store the same amount of energy at a lower temperature and that the more frequent reheating of thinner screed occurs right at the bottom layer, exactly where the most energy can potentially be lost.
However, I think the difference for the overall system is practically negligible because there are advantages and disadvantages on both sides. As I have said before, the main difference remains the cycling time.
My observation is that many heating systems operate more efficiently with longer cycles rather than shorter ones – at least with my geothermal heat pump, testing operating parameters showed that a larger hysteresis (resulting in fewer cycles with longer run times) at the same indoor temperature (thanks to the inertia of the thick screed – temperatures do not fluctuate by 0.1 degrees, I log temperatures in several rooms) clearly reduces energy consumption. Moreover, I am sure the compressor appreciates having several hours of rest between starts instead of running 1-2 times every hour in the long run.
There is certainly a limit to how long cycles can be before efficiency (e.g., heat exchange with the ground) declines, but I was always far from reaching that. I had similar experiences in my previous apartment with a gas heating system, which was also more efficient with longer cycles.

chand1986 schrieb:
But the system never reaches thermodynamic equilibrium, rather a steady state. A small amount of the stored heat also flows downwards. For the entire system at constant temperature, IN = OUT applies.

Equilibrium only exists if the outside temperature is exactly the same as the room temperature, and achieving that requires very long heating periods!

Saruss schrieb:
but only focus on the values relevant to the question.

Which is certainly true in the OP’s sense, and therefore on these points:

The differences in common screed thicknesses won’t make a decisive difference here. Proper installation and a good hydraulic balance are essential!

Saruss schrieb:
We shouldn’t get stuck on pointless theoretical (and practically irrelevant) thought experiments

My fault. I like to break down technical issues to the basic physical principle. And how a steady state of energy IN = OUT behaves when you change parameters is, strangely, always a seemingly non-trivial discussion – admittedly unnecessary for underfloor heating.

Saruss schrieb:
Equilibrium only exists, of course, if the outside temperature exactly matches room temperature; for that, you’d have to heat for a long time!

Just my kind of humor 😀

Merry Christmas to everyone!

So, is a heated ground slab a good option?

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

Hello Christian.
The only useful information on your inquiry so far came from "Alex85"!
Everything else more or less missed the point.
It is correct that a cement-based wet screed in residential construction must have a minimum thickness of 45mm (1.8 inches), at least when the flexural tensile strength is 4 N/mm².
For heated screeds, this thickness refers to the coverage over the heating elements.
The typical diameter of the heating elements is 12mm (0.47 inches) or 15mm (0.59 inches).
These add to the 45mm (1.8 inches), so we end up with an installed screed thickness of about 60mm (2.4 inches). You can see that the 60mm (2.4 inches), which caused some confusion in this discussion, is a standard value for heated screeds according to the relevant standard (DIN 18560 Part 2) and is nothing unusual.
The density of the heating element layout on the insulation, their diameter, and the flow temperature of the heating system ensure sufficient heat output to the room air.
You can safely disregard all other concerns raised in the responses related to this topic!
Regards, KlaRa

I do not understand why a thicker screed should require more energy input and would need an explanation.

That’s not necessary either, because if the system is well insulated downward, the energy goes into the screed and is released into the room air. With a thicker screed, however, thermal inertia increases, which overall leads to shorter operating times for the heat generator.