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

Mycraft schrieb:
Less thick screed also means less heat storage capacity... therefore longer heating times and higher energy demand to maintain temperatures.

No. The opposite is true. Heating times are shorter the less mass there is to store heat from the heating pipes. 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. In theory, you could stop heating sooner because the screed will continue to radiate heat for a while if it is thick enough. However, to achieve the same conditions, you also have to start heating earlier by at least the same amount of time. So, there is no net gain.

At least this is because the insulation below is good but not perfect.

Since I don’t know the values in °C, it could be that we are arguing over very small differences, as it may ultimately be a matter of tenths of a degree. But the physical principle stands.

The most efficient underfloor heating system has as much insulation as possible below and as fast heat transfer as possible above (not to be confused with thermal conductivity). This is even better with thin material that is also a good heat conductor, compared to thicker identical material. The more you optimize in this direction, the lower the required supply temperatures.

chand1986 schrieb:
No. The opposite is true. The heating times are shorter the less mass the heating coils have to warm up. 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 correspondingly long time if it has the appropriate thickness. However, for the same conditions, you also have to start heating earlier. So, in net terms, nothing is gained.

At least this is because the insulation below is good but not perfect.

Since I don’t know the values in °C, it’s possible we’re arguing over trivial differences because in the end it comes down to tenths of a degree. But the physical principle remains.

The most efficient underfloor heating has as much insulation as possible on the underside and as fast heat conduction as possible upwards (not to be confused with thermal conductivity). And with thin material that is also a good conductor of heat, this is even better than with thicker identical material. The more you move in this direction, the lower the supply temperatures can be.

I would say you are thinking too much about temperature and too little about energy. For the energy supplied to the house, the thickness of the screed does not matter; it’s just that a thick screed is slower to respond. The supply temperature and efficiency do not depend on the thickness. A KFW 55-standard house is so well insulated that the heating system can afford to be slow. The only difference should be how often the heat generator cycles. With thin screed, it cycles more, with thick, less. Which is better depends on the unit. But I think a difference of 1-2 cm (0.4-0.8 inches) more or less is relatively irrelevant, especially for heat pumps.

chand1986 schrieb:
In theory, you could turn off the heating earlier because the screed, if thick enough, continues to release heat for a long time afterward.

Yes, I’m not just speaking theoretically but practically... once the system reaches equilibrium, the heating turns off, and with thicker screed it stays off longer than with thinner screed, since 5 tons of concrete can store and release more heat over time than just 3 tons.

chand1986 schrieb:
Since I don’t know the values in °C, it’s possible we are discussing something trivial, as it might just be a matter of tenths of a degree.

That is probably the case.

Isn't there a standard that requires a minimum coverage of 40mm (1.6 inches) for underfloor heating? Adding the 20mm (0.8 inches) thickness of the heating system results in a typical screed thickness of 60mm (2.4 inches).

Saruss schrieb:
The supply temperature and efficiency do not depend on the thickness.

And this is – in principle – not correct. It is probably true that, in practice, the differences in thickness discussed here are unlikely to have a significant effect.

This becomes clearer when you don’t compare just a few millimeters (mm), but instead imagine, as a thought experiment, 1 meter (3.3 feet) versus 60 millimeters (2.4 inches) of screed.

As long as thermal conductivity towards the ground is not zero, the extra thickness will increase heat storage capacity but will also reduce efficiency.

Saruss schrieb:
I would argue that you think too much about temperature and too little about energy. For the energy supplied to the house, the thickness of the screed doesn’t matter,

You need more energy to raise a greater volume of screed to the same temperature than a smaller volume, assuming the material is the same. But I guess you meant something else with that?

Saruss schrieb:
The difference of 1–2 centimeters (0.4–0.8 inches) more or less probably doesn't matter much for heat pumps.

That is probably correct.

Mycraft schrieb:
Once the system reaches equilibrium, the heating turns off and with thicker screed it stays off longer.

However, the system never reaches a thermodynamic equilibrium, but rather a steady state. A small amount of stored heat still flows downward. For the whole system at constant temperature, INPUT equals OUTPUT.

chand1986 schrieb:
The system never reaches a thermodynamic equilibrium

From a purely physical and mathematical perspective, no, but from a technical system standpoint, yes. Therefore, the operating times of the heat generator are only longer during the initial heating phase in autumn when using a thicker screed. In the following winter, the system only needs occasional minor reheating since the thermal mass is greater.

With a thinner screed, heat release is more direct and faster, but the heat generator has to run more frequently, which leads to higher standby losses and accordingly increases maintenance effort.

In conclusion, I maintain the opinion that a thicker screed is the better choice in a modern energy-efficient building.

Of course, we are all right here—it just depends on the perspective.