Underfloor heating beneath floorboards

But then it works, why shouldn't it?

For this simple reason:

For an underfloor heating system to work flawlessly, the installed covering must also conduct heat. The extent to which this occurs is expressed by the thermal resistance (R). For this purpose, the thermal conductivity (λ) is divided by the material thickness (d), and this is done for all layers of the covering. The two values are interdependent because the thicker the covering, the lower the thermal conductivity. For the underfloor heating to operate energy-efficiently, the total value must not exceed 0.15 m²K/W.

I would like to have a floor as warm and natural as possible because it is breathable.

Even at the price of keeping consumption costs higher?

As I said, I didn’t install it myself, but both the wood dealer (selling solid oak planks up to 20mm thick) and the installer (who did our stairs) see no problem there.

They also want to sell material and labor.

Master craftsman painter says clearly, planks and FHZ are energy waste times three. The eco-freak lives on wood and just runs the gas through the burner and the CO2 into the air,

Everything is possible, everything is somehow made doable. But it becomes totally pointless and contradictory – you don’t have to jump on the eco-train here if you want to do it at the expense of consumption and the environment

Except for this last number here:

0.15 m²K/W

I know all that.

And admittedly, I didn’t calculate it for a solid wood plank assumed to be 20mm thick made of oak.

Now, a underfloor heating also has other reasons besides efficiency. Appearance, free furniture placement, etc.

So the question arises for me, how many percent efficiency do you lose and compared to what? If X% compared to glued parquet is worth it to someone, then fine.

So according to what the internet provides, it is generally accepted that most woods up to 22mm thickness comply with 0.15. It is clear that tiles have the better thermal conductivity coefficient. But to say that underfloor heating and solid wood planks do not go together is nonsense; of course, the combination has disadvantages. However, everyone has to decide that for themselves; you just need to choose the type of wood sensibly and also the thickness of the planks.

And admittedly, I did not calculate it for a solid wood plank of assumed 20mm thickness made of oak.

25 mm Douglas fir: 0.025 / 0.13 = approx. 0.19 m² / K/W
27 mm Douglas fir: 0.028 / 0.13 = approx. 0.21 m² / K/W
20 mm oak: 0.020 / 0.13 = approx. 0.14 m² / K/W
27 mm oak: 0.030 / 0.17 = approx. 0.15 m² / K/W

27mm is also possible for oak... Douglas fir should indeed only be 20mm thick.

In order for the underfloor heating to operate energy efficiently, the total value must not exceed 0.15 m²K/W.

Everything is correct, the heat must pass from the pipes into the floor, and the floor must conduct it into the room. The 0.15 is observed, as also wrote, if the floorboards are not too thick (which I already mentioned).

That the heat transfer to the room air is not as good as with tiles is clear, but that this causes heating costs to explode or the fuel demand to increase enormously is nonsense. Where should that come from?

Initially, due to the poorer heat conduction of the wooden floorboards, only the heating output of the surface heating per m² is reduced. This has no influence at all on the amount of heat required to heat the house. So it is correct that with wooden floorboards, the underfloor heating must be designed differently than with tiles. But this also applies to other floor coverings, such as vinyl or parquet.

The only thing that leads to an increase in fuel demand is a possibly necessary increase in the flow temperature to compensate for the poorer heat transfer. With 15 mm parquet, by the way, this is not significantly different from 20 mm floorboards.
For gas heating systems, it does not matter for energy efficiency whether the heating runs at 30/25°C or 40/35°C (radiators sliding up to 55/45°C), this makes maybe about 1% more gas demand at the end of the year.
For heat pumps, of course, this is significantly more, but that has already been mentioned.