Soundproof wall against neighbor's heat pump

and are devices with higher power quieter compared to "weaker" devices, since theoretically they don't have to run at full throttle?

That's how theories usually are

I have some theories too: volume doesn't exist "absolutely," but rather "perceived" - hence such specifications as "db (A)" - which vary according to frequencies.

Theoretically, the frequency of airflow from rotating parts (like fans) is related to their diameter, and their noise level more to the speed.

Practically, what usually matters more is how "unevenly" such a thing runs. Uneven in two dimensions: first, fluctuating speed (which modulates the pitch); second: uneven in the sense of "elliptical, because of an imprecise circle." That comes from imprecise manufacturing quality and shaft alignment, worn bearings due to wear (or too large clearances), etc.

In other words: cheap crap can be significantly louder and more annoying than a device of the same design even with "more power."

Great theory of the community.

The costs jointly.... And who pays if one of them cannot (or does not want to) pay? The other one jointly and severally all? Great idea. Not at all.

Great theory of the community.

No theory. Next year we celebrate the 200th birthday of the community practitioner Friedrich Wilhelm Raiffeisen.

Yes exactly, and the founding costs of a cooperative are around 1500 euros and the ongoing fees of 200 euros per year definitely pay off, just like the audit fees of 800 euros every two years.

With that, the heating is a bargain at 600 euros fixed costs per semi-detached house.

Oh no... that doesn't actually work like that. Because you need at least 7 members for the founding.

And once again the question: liability? Well... limited to the contribution... and the value of the heating system. Certainly great if it gets seized.

And yes, I know there are such concepts with local heating or combined heat and power plants, but that is not a solution for a single semi-detached house.

Yes exactly, and the founding costs of a cooperative society (eG) starting from about 1500 euros and the ongoing contributions of 200 euros per year certainly pay off, just like the audit fees of 800 euros every two years.

I was neither talking about a cooperative society (eG), nor about applying this model to a duplex - you are mixing several things.

My suggestions were: to team up with about half a dozen neighbors, to plan and use your energy stuff together; to start with a duplex by not everyone baking their own little bread; and for example to take an unregistered association. I had explicitly mentioned Raiffeisen as a practitioner - not recommended the eG as a legal form.

also chose gas; because for him it is the cheapest way to keep the house warm.

Good for him; but from a socio-political perspective not really the right path for the successful implementation of the energy transition.

Unfortunately, you are not properly informed. The storage problem is still one of the unsolved puzzles of the energy transition, and the currently assumed solution is still most likely that the gas network will be the winter energy storage.

Gas is not the same as natural gas. Gas can be produced artificially or biologically.

A gas heating system today can still be there in 50 years (well, renewed 1-2 times) and burn biogas, so-called wind gas, or other gases. It then has nothing to do with fossil fuels.

There are already 100%-biogas tariffs for just under 10 cents. That is the current state of the art and this price will decrease in 10, 20 years once this becomes interesting. But even at 10 cents per kWh, one could get along 100% without fossil fuels and continue to operate the gas heating system.

In the future, of course, biogas, wind gas, etc. must become simply cheaper than natural gas and then automatically replace natural gas. Without a system change. Then simply a different gas will be pumped into the virtually infinitely large gas/gas reserve network of Germany. This gas network and the reserves are probably by far the largest energy storage systems worldwide. And can be converted easily and gradually from fossil to completely non-fossil.