Construction costs are currently skyrocketing

Thank you for knowing the sleeping habits of all Germans. Respect. It's not about the temperature but about the fresh air. When the window is closed, my nose closes up and I have to breathe through my mouth. I don't know why that is, but it is very unpleasant.

It's not about personal comfort feelings now, but rather a quite radical transformation. And in that context—wanting to heat during the day and send all the warmth out the window at night—seems somewhat anachronistic. Gas simply will not become cheap again quickly, as some here imagine. And even if it did (in 2 years??), we have to give up fossil energy sooner rather than later anyway, so the change is coming regardless.

Cop 4.0 with air-to-air heat pump in an old building? Oh man!
I don’t know such systems, but please clarify, what supply temperature is the air supposed to have then?

LLHPs are certainly not the best in terms of comfort. But honestly, is an HT radiator the best in the world with its hot surfaces and convection?
By the way, it doesn’t at all justify equating an air-to-air heat pump with a fan heater and saying "that can’t work." Yes, it can, it actually does. You can easily get 6kW of heat from a single air-to-air heat pump indoor unit if you want. Of course, you spread that over more units, causing less draft and lower supply temperature.

The discussion here was about replacing a fossil heating system in an energetically unrenovated old building. This is conceptually always disadvantageous per se because you should first reduce energy consumption before worrying about a heating system that should last more than 20 years. But that was not the discussed scenario.

Yes, the COP suffers with rising supply temperature, that is no different than with an air-to-water heat pump as well.
Just take a look at the datasheets of air-to-air heat pumps, you would certainly be surprised. You won’t find fan heaters there.

I liked sleeping with an open window and my sister did too. Our eldest demands it open as well. Whoever wants to sleep in their “stuffy air” should do so.

Nice when your own opinion is always the right one, right? ;) Other families handle it differently. I don’t understand how that is so hard to accept.

Because that is learned behavior. Nothing that should stand in the way of change.

I liked sleeping with the window open and so did my sister. Our eldest also insists on having it open. Whoever wants to sleep in their "stuffy" room should do so.

It’s not about the temperature but about the fresh air. When the window is closed, my nose closes up and I have to breathe through my mouth. I don't know why that is, but it’s very unpleasant.

That’s exactly why you drill a small hole in the wall and install decentralized ventilation. This system certainly has its justification. Reasonable air quality in the bedroom and no energy waste from the open window. Comfortable temperature on average at 18-20 degrees in the bedroom.

That’s exactly why you briefly drill a hole in the wall and install a decentralized ventilation system. This system definitely has its reason.

Reasonable air quality in the bedroom and no energy waste caused by the open window. Comfortable temperature on average at 18-20 degrees in the bedroom.

And what is different about that compared to a slightly open window?

Because it is learned behavior. Nothing that should stand in the way of change.

Or simply feeling comfortable. I do not want to restrict myself like this for life either but instead adjust the heating and room usage to my life, not the other way around.

And what is different about that compared to a slightly opened window?

There is a heat exchanger installed that recovers most of the energy. When ventilating, the heat is simply lost.