Construction costs are currently skyrocketing

Such ventilation clearly serves comfort.

It serves comfort as long as nothing happens.

Yesterday, I was approached by a colleague who knows that we have a controlled residential ventilation system. He has recently been struggling with mold in the bedroom. Due to our working hours, the bedroom is ventilated incorrectly, and this shows on the exterior walls.

He is now trying to ventilate more carefully and is considering retrofitting a decentralized solution as a way out.

This stress and the health cost something?

That is also again the extreme example. If I air out for 10-15 minutes every morning/evening before leaving the house, I have no mold and have never had problems with air exchange in recent years. A controlled residential ventilation system is a nice gadget, but it is not necessary and certainly not as important as some think. Sure, it provides comfort and is nice, but it is definitely not needed. It works perfectly fine without it. And for those who cannot manage to open all the windows daily, I would recommend not installing any stairs in the house at all, since you have to go up and down them several times a day. That just can't be.

That works wonderfully as well. And whoever doesn't manage to open all the windows daily,

Just walk around and look at houses and apartments. The amount of "decorations" and plants on the windowsills is quite astonishing. Maybe also curtains and drapes. Nobody clears all that stuff away several times a day to properly cross-ventilate. Instead, the windows are left tilted – I've seen some nice mold cultures on window sills that were caused by this. In this respect, a ventilation system is more than just "nice to have," because many people simply do not ventilate properly.

And if you don’t do that, you have to buy it expensively through the [Kontrollierte-Wohnraumlüftung]. I prefer to ventilate and save that money. And I don’t understand those anyway who clutter their floor-to-ceiling windows with everything. Better fewer windows and proper space to place things. But to each their own.

For us, decentralized ventilation was mandatory for the house and cost 6k€. We didn’t upgrade much, but I definitely wanted the central air system for an extra 6k€. Not having to ventilate ever again is a pretty nice thought. When using a fireplace in the living room, is the heat actually transferred to all the other rooms? I only know that through air exchange, pretty much the same climate prevails in all rooms. It would be nice if you could heat the entire house additionally with the fireplace heat.

When using a fireplace in the living room, is the heat actually transferred to all other rooms? I only know that through air exchange, pretty much the same climate prevails in all rooms. It would be nice if you could additionally heat the entire house with the fireplace heat.

No, it is also easily derivable by calculation. (Air 20°, 1.2kg/m3 at 1kj/kg*K)
Rather through heat radiation from walls, e.g. aerated concrete interior walls, provided they were built solidly.