Central ventilation system - is moisture recovery necessary?

That’s exactly the point where the argument starts. There are tons of studies with many different results. Most studies, however, conclude that well-being and concentration ability decrease significantly from 1500 (also Frauenhofer, Max-Planck and not just Brigitte). I don’t know where your 25000 come from, at that level one is still lying tiredly in a corner... I find the air already very unpleasant above 1500 ppm, it just quickly becomes stuffy. When the ventilation system in the bedroom is switched off, the concentration quickly shoots over 4000 ppm and then continues to rise happily, you wake up in the morning with a headache.

But also right there a lot of fuss is made about nothing. If the user fails in converting % into ppm and therefore has no frame of reference for the measured CO2 content, all this is pretty useless.

By the way, this is a prime example of populism. It is completely irrelevant that the numbers are small when expressed in percent...

If you turn off the ventilation system in the bedroom, the concentration quickly shoots up to over 4000 ppm and then happily continues to rise, you wake up in the morning with a headache.

What is a usual value in existing buildings without a ventilation system?

Of course, they are less airtight, but still, how did we survive all those years?!

Of course they are less dense, but still, how did we survive all these years?!

I ask myself that too.
Because of high humidity (tight windows, but old building, hence mold risk) we have to properly ventilate 2 to 3 times a day and open all windows wide. When I tell others this, they always look at me sympathetically and say they ventilate once in the morning and that's it... (mind you, without a ventilation system)

Of course they are less airtight, but still, how did we survive all those years?!

That will probably often be the decisive point. We had wooden windows in our apartment that were so drafty that you probably had an air exchange rate of 0.3 even without manual ventilation... of course, I didn't measure it back then.
"How did we survive all those years" is also a phrase I like to hear from the boomer generation in my professional environment... I can't stand it anymore.
"We used to get regularly beaten - it didn't harm us." Oh really? "We used to have to work 95 hours a week - how did we survive that?" Yeah yeah... blah blah blah

it’s also very subjective, my parents-in-law – house from the 2000s – also don’t ventilate all day when we visit because it gets cold and drafty and the kids catch colds then. After 2 hours you can hardly stay seated and it’s not just because of the stimulating conversations.

Between "we've always done it this way" and this (self)optimization craze of today, there is still something else. Limit values in particular are often chosen arbitrarily. One must be allowed to question the practical relevance. I am still interested in how the CO2 level typically looks in an old building.

I just looked around for CO2 meters for private users. What I clicked on in the last few minutes (from 60 to 150) only measures from 0 - 3,000 ppm, so it can't even measure your morning 4,000 ppm