Atypically high offer for ventilation system

Where did I claim that no energy is lost through ventilation? I even tried to estimate it quantitatively for our household in my posts. Your posts, on the other hand, are sinking to bar talk level.
I don’t know what’s so unprofessional about that. The 20k€ have to come from somewhere. Either you have them left over, save them somewhere else, or have to finance them, and if you then compare your repayment plan before and after the financing, you end up at the bottom in the total line exactly at the difference of over 40k. Of course, inflation works in your favor here. For the investment calculation, one would therefore discount the whole thing.

I don't know what's so unprofessional about it. The 20,000€ have to come from somewhere

The money for beer at the pub, the car, the vacation... it also has to come from somewhere. Either you spend it or you invest it. It's exactly the same logic. No one (except finance influencers with clickbait titles like "your coffee costs you 100,000€") would come up with the idea of calculating it that way. Then you'd also have to factor in your own working time, of course with interest and night and holiday pay ;)

It certainly fits well with your argument that with interest, the controlled residential ventilation becomes even more expensive and thus it is easier to argue that it isn't worth it at all. But it distracts from the real problem, namely that in modern, airtight houses the necessary air exchange rates cannot be achieved with window ventilation. This is shown in every single case by looking at the ventilation concept. Especially not with ventilating once and letting the cat in or out three times. The claim that you could achieve sufficient air exchange with that lacks any physical basis

This is a classic cost comparison calculation for evaluating an investment. But if you think you can completely ignore [BWL], then we don't need to continue this discussion. By doing so, you disqualify yourself.

The claim that this could achieve a sufficient air exchange lacks any physical basis

That's exactly right. From my experience, window ventilators only give the illusion of air exchange because you feel a draft. When you actually measure, the air quality is terrible and the humidity remains a disaster away from the front door. That's why the myths about "dark basements" persist. My basement, also thanks to ventilation, has full living space quality.

It's clear that the simplistic calculation "the house is free but the ventilation costs millions" is nonsense. But you have to cling to something to justify it to yourself. Nothing pays off as well as controlled residential ventilation, so it makes sense to save on many things but not on that.

So I am pro [Kontrollierte-Wohnraumlüftung], but I also don't believe that a [Kontrollierte-Wohnraumlüftung] is economically worthwhile in a single-family house that is owner-occupied. But it doesn't have to be. No one tries to justify the extra cost of a lift-and-slide door system with a business case either. Or large windows in general. Smaller windows are much more efficient. Laminate flooring is the ultimate price-performance champion, yet people try to use something else if they can afford it, even if it doesn't save ongoing costs. A single-family house itself is in most cases not a good economic case. The [Kontrollierte-Wohnraumlüftung] doesn't deserve to be justified only as an energy-saving miracle. It's simply more comfortable with [Kontrollierte-Wohnraumlüftung]. Even more so for allergy sufferers. Whether it's worth it for your well-being and comfort is something everyone has to decide for themselves. But one thing I've noticed: There are hardly any reports from opponents who have actually had one. Mostly just experiences from people who wouldn't want to be without one afterwards.

In an owner-occupied single-family house, many things are done differently than in the industrial/office sector – above all, the user usually counteracts all the technology: The heat pump is set to 70° all year round, "so that the hot water is nice and hot and the mixer lever is in the middle at the comfort temperature," and of course the classic "turning the heating to 5 so it gets warm faster," and with the ventilation, the windows are kept open all year round so that the ventilation definitely doesn’t work. But I can’t blame the technology or advise against it just because I myself am unwilling to operate it correctly.

Yes, the controlled residential ventilation pays off differently than a pure investment object like the feed-in photovoltaic system on the roof, which I calculate with Excel, and if the calculation works, I install it on the roof and then forget about it. The controlled residential ventilation naturally pays off rather slowly through heating cost savings, but it does. And it is, unfortunately this word has been horrifically misused over recent months, technology-open: Just as I can heat my underfloor heating with a system that perhaps hasn’t even been invented yet, I can eventually upgrade my ventilation with a better (control) system. And this is not an "eventually maybe" like with e-fuels, but something that is already being installed, for example in schools, where individual rooms are really ventilated as needed. I am slowly considering that for my home as well.

The main aspect, of course, and I already wrote this at the beginning, is the massive increase in quality of life. When I see what some people install in the way of golden faucets but save on infrastructure – well, everyone is the architect of their own fortune. Yes, in my sauna wellness oasis I treated myself to really pleasant tiles because I walk barefoot there a lot, but in the rest of the basement, there are simply simpler tiles which I have come to like surprisingly well and with which I even ended up with a minimal profit overall on the tiles.
Summer, winter, draft freedom, mold prevention, noise, insect, and burglary protection – simply perfect air 24/7. That is why truly without exception all my acquaintances who have a central controlled residential ventilation have clearly said: Never again without it.