Atypically high offer for ventilation system

a controlled residential ventilation system is in my opinion NEVER economical (did someone claim that?

Here at least 387€ per year is mentioned for an investment of 6000€

My energy consultant calculated my energy consumption with and without controlled residential ventilation.
In the end, a controlled residential ventilation system is usually absolutely not economical. For 144 sqm living space in my case, the annual savings are just 430,-€ minus 43,-€ electricity consumption for the controlled residential ventilation. So just about 387,-€ per year.

The energy consultant must have miscalculated. €430 almost covers the heating bill for a new building of this size. But it’s not that complicated to calculate. To heat 1 cubic meter of air by 1 degree requires 0.33Wh. So let’s calculate his house with 144 sqm and assume that the rooms are 2.5m high everywhere. Also, let’s assume it’s 20 degrees inside and 0 degrees outside. That results in about 2.4 kWh heat loss per air exchange. Furthermore, we take ventilation seriously; we only gave up ventilation because of the money, not because we like sitting in stale air. So 5 air exchanges per day equals 12 kWh heat or 3-4 kWh electricity.

Of course, the calculation is not exact. There is furniture, and ceilings can be higher or lower. One ventilation is also not exactly one air exchange, and outside it is sometimes warmer and sometimes colder than 0 degrees, and inside it’s not always exactly 20 degrees. But the order of magnitude is rather about one third of the amount.

This is really going on here.
I keep reading diligently.

I don't have exact info about the BU yet.
But we will take one and leave out the stove and close the chimney for now as it looks.
And yes, 20k is a lot of money, you first have to save that elsewhere and see.
Our interest rate is 3.66%.
I’d say we earn quite well, but the house is NOT a kfw40 but only the building energy act. 36.5 exterior wall and that’s it.

With controlled residential ventilation I feel safer, also because of mold. I was well advised about that here as well.