20% humidity in KFW 40+ new construction

I would not turn off a central controlled residential ventilation system.

Turning off the controlled residential ventilation system is certainly not in the inventor's intent...

Huh? The purpose of the controlled residential ventilation system is to provide "good" air. For me, that includes three things: temperature, humidity, odors. If you have no problems with temperature and odors, just turn it off if that solves your problem. If there are no further arguments here why one should not turn it off, I would start with that before wasting energy senselessly. (And also running it unnecessarily without demand is wasteful). Otherwise, please keep all the lights on as well, turning them off is certainly not in the inventor's intent ;)

Quite simply, if air is not regularly circulated (in the channels), condensation will sooner or later form and, together with dust, create a perfect breeding ground for mold. So it’s better to keep it running at a minimum even during vacation.

Häää? The purpose of controlled residential ventilation is to ensure "good" air. For me, that includes three things: temperature, humidity, odors.

CO2 is also an important point. However, you can neither see nor smell it. You can measure it, but that is quite expensive. It is simpler and cheaper to just run the ventilation.

If you have no problems with temperature and odors, just turn it off if that solves your problem. If no further arguments come up here as to why it should not be turned off, I would start with that first before wasting energy senselessly. (And running it unnecessarily without demand is also wasteful).

The most important argument has already been mentioned by Tolentino. If air stands still in the pipes, dust, moisture, and thus mold and other unwanted things can accumulate. This is especially problematic with floor outlets or air ducts exposed to cold. And for CO2, see above.

Otherwise, please keep all lights on as well, turning them off is certainly not the inventor’s intention ;)

A particularly bad comparison. For lighting fixtures, even the possible switching cycles and lifespan are specified. From this, you can see that switching is explicitly intended. Our ventilation system (Recovair) cannot even be turned off. You have to unplug it to change the filter.

Since I do not have an extractor hood yet and we enjoy very fresh air, turning off the [Kontrollierte-Wohnraumlüftung] is definitely not an option. We will probably just call Zehnder next week and ask how much an enthalpy exchanger can contribute to increasing the humidity.

I think you could recover a maximum of 70% of the moisture escaping with the exhaust air using the enthalpy exchanger.

Quite simple, if there isn’t regularly air movement (in the ducts), condensate will sooner or later form and create a perfect breeding ground for mold together with dust.
So even during vacation, it’s better to keep it running at a minimum.

The most important argument has already been mentioned by Tolentino.
If air stands still in the pipes, dust, moisture, and thus mold and other unwanted things can accumulate.
This is especially problematic with floor outlets or air ducts exposed to cold.

Our (Recovair) can’t even be turned off.
You have to unplug it to change the filter.

Well, I’m not arguing for keeping it off all the time, but for reducing it. For example, you could run it for 2 hours and then turn it off for 2 hours. Or, since cooking was mentioned here as an argument: turn it off for 4-5 hours at night and run it in the afternoon... whatever fits the habits best.

At the same time, I believe the argument about mold is incorrect: We are actually talking about the case where the humidity is too low. So what moisture would condense here? Especially since the ventilation shouldn’t be touching cold pipes. In a single-family house, it will certainly be installed about 95% in the ceiling. Probably even right next to the underfloor heating ;)

Dust deposits: With floor outlets, yes, okay, dust can fall onto the outlet. And then? It certainly wouldn’t be sucked back in through the outlet.
With inlets, you only get dust ingress when the ventilation is running (I don’t know of any floor inlets).

My ventilation explicitly has control programs to manage this. Even better, the humidity of the outgoing air is monitored! If it’s too low, I can automatically turn off the ventilation. That is even explicitly intended by the inventor.