Controlled residential ventilation and still open windows at night

We also appreciate the controlled residential ventilation in summer... simply because it smells nice and fresh in the morning - even without airing out and it still does when you come back from work. The effect on humidity is of course partly even rather negative (depending on the weather).

What exactly becomes dangerous there?

That's why you can also use one with heat recovery it then doesn't blow cold air in and also transfers part of the warm exhaust air to the fresh air.

A ventilation system is purely a comfort feature anyway. You avoid manual ventilation and prevent mold formation in the living area from the outset. In winter, you also save some heating costs and still always have fresh air. When 3-4 people take a hot shower in the morning, the bathroom gets quite damp. There's no way that 10 minutes of shock ventilation will fix everything immediately. The ventilation system simply ventilates over the hours even when no one is there to remove the moisture. What's really important is that there are no thermal bridges in the exhaust ducts. Otherwise, condensation would form and it becomes dangerous.

A ventilation system is neither a heating system nor an air conditioning system.
It is neither intended nor capable of heating or cooling.
A ventilation system is designed to bring fresh, unspent, oxygen-rich air into the house.

Since the fresh air outside very rarely has the same temperature as inside, manufacturers install a heat exchanger in order not to overheat or overcool the indoor air too much.

In recent years, the ventilation system has become "fashionable" in single-family homes, with the argument "The house is so tight, no air can get in or out otherwise."

The disadvantage of a ventilation system is that even with the best heat recovery rate of 95%, it works against the heating in winter by bringing cooler air into the house and in summer it further heats up the air inside.

Not every builder is aware of this, and salespeople do not advertise it heavily.

Two extremes as examples:
Heat recovery rate of 90% (which is closer to reality)
Summer:
Outside air 31°C
Supply air 25°C
Exhaust air 24°C
Discharge air 30°C
In this case, the fresh supply air is 1°C warmer than the used exhaust air and at least continuously heats the indoor air during the day.

Winter:
Outside air -20°C
Supply air 18°C
Exhaust air 22°C
Discharge air -16°C
In this case, the heating system not only has to compensate for the heat losses of the house but also for the ventilation system.
Another disadvantage is that the system must be heated with the electric pre-heating element (usually 1000 watts) at lower subzero temperatures to prevent the heat exchanger from freezing.

To prevent the system from cooling the house too much in winter months, manufacturers offer electric or hydraulic post-heating elements as options; the electric one consumes even more electricity, the hydraulic one can be connected to the heating system.
In all cases, energy that you tried to save through insulation is wasted again.

Anyone who wants to have a ventilation system should try to supplement it with a ground heat exchanger. Because this reduces the extreme fluctuations of outside air temperature. This means the heat exchanger has to compensate for smaller temperature differences, thus also reducing the temperature difference between supply air and exhaust air.

In our latitudes, the soil temperature at 1 to 2 meters depth is about 2°C in March and about 15°C in September.
In positive ranges, the use of a pre-heating element is unnecessary.

The difference in outside air temperature without a ground heat exchanger was 51 Kelvin
With a ground heat exchanger it was 13 Kelvin
Even if the air in the ground heat exchanger does not take on exactly the same temperature as the soil, this should serve to illustrate and show the sense and nonsense of a ventilation system.

Two extremes as an example:
Heat recovery rate of 90% (rather corresponds to reality)
Summer:
Outside air 31°C
Supply air 25°C
Exhaust air 24°C
Discharge air 30°C
In this case, the fresh supply air is 1°C warmer than the used exhaust air

This is not an extreme, but practically a zero-sum game. The answer here is not 42 but "the waste heat of 3 LEDs." I calculated that above. Air is not a good heat carrier medium, so it doesn’t matter. That’s why ventilating 5 times a day for 10 minutes each is not bad. Nothing cools down because air is not a good heat carrier medium. It’s just... unpleasant because then you have 50 minutes less time per day.

Winter:
Outside air -20°C
Supply air 18°C
Exhaust air 22°C
Discharge air -16°C
In this case, the heating system not only has to compensate for the heat losses of the house but also for the ventilation system.
Another disadvantage is that the system has to be heated with the electric preheater (usually 1000 watts) at lower subzero temperatures so that the heat exchanger does not freeze.

You have to ventilate anyway. The difference is that the temperature comes in at 18 degrees instead of MINUS 22 degrees.

Anyone who wants to have a ventilation system should try to complement it with a ground heat exchanger.

Nope.

Apart from that, we have long, clear subzero temperatures here only in a few regions, where further measures can be considered.

Thanks to the enthalpy exchanger, I can draw in air down to -10 degrees, then the device shuts off due to the risk of freezing.

And then there is this thing - I will hardly suffocate. And a geothermal heat exchanger is a nice thing, but depending on the design, it can also be problematic in terms of hygiene and is not always possible. I have controlled residential ventilation in the attic - drawing air in sheltered from the wind in the north.

Now I have a somewhat different question.
The scenario mainly relates to the winter season.

If I understand correctly, a controlled residential ventilation system is set so that it exchanges the entire room air about twice within 24 hours (at least in theory/calculation).

If I were to ventilate by fully airing out in the morning and evening everywhere, would I then, in theory, also have exchanged the entire room air twice within 24 hours?
If yes: why do many then say that exchanging air twice a day is not enough?
And: why does controlled residential ventilation result in such dry air compared to window ventilation?

In both cases, in my opinion, moist air is blown out and replaced by dry outside air.

I am asking this question mainly against the background that we consider the resulting dry air a very big disadvantage.
Once because we already have major problems with our eyes in the office with air conditioning, and then also based on my brother-in-law's reports about his controlled residential ventilation (it is hard to recover healthily at night when sick because mucous membranes dry out completely; when you run your hand over the veneers of the door there, you can feel warping due to the dryness in the house; the building there dates from 2007; in winter they often have a humidity of 30%).
Yes, there is moisture recovery, but I do not believe it achieves another 30%.
I also do not want to create a tropical forest by plants as a compensation.

My thoughts are therefore:
- From the perspective of dryness, a controlled residential ventilation system seems mercilessly oversized to me.
- Would lowering the setting not bring improvement?
- But exchanging air twice a day is not so much. Twice window ventilation is also twice air exchange but holds more moisture?!
- I am afraid manual ventilation could become too little and might be skipped due to time effort, but controlled residential ventilation seems to go too far in the opposite direction; there should be something reasonable in between (for KfW5).