Ventilation system in summer?

The ventilation system exchanges relatively little air compared to open windows. However, in winter/during the cold season, the constant exchange prevents excessive moisture and mold. In summer, though, it does not work to carry the heat out of the house. The air can absorb too little heat (energy) to cool a building. What is an advantage in winter (little heat is lost) is a disadvantage in summer (little heat is "lost"). Rather, one should ensure that the house does not even become warm inside. With modern insulation, this is not a big problem. In new buildings, a "sun protection" system is even mandatory. This is mostly implemented with roller shutters or pleated blinds or similar. Roller shutters (and similar systems like venetian blinds) are naturally the best, as they block the heat rays outside and reflect a lot. All protective measures inside the house (e.g., pleated blinds) are already significantly worse since the heat is already inside. Some of it is naturally reflected back. Those who want to do it properly install ducts for an air conditioning system right away. In our regions, this is rather the exception; in other countries like the USA, an air conditioning system is mandatory in some states, just like heating is here. An air conditioning system is not that expensive either. However, one should not forget that an AC uses a lot of electricity. Here, an exception would be a photovoltaic system— the more sun, the more electricity, and the more sun, the more heat in the house ^^ (unfortunately, photovoltaic systems do not like heat...). What unfortunately does not work: connecting the air conditioning to the ventilation system's ducts.

We deliberately decided against a controlled residential ventilation system. On the few hot days we’ve had so far, the exterior shutters were lowered and the windows kept closed. Compared to our old house (timber frame), the new one hardly heats up at all. I’m especially surprised in the attic. Late in the evening and early in the morning, we ventilate once, and that’s it. If it’s significantly warmer outside than inside, you might not only get higher temperatures but also moisture into the house. Why the air inside should noticeably worsen if it’s not ventilated for 12-14 hours, I can’t really understand. Are your houses so small, is something off-gassing, do you cook all day, or are you sitting inside with a soccer team?

Not every system has a bypass. For example, the LWZ 504 from Stiebel does not. Instead, there are Styrofoam blocks used as "pseudo heat exchangers". That would practically be a bypass and replacing them is relatively easy.

Turning off the system worsens the air quality, as others have already written. On the other hand, this only happens if you are inside and the air is being used up. If no one is there, no oxygen is lost either. Most of the heat might also be coming in through the windows?

At night, when it is cooler outside, the system of course makes sense again. In this case, Stiebel has at least an exhaust function with the 504. Only air is pushed out and none is drawn in through the duct. However, you have to open windows accordingly for that.

I am not sure whether the systems might also recognize if it is too warm and then regulate themselves down so as not to heat up the interior of the house too much. Here, a specialist would be the contact person.

The controlled residential ventilation with heat recovery works exactly the opposite. The e.g. 30 degrees warm outside air is first cooled down to 23 degrees. If the remaining difference is 1 Kelvin and 225 m3 are exchanged per hour, then the heat input corresponds to 0.08 kWh per hour. To "cool down" against that, an air conditioner with 0.08 kW power would be needed.

Just to illustrate the dimensions: There are mobile air conditioners on Amazon for 280 EUR with 2.6 kW cooling capacity.

Conclusion: The heat input through controlled residential ventilation is absolutely negligible. It could probably not even be measured in an experiment and would fall under measurement error.

Let it run! Normally during the day, at night with bypass. Some devices do this automatically in my opinion.

Example then: If the house should have heated up to 25 degrees and the bypass pushes cool night air with 15 degrees inside, then you have at least 0.8 kW cooling capacity (difference here 10 Kelvin).

If the residual difference is 1 Kelvin

Can you somehow prove that? I’ll take the answer ahead: certainly not

At 90% efficiency (common values are between 85% and 95%; below 80% the systems would not meet the funding conditions of the KfW, and surely one would have read about that) and a temperature difference between inside and outside of 10 Kelvin, exactly 1 Kelvin remains.

Outside 34 degrees, inside 24 degrees, then with 90% heat recovery the supply air comes into the room at 25 degrees. Or in other words, exchanging the air once through the window brings in as much heat as exchanging the air ten times with the [Kontrollierte-Wohnraumlüftung].

In the construction blog of the Christian family there is an entry, just search for [Kontrollierte-Wohnraumlüftung kälterückgewinnung] and this construction blog should be the first entry.

There, it is "only" 30 degrees outside and 1 Kelvin residual difference, but such values are also rounded.

In general, this blog post also quite well describes the concept and how it works.