ᐅ Ventilation system in summer?

The ventilation system exchanges relatively little air compared to open windows. However, in winter or cold periods, the continuous air exchange prevents excessive moisture and mold growth. In summer, though, it does not help to remove heat from the house. The air can absorb far too little heat (energy) to effectively cool a building. What is an advantage in winter (minimal heat loss) becomes a disadvantage in summer (little heat is "lost").

Instead, the focus should be on preventing the house from heating up in the first place. With modern insulation, this is not a major problem. For new buildings, a sun protection system is even required. This is usually achieved through roller shutters, blinds such as pleated blinds, or similar solutions. Roller shutters (and similar systems like venetian blinds) are the most effective since they block and reflect solar radiation outside before it enters. All protective measures inside the house (e.g., pleated blinds) are considerably less effective because the heat has already entered. Some of it will, of course, be reflected back.

For those who want to do it properly, it’s best to install wiring for an air conditioning system from the start. In our areas, this is rather unusual, but in other countries, such as the USA, air conditioning is as common as heating is here. Air conditioning also isn’t necessarily expensive. However, it should not be forgotten that a powerful air conditioner consumes a lot of electricity.

In this context, a photovoltaic system would be useful—more sun means more electricity, but also more heat inside the house (unfortunately, photovoltaic systems do not like heat...).

What is not possible, unfortunately, is to connect the air conditioner to the ventilation system’s ductwork.

We deliberately decided against a mechanical ventilation system. During the few hot days we've had recently, we lowered the exterior shutters and kept the windows closed. Compared to our old house (wood frame), the new one hardly heats up at all. I’m especially surprised by the top floor. We ventilate briefly late in the evening and early in the morning, and that’s enough. If it’s significantly warmer outside than inside, you might bring in not only higher temperatures but also moisture. I don’t really understand why the indoor air quality should noticeably worsen if you don’t ventilate for 12 to 14 hours. Are your houses really that small, is something off-gassing, are you cooking all day, or do you have a whole soccer team inside?

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." This basically acts as a bypass, and replacing them is relatively easy.

Turning off the system, as others have mentioned, worsens the air quality. However, this only applies if you are inside and using up the air. If nobody is there, no oxygen is lost. Most of the heat probably comes through the windows anyway.

At night, when it is cooler outside, the system obviously makes sense again. In this case, Stiebel’s LWZ 504 at least has an exhaust function. It only expels air and does not draw in air through the duct. For this, you need to open the windows accordingly.

I’m not sure if the systems might also detect when it gets too warm and then adjust to avoid overheating the interior. A professional would be the best person to consult on this.

The controlled residential ventilation system with heat recovery works in the opposite way as well. For example, outside air at 30°C (86°F) is initially cooled down to 23°C (73°F). If the remaining temperature difference is 1 kelvin and 225 m³ (approximately 7960 ft³) of air is exchanged per hour, the heat input corresponds to 0.08 kWh per hour. To actively cool that amount of heat, an air conditioning unit with a power of 0.08 kW would be required.

Just to illustrate the scale: there are portable air conditioners available on Amazon for 280 EUR with a cooling capacity of 2.6 kW.

Conclusion: The heat input from a controlled residential ventilation system is absolutely negligible. It probably could not even be measured in an experiment and would fall within the margin of measurement error.

Let it run! Operate normally during the day, and use the bypass at night. Some units, in my opinion, do this automatically.

For example: If the house heats up to 25°C (77°F) and the bypass brings in cool night air at 15°C (59°F), then you effectively have 0.8 kW of cooling power (temperature difference here is 10 kelvin).

Grym schrieb:
If the remaining difference is 1 Kelvin

Can you provide any evidence for that? My answer in advance: definitely not

With 90% efficiency (typical values range from 85% to 95%; below 80% the systems would not meet the funding requirements of KfW, which you have probably read about), and a temperature difference between inside and outside of 10 Kelvin, exactly 1 Kelvin remains.

Outside 34 degrees, inside 24 degrees, with 90% heat recovery the supply air enters the room at 25 degrees. In other words, exchanging the air once through an open window brings in as much heat as exchanging it ten times with controlled ventilation.

In the building blog of family Christian, there is a post—just search for
Controlled residential ventilation heat recovery
and this blog should be the first result.

There, the outside temperature is “only” 30 degrees and the residual temperature difference is 1 Kelvin, but those values are rounded.

Overall, this blog post also explains the concept and how it works quite well.