Controlled residential ventilation and still open windows at night

Without considering the inventory, in a 150m² house with a ceiling height of 2.70m, you have about 400m³ of air. With 225m³ circulation and taking the inventory into account, you exchange the air roughly every 1.5 hours and increase the temperature each time by 0.5-1°

Sorry, but that is completely wrong. That’s why the calculation.

Heat or cold is stored in the building components and not in the air. You have to calculate energy input. In such great houses, which are heated with air-to-air heat pumps, the air is partly blown in at 50 degrees and more, and at significantly higher volumes per hour. Nevertheless, people have problems getting the place warm. Even KfW40 or KfW55 houses. It is not like the entire air is exchanged after half an hour and suddenly it’s 50 degrees in the room.

For comparison, the energy input of a south-facing window irradiated by the sun, direct and diffuse radiation, about 1kW/m² (of which 650-700 watts direct and the rest diffuse radiation). 2.00 meters by 2.30 meters = 4.6m² = 4.6kW. Of this, triple glazing lets through 55 percent, so 2.53kW.

Controlled ventilation living room: 0.0136 kW
South-facing window living room: 2.53 kW

Running controlled ventilation for one hour: 0.0136 kWh energy input
An unshaded single south-facing window for 19.35 seconds: 0.0136 kWh energy input

Conclusion: The controlled ventilation with heat recovery has no noticeable heat input. The waste heat from 3 LEDs with 5.5 watts each is higher than the energy input of the controlled ventilation.

Are you now trying to prove/derive that a house only heats up slightly more due to controlled residential ventilation and that turning off the system would therefore only have minimal effect?

I still think that those who don’t have an extra air conditioner or cooling can still turn off the system. How much warmer it would get is debatable. But the fact that it would be at least marginally warmer and that you can notice slight temperature differences shows that you could still turn off the systems to create "cooling." Even if it’s just 1-2°. With nighttime bypass, you can double the supply air temperature and that already feels like a lot from a bodily perception.

I did some googling on the specific heat capacity of solid houses and found values between 30 and 70 kWh/K. Let’s calculate with 30 kWh/K, then you need to supply 30,000 Wh to warm the house by 1 Kelvin. If we take the 61.2 watts for the whole building from above and assume that it’s that warm from 12 pm to 8 pm on average, that is 489.6 Wh, or just under 0.5 kWh. The house would warm up by less than 0.02K due to the ventilation. That’s really peanuts.

In comparison, an unshaded south-facing window as above, also exposed to sunlight for 8 hours and unshaded, results in an energy input of 20.24 kWh. The temperature through a single unshaded window would rise by 0.67K. Already a bit more than less than 0.02K, and you only have controlled residential ventilation, but usually many windows.

The only disadvantage is that no air exchange takes place when turned off. And opening windows probably brings more warm air into the house than controlled residential ventilation.

If by opening windows you get a temperature difference of 8K instead of 1K, then you have eight times the energy input. So you would only have to ventilate an eighth as much and thereby fall below the minimum ventilation required for moisture protection.

On the other hand, we currently live in an attic apartment. In summer with lots of sun, we open everything for cross-ventilation and hope for a breeze. You wouldn’t have that with controlled residential ventilation.

Hope. I see...

Personally, the calculation doesn’t help me; in practice it’s probably different anyway because of other circumstances and because it’s not a lab experiment.

In practice, you won’t notice a temperature difference between the exhausted and blown-in air with controlled residential ventilation in summer. It will feel like fresh air is exchanging the used air at exactly the same temperature.

All your calculations are based on a 90% efficiency, which is probably just as doctored as VW's emission values.

Oh dear, and if the heat recovery were 85 percent. That doesn't change anything about the dimensions.

Controlled residential ventilation living room: 0.0136 kW
1 south-facing window living room: 2.53 kW

Then let's say 80 percent (minimum heat recovery to obtain KFW status) and let's say there are 3 south-facing windows:

Controlled residential ventilation living room: 0.0272 kW
3 south-facing windows living room: 7.59 kW

One hour of controlled residential ventilation with 80% heat recovery brings as much heat into the room as comes through the windows in 13 seconds.

Don’t cling to any after-after-after decimal places. Air is not a good heat transfer medium and heat recovery with 80% or 90% "destroys" most of the temperature difference again. Practically, controlled residential ventilation contributes nothing to warming.

On the other hand, you can connect a brine exchanger to the controlled residential ventilation and at best blow 5-7 degrees cold air into the room. It’s not about 1-2K higher temperature but about a 17K difference downward. But even there many say it hardly does anything. With ten times the temperature difference!

And if you see it that critically, then reduce the controlled residential ventilation to 100 m3. Then you have a heat input in the worst case of just under 6 seconds of unshaded windows. If the heat recovery just corresponds to the minimum standard KFW55 and not the over 90 percent as often advertised.

I also do not think that controlled residential ventilation manufacturers have developed software that finds out when the system is being tested and then provides special 90 percent heat recovery there and in normal operation only 80 percent heat recovery or so. If the 90 percent is correct and you reduce the system to 100 m3, then the heat input from one hour of ventilation corresponds to less than 3 seconds of unshaded south-facing windows...

Oh Grym or should I say BauGrym

How often have you changed your opinion over the years since you started dealing with house construction?

Quotes from July 2015

For a long time I was very much in favor of Controlled Residential Ventilation, but I have now revised my opinion.

Since we currently live in an old building and our ventilation system is a permanently tilted window summer and winter: There is no icy cold air. It mixes so quickly with the rest of the air, we don’t notice anything. That is theoretical nonsense from the Controlled Residential Ventilation sellers

So... spring, summer, and autumn the ventilation is unnecessary, since usually several windows in the house / apartment are permanently open anyway (if it is 5-10 degrees outside and a window is tilted, the ventilation losses are already minimal compared to the transmission losses) and in winter there is too dry air. Great thing.

This is not an opinion, but a calculation. The controlled residential ventilation system practically does not contribute to warming.

And my realization back then that there is no ice-cold air when a tilt window is open is also correct. Why? Because air is not a good heat transfer medium...

But I don’t really know what the three quotes now have to do specifically with the fact that a controlled residential ventilation system does not contribute to warming in the summer? The three quotes contribute as much to the topic as the controlled residential ventilation system does to warming in the summer.

So, whether you run the system continuously or not is irrelevant from a heating perspective. However, moisture protection is ensured, fresh air as well, and some see turning it off critically. The constant, slight flow in the supply ducts ensures that no dirt can get in there. By turning it off completely, you forgo this protection.

I would always run the system, summer and winter, and open windows as desired (so generally allow it, because the air is fresh and clean anyway). Turn on the exhaust hood and at the same time open windows in the usual way. And done...