Newly built apartment (KfW-55) cannot be cooled, always at least 25 °

And now I am really a bit confused because you are supposed to keep the WRL running even now.....

You don't have to be confused. At night, the controlled residential ventilation does what it is supposed to do (in regeneration mode). So, like in winter, the "cold" air stays outside and the warm air inside. During the day it changes because it is warmer outside than inside. So the function of the controlled residential ventilation does not actually change, but this time it keeps it nice and warm outside and lets the cooler air in the apartment.

You don't have to be confused. At night, the controlled residential ventilation does what it is supposed to do (in regeneration mode). So, like in winter, the "cold" air stays outside and the warm air stays inside. During the day it changes, because it is warmer outside than inside. So the function of the controlled residential ventilation does not change, but this time it keeps it nice and warm outside and lets the cooler air inside the apartment.

Ok, understood. However, my tenant seems to cross-ventilate at night, so the controlled residential ventilation probably no longer has any influence because individual windows are then open. But if the windows remain closed at night, what setting would be best for the controlled residential ventilation? Regeneration mode is not suitable, as then the cold air stays outside. Most likely permanently on supply air, because then cold air is sucked in. Although heat recovery is deactivated in this case. The problem could be that other problems might occur with continuous supply air...

Ventilating at night is probably the best tip one can give and will likely work well in a single-family house. But in a rental apartment?

I do know cases where in winter an apartment needs little heating because the neighbors above and below already heat well and the warmth spreads across apartments. It should actually be the same in summer, so actually all residents in the building would need to cross-ventilate their apartments for it to have a significant effect. Otherwise, the building will not cool down substantially overnight.

Well, I’m building a bunker here with soundproof windows, etc. And then leaving the window open in the evening?! Besides, it’s bright in the room then, so I can’t sleep…. That’s why I moved to the basement… but it’s reassuring to hear that you do it that way and that it’s probably just normal…

Cross-ventilating would be possible with the rooms in the west, but all the windows there are floor-to-ceiling, and my husband is afraid to leave them open because of the risk of burglary. And on the ground floor you can’t leave anything open at night for the same reason, and when I get up at 8 it’s already been too warm again, or sometimes I’ve gotten up at 5 on purpose and ventilated for half an hour, but then I stopped doing that because after 30 minutes the temperature was back to where it was before anyway.

I’ll additionally throw in the fan as a symptom relief. I know, a bit old school. But it works wonders for the perceived temperature.

Other than that, 25 degrees in the room at this weather is not much, but normal.

Well, I’m building a bunker here with soundproof windows, etc. And then I leave the window open in the evening?! Plus, it’s bright in the room, I can’t sleep…. That’s why I moved to the basement… but it’s reassuring to hear that you do it like that and that it’s probably just normal… Cross-ventilation would work with the rooms in the west, but all the windows there go down to the floor, my husband is afraid to leave them open because of the risk of burglary. And on the ground floor, you can’t leave anything open at night for the same reason, and if I get up at 8, it’s already too warm again, or sometimes I even got up at 5 and ventilated for half an hour, but then I gave up because after 30 minutes the temperature was back to where it was before anyway.

Your husband is afraid of burglary because of the windows upstairs? Where do you live? Where it’s also bright at night and you need soundproof windows at night? That’s obviously unfortunate, but it probably shouldn’t be the norm for single-family houses. By the way, we have a bungalow, and the bedroom windows are also on the ground floor. That’s why we have shutters down to 20 cm and the window open. Burglars don’t want to get caught, they almost certainly wouldn’t break into an occupied bedroom. That only happens in crime stories.