According to the SE technician, the filters are correctly installed; he also removed them, vacuumed them, and explained to me that I should never turn them over, but always reinstall them with the same side facing up after cleaning - we also put markers on the system indicating which side must be on top. I have now slightly lowered the hot water temperature to 45 degrees. However, this does not affect the ventilation. I still need to look more closely at the temperatures (base temperature, slope, etc.) ... it always takes 2-3 days to really notice a change. Overall, I currently consider the entire system to be a complete design failure - the nightmare of one’s own four walls. The only option for us at the moment is to run the heating and ventilation only when we are out of the house, but that cannot be a long-term solution. I will get back to you when I have a statement from the installing plumbing company. Best regards A. Rösner