Warmest bedroom (north side)?

Hello everyone,

I have a somewhat strange topic to bring to you that I can't make sense of. Our house (built in 2021, KfW55) has a central ventilation system with a geothermal heat exchanger, no air conditioning. The temperatures inside the house are quite bearable even at higher outdoor temperatures. According to the display, the ventilation system blows air into the house at 16 - 21°C (depending on the outside temperature) and at full power. The thermostats throughout the house are turned down extremely low so that practically no heating is used, not even at night.

We always assumed that it would generally be warmest on the south side (due to solar radiation), even with appropriate shading. There, at current outdoor temperatures (~25°C), we usually have a maximum of 23.5°C. However, our warmest room is strangely the bedroom, which is located on the north side on the first floor. The room is only directly exposed to sunlight early in the day and then only on the insulated exterior walls, not on the window. When I hold my hand close to the ventilation outlet in the room, the incoming air feels cool, just as indicated by the ventilation system. Nevertheless, the temperature never drops below 23.5°C, often even rising above 24.0°C at night.

I have already checked the distribution of the underfloor heating but could never detect any flow there either. Also, beneath the bedroom there is only the (unheated) guest WC and the (non-heatable) pantry, with an insulated ceiling above. Does anyone have any idea how this phenomenon can occur? Many thanks in advance.

Regards Abzug86

Is the technical room or any other heat source directly below? How large is the room?

No, as mentioned only two unoccupied rooms in which not much happens. The bedroom is about 18 m² in size.

Are you sure that no sun is coming through the window? Low sun in the morning and evening also penetrates deep into the rooms and heats them up. Which other rooms are adjacent and could transfer the heat through the interior walls? A floor plan would be best. Have you checked that heating is really turned off (valves closed, supply cold, preferably the entire heating system off)? Do you have the possibility for proper ventilation in the evening and morning?

Presumably, recordings from a thermal imaging camera of all walls, floors, and ceilings would help to locate the heat entry. It can also be rented ...