ᐅ Warmest Room: Bedroom (North Side)?

Hello everyone,

I have a somewhat unusual question that I can’t quite figure out. Our house (built in 2021, KfW55 standard) has a central ventilation system with a ground heat exchanger, but no air conditioning. The indoor temperatures remain comfortable even during warmer outdoor conditions. According to the display, the ventilation system blows air into the house at 16 - 21°C (61 - 70°F) depending on the outside temperature, and operates at full capacity. All thermostats in the house are turned down very low, so essentially no heating is used, not even at night.

We always assumed that, due to solar radiation, the south side would generally be the warmest, even with some shading. Under current outside temperatures of around 25°C (77°F), the maximum temperature on that side is usually about 23.5°C (74°F). Oddly, our warmest room is the bedroom, which is located on the north side on the first floor. This room only receives direct sunlight early in the day, and even then, only on the insulated exterior walls, not on the window. When I hold my hand near the ventilation outlet in this room, the incoming air feels cool, as indicated by the ventilation system. However, the temperature never drops below 23.5°C (74°F) there and often rises above 24°C (75°F) at night.

I have already checked the underfloor heating distribution, but could never detect any flow there. Underneath the bedroom are only the (unheated) guest bathroom and the (non-heatable) pantry, with an insulated ceiling above them. Does anyone have any idea how this phenomenon might occur? Many thanks in advance.

Best regards,
Abzug86

Is the utility room or any other heat source located directly underneath? How large is the room?

No, as mentioned, only two unoccupied rooms where not much is happening. The bedroom is approximately 18 m² (194 sq ft) in size.

Are you sure that no sunlight comes through the window? Low-angle sun in the morning and evening can also penetrate deep into the rooms and warm them up. Which other rooms are adjacent and could transfer heat through the interior walls? A floor plan would be best.
Have you checked that the heating is really off (valves closed, supply cold, preferably the heating system completely switched off)?
Do you have the possibility to ventilate properly in the evening and morning?

Thermal imaging of all walls, floors, and ceilings would probably help to locate the heat input. You can also rent thermal cameras...