Placement of External Temperature Sensors for TRVs

I have now switched to Better Thermostat (BT) with an external temperature (and window) sensor per room. So far this is working great with BT being able to hit and hold the setpoint temperatures relatively reliable. [1]

My problem is that sensor placement seems to be pretty important here. So far I tried to place the sensor top center in the middle of the ceiling. The thinking being that this is the closest I can get to “in the middle of the room”.

Still, this does not seem to be ideal, as there is a 1-2 °C temperature gradient between the ceiling sensor and the sensor at the radiator (with the ceiling temperature being higher). Setting BT to 20°C this way actually feels to cold in the room.

Now my question here is: What are your thoughts on temperature sensor placement?

My thoughts: Can’t place the sensors next to doors as that places the sensors to close to other rooms and opening and closing the door messes with the readings. Would I prefer inner vs. outer walls, ideally I wouldn’t want to mount to close to the walls at all. Ideally I would to measure “where the humans are”, e.g. on the living room table, but this is often not possible for neatness reasons.

So … where do you mount your external temperature sensors?

[1] There are several instances where a “sudden” hour of direct sunlight raises the temperature in the south facing rooms by several degrees Celcius, but BT can’t do anything about that.

opposite wall from where the window is at waist height.
Generally that means next to the light switch here.

That’s what I started with, especially with the sensors with a display, that makes a lot of sense.

The problem with that is in our bedroom the temperature across the room, next to the open door is about 1°C higher than the room temperature. Using that as a reference results in either too cold room temperatures (once it has settled) or to warm temperatures (while the radiator is on).

I am not sure what I am getting at with this, right now I feel like the problem is a lot more complicated than it appears on the surface.