Here are some tips for positioning a room thermostat:
The positioning of the room thermostat is critical to the correct operation of the system. It should have overall control of the system, because it will save energy by shutting everything down when the ideal temperature is reached.
The room thermostat should be in a room that is not effected by other sources of heat such as an open fire, cooking equipment, or direct sunlight etc. It should not be somewhere drafty or too close to a window, especially if it's one that is opened in the winter.
Secondly it should be in a cool room, ideally the last to warm up. That way the other rooms will be warm enough, (but not overheated if fitted with TRV’s) before the room thermostat switches the entire heating system off. [To achieve this, sometimes it may involve downsizing a radiator in the room where the room stat is located]
The radiator in the room with the thermostat installed should not have a TRV fitted. Otherwise the TRV could interfere with the operation of the room thermostat and prevent it from working properly.
The thermostat should be mounted about 1.5 metres from the floor, and not close to, or above a radiator.
Preferably it should not be on an outside wall, (although if you have well insulated walls, this is not quite so crucial.)
It should be in an area where air can circulate easily, not in a corner, or hidden behind curtains, or furniture.