The Cozy senses the room temperature using a thermopile ( link: Thermal Infrared Thermopile by Melixis ) because the temperature at the Cozy could be very, very hot when the radiator is on, so we cannot use the temperature reading around the radiator itself. (It is typical to use a thermocouple to measure the ambient temperature around an object. A thermocouple would be found in a traditional thermostat mounted on a wall.)
The thermopile works similarly to a non-contact thermometer, where it sends out an infrared light signal to record the temperature remotely. The temperature recorded might be skewed if the sensor is pointing at something that itself is cold or hot (e.g., it's pointed at a wall that has steam pipes behind it) or if it is is pointed at something that is more reflective and/or retains heat (e.g., low emissivity, see Note 1 below).
Using your knowledge of how the Cozy senses the room temperature, set your target temperature accordingly.
Note 1: Emissivity measures how efficiently a surface emits thermal radiation compared to a perfect emitter, called a blackbody, at the same temperature. An emissivity value of 1 indicates a perfect emitter that radiates all its thermal energy, while a low emissivity number (close to 0) means the surface emits less thermal radiation and reflects most of it. Materials with high emissivity are better at emitting heat, whereas those with low emissivity are more reflective and retain heat.