The accuracy of a thermostat controlling a heating system can be measured by comparing its reading against an accurate atmospheric thermometer. The temperature recorded by the thermostat must not deviate from the reference thermometer's reading by more than one degree.
When testing a thermostat, the reference thermometer should first be mounted beside it using sticky tape. Mounting the thermometer close to the thermostat being tested allows a reading to be obtained that takes into account the different factors affecting that thermostat, such as direct sunlight or hot air blowing against it that might be skewing its reading. Care must be taken to insulate the thermometer from the wall using paper towels or similar material. This ensures that reference readings correctly measure the temperature of the air and are not influenced by the temperature of the wall. Additionally, before taking a reading, the reference thermometer should be left for 15 minutes to allow it to settle into the room's ambient temperature.
If the test reveals that the temperature reading obtained by the thermostat is incorrect, this might indicate that built-up dirt is insulating the temperature sensor; cleaning the thermostat should remedy the issue. With older, mechanical thermostats, incorrect readings can occur if the device is not mounted level on the wall, and a quick adjustment to level it should allow it to work properly again.