Last time I looked at a thermostat (it's been a while!) I only needed the switch; now my Honeywell T6360B has suddenly started to need to be turned up to 25 instead of 16-17 to come on. I've never been sure what the heater inside is for, but might this be faulty?
The little heater in the thermostat is to anticipate the heat that will be emitted from a radiator after the switch has opened. In other words, if the stat were to switch off at the set temp, the radiator will still be hot, and the room temp will still increase. The heater avoids this problem, and can make the stat surprisingly accurate. The only correct way of checking is to place a thermometer near to the stat, and see what you get. A lot of the latest stats work electronically, and have only two wires, the neutral not being required.
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