Saunier Thermistors
The thermistor decreases the circuit resistance as it gets hotter. The electronic control sends a current to the thermistor and can tell whether a decrease or increase in temperature is needed by the resistance in the thermistor.
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Saunier Duval S5739800 Temperature Sensor - Isofast F/Isomax F/Themaclassic
Part No: 1416330 / MPN: S5739800