Glowworm Thermistors
The thermistor decreases circuit resistance as the temperature of the thermistor rises. The printed circuit board sends a current to the thermistor and can tell whether a decrease or increase in heat is needed by the resistance in the thermistor.
Glow-worm 0020079644 Central Heating Pressure Sensor - Flexicom/Ultracom
Part No: 1337421 / MPN: 0020079644
Glow-worm 0020014160 NTC Sensor - DHW Outlet Pipe - Flexicom/Ultracom
Part No: 1337363 / MPN: 0020014160