The thermistor decreases the current resistance in a manner that is inversely proportional to the temperature of the thermistor. The electronic control board sends a current to the thermistor and can tell whether a decrease or increase in heat is needed by the relative resistance provided by the thermistor.
Fits a number of Ariston Boilers
Technical specification
Waist Sizeac98cj94gxToilet Cistern CapacityFTW--Supplier Part Number998458Brand NameAriston
Reviews
Ariston 998458 Domestic Hot Water Temperature Probe