Thermistors are used to sense/measure temperature. You will find them in ductless splits to detect evaporator coil or condenser temperature, temperature measuring devices such as a digital thermostat, ambient air temperature, PTC start relay, and supply or return air temperature. The name thermistor is a combination of “thermal” and “resistor.”

A thermistor changes electrical resistance with a change in temperature. The difference in electrical resistance made is dependant on the type of thermistor used. There are two types of thermistors. PTC (positive temperature coefficient) and NTC (negative temperature coefficient), but measurements are usually in the thousands of Ohms.

A PTC thermistor increases resistance with an increase in temperature, whereas an NTC thermistor decreases resistance with an increase in temperature. NTC is most common in the HVAC industry.

Example: When used to sense freezing conditions on an air conditioning evaporator coil, as the coil freezes, the NTC thermistor would increase in resistance and not allow current to flow once the resistance is too high. The air conditioner would then not function.