What Is a Thermistor & How Does It Work?

Modern HVAC systems today rely on thermistors for proper sensing of air and liquid temperatures to be used by system control boards for adjustments to operational speeds. As temperature conditions change, the thermistor reports these temperatures to the control boards which then adjust the compressor and blower speeds to compensate for these changing conditions.

• Thermistor type selection varies according to the use needed or application required.
• HVAC thermistors resistance ranges used by CBP are the 10k-ohm & 50k-ohm varieties.
• These are called NTC Thermistors, as they respond inversely to the temperature chance.
• If the measured temperature rises, then the thermistor resistance value drops.
• Likewise, if the measured temperature drops, then the thermistor resistance value rises.

Glass Encapsulated NTC Thermistors

These are NTC temperature sensors sealed in an airtight glass bubble. They are designed for use with temperatures above 150°C, or for printed circuit board mounting, where ruggedness is a must. Encapsulating a thermistor in glass improves the stability of the sensor as well as protecting the sensor from the environment. They are made by hermetically sealing bead type NTC resistors into a glass container. The typical sizes range from 0.4 – 10mm in diameter.

NTC Thermistors

An NTC thermistor is a thermally sensitive resistor whose resistance exhibits a large, precise and predictable decrease in resistance, as the core temperature of the element increases over the operating temperature range.

Characteristics of NTC Thermistors

Unlike RTDs (Resistance Temperature Detectors), which are made from metals, NTC thermistors are generally made of ceramics or polymers. Difference materials used result in different temperature responses, as well as other characteristics.

Temperature Response

While most NTC thermistors are typically suitable for use within a temperature range between -55°C (-67°F) and 200°C (392°F), where they give their most precise readings, there are special families of NTC thermistors that can be used at temperatures approaching absolute zero (-273.15°C (-459.67°F) as well as those specifically designed for use above 150° C (302°F).

Thermistors are of two types, NTC (negative temperature coefficient) and PTC (positive temperature coefficient types). As their name indicates, the resistance of an NTC thermistor will decrease with temperature rise and the resistance of a PTC thermistor will increase with temperature rise.



Both PTC as well as NTC thermistors can be checked by using an analogue or digital multimeter. Keep the analogue or digital multimeter in resistance mode. Connect the multimeter leads to the thermistor leads. Polarity is not an issue here. Now heat the thermistor by exposing to a known temperature source. You can see the multimeter reading smoothly increases or decreases depending on whether the thermistor under test is PTC or NTC. This will happen only for a healthy thermistor.

For a faulty thermistor, following observations are possible: The change in reading will not be smooth or there will not be any change. A shorted thermistor will always read zero while an open thermistor will always read infinity.

Using an ohmmeter to test the thermocouple will determine if the thermocouple sensor is defective or not.



If a thermistor goes out of calibration, shorts, or is open, then the HVAC system cannot adjust operational speed according to the changing load condition. A simple test meter can be used for testing of the thermistor.

Using the following 10K-ohms chart, and based on the thermistors associated location temperature, then ohm out the thermistor quickly and compare the ohms value from the chart to the actual ohms reading on the meter.

That is all there is to testing the thermistor, reading should be with-in 5-10 percent depending on the thermistors tolerance value.