THERMISTOR Operation and Testing

Thermistors are essential components in HVAC systems, particularly in mini-split systems, as they help monitor air temperature and refrigerant temperature. Understanding their operation and how to test them is crucial for HVAC technicians. In this article, we will explore the basics of thermistors, their types, and the process of testing them.

What is a Thermistor?

A thermistor is a type of temperature sensor that measures temperature changes by detecting the electrical resistance of a semiconductor material. It consists of a wire connected on both sides of the semiconductor in the middle, with two wires coming out. Thermistors are commonly used in HVAC systems to monitor and regulate temperature.

Types of Thermistors

There are two main types of thermistors used in mini-split systems: bead thermistors and tube thermistors.

Bead Thermistors

Bead thermistors are typically used to measure air temperature. They are mounted in a plastic clip or a clear vinyl tube at the inlet of the indoor and outdoor coils. Bead thermistors have a negative temperature coefficient (NTC), meaning their electrical resistance decreases as the temperature increases. They are calibrated to provide accurate temperature readings.

Tube Thermistors

Tube thermistors, on the other hand, are used to measure the temperature of the refrigerant traveling through the indoor and outdoor coils. They are temporarily mounted next to the refrigerant tube and held in place by a spring clip or inside a hollow copper tube. Tube thermistors have an increased surface area, which results in a slower change in electrical resistance with temperature compared to bead thermistors. However, they use the same electrical resistance charts for calibration.

Testing Thermistors

Testing thermistors is a straightforward process that can be done with the power off. Here’s a step-by-step guide:

  1. Turn off the power to the mini-split system and put it in standby mode.
  2. Disconnect the thermistor connector from the PCB.
  3. Measure the electrical resistance of the thermistor using a multimeter. Ensure that the probes make proper contact without accidentally opening the electrical connection.
  4. Simultaneously measure the temperature at the thermistor using a temperature sensor or a tool specifically designed for this purpose.
  5. Compare the measured electrical resistance with the manufacturer’s electrical resistance chart for the corresponding temperature.
  6. If the measured resistance is within an acceptable range (usually within a few percentage points of the chart value), the thermistor is considered calibrated correctly.
  7. If the measured resistance deviates significantly from the chart value (e.g., more than 7-10%), the thermistor may be faulty and should be replaced.

It’s important to note that each manufacturer may have specific instructions and resistance charts for their thermistors. Always refer to the manufacturer’s guidelines for accurate calibration and testing.

Importance of Accurate Thermistor Calibration

Accurate thermistor calibration is crucial for the proper functioning of HVAC systems. If a thermistor is not calibrated correctly, the system may work inefficiently, leading to inaccurate temperature monitoring and potential energy wastage. Therefore, regular testing and calibration of thermistors are essential to ensure optimal system performance.


Thermistors play a vital role in monitoring air temperature and refrigerant temperature in HVAC systems. Understanding their operation and knowing how to test them is essential for HVAC technicians. By following the manufacturer’s guidelines and using the appropriate resistance charts, technicians can ensure accurate calibration and reliable performance of thermistors in mini-split systems.

Leave a Reply

Your email address will not be published. Required fields are marked *