What is a Thermistor and How to Test it?

 April 12, 2022      Jeff Rosenblum

As our industry changes and becomes more reliant on circuit boards to control the functions of our equipment, the way temperature is monitored has also changed. We need a way to provide needed inputs to the control boards so they can change the operating parameters of the equipment, as needed, to provide comfort in changing conditions. So, we have made the change to thermistors.

What exactly is a thermistor? A Thermistor is an electrical, variable resistance, temperature sensor. The resistance through it changes as the temperature changes. The Thermistor wires plug into the control board and provide the temperature inputs to the board. These inputs allow the control board to make the necessary temperature changes, in order too, provide comfort conditions to the controlled space. Thermistors provide faster response time, are more accurate and are more reliable than previous methods.

Thermistors, like all controls need to be periodically checked. Testing a Thermistor is quite easy.

First, you will need the correct thermistor chart. (See the sample below) There are several different thermistors ranges so choosing the correct chart will ensure you diagnose the thermistor correctly.

Next, measure the temperature that the thermistor senses. This may be air temp, line temp etc.

Next, read the resistance through the two wires and compare to the readings on the chart.

That's it! If your reading falls outside the range, the thermistor would require replacing.

As always if you need any additional help please call the Famous Supply HVAC Tech Department at 800-362-8230.