When electronics found their way into the HVAC field they brought thermistors along for the ride. Now they’re lurking everywhere from simple residential thermostats to centrifugal chillers.
What are they? What do they do? What’s NTC and PTC mean? What the hell does 5K, 7K and 10K mean? How do I know if they’re failed or not? Is it time for lunch?
Read on and we’ll answer all these questions and more.
(except for lunch…it’s a time zone thing)
A thermistor is a resistor that varies its resistance with changes in temperature. They’re used to sense air temperature, water temperature, refrigerant temperature, motor winding temperature…you name it, they sense it.
There are two styles of thermistors: NTC and PTC.
NTC stands for “negative temperature coefficient” meaning its resistance decreases as temperature increases.
PTC stands for “positive temperature coefficient”, its resistance increases as temperature increases.
What about the ‘K’ thing?
3K, 5K, 10K, 30K, whatever K is the thermistor’s resistance (in thousands of ohms) at 77 degrees Fahrenheit.
The device “reading” the thermistor is calibrated for a particular resistance (the K thing) and either PTC or NT.
Thermistors can (usually) be checked “in-circuit” or “out of circuit”
First, the out of circuit check. Power is shut off, the thermistor leads are disconnected, and the resistance checked with a multi-meter. Then the temperature and resistance is checked against a chart.
For the in circuit check you set your meter to volts and check the voltage drop across the thermistors leads (while connected) and compare against the chart for that system.
-Words of caution-
When replacing a thermistor, use the one specified for the machine. Why? Besides the “K” and PTC / NTC ratings there’s also the curve.
What’s the curve? It’s the thermistors sweet spot, the temperature range where it needs to be most accurate. Supply chilled water thermistor? Supply air for a furnace? The thermistors for each will likely have different sweet spots.
That’s pretty much all you need to know about HVAC thermistors.