‘Tis heating season once again. It’s getting colder outside, and the heat pumps are starting to frost up outdoors. The defrost cycle helps get rid of the frost, and there are a few different ways we can do that; the methods themselves may differ between systems (time & temperature vs. demand defrost), as may the hardware. The coil temperature sensors may come in two main varieties: snap-action thermostats or thermistors.

These two sensor types vary in the way they send information to the defrost board, and we’ll cover the differences in appearance, function, and testing.

How to Recognize Thermostats and Thermistors

The two types of coil temperature sensors look quite distinct from one another, so they’re quite easy to recognize in the field.

Thermostats

The first type, the snap-action thermostat sensor, usually consists of a round disc with two different metals (a bimetallic disc) that clips onto the lowest possible point of the coil. This sensor type doesn’t really measure temperature; the metal just warps and closes the thermostat once the coil temperature drops below a certain point. 

In the case of Carrier defrost boards, a thermostat will close once the coil reaches 32°F. When it closes, it completes the circuit back to the board, which initiates a timer. This timer will last 30, 60, or 90 minutes as determined by the selected pin on the board. If the compressor has run for the entire duration (i.e., the indoor thermostat has NOT reached its setpoint) by the end of that cycle, that means that the coil is still below freezing, and the unit goes into defrost. If the indoor thermostat is satisfied in the meantime, the system will NOT go into defrost; it will just turn off as usual.

The metals will flex back into place as the coil warms up during the defrost cycle, which will open the thermostat once again (if it opens before the maximum defrost duration, of course). The exact temperature will vary by manufacturer, but for Carrier, this is about 65°F.

Thermistors

Thermistors are a bit different because they don’t just open and close; their electrical resistance increases or decreases with changes in coil temperature. A thermistor that increases resistance with a rise in temperature is a positive temperature coefficient (PTC) type. One that decreases resistance with a rise in temperature is a negative temperature coefficient (NTC) type. We tend to use NTC thermistors for coil sensors.  

The resistance corresponds to a specific temperature, which will tell the defrost board whether the coil is cold enough for the defrost cycle to begin. The exact temperature will depend on the manufacturer (if an OEM board is used) or its configuration (if a universal board is used).

Since the thermistor’s resistance is constantly checked by the defrost board, the thermistor’s resistance will also notify the defrost board when the coil is warm enough for the unit to come out of defrost within the set time limit.  

Thermistors come in different varieties based on their resistance. The most common type is a 10k (kilohm or kΩ) thermistor. The OEM may have a chart with the ohm readings and corresponding temperatures in their respective diagnostic or tech support app, but they’re quite easy to find online as well. 

Many units that use thermistors as coil temperature sensors will actually have two: one that measures coil temperature and one that measures the ambient temperature. Systems that use demand defrost look for differentials between the two to determine when to put the unit into defrost. The White-Rodgers Universal Defrost Control (47D01U-843) is one such example; its thermistors can also replace a thermostat.

Common Sensor Issues

Coil temperature sensor failures are relatively uncommon. However, they’re not impossible, and there may be other issues to look out for, including wiring problems.

Thermostats in particular don’t often fail on their own. They’re not very prone to drifting, so most of the time, the issues are actually with loose or rubbed-out wires. A visual inspection will be able to tell you if there is a wiring issue. These can be resolved fairly easily by making sure the new wiring is tight, secured properly, and doesn’t rub against anything. On the flipside, they are often not as accurate or precise as thermistors. 

Thermistors are a bit more prone to drifting than thermostats. When possible, the sensor itself should be on the underside of the coil so that the sun doesn’t affect its readings. Radiant heat from the sun could cause the thermistor to pick up more heat than what’s actually on the coil, thus inflating the resistance and preventing the thermistor from putting out a low enough resistance value for the board to recognize a need for defrost.

In any case, you can still test the thermostats and thermistors to see if they’re in good shape or need replacement. Some electronic boards, like the White-Rodgers Universal Defrost Control, may also display a fault code when they detect an issue with a sensor. Further diagnostics will be necessary to determine what is wrong with the sensor, but it gives you a solid start by identifying the coil sensor as the problem. 

Testing Coil Temperature Sensors

You can test the coil temperature sensors with an ohmmeter or an electrical multimeter that can read in the kilohms scale. The methods for testing the two are slightly different.

Testing a Thermostat

When we test a thermostat, we’re looking for a closed or open path from one side of the thermostat wire to the other. The thermostat sends out a 24V signal on that wire, and it won’t receive it if the thermostat is open. It will when the thermostat is closed, so we can measure ohms from one side of the wire to the other and look for OL when the coil temperature is clearly above the manufacturer’s defrost initiation temperature. Again, that’s 32°F on Carrier systems, but it may vary by OEM. 

To make the coil freeze quickly, you can run the heat pump in heat mode and unhook the fan. The fan pushes air (and its heat) over the coil, which prevents moisture from freezing onto it. When we get rid of that, the cold refrigerant moves through at near-freezing temperatures, causing any moisture on the coil to start freezing.

Since a thermostat should be open when the coil is not frozen, we want to remove the thermostat plug from the board and look for OL when the coil is above the defrost temperature. Once the thermostat sensor closes, you’ll see a proper ohm reading if you measure the circuit’s resistance (it may not be very much, like 0.1 ohms). 

You can watch the reading change in real time if you run the heat pump in heat mode without the fan, use a liquid line temperature clamp near the thermostat to monitor the line temperature, and put your meter leads in the sockets on the sensor’s plug. Keep in mind that it takes some time for the thermostat to close after the coil drops below the sensor’s threshold.

Testing a Thermistor

Instead of looking for an OL when measuring resistance on a thermistor, you will always see a resistance value unless the thermistor has failed or has a severe wiring issue. Have a reliable thermometer on hand to measure the ambient temperature, allow the thermistor to acclimate (if it hasn’t already), and then measure resistance across the plug. Keep in mind that heat from the sun or your hand can heat the thermistor above the ambient temperature.

Take the ohm reading and consult a chart that shows the resistance readings and their corresponding temperatures. Your meter’s resistance reading should be pretty darn close to the chart’s resistance value for the ambient temperature measured on your thermometer. Note that your meter will most likely auto-range to the kilohm scale (shown below), so you may need to multiply it by 1000 to interpret the reading on the chart. In this case, the ambient temperature was just above 77°F.

If you don’t have a thermometer on hand, you can use the same method one could use to calibrate temperature clamps. Get a bowl of distilled water, fill it with ice cubes, allow it to sit for a little while (there should still be an ice-water mixture), and then put the thermistor in there. The resistance readings should correspond to the value listed for 32°F. We know it’s 32°F because ice and liquid water exist together, and the temperature can’t drop until we have 100% liquid. Just like refrigerant at saturation, any added heat goes towards the phase change. (That’s the latent heat of fusion when a solid-liquid mixture is involved.)

If the values don’t match up (or aren’t at least within a reasonable 1–2°F tolerance), that indicates that the sensor has drifted and will need to be replaced.

Can Thermostats Be Replaced With Thermistors (or Vice Versa)?

It really depends on the situation. Let’s say you have a wire rubout on a thermostat-type coil sensor and want to keep using the OEM board. In that case, you’d just replace it with the OEM part. However, if you’re using a universal board (such as if the customer wants to upgrade a time & temperature defrost to demand defrost or there’s an issue with the board as well), you will have to use the sensor type that comes with it. 

The bottom line is that these coil temperature sensors work in different ways, which we need to understand to diagnose them properly.