Arkansas HVACR NewsMagazine January 2026

HVACR NewsMagazine January 2026

Tech News

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