Arkansas HVACR NewsMagazine January 2026

HVACR NewsMagazine January 2026

Tech News

coil reaches 32°F. When it closes, it completes the circuit back to the board, which initiates a timer. This timer will last 30,

Coil Temperature Sensors:

Thermostats vs Thermistors Bryan Orr, HVAC School by Techs for Techs

'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.

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).

In the case of Carrier defrost boards, a thermostat will close once the