Heat Pump Defrost Sensor vs. Thermostat

carrier_defrost_thermostat

When you work on a heat pump system and you want to test defrost there are so many different test procedures to follow to test the board and sensors. Most involve “forcing” a defrost by shorting out pins on the board or advancing the time on the defrost initiation by shorting our pins or installing a factory provided pin jumper.

Lots of pins and jumping involved.

But one thing to need to be be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost.

A thermostat is an open and closed switch, they are usually round in shape like the one shown above and they open within a set temp range and they close within a set temp range. The one shown above is a Carrier Defrost Thermostat and it closes at 30 degrees +/- 3 degrees and it opens at 65 degrees +/- 5 degrees. In this case because this particular sensor closes in colder than 32 degree temps you can’t even use an ice bath to test it. If it is below 32 outside it is easy to test (duh) otherwise you can just run it in heat mode with the cond fan off and see when it closes by using an Ohmmeter.

On a defrost thermostat you can also easily jump it out to test the board since it is just open an closed.

A defrost sensor is a thermistor. A thermistor changes resistance based on the temperature it is exposed to. In order to test you can measure the ambient temperature, make the the sensor is removed and acclimated, measure the Ohms  of resistance and compare to the manufacturer chart.

Thermistor

You CANNOT jump out a thermistor.

— Bryan

P.S. – A new podcast about Heat pumps is out today HERE

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