A system for transferring heat via a refrigerant between an indoor heat exchange coil and an outdoor heat exchange coil of an HVAC system of a building includes an expansion valve and a compressor. The system includes a compressor discharge refrigerant temperature sensor, an outdoor air temperature source, a controller operatively coupled to the compressor discharge refrigerant temperature sensor and the outdoor air temperature source. The controller identifies a difference between the temperatures indicative of the temperature of the compressed refrigerant at or near the output of the compressor and the measure of outdoor air temperature source, identifies a change in the identified difference over time, and reports a fault when the change in the difference exceeds a threshold.