A fault detection and diagnostics (FDD) system is provided for a refrigeration circuit having an evaporator and a compressor configured to circulate a refrigerant through the evaporator. The FDD system includes a communications interface configured to receive a measurement of a thermodynamic property affected by the refrigeration circuit and a processing circuit having a processor and memory. The processing circuit is configured to use the measured thermodynamic property to determine an expected suction entropy of the refrigerant at a suction of the compressor, use the expected suction entropy to determine an expected thermodynamic discharge property of the refrigerant at a discharge of the compressor, determine an actual thermodynamic discharge property of the refrigerant at the discharge of the compressor, and detect a fault in the refrigeration circuit by comparing the expected thermodynamic discharge property with the actual thermodynamic discharge property.