Water heater system diagnostics

A heat pump water heater, comprising:

• a condenser;
• a compressor upstream of the condenser that compresses a refrigerant;
• a first sensor communicating a first indication of a temperature upstream of the compressor;
• a second sensor communicating a second indication of a temperature of the refrigerant upstream of the condenser and downstream of the compressor;
• an expansion device downstream of the condenser that controls a pressure of the refrigerant;
• and
• control circuitry configured to:
• store a flow coefficient of the expansion device;
• determine, based on performance data of the compressor and the first and second indications, a coefficient estimate of the expansion device;
• compare the flow coefficient to the coefficient estimate;
• and
• determine a condition of a heat pump based on the comparison.

A method of operating a water heater system that includes a heat pump, the method comprising:

• compressing a refrigerant via a compressor;
• exchanging heat from a condenser of the heat pump to a tank of the water heater system;
• determining a position command for an expansion device based on at least one operating temperature of the heat pump to control a pressure of the refrigerant;
• controlling the expansion device based on the position command;
• determining a saturated suction temperature (SST) of the refrigerant using a temperature upstream of the compressor;
• determining a saturated condensing temperature (SCT) of the refrigerant using a temperature downstream of the compressor and upstream of the condenser;
• determining a position estimate of the expansion device based on the SST and the SCT;
• comparing the position command to the position estimate;
• and
• determining a condition of the heat pump based on the comparison.