Heat pump water heater appliance and a method for operating the same

A water heater appliance, comprising:

• a tank defining an interior volume;
• a sealed system comprising a compressor, a condenser and an evaporator, the compressor operable to compress refrigerant, the condenser in fluid communication with the compressor such that refrigerant from the compressor is received by the condenser, the condenser thermally coupled to the tank in order to heat water within the interior volume of the tank with energy from the refrigerant;
• a fan positioned adjacent the evaporator of the sealed system, the fan configured for selectively directing a flow of air across the evaporator of the sealed system;
• an ambient temperature sensor configured for measuring an ambient temperature, ta;
• an outlet temperature sensor positioned adjacent an outlet of the evaporator and configured for measuring a refrigerant outlet temperature, to, of the evaporator;
• a controller in operative communication with the compressor and the fan, the controller configured for
• establishing that the evaporator of the water heater appliance is frosted over by:
• comparing the ambient temperature ta to a threshold temperature, tt;
• and
• determining whether a difference between the ambient temperature ta and a reference temperature, tr, is less than the refrigerant outlet temperature to, the reference temperature tr being adjustable based on the ambient temperature ta;
• initiating a defrost cycle;
• deactivating the compressor of the sealed system during the defrost cycle;
• and
• operating the fan during the defrost cycle in order to direct the flow of air across the evaporator of the sealed system.