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

1. A method for defrosting an evaporator of a water heater appliance, comprising:

establishing, by use of a controller in communication with an ambient temperature sensor and an outlet temperature sensor positioned adjacent an outlet of the evaporator, that the evaporator of the water heater appliance is frosted over by:

measuring an ambient temperature, t a , at the water heater appliance;

comparing the ambient temperature t a to a threshold temperature, t t ;

gauging a refrigerant outlet temperature, t o , of the evaporator; and

determining whether a difference between the ambient temperature t a and a reference temperature, t r , is less than the refrigerant outlet temperature t o , the reference temperature t r being adjustable based on the ambient temperature t a ;

initiating a defrost cycle of the water heater appliance;

deactivating a compressor of the water heater appliance during the defrost cycle of the water heater appliance; and

operating a fan of the water heater appliance during the defrost cycle of the water heater appliance, the fan of the water heater appliance drawing a flow of air across the evaporator of the water heater appliance during said step of operating.

2. The method of claim 1 , wherein the threshold temperature t t is about forty-five degrees Fahrenheit and the reference temperature t r is about fourteen degrees Fahrenheit.

3. The method of claim 1 , wherein refrigerant from a condenser of the water heater appliance flows to the evaporator of the water heater appliance during said step of operating.

4. The method of claim 3 , wherein the refrigerant from the condenser of the water heater appliance has a temperature greater than about ninety degrees Fahrenheit.

5. The method of claim 1 , further comprising opening a throttling valve of the water heater appliance after said step of deactivating.

6. The method of claim 5 , wherein said step of opening is performed more than about three minutes and less than about ten minutes after said step of deactivating.

7. The method of claim 1 , further comprising terminating the defrost cycle of the water heater appliance.

8. The method of claim 7 , wherein said step of terminating comprises

determining whether a refrigerant outlet temperature, t o , of the evaporator is greater than a difference between an ambient temperature, t a , at the water heater appliance and a predetermined temperature, t p ; and

establishing whether a refrigerant inlet temperature, t i , of the evaporator is greater than the difference between the ambient temperature t a and the predetermined temperature t p .

9. The method of claim 8 , further comprising activating a heating element at the evaporator of the water heater appliance if the difference between the ambient temperature t a and the predetermined temperature t p is greater than the refrigerant outlet temperature t o of the evaporator at said step of determining or the difference between the ambient temperature t a and the predetermined temperature t p is greater than the refrigerant inlet temperature t i of the evaporator at said step of establishing.

10. The method of claim 8 , wherein the predetermined temperature is about two and one-half degrees Fahrenheit.

11. The method of claim 7 , wherein said step of terminating is performed at least ten minutes after said step of deactivating.

12. 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, t a ;

an outlet temperature sensor positioned adjacent an outlet of the evaporator and configured for measuring a refrigerant outlet temperature, t o , 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 t a to a threshold temperature, t t ; and

determining whether a difference between the ambient temperature t a and a reference temperature, t r , is less than the refrigerant outlet temperature t o , the reference temperature t r being adjustable based on the ambient temperature t a ;

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.

13. The water heater appliance of claim 12 , wherein the sealed system further comprises a throttling valve configured for regulating a flow of refrigerant through the sealed system, the controller in operative communication with the throttling valve and further configured for opening the throttling valve after said step of deactivating.

14. The water heater appliance of claim 13 , wherein the throttling valve is adjusted to a fully open position at said step of opening.

15. The water heater appliance of claim 12 , further comprising a heating element positioned at the evaporator of the sealed system, the controller in operative communication with the heating element, the controller further configured for activating the heating element if a difference between an ambient temperature, t a , at the water heater appliance and a predetermined temperature, t p , is greater than a refrigerant outlet temperature, t o , at the evaporator or the difference between the ambient temperature t a and the predetermined temperature t p is greater than a refrigerant inlet temperature, t i , of the evaporator.