Defrost systems and methods for heat pump water heaters

Abstract

A heat pump water heater can include a refrigerant circuit, an evaporator coil in fluid communication with the refrigerant circuit, a fan configured to move air across the evaporator coil, one or more temperature sensors, a heating element located proximate an air flow path between the fan and the evaporator, and a controller. The controller can be configured to receive temperature data from the one or more temperature sensors and, in response to the temperature data, output instructions for the fan to move air across the heating element to the evaporator coil.

What is claimed is: 
     
         1 . A heat pump water heater system, comprising:
 a refrigerant circuit;   an evaporator coil in fluid communication with the refrigerant circuit;   a fan;   an ambient air temperature sensor; and   a controller configured to:
 receive temperature data from the ambient air temperature sensor; and 
 in response to the temperature data, output defrost instructions for:
 the heat pump water heater system to stop heating the water; 
 the fan to move ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water and based on the temperature data at a first time indicating that a first temperature of the ambient air is below a first temperature threshold; and 
 the fan to deactivate based on at least one of expiration of a period of time or the temperature data at a second time after the first time indicating that a second temperature of the ambient air is above a second temperature threshold. 
 
   
     
     
         2 . The heat pump water heater system of  claim 1 , wherein the controller is further configured to:
 determine that the first temperature exceeds a third temperature threshold less than the first temperature threshold,   wherein to output the defrost instructions for the fan to move the ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water is further based on determining that the first temperature exceeds the third temperature threshold.   
     
     
         3 . The heat pump water heater system of  claim 1 , wherein the heat pump water heater system continues to not heat the water during the period of time. 
     
     
         4 . The heat pump water heater system of  claim 1 , wherein the controller is further configured to:
 determine that a third temperature of the evaporator coil at the first time is below a third threshold temperature,   wherein to output the defrost instructions for the fan to move the ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water is further based on determining that the third temperature is below the third temperature threshold.   
     
     
         5 . The heat pump water heater system of  claim 4 , wherein the controller is further configured to:
 determine that the third temperature of the evaporator coil exceeds a fourth threshold temperature less than the third temperature threshold,   wherein to output the defrost instructions for the fan to move the ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water is further based on determining that the third temperature exceeds the fourth temperature threshold.   
     
     
         6 . The heat pump water heater system of  claim 1 , further comprising
 a housing,   wherein the evaporator coil is positioned within the housing, and   wherein the ambient temperature sensor is positioned within the housing.   
     
     
         7 . The heat pump water heater system of  claim 1 , further comprising
 a housing,   wherein the evaporator coil is positioned within the housing, and   wherein the ambient temperature sensor is positioned on the housing.   
     
     
         8 . The heat pump water heater system of  claim 1 , further comprising
 a housing,   wherein the evaporator coil is positioned within the housing, and   wherein the ambient temperature sensor is positioned exterior to and remote from the housing.   
     
     
         9 . A non-transitory, computer-readable medium having instructions stored thereon that, when executed by one or more processors, causes a heat pump water heater controller to:
 receive temperature data from an ambient air temperature sensor; and   in response to the temperature data, output defrost instructions for:
 a heat pump water heater system to stop heating water; 
 a fan to move ambient air across an evaporator coil after the heat pump water heater system has stopped heating the water and based on the temperature data at a first time indicating that a first temperature of the ambient air is below a first temperature threshold; and 
 the fan to deactivate based on at least one of expiration of a period of time or the temperature data at a second time after the first time indicating that a second temperature of the ambient air is above a second temperature threshold. 
   
     
     
         10 . The non-transitory, computer-readable medium of  claim 9 , wherein the instructions, when executed by the one or more processors, further cause the heat pump water heater controller to:
 determine that the first temperature exceeds a third temperature threshold less than the first temperature threshold,   wherein to output the defrost instructions for the fan to move the ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water is further based on determining that the first temperature exceeds the third temperature threshold.   
     
     
         11 . The non-transitory, computer-readable medium of  claim 9 , wherein the heat pump water heater system continues to not heat the water during the period of time. 
     
     
         12 . The non-transitory, computer-readable medium of  claim 9 , wherein the instructions, when executed by the one or more processors, further cause the heat pump water heater controller to:
 determine that a third temperature of the evaporator coil at the first time is below a third threshold temperature,   wherein to output the defrost instructions for the fan to move the ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water is further based on determining that the third temperature is below the third temperature threshold.   
     
     
         13 . The non-transitory, computer-readable medium of  claim 12 , wherein the instructions, when executed by the one or more processors, further cause the heat pump water heater controller to:
 determine that the third temperature of the evaporator coil exceeds a fourth threshold temperature less than the third temperature threshold,   wherein to output the defrost instructions for the fan to move the ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water is further based on determining that the third temperature exceeds the fourth temperature threshold.   
     
     
         14 . The non-transitory, computer-readable medium of  claim 9 , wherein the evaporator coil is positioned within a housing, and wherein the ambient temperature sensor is positioned within the housing. 
     
     
         15 . The non-transitory, computer-readable medium of  claim 9 , wherein the evaporator coil is positioned within a housing, and wherein the ambient temperature sensor is positioned on the housing. 
     
     
         16 . The non-transitory, computer-readable medium of  claim 9 , wherein the evaporator coil is positioned within a housing, and wherein the ambient temperature sensor is positioned exterior to and remote from the housing. 
     
     
         17 . A method for operating a heat pump water heater system, the method comprising:
 receiving, by processing circuitry of a heat pump water heater system, temperature data from an ambient air temperature sensor; and   in response to the temperature data, outputting, by the processing circuitry, defrost instructions for:   the heat pump water heater system to stop heating water;   a fan to move ambient air across an evaporator coil after the heat pump water heater system has stopped heating the water and based on the temperature data at a first time indicating that a first temperature of the ambient air is below a first temperature threshold; and   the fan to deactivate based on at least one of expiration of a period of time or the temperature data at a second time after the first time indicating that a second temperature of the ambient air is above a second temperature threshold.   
     
     
         18 . The method of  claim 17 , further comprising:
 determining that the first temperature exceeds a third temperature threshold less than the first temperature threshold,   wherein outputting the defrost instructions for the fan to move the ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water is further based on determining that the first temperature exceeds the third temperature threshold.   
     
     
         19 . The method of  claim 17 , wherein the heat pump water heater system continues to not heat the water during the period of time. 
     
     
         20 . The method of  claim 17 , further comprising:
 determining that a third temperature of the evaporator coil at the first time is below a third threshold temperature,   wherein outputting the defrost instructions for the fan to move the ambient air across the evaporator coil after the heat pump water heater system has stopped heating the water is further based on determining that the third temperature is below the third temperature threshold.