Demand defrost with frost accumulation failsafe

A defrost method for a heat pump system, the defrost method comprising, by a controller comprising a central processing unit and memory:

• running the heat pump system in a heating state a first time and determining if a calibration state has been previously run;
• responsive to a determination that the calibration state has not been previously run, running the heat pump system in the calibration state;
• running the heat pump system in the heating state a second time and determining if a difference in temperature between a clear coil temperature of an evaporator coil of the heat pump system and a current temperature of the evaporator coil is greater than a temperature threshold value;
• responsive to a determination that the difference in temperature between the clear coil temperature and the current temperature is less than or equal to the threshold temperature value, running the heat pump system in an initializing defrost state;
• responsive to a determination that the difference in temperature between the clear coil temperature and the current temperature is greater than the threshold temperature value, running the heat pump system in a defrost state;
• monitoring an amount of time that the heat pump system runs in a defrost mode;
• determining a ratio of the amount of time to an initial defrost time;
• and
• responsive to a determination that the ratio is less than a second threshold value, increasing a defrost failsafe time to run the defrost state less frequently.

A heat pump system comprising:

• an evaporator coil;
• a condenser coil coupled to the evaporator coil to permit a fluid to cycle between the evaporator coil and the condenser coil;
• a compressor coupled between the evaporator coil and the condenser coil;
• a reversing valve configured to reverse a direction of flow of the fluid through the heat pump system;
• and
• a controller for initiating a defrost cycle of the heat pump system, the controller comprising a central processing unit and memory configured to:
• run the heat pump system in a heating state a first time and determining if a calibration state has been previously run;
• responsive to a determination that the calibration state has not been previously run, running the heat pump system in the calibration state;
• run the heat pump system in the heating state a second time and determining if a difference in temperature between a clear coil temperature of the evaporator coil of the heat pump system and a current temperature of the evaporator coil is greater than a temperature threshold value;
• responsive to a determination that the difference in temperature between the clear coil temperature and the current temperature is less than or equal to the threshold temperature value, run the heat pump system in an initializing defrost state;
• responsive to a determination that the difference in temperature between the clear coil temperature and the current temperature is greater than the threshold temperature value, run the heat pump system in a defrost state;
• monitor an amount of time that the heat pump system runs in a defrost mode;
• determine a ratio of the amount of time to an initial defrost time;
• and
• responsive to a determination that the ratio is between a first threshold value and a second threshold value, add no time to a defrost failsafe time.