A defrost method for a heat pump system includes running the heat pump system in a heating mode to provide heat to an enclosed space and determining if an outdoor temperature is less than an outdoor threshold temperature. Responsive to a determination that the outdoor temperature is below the outdoor threshold temperature, 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. Responsive to a determination that the calibration state has been previously run, determining if a temperature difference between a temperature of an evaporator coil of the heat pump system and the outdoor temperature exceeds a temperature threshold value. Responsive to a determination that the temperature difference between the evaporator coil and the outdoor temperature is greater than the temperature threshold value, running the heat pump system in a defrost state.