A method of operating a reversible heat pump system includes steps of sensing an outdoor air temperature with a first sensor and sensing at least one condition of an outdoor heat exchanger coil with a second sensor. Input from the first and second sensors is then electronically analyzed to predict when frosting may occur on an outdoor heat exchanger coil. A proportioning reversing valve is modulated in response to this analysis in order to direct a controlled proportional backpressure flow of pressurized refrigerant from a pressure side of a compressor into the outdoor heat exchanger coil while pressurized refrigerant is continued to be supplied to an indoor heat exchanger coil. As a result, the pressure and temperature of refrigerant within the outdoor heat exchanger coil is raised to at least an extent necessary to prevent frosting on the outdoor heat exchanger coil. The steps are preferably performed continuously in a feedback control loop during operation of the reversible heat pump system as a heat pump.