An improvement in defrosting an air-to-air heat pump system when in the heating mode. A bypass loop transfers refrigerant at a higher temperature and pressure than is normally cycled through the outdoor unit to an outdoor coil to defrost ice on the outdoor coil, The bypass loop includes a valve movable between closed and open positions. A sensor monitors a preselected condition indicative of outdoor coil performance. A controller communicates with the valve and the sensor. Once the controller determines that a preselected set point of a preselected condition indicative of deteriorating performance has been reached, based on received sensor signals, the controller sends a signal to open the valve, allowing warm refrigerant to bypass expansion valves and flow to the outdoor unit to defrost the outdoor unit. Once defrosting is accomplished, the valve can be moved to a closed position to resume normal operation of the heat pump unit.