A system for defrosting an air-to-air heat pump system is disclosed. 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, 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.