Abstract: A self-contained window-mounted, heat/cool room air conditioner for automatically maintaining a predetermined indoor room temperature over a wide range of outdoor ambient temperatures. The air conditioner includes a reverse cycle refrigeration system having an inside heat exchange coil and an outside heat exchange coil, the coils being series-connected with an associated refrigerant compressor via a reversing valve wherein the system in addition to functioning in a conventional room air cooling mode can also function as a heat pump to heat room air over a limited range of outdoor ambient temperatures. Low and high wattage resistance heating elements supplement or replace the heating action of the reverse cycle refrigeration system under certain operating conditions. Four thermistor temperature sensors are used to provide precise control of the air conditioner for maximum efficiency.