An improved air conditioning system that cools or heats air very rapidly after being started. The system includes a main heater and a cooler which also functions as an auxially heater. The cooler includes a variable displacement compressor for compressing refrigerant gas. The compressor has a crank chamber and a discharge chamber. A crank mechanism is accommodated in the crank chamber. Compressed refrigerant gas is supplied to an external refrigerant circuit via the discharge chamber. The discharge chamber is connected to the external refrigerant circuit by a passage. A throttle valve is located in the passage. The throttle valve closes the passage immediately after the compressor is started, which quickly increases the pressure of the discharge chamber. As a result, the displacement of the compressor is increased quickly, and rapid heating or cooling results.