The system has an air conditioning housing (11) which forms an air duct with a headroom opening (21) for discharging air into the upper region of the passenger compartment. It also has a discharge opening (25) for the foot wells and a defroster opening for blowing air onto the windscreen. The housing contains an engine cooling water heated heat exchanger (13) with a temperature sensor which controls the flow of air in order to control the temperature of the air discharged into the passenger compartment. An electrical auxiliary heater (16) heats at least the air discharged from the foot well opening and the discharge control system always ensures that the foot well opening is open. The auxiliary heater is controlled so that the air discharged into the foot wells is warmer than that from the headroom opening by a bi-level mode of control which utilizes the electrical heater output in that both head room and foot well openings are open
