Abstract: A heating and air conditioning unit of a motor vehicle has a heat exchanger that can be supplied with coolant of the engine and a compressor disposed in the refrigerant circuit of the air conditioning unit for the compression of a gaseous refrigerant. The heat exchanger 3, apart from the exchanger region 6 through which the coolant flows, has a separate exchanger region 7 for the heated refrigerant, which can be supplied with heated refrigerant via a bypass line 14 branching on the pressure side of the compressor 8 from the refrigerant circuit. The bypass line 14, after passing though the heat exchanger 3, discharges on the suction side of the compressor 8 again into the refrigerant circuit.