An air conditioning system for the treatment of air to a passenger compartment (C) of a vehicle, wherein the system comprises a housing structure with a first air flow channel for a first flow of air (F1), a second air flow channel for a second flow of air (F2), a third air flow channel , and a fourth air flow channel , wherein a first heat exchanger is arranged in the first air flow channel and a second heat exchanger is arranged in the second airflow channel , wherein the system further comprises a door unit , wherein the door unit comprises an inner screen part stationary arranged in relation to the housing structure , an outer casing part rotatably arranged between different operational positions in relation to the inner screen part and the housing structure , and a flap member attached to the outer casing part , wherein the outer casing part comprises a first inlet opening and a second inlet opening , wherein the door unit is configured for directing the first flow of air (F1) in the first air flow channel and the second flow of air (F2) in the second air flow channel to the third air flow channel , or to the third air flow channel and the fourth air flow channel , wherein the flap member is configured for blocking the first air flow channel or the second air flow channel , wherein the outer casing part is configured for directing the first flow of air (F1) to the fourth air flow channel via the first inlet opening and for directing the second flow of air (F2) to the fourth air flow channel via the second inlet opening , characterized in that the door unit is configured for directing the first flow of air (F1) and the second flow of air (F2) to the passenger compartment (C) via the third air flow channel , or to the passenger compartment (C) via the third air flow channel and the ambient environment (E) via the fourth air flow channel , wherein an internal volume of the door unit enclosed by the inner screen part and the outer casing part is forming the fourth air flow channel . wherein the system further comprises a door unit , wherein the door unit comprises an inner screen part stationary arranged in relation to the housing structure , an outer casing part rotatably arranged between different operational positions in relation to the inner screen part and the housing structure , and a flap member attached to the outer casing part , wherein the outer casing part comprises a first inlet opening and a second inlet opening , wherein the door unit is configured for directing the first flow of air (F1) in the first air flow channel and the second flow of air (F2) in the second air flow channel to the third air flow channel , or to the third air flow channel and the fourth air flow channel , wherein the flap member is configured for blocking the first air flow channel or the second air flow channel , wherein the outer casing part is configured for directing the first flow of air (F1) to the fourth air flow channel via the first inlet opening and for directing the second flow of air (F2) to the fourth air flow channel via the second inlet opening , characterized in that the door unit is configured for directing the first flow of air (F1) and the second flow of air (F2) to the passenger compartment (C) via the third air flow channel , or to the passenger compartment (C) via the third air flow channel and the ambient environment (E) via the fourth air flow channel , wherein an internal volume of the door unit enclosed by the inner screen part and the outer casing part is forming the fourth air flow channel . <EOS>