Air conditioning system

Air conditioning system including an outdoor unit, a compact indoor unit (10), air supply duct (20), air discharge duct (30), and preheat exchanger(40). An indoor heat exchanger (100) in the indoor unit (10) has a space therein in communication with a room, a fan (200) in the indoor unit (10) provided in the space draws air and discharges through the indoor heat exchanger (100). A ventilation guide duct (300) in the indoor unit (10) on an underside of the indoor heat exchanger (100) has a partition wall (316) for separating external air supplied from an outside of the room, and room air, for guiding the external air to the room through the fan (200), and the room air to the outside of the room. The air supply duct (20) and the air discharge duct (30) have one ends connected to the ventilation guide duct (300) for guiding the external air to the room, and the room air to the outside of the room, respectively. The preheat exchanger (40) is provided in the middle of the air supply duct (20) and the air discharge duct (30), for indirect heat exchange of the external air and the room air passing through the air supply duct (20) and the air discharge duct (30).