A heat exchanger included in an air conditioner includes a first header pipe to have a refrigerant, compressed by a compressor, to flow therein and a first heat exchange unit coupled to the first header pipe to receive the refrigerant flowing in the first header, a second header pipe to have the refrigerant to flow therein and a second heat exchange unit coupled to the second header pipe to receive the refrigerant flowing in the second header pipe in the air cooling operation. A bypass pipe couples the first heat exchange unit with the second header pipe and a bypass valve controls a flow of the refrigerant through the bypass pipe. A controller controls the bypass valve such that the refrigerant is allowed to flow from the first heat exchange unit to the second header pipe in the air cooling operation.