An air includes: a compressor for compressing a refrigerant; an indoor heat exchanger for cooling or heating indoor with a refrigerant; an outdoor heat exchanger for exchanging heat of a refrigerant and outdoor air; an outdoor fan for ventilating outdoor air with the outdoor heat exchanger; a cooling/heating switching valve for switching cooling/heating; a first expansion device installed between the indoor heat exchanger and the outdoor heat exchanger; a CNT heater module installed between the outdoor heat exchanger and the cooling/heating switching valve to evaporate a refrigerant; a second expansion device installed between the outdoor heat exchanger and the CNT heater module; and a controller for sustaining a heating mode of the cooling/heating switching valve.