An air conditioning system comprising: an outdoor unit having a compressor, an accumulator, a first heat exchanger, a first expansion valve, an air to air, A2A, valve and a flow path switching valve; a first indoor unit having a second heat exchanger and a second expansion valve, configured to perform a cooling/heating operation through heat exchange between a refrigerant and air; and a second indoor unit having a third heat exchanger and a third expansion valve, configured to perform the cooling/heating operation through heat exchange between the refrigerant and water; wherein the flow path switching valve comprises a first port connected to an outlet of the compressor, a second port connected to the first heat exchanger, a third port connected to the second heat exchanger and the third heat exchanger, and a fourth port connected to the accumulator of the outdoor unit which is connected to the compressor, wherein the first heat exchanger is connected to the second heat exchanger and the third heat exchanger, wherein the second heat exchanger and the third heat exchanger are connected in parallel, and wherein the A2A valve is installed in a refrigerant pipe between the third port of the flow path switching valve and the second heat exchanger, configured to change a refrigerant cycle formed between the outdoor unit and the first indoor unit and the second indoor unit, wherein the air conditioning system further comprises: a controller configured to control the A2A valve to prevent flow of the refrigerant from the third port of the flow path switching valve to the second heat exchanger when the first indoor unit is stationary and a heating operation of the second indoor unit where the first heat exchanger operates as an evaporator and the third heat exchanger operates as a condenser. an outdoor unit having a compressor, an accumulator, a first heat exchanger, a first expansion valve, an air to air, A2A, valve and a flow path switching valve; a first indoor unit having a second heat exchanger and a second expansion valve, configured to perform a cooling/heating operation through heat exchange between a refrigerant and air; and a second indoor unit having a third heat exchanger and a third expansion valve, configured to perform the cooling/heating operation through heat exchange between the refrigerant and water; wherein the flow path switching valve comprises a first port connected to an outlet of the compressor, a second port connected to the first heat exchanger, a third port connected to the second heat exchanger and the third heat exchanger, and a fourth port connected to the accumulator of the outdoor unit which is connected to the compressor, wherein the first heat exchanger is connected to the second heat exchanger and the third heat exchanger, wherein the second heat exchanger and the third heat exchanger are connected in parallel, and wherein the A2A valve is installed in a refrigerant pipe between the third port of the flow path switching valve and the second heat exchanger, configured to change a refrigerant cycle formed between the outdoor unit and the first indoor unit and the second indoor unit, wherein the air conditioning system further comprises: a controller configured to control the A2A valve to prevent flow of the refrigerant from the third port of the flow path switching valve to the second heat exchanger when the first indoor unit is stationary and a heating operation of the second indoor unit where the first heat exchanger operates as an evaporator and the third heat exchanger operates as a condenser.