An air conditioning system 1 for a cabin 10 including a pressurized air source 2, a cooling air source 3, a main heat exchanger 4, a compressor 5, a manifold 6 and a mixing chamber 7. An inlet 43 to a cold side of the main heat exchanger 4 is in fluid communication with the cooling air source 3. An inlet 41 of a hot side of the main heat exchanger 4 is in fluid communication with the pressurized air source 2. An inlet 51 of the compressor is in fluid communication with the outlet 44 of the cold side of the main heat exchanger 4.