An air conditioning system having a cooling mode and a free-cooling mode, comprising: a refrigeration circuit having a compressor and a pump; and a controller for selectively operating in the cooling mode by circulating and compressing a refrigerant through said refrigeration circuit via said compressor but not said pump or operating in the free-cooling mode by circulating said refrigerant through said refrigeration circuit via said pump but not said compressor; characterised by: a first pressure sensor at an inlet of said pump; a second pressure sensor at an outlet of said pump; and a pump-protection sequence resident on said controller, said pump-protection sequence, when operating in the free-cooling mode, turning said pump to an off state based at least upon a differential pressure determined by said controller from pressures detected by said first and second pressure sensors: wherein in a first comparison step said pump-protection sequence turns said pump to said off state when said differential pressure is less than a predetermined pressure differential threshold; and wherein in a second comparison step said pump-protection sequence turns said pump to said off state when a standard deviation average of differential pressure is greater than a predetermined standard deviation average threshold. a refrigeration circuit having a compressor and a pump; and a controller for selectively operating in the cooling mode by circulating and compressing a refrigerant through said refrigeration circuit via said compressor but not said pump or operating in the free-cooling mode by circulating said refrigerant through said refrigeration circuit via said pump but not said compressor; characterised by: a first pressure sensor at an inlet of said pump; a second pressure sensor at an outlet of said pump; and a pump-protection sequence resident on said controller, said pump-protection sequence, when operating in the free-cooling mode, turning said pump to an off state based at least upon a differential pressure determined by said controller from pressures detected by said first and second pressure sensors: wherein in a first comparison step said pump-protection sequence turns said pump to said off state when said differential pressure is less than a predetermined pressure differential threshold; and wherein in a second comparison step said pump-protection sequence turns said pump to said off state when a standard deviation average of differential pressure is greater than a predetermined standard deviation average threshold. a first pressure sensor at an inlet of said pump; a second pressure sensor at an outlet of said pump; and a pump-protection sequence resident on said controller, said pump-protection sequence, when operating in the free-cooling mode, turning said pump to an off state based at least upon a differential pressure determined by said controller from pressures detected by said first and second pressure sensors: wherein in a first comparison step said pump-protection sequence turns said pump to said off state when said differential pressure is less than a predetermined pressure differential threshold; and wherein in a second comparison step said pump-protection sequence turns said pump to said off state when a standard deviation average of differential pressure is greater than a predetermined standard deviation average threshold. wherein in a first comparison step said pump-protection sequence turns said pump to said off state when said differential pressure is less than a predetermined pressure differential threshold; and wherein in a second comparison step said pump-protection sequence turns said pump to said off state when a standard deviation average of differential pressure is greater than a predetermined standard deviation average threshold.