A refrigerant cycle comprising: a compressor for compressing a refrigerant; a first heat exchanger downstream of said compressor, said first heat exchanger receiving refrigerant from said compressor, and said first heat exchanger further changing the temperature of a second fluid to be conditioned by said refrigerant in said first heat exchanger; an expansion device downstream of said first heat exchanger; a second heat exchanger downstream of said expansion device, and said refrigerant flowing from said compressor to said first heat exchanger, through said expansion device, said second heat exchanger, and back to said compressor, and characterised by further comprising: a control for monitoring a variable condition of said refrigerant, and a variable condition of the second fluid at said first heat exchanger, said controller determining a desired value for each of said two variable conditions, and monitoring an actual value of each of said two variable conditions, said control having an error correction algorithm for determining a correction factor for each of said two variable conditions, each of said error correction algorithms utilizing a determined error for both of said variable conditions, wherein said variable condition of said refrigerant is discharge pressure downstream of the compressor; and wherein said second fluid is water and wherein said variable condition of said second fluid is a leaving water temperature leaving said first heat exchanger. a compressor for compressing a refrigerant; a first heat exchanger downstream of said compressor, said first heat exchanger receiving refrigerant from said compressor, and said first heat exchanger further changing the temperature of a second fluid to be conditioned by said refrigerant in said first heat exchanger; an expansion device downstream of said first heat exchanger; a second heat exchanger downstream of said expansion device, and said refrigerant flowing from said compressor to said first heat exchanger, through said expansion device, said second heat exchanger, and back to said compressor, and characterised by further comprising: a control for monitoring a variable condition of said refrigerant, and a variable condition of the second fluid at said first heat exchanger, said controller determining a desired value for each of said two variable conditions, and monitoring an actual value of each of said two variable conditions, said control having an error correction algorithm for determining a correction factor for each of said two variable conditions, each of said error correction algorithms utilizing a determined error for both of said variable conditions, wherein said variable condition of said refrigerant is discharge pressure downstream of the compressor; and wherein said second fluid is water and wherein said variable condition of said second fluid is a leaving water temperature leaving said first heat exchanger. a control for monitoring a variable condition of said refrigerant, and a variable condition of the second fluid at said first heat exchanger, said controller determining a desired value for each of said two variable conditions, and monitoring an actual value of each of said two variable conditions, said control having an error correction algorithm for determining a correction factor for each of said two variable conditions, each of said error correction algorithms utilizing a determined error for both of said variable conditions, wherein said variable condition of said refrigerant is discharge pressure downstream of the compressor; and wherein said second fluid is water and wherein said variable condition of said second fluid is a leaving water temperature leaving said first heat exchanger.