A refrigerant compressor for use in a chiller-heater system that can provide chilled water at less than 10 DEG C and hot water at more than 80 DEG C without the need for additional heat input, the compressor being configured to provide a compression ratio of at least 18: 1 and a temperature lift across the compressor of at least 80K, and the refrigerant fluid being selected to give the desired temperature lift without the pressure at the compressor outlet exceeding 25 bar.