To increase the heating capacity of air-source heat-pump in cold regions, an improved heat pump cycle with scroll compressor economizer was developed. The measured results of the prototype demonstrate that it can provide a high temperature and high-capacity water supply even under a low ambient temperature of -10 similar to -15 degreesC. The performances of the improved heat-pump cycle are comprehensively analyzed with aid of the proposed thermodynamic model in this paper. The design parameters and the components of the cycle are optimized and the predicted results. The predicted results discussed indicate that the improved heat pump has high efficiency under all weather conditions and that this improvement is conducive to enlarging the running region of the heat pump and providing an alternative heating service for cold regions. (C) 2003 Elsevier Ltd. All rights reserved.