In this paper, we report on an underground water-source water-loop heat-pump (UWSWLHP) air-conditioning system for a tall apartment building in Beijing. Water at 14 degrees C was used as an external low-temperature heat-source. By analyzing field-test data for two years, the authors assessed the operating and controlling conditions of the system. Based on the electricity consumed by public-use air-conditioning power equipment and terminal heat-pumps, the authors put forward an overall evaluation of the energy-conservation characteristics of the system. (c) 2005 Elsevier Ltd. All rights reserved.