Ground-source heat-pump (GSHP) systems are becoming more and more attractive for air conditioning owing to their high energy efficiency and reliable operation capability. However, when the technology is used in buildings in a cooling-load-dominated area, the heat rejected into the ground by the GSHP system will accumulate around the ground heat exchangers (GHEs), resulting in the degradation of system performance and an increase in the system operating costs during the summer. Hence, hybrid GSHP (HGSHP) systems are used to overcome this problem by using additional or supplemental heat rejecters such as a cooling tower (CT) to reject the condensation heat. This paper presents the theoretical design for a practical HGSHP experimental system considering the borehole distance, borehole depth, and backfilled material. Furthermore, the optimized combination strategy is determined through a theoretical analysis for almost all operational strategies for the HGSHP system. The results show that the HGSHP system can effectively solve the heat accumulation problem and the decrease in system performance in the long run. (C) 2014 Elsevier B.V. All rights reserved.