Ground-source heat pump (GSHP) systems for HVAC have aroused more and more interest in China in recent years because of their higher energy efficiency compared with conventional systems. The design and performance simulation of the geothermal heat exchangers is vital to the success of this technology. In GSHP systems, the load of the geothermal heat exchanger varies greatly and is usually discontinuous even during a heating or cooling season. This paper outlines a heat transfer model for geothermal heat exchangers. The model was used to study the influence of the discontinuous operation of the heat pumps on the performance of the geothermal heat exchangers. A simple and practical approach is presented for sizing the geothermal heat exchangers.