Horizontal ground heat exchangers have been widely used in many countries as the heat source for ground-source heat pump systems. When abundant land space is available for the installation of ground heat exchangers, the horizontal ground heat exchangers provide a cost-effective choice because the installation costs of horizontal ground heat exchangers are much lower than those of vertical ground heat exchangers. Slinky-coil horizontal ground heat exchangers, which are basically coiled ground heat exchangers, require less land space than conventional straight horizontal ground heat exchangers because of the higher installation density of heat exchange pipes per given area. However, at present, the slinky-coil horizontal ground heat exchangers tend to be over-or undersized because of the lack of an appropriate design method for them. In this research on the optimum design of slinky-coil horizontal ground heat exchangers, a commercial finite-element simulator, FEFLOW, was used to simulate the performance of slinky-coil horizontal ground heat exchangers taking into account the energy balance at the land surface. The accuracy of the developed simulation model was validated through history-matching calculations on the basis of the results of thermal response tests and a long-term air-conditioning test under various test conditions. In history matching, reasonable agreement was obtained between the measured and simulated values of the heat medium and ground temperatures, demonstrating the reliability of the developed numerical simulation model using FEFLOW. (C) 2011 Elsevier Ltd. All rights reserved.