A downhole heat exchanger (DHE) geothermal system has been widely used for space heating in commercial and residential buildings. The DHE is important in heat extraction performance of the geothermal system. Nevertheless, to the best of our knowledge, the single U-tube is the most common configuration for a DHE. An investigation of the heat extraction performances of various DHE configurations is important. In this study, a 3D unsteady-state numerical model was established to describe the fluid flow and thermal processes of a DHE system. The model was validated by field experimental data. Three DHE configurations were evaluated, including a spiral tube, multiple-tubes with a parallel connection, and multiple-tubes with a serial connection. Subsequently, the influences of the key geometrical parameters for the three configurations on the DHE performance were analyzed. The DHE performances of the three configurations were compared. The simulation results showed that the outlet temperature and thermal power of the serial connection DHE were higher than those of the parallel connection with an equal number of tubes. Among the three configurations, the spiral tube DHE showed the greatest heat extraction performance. The key findings of this study could offer guidance for the optimisation and design of a DHE. (C) 2017 Elsevier Ltd. All rights reserved.