This paper proposes an optimum operation method for open-loop ground heat exchangers (GHEX) considering the subsurface temperature gradient. A series of thermal response tests and artificial heating/cooling operations was carried out along with monitoring temperatures in the standing column well. The underground temperature naturally increases with depth, but a switch between the cooling and heating modes can alter the temperature distribution. The effect of the mode change was evaluated by performing logarithmic mean temperature difference (LMTD) and computational fluid dynamics (CFD) analyses for a reduced (or physical) model with the well depth of 150m. As a result, in the cooling mode, the upstream operation is more efficient than the downstream operation and reduces entering water temperature (EWT) by 2.26 degrees C. On the other hand, in the heating mode, the downstream operation is advantageous over the upstream operation and increases EWT by 3.19 degrees C. According to the results of the LMTD and CFD analysis, the thermal conductivity of the ground formation and the flow direction of water are the most important factors in the open-loop GHEX. Finally, an optimum flow direction with respect to each operation is proposed to enhance its efficiency; thus, a new GHEX system is flexible to a change in the flow direction. Copyright (c) 2015 John Wiley & Sons, Ltd.