This paper proposes an optimization design methodology for borehole heat exchangers coupled with heat pump units by controlling maximum/minimum inlet fluid temperature in the life cycle of ground coupled heat pump system. Base on the hourly performance simulation of ground coupled heat pump system, the optimal combination of the distance between boreholes, borehole depth and borehole number under a given annual cooling/heating load is determined using the Hooke-Jeeves pattern search algorithm. A case study is presented to validate the effectiveness and feasibility of the proposed method for optimal design of borehole heat exchangers. When minimum inlet fluid temperature is decreased from 5 degrees C to 0 degrees C, total borehole lengths and total areas required for the installation of single U-pipe borehole heat exchangers are reduced by 13.3% and 75.4% respectively, for double U-pipes borehole heat exchangers, the corresponding reductions are 11.5% and 73.1% respectively. For the same minimum inlet fluid temperature (0 degrees C), the minimum reduction of total areas required for borehole heat exchangers installation is still up to 21% when the type of borehole heat exchangers is changed from single U-pipe to double U-pipes. (C) 2016 Elsevier Ltd. All rights reserved.