This paper presents the results from experimental study of borehole heat exchangers (BHE) installed in a ground source heat pump (GSHP) system in Nuremberg, Germany. The BHE was constructed with 18 boreholes that can be grouped into three blocks: block I of 121 mm, block II of 165 mm and block III of 180 mm. The operation of the GSHP system had been monitored between March 2009 and October 2012 for analysis. Over years of the system operation, the thermal performance of BHEs is observed to be slightly improved for larger borehole diameters (block II and III). For the comparison among the three blocks, thermal loads of block II is 1.64% and block III is 3.45% larger than that of block I, respectively. Furthermore, saving-to-investment ratio (SIR) is evaluated for two parts: the basic part (SIRI) for block I: the extra parts (SIRII-I, SIRIII-I) for block II and block III. Over a thirty-year period the SIR is estimated to be 4.80, 2.14 and 3.18 for SIRI, SIRII-I and SIRIII-I, respectively. These findings suggest that block I has the highest economic profitability. Based on these evaluations, the acceptable drilling costs for block II and block III are also analyzed and discussed. (C) 2013 Elsevier B.V. All rights reserved.