In our era ground-source heat pumps are known as energy-efficient air conditioning systems. However, their high initial costs are a major obstacle to the widespread use of such systems. In this study, the effects of using Al2O3/water nanofluid as heat transfer fluid on reducing the bore length of a ground heat exchanger in a vertical ground-source heat pump are examined. For this purpose, the effective thermal conductivity and the effective viscosity of the nanofluid, which play prominent roles in the convective heat transfer, are optimized via using multi-objective Flower Pollination Algorithm. In this algorithm, the logic of the second version of non-dominated sorting is utilized to deal with the two objectives of this optimization. Then some of the best possible combinations of the thermal conductivity and the viscosity of the nanofluid, extracted from the obtained Pareto front, were used to compute the required bore length. The results were compared with the required bore length calculated using pure water as the heat transfer fluid in the ground heat exchanger. The comparison demonstrated that employing Al2O3/water nanofluid instead of water as heat transfer liquid reduced less than 1.3% of the bore length. Furthermore, investigating the reason of this low reduction in the bore length revealed that grout has the most potential to reduce the bore length among the heat transfer fluid, tubes, and grout. (C) 2016 Elsevier Ltd. All rights reserved.