This article reports on the performance of a ground source heat pump system installed in a New Municipality Hall in Northern Greece over an eight-year operation period. The system consists of a vertical ground heat exchanger, 21 boreholes in 80 m depth, 11 water-to-water heat pump units. Basic parameters of its operation are continuously monitoring by a data acquisition system. Based on these recordings, heat transfer flows from/to the building and the ground were calculated in order to estimate the performance of the system. It is found that the maximum ground heat exchanger load reaches 50 W/m in heating operation while in cooling mode it ranges between 20 and 210 W/m. The Weekly Performance Factor of the heat pumps as well as the Seasonal Energy Efficiency Ratio were found to be between 5.0-6.2 and 4.5-5.5 in heating mode and 4.1-5.9 and 3.6-4.5 in cooling mode, respectively. Compared to a conventional heating and cooling system for this building, the ground source heat pump consumes 25.7% less primary energy and emits lower CO2 and NOx emissions by 22.7% and 99.6% respectively, but its SO2 emissions are 18.4% higher. (C) 2012 Elsevier Ltd. All rights reserved.