Ground source heat pumps (GSHPs), often referred to as geothermal heat pumps (GHPs), offer an attractive option for heating and cooling residential and commercial buildings owing to their higher energy efficiency compared with conventional systems. GSHPs have been used for four years in the Turkish market, although they have been in use for more years in developed countries. The purpose of this study is to present an energy and exergy analysis of a GSHP system with a 50 m vertical 1.25 in. nominal diameter U-bend ground heat exchanger. This system was applied to a 65 m(2) room in the Solar Energy Institute, Ege University, Izmir, for the first time at the university level in Turkey. The Institute, built in 1986, has a livable floor area of 3000 m(2) and uses passive solar techniques. The heating and cooling loads of the room studied were 3.8 and 4.2 kW at design conditions, respectively. The system was commissioned in May 2000, and performance tests have been conducted since then. The exergy transports between the components and the consumptions in each of the components of the GSHP system were determined for the average measured parameters obtained from the experimental results in February 2001. The exergy diagram (the Grassmann diagram) was also presented for the GSHP system studied to give quantitative information regarding the proportion of the exergy input that is dissipated in the various system components. (C) 2003 Elsevier Ltd. All rights reserved.