To evaluate the efficiency of ground source heat pump (GSHP), an application is studied in a residential building divided in two independent apartments blocks. The numerical results are produced by using the software tool TRNSYS 16. The results include the trends of the thermo-climatic variables and a comparison between conventional and geothermal systems analyzing operational and capital costs. The GSHP studied in this paper produces a thermal power output of 29 kW in winter and a cooling power output of 26 kW during summer time. The numerical simulations have highlighted that with such a system, the GSHP has a winter coefficient of performance of 4.9, and a warm season coefficient of performance which reaches 2.3. The simulations show that the best plant configuration consists of a winter heating GSHP system and a summer free-cooling bypass. The thermo-climatic variables inside the building provide good comfort conditions.