Geothermal energy is a renewable resource that nature provides in a constant and lasting way throughout the year. An analysis method based on the use of computational fluid dynamics has been developed to predict the behaviour of a horizontal air-ground heat exchanger. The air-ground heat exchanger consists of a buried horizontal pipe inside which water or air passes, cooling in summer and heating in winter. The system exploits the thermal inertia of the ground whose temperature remains constant throughout the year at a certain depth. The model has been validated by comparing the temperature values of a heat exchanger built in the province of Turin. The results showed that the geothermal system is more influenced by the burial depth rather than by the conductivity of the soil due to the low thermal capacity of the air.