Soil can be used as heat reservoir for building heating or cooling purposes, as the underground temperature is different of the ambient air temperature. Ambient air is heated or cooled when flowing along a tube installed underground, and this air with changed temperature is introduced in the building. This problem can be studied in many ways, the main differences being the objectives and the considered details of the flow and temperature fields. In the present work a thermodynamic analysis is made, and important criteria are obtained in what concerns both the construction parameters (diameter and length of the tube installed underground) and the operation parameters (mass flow rate and heating or cooling effect). For such an analysis it is of crucial importance the consideration of the entropy generation minimization criterion which leads to the best thermodynamic performance of the system. Results are presented for three different specified operating conditions, which can be used for a better understanding of the heating or cooling systems using the soil as heat reservoir, as well as to give guidelines and limits when more detailed analysis are to be made. (c) 2006 Elsevier Ltd. All rights reserved.