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This paper presents heat transfer analysis of a passive summer cooling and winter preheating technique, which utilizes the underground temperature at a depth of 6 ft (1.83m) or more. In this study, a computer model solution has been developed for predicting the air temperature at any point, at any time while passing through the underground pipe. The air temperature is a function of the inlet air temperature and humidity, soil temperature, soil properties, the pipe (length, diameter, shape, and material), and the air flow velocity. Experiments were performed to measure air temperature, soil temperature, and pipe surface temperature at different lengths and different intervals of times. The results of the experiments are compared with theoretical calculations, which showed very good agreement. Thus the theoretical model may be used for predicting the performance of such systems in the future.