Nowadays, one of the greatest energy consumption sources in the world is recognized as HVAC systems, which are used vastly not only for industrial productions but also for creating comfort condition in dwellings. Since beginning of warm or cold season, we will need a source of energy to reach desirable thermal conditions inside a building. Also, earth is naturally a great thermal source, which exists under buildings, courtyards and city squares and is easily within reach. Using annually stable temperature within the earth, one can heat or cool the air by allowing ambient air to pass through soil context under the building. This is possible according to thermal transfer principle and temperature difference between soil and ambient air. In this way, heat is transferred to or from the surrounding soil by conduction through the pipe wall and convection with the tunnel air, tempering the air as it flows through the pipe. In cycle’s continuation, we can transfer mixed air of the room and that of the exiting from pipes to air conditioning unit in order to augment the efficiency of unit. Knowledge of the ground temperature distribution is required for the first order calculation of heat exchange between earth and buildings (partially or fully underground). The variation of the ground temperature with depth depends, however, on the condition of surface. In this paper, the energy performance achievable is using an earth-to-air heat exchanger for an air-conditioned building has been evaluated for both winter and summer. Moreover, soil temperature in Tehran’s climate is calculated by using a thermal model and by considering soil thermo physical and climatic condition of Tehran during 10 past years. Predictions of soil temperature exhibit a sinusoidal pattern due to the annual fluctuations. It has been found that at the depth of about 4-6 m, the earth provide a stable thermal environment. In addition, by using simple model of underground pipe systems, we can calculate heating or cooling potential of so-called system for three-story educational building with 12 classes which is located in Tehran. Also, we can estimate the percentage of energy conservation for this specific building. In this system, three rows of 45 meter long pipes with the radius of 50 centimeters are located 6 meter underground, have 53% energy conservation in summer and 63% energy conservation in winter. These numbers show a good potential for reducing cooling and heating energy demand in a typical educational building. The most important problem which is not considered in majority of papers is the change of soil temperature surrounding the underground pipes during cold and hot season. This will happen by passing of time and will decrease the efficiency of system at the end of cold and hot season. In conclusion, According to transient conduction theory in so-called model, the longest period and the optimum condition in which this system is of the highest efficiency when it starts to work, is 115 days in warm season and 150 days in cold season.