Turkey has a great potential of solar energy, which is the primary source of renewable energy; however, sufficient benefit cannot be obtained from this clean energy source. In Turkey, 36% of the total energy consumed in buildings is used for heating. Due to the lack of regulations in encouraging the solar energy utilization in buildings, the heating energy consumption plays an important role in the energy economy of our country. Therefore, energy conservation methods become necessary. Energy conservation is important for the existing buildings as for the new buildings. In this study, the south facade of a flat in an existing building in Istanbul in Turkey is recommended to be renovated by the application of Trombe wall principle, which is a well-known indirect passive solar gain system. An approach is proposed for this application and the comparison of the existinsg facade with the renovated facades has been made according to thermal performances and hourly variations of wall interior temperatures. The results of this theoretical application study are given in this paper.