The objective of this study is to investigate the temperature behavior of a building integrated with a thermal storage wall and a window for direct solar gain for supplying heat in winter. This is achieved by modeling and simulation with the TRNSYS (Transient System Simulation) program. A hypothetical two story building with a total floor area of 120 meter square has been modeled with the program. A Trombe wall is used on the south fa‡ade of the ground floor. Additionally a direct gain window of area 6.5 meter square is placed on the south fa‡ade of the First floor. The hourly indoor temperature variations on the ground floor are compared with that of the First floor. It is assumed that the model building is located in Cyprus and Larnaca climatic conditions are used for simulation.