Nowadays two important discussions in academic parties all over the world are energy crisis and environment crisis. One of the strategies to deal with these crisises is using clean, renewable and domestic energy sources. One of the best renewable energies that can be used in different scales is solar energy. In Iran which is considered among the greatest countries that receive huge amount of solar radiation, it can take the best advantages of this free blessing. Solar energy can be used in two ways: direct and indirect (after convert to electricity). In buildings solar heating can be achieved by two methods: active (like using solar collectors) and passive (like using greenhouses). Solar collectors are widely used in buildings to take advantages of solar radiant and its heat directly. These collectors include an absorbent surface that receives heat of solar beam and diffuse radiation and transfers it to the intermediate fluid which transfers this heat where it must use. A solar combisystem that use solar heating to provide space heating and domestic hot water includes solar collector too. In this system heat that gains in solar collector, inters to hot water storage that act as a small auxiliary storage for space heating. The performance of a solar collector is highly influenced by its orientation and its tilt angle. This is due to this fact that orientation and tilt angle both change the solar radiation reaching the surface of collector. Generally, systems installed in the northern hemisphere are oriented due south. The other important parameter for optimum usage of these collectors is determination of appropriate tilt angle to the south that maximizes the absorption of solar radiation. Many investigations have been carried out to estimate the optimum tilt angle for solar systems. This paper deals with calculation of optimum tilt angle for solar collectors in Tehran (capital of Iran) in a mathematical model and discussion about it. For this purpose, monthly solar radiation on a flat solar collector calculates with different angles and results are compared and evaluated to select the optimum tilt angle for a solar combisystem. First, daily extraterrestrial radiation on a horizontal surface must calculate and then due to the number that obtained, monthly average clearness index can be achieved. Afterward proportion of diffuse radiation to total radiation should estimate. Then proportion of beam radiation on sloped surface to horizontal surface can calculate. Due to this number proportion of total radiation on sloped surface to horizontal surface calculates. At the end by multiply the obtained number to daily average radiation in each month, total radiation on sloped surface is calculated. These processes should be done for different angles from 0 to 90 degrees and finally results are compared with collectors that install on the southern facade (with 90 degrees in fact) and advantages of facade collectors for solar combisystem are expressed.