The natural heat transfer in ducts depend on generating an induced flow due to the difference in density between the heated fluid and it is environment. The effect of reduce cross sectional area of a vertical isothermal duct by adding half circular obstacles to the inner side of the duct. The effect of the induced flow and obstacles were study using COMSOL software and compared to heat through same duct with no flow. The results that obstacles increase induced flow by 16% compared to normal vertical duct, Also shows a reduction in value of the maximum temperature of the hot surface at the solar peak heat flux (3:00 PM).