In recent years, numerical simulation heat transfer became a widely used for the chimneys simulation with the natural boundary. This simulation has been performed using an iterative technique line-by-line, which is based on Thomas algorithm or TDMA (3) (tri-diagonal matrix algorithm) and using the computational fluid dynamics (CFD) Fortran 90 code. CFD has been used to study natural ventilation. The main advantages of the CFD approach are that it enables to analyze complex gases flow patterns for fireplaces of buildings and that it can be used to simulate the chimneys flow performance for a lot of imaginary buildings for design purposes. The results are included the figures of the temperature distribution, as well as heat flow lines and vectors. The overall heat loss through the chimney walls has reached for chimneys 1 and 4 to 2876 and 2871 (W/m) respectively. These values have been generated by the post-processing program (Tecplot 360). The comparative assessment of mesh sizes shows that the results are close to the ideal situation which say that a mesh independent solution is one which does not change with further mesh refinement. This study has demonstrated that CFD is a powerful tool capable of capturing the complex heat transfer process (with some assumptions) in gas-solid structures with satisfactory results and is a useful tool in the design of other structures reactors.