The results of investigation on improving thermal output of having- chimney gaseous heater by an external installable system have been presented in this research. Decreasing the amount of primary and secondary air which are mixed by natural gas or combustion products, changing the protuberances which are inside the smoke-stack of gaseous heater or using forced convection for heater without using server channels , have bee cases that researcher focused to them before this issue. While, all of these researches seem to be proficient for increasing the heater output, they are less welcomed by industry for some reasons.the chimney increases the energy loss of having- chimney heater, on the other hand, we have to use chimney in this heater for exiting the combustion products. However, these combustion gases can be return to the environment by installing a channel to pass high speed air on the chimney, using metallic fins, or changing the geometry of chimney heater. This can be performed by various ways. To achieve permissible condition and other data, initial design was made on the basis of plainness of construction and examination, after evaluating available limits in test of desired system. Then, primary tests are performed for gaining practical conditions and output data (entrance and exit temperature of chimney, gas speed inside of the chimney, environment temperature). Finally, thermal flow and transfer of gas inside the chimney are studied by using ANSYS –FLUENT software for several suggested geometry. Additional to using half of geometries and using symmetry condition, their correspondening tests and simulatio are designed in two part for decreasing number of simulation times, due to several unknow available parameters. First part, chimney contains fin plates without present of more speed air channel is tested and simulated by the condition of natural displacement to compare gained results , all unknown parameter correctly obtain by the approximate heat transfer coefficient . next part, using the numerical method of functional hypotenuse , guess and error i performed by le frequency for gaining the heat transfer
