The optimization of the fin and protrusion dimensions and shapes of finned tube gas to liquid heat exchangers are performed numerically in this thesis. The dimensions of a commercially available heat exchanger of a combi boiler apparatus are taken as basic dimensions and various parameters about plate fin and three different protrusions, namely balcony, winglet and imprint, are examined step by step. The study is realized within the geometrical dimension range used in actual applications. The numerical analyses of several heat exchangers are performed using Fluent CFD software. The boundary conditions used in the numerical investigation are taken from the actual operating conditions of the combi boiler heater apparatus. Firstly, the best plate fin geometry is determined. Secondly, the best protrusion dimensions are obtained. Finally, the effects of three protrusion placement on plate fin surface are examined. The fin I5B2W3 is determined as the best among the investigated fins. The use of actual operating conditions as boundary conditions and the investigation of the individual and cumulative effects of three different protrusions are the novelties of the study. In order to validate the numerical models, a comparison with a computational and experimental study performed by Wu and Tao is also made. The results of the models created for comparison purpose show good agreement with the results given by Wu and Tao. It is also observed that results obtained from the two alternatives for the modeling, one with symmetrical and the other with periodic boundary condition, are very close. According to these results, it is concluded that the numerical outcomes obtained in the present study are reliable.