In this study, performance of a heat exchanger used in combi boilers was investigated numerically for different fin geometries. Analyses were performed at the boiler operation conditions. A commercial CFD software package, FLUENT, was used for numerical simulations. The 3-D steady-state turbulent flow field analysis was carried out and k-? model was preferred as the turbulence model. In the analysis, it was assumed that the heat transfer phenomenon occurred both by conduction and convection. Flat fin geometry was taken as a reference for the investigation. Variation of the heat transfer and pressure drop values for the wavy fin were compared with the reference geometry. The wave angle and wave radius were taken as the parameters for the wavy fins. For different fin geometries: the outlet temperaTure of the combustion gases, the heat transfer to the water, and the pressure drop were calculated and the results were presented. Compared with flat fin, average decrease for the outlet temperature of hot gases was obtained as 4 K and average increase for the heat transfer to the water was calculated as 0.68 W. On the other hand, the average pressure drop in the heat exchanger with wavy fins was about 70% higher than the flat fin.