Large installations in which heat is generated via the combustion of fossil or renewable fuels, such as boilers, furnaces, and gas turbines, are an important aspect of the production industry. Due to the ever-increasing concerns over climate change and the rising energy prices, increasing the efficiency of these installations is a top priority. In an air preheater, the incoming combustion air is preheated with recovered heat from the exhaust flue gases before they are expelled through the stack. In this way, the efficiency of the overall system is improved. Effective use of an air preheater can lead to significant efficiency increases which in turn saves fuel. The additional heat extracted from the flue gasses also results in lower flue gas temperatures, making the stack design less complex and expensive. A well-designed air preheater effectively transfers the heat from the flue gases to the air, is compact, and induces a limited pressure drop. These requirements are conflicting with one another as a more compact design inherently induces a larger pressure drop. To increase the heat transfer while attaining a compact size, the use of fins is widely employed. The air preheater investigated in this paper is built up of hundreds of modules that can be placed both in series and/or in parallel. The modules have fins mounted on both the flue gas- and airside in a cross-flow orientation. However, only the fins on the flue gas side are investigated within this study. Two fin geometries for a modular cast-iron air preheater are compared with respect to their thermal performance and the induced pressure drop. The existing fin geometry, currently on market, is experimentally compared to a CFD-optimized alternative. The pressure drop and the thermal resistance are measured by an experimental setup, for different volumes flow rates and heat fluxes. The measured pressure drop over one fin assembly using the CFD-optimized fins is around 5% lower than the commercially available geometry. Secondly, the temperature profiles on the baseplate for the same boundary conditions are similar. The thermal resistance stays almost the same, depending on the boundary condition it either decreases by a maximum of 5% or increases maximally by 3%. The increases in thermal resistance are most notable for higher flow rates. Both these results are likely due to the fin geometry near the baseplate. It is speculated that the CFD-optimized fins shed their boundary layers later, resulting in lower pressure drops but slightly higher thermal resistances. Another impact of the fin geometry is that the temperature difference between the air and the top of the fins is consistently lower for the CFD-optimized fin. These results imply that these new fins will enable higher heat recovery rates for the same pressure drop in the flue gasses.