The paper presents an analysis of the heat transfer process on heating surfaces made with different materials in a fire tube water boiler supplied by natural gas. The analysis was based on analytical calculations and numerical simulations using ANSYS Fluent software. The analytical calculations were performed to investigate the flue gas composition as a product of the natural gas combustion process and to apply these results in numerical modeling of heat transfer phenomena between hot exhaust gas and water in the boiler's heat exchanger. The obtained results show that optimization of heat exchanger design is crucial for efficient heat transfer process in every boiler, which has important implications for the energy efficiency of those systems and environmental protection due to fuel consumption reduction. This research provides valuable insights into the importance of heat exchanger design and its impact on the efficiency of water boilers.