In the present work a computational model is being developed and corroborated by verification tests for the proper selection and design of tubular heat exchangers. Exhaust outlet temperatures can be estimated by the developed calculation model, which is programmed in VBA. Based on the calculations and the experiments, the aluFer® heat exchanges patented by Hoval for condensing boilers and heat exchangers with beads are being compared and evaluated. Therefore, the characteristic lengths for both heat exchangers are being defined due to the importance to calculate the heat exchangers. The objective is to find an alternative heat exchanger for the existing aluFer® heat exchangers. It does not seem appropriate to replace the aluFer® heat exchangers with heat exchangers with beads because of their lower heat transfer. The combination of the two heat exchangers results in a promising alternative. First calculations based on the developed calculation model and experiments have shown reliable results. However, before a combined heat exchanger can be built, the high pressure losses must be reduced by other beadshapes.