This paper focused on the hydrogen-methane mixtures (H(2)NG) implications on end-user devices such as boilers, so as to provide a useful tool for maintenance operators. In detail, according to current standards, H2NG blends characteristics parameters for boilers combustion efficiency measurement are calculated. Then, the Ostwald chart for each blend is built by the analytical procedure and is plotted. Additionally, the chemical equations as well as the blended gas characteristic values are computed in order to implement them within software library of commercial gas analysers. Indeed, they are commonly based on Ostwald combustion equation for correlating CO, CO2 and O-2 concentrations. The results can be applied to analyze conventional and widespread domestic boilers fueled with H(2)NG addressing all of the technical implications for their performance certification when they are operating also in condensing mode. Air excess and relative humidity are chosen as variables to evaluate their effects on boilers performance. The oxygen concentration ranges from 0% vol. up to 15% vol. The specific heat values drop non linearly down for each mixture, showing a maximum reduction equal to 9.825%, for H(2)NG@30%. A maximum Energy Fraction of Condensation (EFC) enhancement of 0.9% points is noticed for H(2)NG@30% vol. While, the highest EFC gain, 4.8% points, occurred at relative humidity of 90% compared to the assumed reference values for calculations. (C) 2017 Elsevier Ltd. All rights reserved.