Natural gas is currently a hotly debated topic both from an environmental point of view due to the production of CO2 and from an economic point of view. The article is focused on the analysis of selected energy factors when burning natural gas with a gradual increase in the hydrogen content in the fuel mixture. In the article, the authors focused on modelling combustion and the influence of hydrogen concentration on heat flows from flue gas in a thermal aggregate. The aim of the authors is to point out changes combustion in the thermal aggregate using document analysis and calculations. By gradually increasing hydrogen in natural gas, it is possible to prove the effect on heat flows during the exchange of heat from the flue gas to the heated material. According to the results, it can be concluded that increasing the content of methane in natural gas changes the nature of the combustion of the mixture. The combustion temperature increases, heat flow density, on the other hand, the volume of flue gas converted to 1 m3 of fuel decreases. An important factor that positively decreases in this is the CO2 emission factor.