As CO2 levels increase in a gas furnace's flue gas, what happens to the O2 levels in the flue gas?

When CO2 levels increase in the flue gas of a gas furnace, it indicates that combustion is occurring effectively with sufficient fuel, and the oxygen (O2) that originally supported the combustion process is being consumed. As fuel burns, it combines with oxygen to produce carbon dioxide and water vapor.

Hence, as more CO2 is formed from the combustion process, this typically reflects a relative depletion of O2 in the flue gas. Therefore, as CO2 levels rise due to the combustion of hydrocarbons, the levels of O2 concurrently decrease, demonstrating the transition of reactants into products during combustion. This relationship is a crucial aspect of understanding combustion efficiency and the performance of gas-fired heating systems.