A fuel-air mixture in a gas furnace containing natural gas below 4% is considered what for combustion?

A fuel-air mixture in a gas furnace containing natural gas below 4% is classified as lean because it refers to a condition where there is an excess of air in relation to fuel. A lean mixture typically has more air than what is necessary for complete combustion of the fuel. This translates into a lower concentration of fuel, as evidenced by the percentage being under 4%.

For combustion, lean mixtures can provide more efficient burning and lower emissions, often resulting in cooler flame temperatures and reduced production of carbon monoxide. In contrast, a rich mixture would involve a higher concentration of fuel and insufficient air, leading to incomplete combustion and potentially harmful emissions. Understanding this balance is crucial in ensuring efficient and safe operation of gas furnaces.