What causes an almost totally yellow flame in a gas appliance?

An almost totally yellow flame in a gas appliance is typically caused by an absence of primary air. In gas combustion, air is necessary for the complete burning of the gas fuel to produce a clean and efficient flame. When primary air is insufficient, it leads to incomplete combustion, which can result in a yellow flame due to the production of soot and carbon particles.

In a properly functioning gas appliance, an adequate mix of gas and primary air ensures that the flame burns a bright blue, indicating efficient combustion with minimal pollutants. Conversely, when the primary air supply is cut off or significantly reduced, there is not enough oxygen to support complete combustion; this results in a cooler flame that primarily emits yellow light due to the glowing carbon particles, indicating incomplete combustion.

Other factors like the lack of secondary air, excessive fuel, or insufficient pressure can influence combustion, but the specific issue of an almost totally yellow flame directly points to the inadequate supply of primary air. Understanding the role of primary air helps in diagnosing combustion problems and ensuring gas appliances operate efficiently and safely.