Which statement best describes why a furnace flame might appear yellow instead of blue?

• A. Excess air causing a yellow flame.
• B. The flame color is unaffected by air-fuel ratio.
• C. Incomplete combustion due to improper air-fuel ratio, burner misadjustment, or blockage.
• D. The burner fuel type determines flame color.

Answer: (C)

The main idea is that flame color shows how completely the fuel is burning and whether there’s the right amount of air for that burning. A blue flame indicates efficient, complete combustion with enough oxygen. A yellow flame means incomplete combustion, typically caused by a fuel-rich mixture where not all fuel burns, producing soot particles that glow and give a yellow color.

This happens when the air–fuel ratio is off, the burner is misadjusted, or there’s a blockage. Those issues prevent proper mixing or airflow, leading to incomplete combustion and a yellow, sooty flame. The other statements don’t fit because excess air usually yields a bluer, leaner flame; flame color isn’t unaffected by the air–fuel ratio; and while fuel type can influence flame characteristics, the conditions described (air–fuel balance and burner condition) are what mainly drive the yellow color in this scenario.