White smoke in the flue from a gas-fired burner indicates the fuel-air mixture is too lean.

• A. True
• B. False
• C. Not related
• D. Cannot determine

Answer: (A)

White smoke in the flue signals improper combustion related to the air-fuel balance. When the mixture is too lean, there’s excess air and the flame runs cooler. That lower flame temperature means the water vapor produced during burning is more likely to condense in the cooler flue, showing up as visible white smoke. So the presence of white smoke points to a lean (too much air) condition rather than a rich one. If the burn were too rich, you’d expect signs like soot or a yellowish flame rather than white steam.