In a flue gas analysis showing O2 5%, CO2 12%, CO 0.02%, what does this indicate about combustion?

• A. Moderate excess air with relatively good combustion; CO very low and adjust to optimize efficiency
• B. Excess fuel and poor combustion
• C. Perfect combustion with no action needed
• D. Dangerously high CO

Answer: (A)

In flue gas analysis, the amounts of leftover O2, CO2, and CO tell you about the air–fuel mix and how complete the burn is. Here, there’s about 5% O2 in the flue, which means there’s some excess air in the combustion—enough to ensure complete burning but not so much that you’re wasting heat. The CO is very low (0.02%), so almost all the carbon is being burned to CO2, indicating excellent combustion quality. The CO2 level at 12% aligns with a burn that is reasonably complete but not overly lean; with this combination of moderate excess air and almost no CO, the system is operating efficiently and safely.

So this indicates moderate excess air with relatively good combustion; CO is very low, suggesting you’re close to optimal efficiency, though you could nudge the air level down slightly if you’re aiming for a touch higher efficiency while keeping CO from rising. The other options don’t fit because high CO would point to incomplete combustion, no O2 would suggest perfect stoichiometry, and a combination of excess fuel with poor combustion would show higher CO and likely lower CO2.