Washington DC 1st Class Boiler Practice Test

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.