In a flue gas analysis, which reading combination best signals good combustion efficiency?

• A. CO near zero and CO2 at a moderate level
• B. High CO and low CO2
• C. High O2 with low CO2
• D. No O2 and no CO

Answer: (B)

When checking combustion efficiency with flue gas, the goal is to have the fuel burn completely so carbon leaves as CO2 rather than as CO. Carbon monoxide is a sign of incomplete combustion, while carbon dioxide shows that the carbon in the fuel is being fully oxidized. So a reading where carbon monoxide is near zero and carbon dioxide is present at a healthy level signals that most of the fuel’s carbon is turning into CO2 and not into CO.

The other scenarios point away from good efficiency. High CO means incomplete combustion still occurring, with oxygen not fully meeting the fuel’s needs. High O2 in the flue gas indicates excess air, which cools the flame and wastes heat, reducing overall efficiency. No oxygen and no CO would imply little or no combustion happening, which is not efficient either.

Thus the combination of very little CO and a reasonable level of CO2 best indicates efficient, complete combustion.