Optimising excess air on gas-fired furnaces: For a gaseous-fuel-fired furnace operating efficiently with complete combustion and minimal losses, the ideal oxygen percentage measured in the flue gas is typically about…

Introduction / Context:
Excess air in combustion ensures completeness but also carries heat up the stack. The flue-gas oxygen reading is a convenient proxy for excess air. The objective is to maintain enough O2 to avoid CO/soot while keeping it low enough to limit sensible stack losses.

Given Data / Assumptions:

• Gaseous fuel (well-mixed, clean-burning) is being fired.
• Modern burners and controls allow reasonably tight excess-air management.
• Target is efficient, safe operation with negligible CO.

Concept / Approach:
For gas-fired furnaces, typical economical flue-gas O2 setpoints often lie in the low single digits. Values below about 2% risk instability or CO formation under disturbances, while values near 5% strike a practical balance between complete combustion and reduced stack loss. Higher O2 levels correspond to more excess air and efficiency penalties.

Step-by-Step Solution:
Consider safety and completeness: maintain some positive O2 margin.Minimize heat loss: avoid unnecessarily high O2 which increases nitrogen ballast and stack loss.Industry practice converges near a few percent O2; hence “less than 5%” is the most appropriate target band.

Verification / Alternative check:
Combustion tuning guides for process heaters and boilers frequently recommend O2 control between about 2% and 4% (dry) for natural gas when conditions allow, aligning with “< 5%”.

Why Other Options Are Wrong:
< 2%: can be risky, leaving little margin before CO appears.< 8% or < 10%: too loose; allows avoidable efficiency loss.~12% or higher: indicates excessive excess air and significant stack loss.

Common Pitfalls:
Setting a fixed O2 value without considering draft, load swings, or burner condition; O2 trim systems can maintain the chosen band dynamically.

Final Answer:
< 5%