Which of these best describes signs of suboptimal combustion in a fuel-fired boiler?

• A. Sooty smoky flame, high CO or CO/HC levels, and excessive stack gas oxygen indicating poor efficiency or misalignment.
• B. Blue flame with low CO indicates good combustion, and a quiet boiler operation.
• C. Clean burn with no CO and CO2 equal to ambient air.
• D. High NOx and low oxygen always indicate perfect combustion.

Answer: (A)