🔍 Key Concepts

• Relationship between CO₂ percentage and excess air in boiler combustion • The difference between theoretical maximum CO₂ and the range used for practical, efficient operation • What too low and too high CO₂ readings usually indicate about combustion

💭 Think About

• Think about what a very low CO₂ reading in the flue gas tells you about how much air is going through the furnace—are you wasting heat up the stack? • Consider what happens to CO₂ concentration if you don’t supply enough air for the fuel—what other gas starts to appear and what does that do to efficiency and safety? • Recall roughly where the theoretical maximum CO₂ for fuel oil combustion lies, and then think about how much lower the optimum efficient operating range must be to allow a safe margin of excess air.

✅ Before You Answer

• Make sure the range you choose is not so low that it implies very high excess air and wasted heat. • Make sure the range you choose is not so high that it suggests near-zero excess air and a risk of incomplete combustion or CO production. • Compare each option against a remembered approximate theoretical max CO₂ value for oil firing (around the mid‑teens by volume) and pick the band that leaves a realistic excess‑air margin.