Combustion Analysis and Fuel Combustion Fundamentals
What this domain tests
Combustion analysis questions check whether you can read flue gas as chemistry in action: how much air the burner actually received (O2, excess air), how completely the fuel burned (CO, CO2), and what stack temperature says about loss up the flue.
Core rules and numbers
- Methane burns as CH4 + 2O2 -> CO2 + 2H2O. Theoretical air is about 9.5 ft3 per ft3 of natural gas, rounded in the field to 10:1.
- Maximum dry CO2 is about 11.7% for natural gas, 13.7% for propane, and near 15% for No. 2 oil.
- O2 and CO2 move inversely, and CO2 + O2 approximates CO2max for a given fuel.
- Excess air % = O2 / (20.9 - O2) x 100. A 5.0% O2 reading is roughly 31% excess air.
- Air-free CO = measured CO x 20.9 / (20.9 - O2). Only air-free values compare to a standard.
Worked example
The meter shows 2.0 ft3 of gas in 60 seconds at 1,020 Btu/ft3. Input = 2 x 1,020 x 60 = 122,400 Btu/h. The analyzer reads 6.0% O2 and 60 ppm CO. Air-free CO = 60 x 20.9 / 14.9 = about 84 ppm, under the 100 ppm target many commissioning specs use but worth watching. Excess air = 6 / 14.9 = about 40%. Stack 380F minus 70F ambient gives a 310F net stack temperature, where most of the loss lives.
Traps that cost marks
- Quoting diluted CO. Ten ppm in a heavily diluted flue can be worse than 100 ppm in a tight one.
- Comparing CO2 across fuels; 9% CO2 is rich for propane but normal for gas.
- Sampling before steady state or before the blower runs.
- Treating 0 ppm CO as proof the heat exchanger is sound.