Flue Gas Analysis: Reading O₂, CO₂, CO and Stack Temperature
A flue gas analysis is the fastest health check a boiler can have. Four numbers — oxygen, carbon dioxide, carbon monoxide and stack temperature — tell you whether the fuel is burning completely, how much air is being wasted, and how much heat is leaving up the chimney.
You'll learn
- Know what each flue gas reading means
- Diagnose combustion from O₂ and CO together
- Read stack temperature as a heat transfer indicator
- Use a published efficiency table
Before this: Air–fuel ratio and excess air
The four readings
| Reading | What it shows |
|---|---|
| Oxygen (O₂) | How much air is left over: the measure of excess air |
| Carbon dioxide (CO₂) | The product of complete carbon burning; highest at the right air setting |
| Carbon monoxide (CO) | Incomplete combustion: fuel leaving unburnt |
| Stack temperature | How much heat the flue gas carries away |
Portable analysers take a sample through a probe pushed into the flue duct; fixed oxygen analysers sit permanently in the duct and feed the boiler’s controls. The sampling point should be after the last heat recovery surface and before any point where air can leak in.
Reading the numbers together
- High O₂, low CO: too much air. Trim it.
- Low O₂, high CO: too little air, or poor mixing. Add air or fix the burner or grate.
- High O₂ and high CO together: poor mixing, a damaged burner, or air leaking in after a fuel-rich zone. More air will not help.
- Rising stack temperature at the same load: heat transfer surfaces are fouling — soot on the gas side or scale on the water side.
| O₂ % | ΔT 111 °C | ΔT 167 °C | ΔT 222 °C | ΔT 278 °C | ΔT 333 °C |
|---|---|---|---|---|---|
| 2 | 85.4 | 83.1 | 80.8 | 78.4 | 76.0 |
| 3 | 85.2 | 82.8 | 80.4 | 77.9 | 75.4 |
| 5 | 84.7 | 82.1 | 79.5 | 76.7 | 74.0 |
| 7 | 84.1 | 81.2 | 78.2 | 75.2 | 72.1 |
| 10 | 82.8 | 79.3 | 75.6 | 71.9 | 68.2 |
ΔT is flue gas temperature minus combustion air temperature, converted from the published °F. US DOE Steam Tip Sheet #4.
Read across a row and the stack temperature effect is plain: at 3% O₂, going from 111 to 333 °C above ambient costs 9.8 points of efficiency. Read down a column and excess air costs less but still counts: at 222 °C, 10% O₂ instead of 2% costs 5.2 points.
Getting a reading you can trust
- Wet or dry basis: most analysers report on a dry basis, after the water vapor is removed from the sample. Formulas and tables must use the same basis.
- Stratification: in a large duct, gas composition and temperature vary across the section. Traverse the duct or sample at several points before trusting a single reading.
- Steady load: readings taken while load is swinging tell you about the swing, not the boiler.
- CO limits: very low CO is the target; CO climbing sharply at a given O₂ marks the edge of good combustion, and the best excess air sits a little above it.
- Calibration: oxygen cells drift; fixed analysers need regular checks against a reference, or the trim control trims to a wrong number.
Flue gas analysis is also what the indirect efficiency method and emission compliance both rest on. The same O₂ reading is used to correct measured pollutant concentrations to a reference oxygen level, so an analyser that reads wrongly distorts the efficiency report and the emission report together.
Key takeaways
- O₂ measures excess air; CO measures incomplete burning; CO₂ peaks near the right air; stack temperature measures heat lost.
- Read O₂ and CO together: high O₂ with high CO means mixing problems, not air quantity.
- Rising stack temperature at constant load means fouling.
- Sample after heat recovery, before leaks; mind wet versus dry basis.