Combustion and Flue Gas Analysis
The combustion story
Complete combustion of oil produces carbon dioxide and water vapour. Incomplete combustion produces carbon monoxide, soot and unburnt hydrocarbons. Your job is to get enough excess air for complete combustion without excessive dilution.
Measurements and what they mean
- CO₂ and O₂ are inverse indicators of excess air; high O₂ means too much air, low O₂ with rising CO means too little.
- Flue temperature and ambient temperature give the sensible heat loss.
- CO is the safety-critical reading; even small concentrations matter.
- Smoke number (0–9 scale) indicates sooting; 0–1 is the target for pressure-jet burners.
- Draft/draught at the flue and at the appliance, and the effect of a blocked or leaking flue.
- Efficiency calculation: net or gross efficiency from CO₂/O₂ and flue temperature, using the appliance's stated figures.
Traps in questions
- Treating low CO₂ as good - it usually means too much excess air and wasted heat.
- Confusing smoke number with CO; a clean flame can still produce CO.
- Ignoring ambient temperature when interpreting flue loss.
- Forgetting that draught affects readings and that instruments must be zeroed and calibrated.
Worked example
A pressure-jet boiler reads 9% CO₂, smoke 1, flue 210 °C, ambient 20 °C. Rule of thumb: raise CO₂ toward the manufacturer's figure by reducing excess air, then re-check smoke and CO. Never chase CO₂ at the expense of a rising smoke number or CO - that trades efficiency for safety.