Reading the Chimney: The Science of Flue Gas Analysis
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Open the Combustion Efficiency Calculator →Combustion efficiency can be estimated from what leaves the chimney — the temperature and composition of the flue gas — without ever measuring the useful heat directly. This clever shortcut rests on a long tradition of chemically analyzing exhaust gases, reading the story of a fire in the smoke it produces.
The Exhaust Tells a Story
Every fire writes a record in its flue gas. Complete, efficient combustion produces mostly carbon dioxide and water vapor; incomplete or poorly tuned burning leaves carbon monoxide, unburned fuel, and excess air. By analyzing the gas going up the stack, an engineer can infer how well the combustion below is proceeding — diagnosing the fire from its breath.
The Orsat Apparatus
Before electronic analyzers, engineers used the Orsat apparatus — a set of glass vessels holding chemical solutions that each absorbed one component of the flue gas in turn, revealing its percentage by the change in volume. Measuring carbon dioxide, oxygen, and carbon monoxide this way let operators tune a furnace for efficient combustion. It was chemistry as a field diagnostic.
| Reading | Indicates |
|---|---|
| High flue temperature | Heat escaping up the stack |
| Low CO2 | Too much excess air |
| Carbon monoxide | Incomplete combustion |
Heat Up the Stack
The single biggest loss in most combustion is the heat carried away by hot flue gas. The hotter the exhaust relative to the incoming air, the more energy is escaping unused. This is why a flue-gas method keys on the temperature difference between exhaust and ambient air — it directly measures the largest avenue of loss, scaled by how much carbon dioxide the burning produced.
The Excess Air Balance
Efficiency also hinges on air. Too little air and fuel burns incompletely, wasting energy and producing dangerous carbon monoxide; too much air and the surplus is heated uselessly and swept up the chimney, cooling the fire and carrying heat away. The carbon dioxide percentage in the flue gas reveals this balance, which is why it anchors the efficiency estimate.
Estimating Combustion Efficiency
To estimate efficiency from flue-gas readings, use the Combustion Efficiency Calculator. Compare it with the direct method in the Boiler Efficiency Calculator, and quantify total losses with the Heat Loss Calculator.
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