A combustion analyzer reading only becomes useful when it is turned into an efficiency number you can act on. This tool applies the Siegert dry-gas-loss method to your flue-gas oxygen or carbon dioxide reading and stack temperature to estimate the appliance’s gross stack efficiency and its excess air, so you can tune a gas boiler or furnace for clean, economical combustion.
How it works
Stack efficiency is one hundred percent minus the dry flue-gas loss, which the Siegert equation derives from temperature and the oxygen reading:
Q_L = A × (stack temp − ambient temp) / (21 − O₂%) [Siegert]
efficiency = 100 − Q_L
excess air = O₂ / (21 − O₂) × 100 [dry basis]The constant A is fuel-specific. When you enter CO₂ instead of O₂, the tool
converts using the fuel’s stoichiometric maximum CO₂, so either analyzer reading
gives a full result. Excess air is computed from the oxygen content on a dry basis.
What good combustion readings look like
For natural gas in a residential boiler or furnace, a well-tuned condition typically shows:
| Parameter | Target range |
|---|---|
| O₂ in flue | 3–5% |
| Excess air | 15–30% |
| Stack temperature | 120–200 °C (250–400 °F) |
| Gross stack efficiency | 82–93% |
| CO in flue (air-free) | Under 100 ppm |
Too little excess air (below 15%) risks incomplete combustion and carbon monoxide production. Too much excess air (above 50%) wastes heat sending unnecessary air up the flue. The sweet spot for natural gas is modest excess air — enough to ensure complete combustion without excessive heat loss.
The two heat losses the Siegert method captures
The Siegert dry-gas-loss formula specifically estimates the sensible heat carried away by the flue gas — the heat needed to raise that gas from ambient to stack temperature. This is the dominant stack loss in a standard (non-condensing) boiler.
It does not capture the latent heat loss from water vapour in the flue gas. When natural gas burns, hydrogen in the fuel oxidises to water vapour, which carries away latent heat as it exits the stack. In a non-condensing boiler this latent heat is lost. In a condensing boiler, the heat exchanger extracts it as the vapour condenses, which is why condensing boilers can achieve AFUE ratings above 90% while their gross stack efficiency during a single measurement might appear similar to a well-tuned non-condensing unit.
Example and notes
Natural gas firing with 4.5% O₂ in the flue, a 180 °C stack, and 20 °C ambient air gives a dry-gas loss of roughly 6.5%, a stack efficiency near 93%, and about 27% excess air — a well-tuned condition for a non-condensing boiler. Lower the stack temperature and you raise efficiency further; raising the O₂ content too high wastes heat on excess air.
Always confirm carbon monoxide is within limits with the same analyzer before signing off a tune-up. A reading that shows excellent efficiency but elevated CO means the burner is out of adjustment in a dangerous direction — incomplete combustion is not just an efficiency issue.