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.
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. Two cautions: this is gross stack efficiency, so it sits below a condensing unit’s AFUE because it ignores the latent heat recovered from water vapor, and it is a snapshot of one operating point. Always chase both a cool stack and modest excess air, and confirm carbon monoxide is within limits with the same analyzer before signing off a tune-up.