Flue Gas Loss Calculator (Siegert Formula, Boiler Combustion Efficiency)
Heat lost up the chimney as a percentage of the fuel's energy, and the combustion efficiency, from the flue gas temperature, the oxygen reading and the fuel.
Results
The dashed line marks the value you entered.
What this tool does
The Siegert formula is what a combustion analyser uses: the loss grows with how much hotter the flue gas is than the combustion air, and with how much excess air the burner uses, measured through the oxygen left in the flue gas. Cutting the oxygen from 6 % to 3 % can save a visible percentage point on an otherwise unchanged boiler, but too little air produces carbon monoxide and soot, so the burner must be tuned with a CO reading. The result covers the chimney loss only, not heat lost through the boiler casing, and a condensing boiler recovers latent heat that this formula does not credit.
Formula
q_A = (t_flue − t_air) × (A₂ ÷ (21 − O₂) + B) (Siegert; A₂ = 0.66, B = 0.009 gas · A₂ = 0.68, B = 0.007 oil) · λ = 21 ÷ (21 − O₂)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
tf | Flue gas temperature | °C |
ta | Combustion air temperature | °C |
o2 | Oxygen in the flue gas (dry) | % |
fu | Fuel | — |
QA | Flue gas loss | % |
EF | Combustion efficiency (on the lower heating value) | % |
LA | Air ratio λ | — |
EX | Excess air | % |
Worked example
- Flue gas temperature140 °C
- Combustion air temperature20 °C
- Oxygen in the flue gas (dry)4 %
- Fuel1
- Flue gas loss5.74 %
- Combustion efficiency (on the lower heating value)94.26 %
- Air ratio λ1.235
- Excess air23.5 %
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.