What is the indirect (heat loss) method?

The indirect method calculates efficiency by measuring each category of heat loss and subtracting their sum from 100%. It is more accurate than the direct (input-output) method because it does not require precise fuel flow measurement.

What does CO2% vs O2% tell me?

Both indicate excess air in the flue gas. O2% is easier to measure accurately with modern analyzers. CO2% peaks at stoichiometric combustion and decreases with excess air; O2% increases with excess air. Either value lets the calculator derive the air/fuel ratio.

What is a typical boiler efficiency?

Well-tuned natural gas fire-tube boilers run 80–85% (gross/HHV basis). High-efficiency condensing boilers reach 90–98%. Older coal or oil units may fall below 75%. Efficiency drops with high excess air, high stack temperature, or excessive radiation losses.

What is the dry flue gas loss?

Heat carried away by the hot dry combustion products leaving the stack. It is typically the largest single loss (5–20%) and depends on stack temperature and excess air level.

Why does moisture increase losses?

Water vapor in flue gas (from hydrogen in the fuel and any moisture in the fuel) leaves the stack as steam, carrying latent heat that cannot be recovered in a non-condensing boiler.

How do I reduce radiation and convection losses?

Ensure boiler insulation and casing are intact, repair damaged refractory, and keep the boiler room reasonably warm. These losses are typically 0.5–2% for well-maintained industrial boilers.

Boiler Efficiency Calculator

Name: Boiler Efficiency Calculator
Availability: InStock
Author: Nham Vu

Calculate boiler thermal efficiency using the indirect (heat loss) method — stack losses, moisture losses, radiation, and unburned combustibles.

Boiler Inputs

Fuel Type

Stack (Flue Gas) Temperature

Ambient Air Temperature

Same unit as stack temperature above.

Flue Gas Measurement

O₂ % in dry flue gas (typical 2–5% for natural gas).

Radiation & Convection Loss (%)

Typically 0.5–2% for well-insulated industrial boilers.

Unburned Combustibles Loss (%)

Near 0% for gas/oil; 1–3% for solid fuels. Enter CO ppm measurement or leave 0.

Thermal Efficiency

—

Gross (HHV basis)

0% 50% 100%

Enter values and click Calculate.

Heat Loss Breakdown

Loss Component	% of Input
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Total Losses
Boiler Efficiency

Summary