Boiler Efficiency Calculator (ASME PTC 4)
Boiler efficiency is calculated two ways per ASME PTC 4. 1) Direct (input-output) method: η = (m_steam × Δh) / (m_fuel × HHV) where m_steam is steam mass flow, Δh is steam enthalpy...
Formula
Source: ASME PTC 4 (Fired Steam Generators); EN 12953-11 (Shell Boilers); ASHRAE 90.1 | Last reviewed: July 3, 2026
Examples
50000 steam_flow
= 0.83 efficiency
- steam_pressure = 150
- feedwater_temp = 220
- fuel_type = natural_gas
- o2_percent = 3
- flue_temp = 350
- ambient_temp = 70
- co_ppm = 50
50000 lb/hr 150 psig boiler, 3% O2, 350°F flue → 83% efficiency
200000 steam_flow
= 0.88 efficiency
- steam_pressure = 600
- feedwater_temp = 280
- fuel_type = natural_gas
- o2_percent = 1.5
- flue_temp = 280
- ambient_temp = 70
- co_ppm = 20
200000 lb/hr 600 psig watertube, 1.5% O2, 280°F flue → 88% efficiency
Quick Reference Table
| Loss Component | Natural Gas | #2 Oil | Coal (Bituminous) |
|---|---|---|---|
| Dry flue gas sensible heat | 8-12% | 10-14% | 5-8% |
| Moisture from combustion | 10-15% | 8-12% | 2-4% |
| Moisture in fuel | — | 0-1% | 2-5% |
| Unburned CO (incomplete combustion) | 0.1-0.5% | 0.2-1% | 0.5-2% |
| Unburned hydrocarbons | <0.1% | 0.1-0.3% | 0.5-1% |
| Radiation & convection (surface loss) | 0.5-2% | 0.5-2% | 1-3% |
| Unaccounted / other | 0.5-1.5% | 0.5-1.5% | 1-2% |
| TOTAL LOSSES (typical) | 20-30% | 20-30% | 15-25% |
| EFFICIENCY (typical) | 70-80% | 70-80% | 75-85% |
Where is this used?
Used for: 1) Field efficiency audits (indirect method, ASME PTC 4).
2) Manufacturer spec verification (direct method, ASME PTC 4).
3) Operational benchmarking (compare across units).
4) Energy cost analysis (lower efficiency = higher fuel cost).
5) Emissions calculations (CO2, NOx, SO2 are functions of fuel consumption).
Typical efficiencies: 70-82% for old cast iron hot water boilers, 80-86% for firetube steam boilers, 85-90% for watertube boilers, 92-98% for condensing boilers, 80-85% for HRSG (with gas turbine).
Each 1% efficiency improvement on a 100,000 lb/hr boiler saves ~$50,000-$200,000/year depending on fuel cost.
Real-World Usage Scenarios
Boiler Efficiency Audit at 100,000 lb/hr Plant
A 100,000 lb/hr, 250 psig firetube boiler is audited per ASME PTC 4. Flue gas analysis: O2 = 4%, CO = 100 ppm, flue temp = 380°F, ambient = 70°F. Indirect method: Dry flue loss = 0.068 × (380-70) × %CO2_corr ≈ 7.5%. H2O loss (from combustion of natural gas, 11% H2O by volume in products) = 0.094 × 0.11 × (380-70) ≈ 3.2%. CO loss = 0.001 × 100 × 10,200 / 100 = 0.02%. Surface loss (estimate from boiler size) = 1.5%. Total losses = 7.5 + 3.2 + 0.02 + 1.5 = 12.2%. Efficiency = 100 - 12.2 = 87.8%. The plant engineer notes the O2 is 4% (1-2% optimal), indicating excess air should be reduced — at 2% O2, dry flue loss would be ~6%, saving ~1.5% efficiency = $80,000/year at $8/MMBTU gas.
Common Mistakes to Avoid
Using LHV instead of HHV for natural gas
The latent heat of water vapor in the flue gas is significant for natural gas (~10% of HHV). Using LHV (which excludes this) gives an artificially high efficiency (~10% too high). ASME PTC 4 standard is HHV. EU standards often use LHV. Verify which one your spec requires.
Ignoring surface radiation loss
The surface loss (1-2% for a small boiler, 0.5-1% for a large boiler) is sometimes ignored in field audits, leading to overestimated efficiency. ASME PTC 4 requires surface loss estimation based on boiler surface area and temperature.
Industry Standards Referenced
Frequently Asked Questions
What is the difference between combustion efficiency and thermal efficiency?
Combustion efficiency is the indirect method result (% heat input retained in combustion products). Thermal efficiency is the direct method result (heat absorbed by water/steam / heat input). For most boilers, the two are within 1-2% of each other. Combustion efficiency is more practical for field measurements; thermal efficiency is more accurate for manufacturer spec verification.
What is a good boiler efficiency?
Modern condensing boilers: 92-98%. Modern non-condensing boilers: 80-90%. Old cast iron boilers: 65-75%. Per ASHRAE 90.1, the minimum efficiency for new commercial boilers is 80% (firetube) or 84% (watertube). Condensing boilers are 95%+ required in some jurisdictions.
What fuel HHV values should I use?
Natural gas: ~1,030 BTU/scf. Propane: 2,516 BTU/scf. #2 oil: 138,500 BTU/gal. Bituminous coal: 12,500 BTU/lb. Use HHV per ASME PTC 4, not LHV.
Reviewed for accuracy
Cross-referenced against ASME PTC 4 and EN 12953-11 · Last reviewed: July 3, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.