Why Can the Thermal Efficiency of a Boiler Exceed 100%?​

1. Understanding Boiler Thermal Efficiency

Thermal efficiency (η) is defined as the useful heat output divided by the energy input, typically expressed as a percentage.
When based on the HHV—which assumes all combustion products, including water vapor, are fully condensed—η cannot exceed 100%.

2. Heating-Value Basis: HHV vs. LHV

2.1 High vs. Low Heating Value

• HHV (Higher Heating Value) includes the latent heat released when water vapor condenses.

• LHV (Lower Heating Value) excludes that latent heat.

Because LHV omits the energy from vapor condensation, dividing the useful heat (which does include latent heat recovery in condensing boilers) by the smaller LHV denominator can yield efficiencies >100%.
By convention in North America, condensing appliances often report LHV-based efficiencies to highlight latent-heat gains, whereas HHV-based ratings cap at 100%.

3. Condensing Boilers and Latent Heat Recovery

3.1 Latent Heat Extraction

Burning hydrogen-rich fuels produces water vapor; condensing it releases roughly 1,000 Btu per pound of water.
Modern condensing boilers leverage a secondary heat exchanger to cool flue gases below the water dew point, capturing this latent heat.

3.2 Design Considerations

• Heat exchangers must resist acidic condensate, so stainless steel or aluminum/silicon alloys are common.

• Return water temperatures must stay below ~55 °C (131 °F) to maintain condensation.

3.3 Efficiency Gains

Condensing technology can boost overall efficiency by 10–12% over non-condensing designs when measured on the same LHV basis.
In practical testing (120 °F return, 140 °F supply), annual fuel utilization efficiencies (AFUE) for condensing boilers often reach 90–95%.

4. Measurement Methods and Ratings

4.1 AFUE and Test Conditions

AFUE ratings are determined under standardized laboratory conditions; deviations in return/supply temperatures affect the degree of condensation and thus reported efficiency.

4.2 System-Level Efficiency

Adding a condensing economizer to reclaim flue-gas heat can improve overall boiler system efficiency by up to 10%.

4.3 Real-World Performance

While laboratory AFUE may top 95%, on-site efficiencies depend on load profiles, cycling losses, and radiation/conduction losses—often yielding lower annual averages.

5. Conclusion

Efficiencies exceeding 100% do not defy physics but reflect the choice of heating-value basis and the exploitation of latent heat in condensing boilers. By understanding HHV vs. LHV conventions, condensate-recovery mechanics, and testing standards, engineers and homeowners can make informed decisions and accurately compare appliance performance.

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