Understanding the HHV vs LHV difference: simple definitions and units
HHV: what you count when all the water vapour condenses
HHV, the higher heating value, corresponds to the energy released by combustion including the heat that can be recovered if the water formed in the flue gases is brought back to a liquid state. In plain terms, you add the latent heat of condensation. This is the basis used when you reason in terms of “total condensation”, typically to express the delivered energy of a fuel.
LHV: the basis most commonly used to talk about efficiency in France
LHV, the lower heating value, excludes that condensation heat. In the field, most boiler efficiencies are expressed on an LHV basis. That is why a condensing boiler can show > 100% on LHV without “creating” energy. When you are asked about the HHV vs LHV difference, the key issue is mainly to check which basis is used on the technical datasheet, the quote and the commissioning report.
Heating value, kWh, MJ: speaking the same language on site and on paper
Heating values are generally expressed in kWh/kg, kWh/Nm³ or MJ/kg. Remember the equivalence 1 kWh = 3.6 MJ. On documents, always specify the unit and the basis (HHV or LHV) to avoid distorted comparisons between calculations, gas bills and claimed efficiency. For more detail, see our article on fuel conversion ratios.
The physical HHV/LHV difference: latent heat and combustion flue gases
Where the gap comes from: water vapour in flue gases and latent heat of condensation
The HHV vs LHV difference comes from a simple physical point. Combustion produces CO2 and, above all, water vapour in the flue gases, formed from the hydrogen in the fuel and, depending on the case, its moisture content. HHV includes the energy that can be recovered if this vapour condenses and releases its latent heat. LHV does not include this recovery; it assumes the vapour is exhausted without condensation.
What a condensing boiler recovers, and what it does not create
A condensing boiler recovers part of that latent heat by lowering flue-gas temperature below the dew point, using a heat exchanger and a sufficiently cool heating return. It does not “create” energy. Efficiencies above 100% appear only when efficiency is expressed on an LHV basis, which is common practice in French technical data.
When the gap increases: hydrogen-rich fuels, moisture and operating conditions
The HHV/LHV gap increases with hydrogen-rich fuels (natural gas, LPG) and with moisture content (wood, pellets, biogas). On site, actual recovery also depends on operating conditions. If the return temperature is too high or excess air is significant, condensation becomes marginal: this is exactly what a combustion analysis helps you verify.
2026 table: HHV/LHV ratio for common fuels (benchmarks for your calculations)
| Fuel | HHV/LHV ratio | Site benchmark |
|---|---|---|
| Natural gas | 1.10 to 1.12 | Billing often on HHV, efficiency often on LHV |
| Propane | 1.08 to 1.09 | Stable benchmark, useful for sizing calculations |
| Butane | 1.08 to 1.09 | Close to propane, pay attention to supplier units |
| Heating oil | ≈ 1.06 | Smaller gap, but basis still needs to be stated in your notes |
| Wood (dry) | ≈ 1.08 | Highly sensitive to the actual moisture content of the fuel |
Natural gas: order of magnitude of the ratio and points to watch with supplier indexing
For gas, the HHV vs LHV difference becomes obvious as soon as you compare boiler performance and a bill. Note the basis used in the contract and in the kWh indexing. In the field, keep one reference basis in your statements and convert everything else.
Propane/butane: useful benchmarks for heating and domestic hot water
The ratio is less variable. In quotes and calculation notes, lock down the unit (kg, L, kWh) and the HHV or LHV basis. That is what avoids discrepancies in consumption estimates and supporting documents.
Heating oil and wood (logs/pellets): effect of moisture and cautions when interpreting data
For heating oil, the HHV/LHV gap is moderate. For wood, it is the opposite. As moisture rises, LHV drops. State the moisture assumption and avoid comparing efficiencies or energy costs without specifying the HHV/LHV basis. For more detail, see actual fuel moisture content.
Boiler efficiencies: why you sometimes see more than 100% (with no physical trick)
Efficiency expressed on LHV: the French convention and what it implies
In France, boiler efficiencies are often quoted on an LHV basis. This basis does not count the latent heat of the water vapour contained in the flue gases. A condensing boiler recovers part of it through the heat exchanger. The displayed efficiency can therefore exceed 100% on an LHV basis, without any energy being created. On an HHV basis, it remains mechanically below 100%.
Converting an efficiency from LHV to HHV: method and concrete condensing example
The conversion is straightforward. ηHHV = ηLHV × (LHV/HHV). For natural gas, the LHV/HHV ratio is around 0.90 depending on gas quality.
| LHV (kWh/Nm³) | HHV (kWh/Nm³) | LHV/HHV (1) | Efficiency on LHV (%) | Efficiency on HHV (%) |
|---|---|---|---|---|
| 10.35 | 11.65 | 0.89 | 108 | 96 |
Avoiding the sales mistake: explaining to the customer what “> 100%” means
In quotes, state the expression basis and keep it consistent between offers. If you compare against a gas bill, remember that delivered energy is on an HHV basis. When the discussion turns to the HHV vs LHV difference, translate it immediately into HHV versus LHV. It is a matter of convention and latent heat recovery, not a physical efficiency above 100%. To also frame the contractual side for the customer, you can rely on a clear boiler maintenance contract.
Billing and quotes: securing your HHV/LHV comparisons across documents
Gas bill in 2026: energy billed on HHV (coefficients, conversion to kWh)
In 2026, a gas bill expresses energy in kWh from a volume in m³, corrected and multiplied by a conversion coefficient based on HHV. Record the coefficient (kWh/m³) and its validity period. This is the reference when you cross-check with an audit or a consumption estimate.
| Item | Unit | Where to find it |
|---|---|---|
| Consumed volume | m³ | Meter reading line |
| Conversion coefficient | kWh/m³ | Calculation breakdown |
| Billed energy | kWh | Total energy |
Quotes, manufacturer manuals, audits: spotting whether values are on HHV or LHV
In quotes, manuals and audits, look for the calculation basis. Mentions “on LHV” or “on HHV”, or the notations Hi (LHV) and Hs (HHV). Quoted efficiencies can exceed 100 only on LHV. Without a written basis, the HHV vs LHV difference can trip you up between documents.
- Write the basis in the line item title, not in a footnote.
- Align the units in your work schedule, kWh LHV or kWh HHV, and then keep the same basis everywhere.
Practical case: comparing an estimated consumption (LHV) with an actual bill (HHV)
Audit at 12,000 kWh LHV, bill at 13,300 kWh HHV. Before drawing conclusions, bring everything onto the same basis. For natural gas, an HHV/LHV ratio of around 1.11 gives a quick conversion. Simple conversion: 12,000 × 1.11 = 13,320 kWh HHV. You can then compare properly with the bill.
