HHV/LHV Heating Value

The calorific value of a fuel expresses the energy released by its combustion. A distinction is made between the HHV (higher heating value), which includes the heat of condensation of the water vapor formed, and the LHV (lower heating value), which excludes it (it is assumed that the vapor exits uncondensed in the flue gases). Simply put: HHV ≥ LHV. The difference depends primarily on the hydrogen content of the fuel (the higher the hydrogen content, the more water vapor is produced → a greater difference between HHV and LHV).

PCS vs PCI: definitions, relationships and examples

PCS : total energy if we also recover the heat of vaporization of the water from the flue gases.

LHV : useful energy if this latent heat is not recovered (case of non-condensing boilers).

Practical relationship : LHV = HHV − condensation losses (related to the water contained/produced).

Orders of magnitude (indicative):

Methane : PCS ≈ 55.5 MJ/kg, PCI ≈ 50.0 MJ/kg (≈ –10%).

Propane : PCS ≈ 50.3 MJ/kg, PCI ≈ 46.4 MJ/kg (≈ –8%).

Wood pellets (dry ~8–10 % H₂O): PCS→PCI difference typically -6 to -9%; the wetter the wood, the greater the difference.

Common units : kWh, MJ, kWh/m³ (gas) or kWh/kg (solids/liquids). In France, gas billing is expressed in kWh (often based on the lower heating value), while some appliance efficiencies are indicated based on the lower heating value — a frequent source of confusion.

Advantages, limitations and points to consider (regulations, returns, comparisons)

Interests

• Efficiency: a non-condensing boiler converts energy based on LHV; a condensing boiler recovers latent heat and approaches LHV.
• Fuel comparison: putting all values ​​on the same basis avoids errors in choice (cost/kWh useful, sizing).
• Audits & contracts: clarify the basis (PCS or PCI) for kWh delivered, SLA and CPE.

Boundaries

• Values ​​dependent on humidity and composition: wetter wood lowers the useful LHV.
• Efficiencies “>100%” (on LHV) of condensing boilers: no magic, it’s a unit effect.
• Hasty conversions: applying a fixed coefficient may be wrong if fuel quality varies.

Points to consider

• Always state the basis: "kWh PCS" or "kWh PCI" on quotes, technical data sheets, reports.
• Condensation: to benefit from it, aim for cold returns (soft water law, sufficient ΔT, low temperature emitters).
• Biomass: require the contractual water content (e.g. ENplus A1 pellets), otherwise the LHV drops.
• Gas/propane: check the efficiency reference (LHV vs HHV) before comparing boilers.
• Cogeneration / heat recovery: if the installation actually condenses the fumes, consider the PCS for the recoverable energy.

Anecdote — "The percentage that cost Saint-Étienne dearly"

In Saint-Étienne, a building manager was comparing two offers: one for a condensing boiler advertised at 109% efficiency and another at 97%. Panic ensued… until clarification was provided: the first was based on the lower heating value (LHV), the second on the higher heating value (HHV). After conversion to the same basis, the two units were actually very similar. The choice then came down to hydraulic criteria (possibility of colder return temperatures), building management system (BMS), and maintenance, not on a percentage inflated by the unit price.