How this instrument works
When a hydrogen-containing fuel burns, one of its combustion products is water — and that water comes out as vapor at typical exhaust temperatures, carrying away a real chunk of energy as its latent heat of vaporization. Heat of combustion is reported two different ways depending on whether that vaporization energy is counted as recoverable or not: the lower heating value (LHV) assumes the water leaves as vapor and that energy is lost, while the higher heating value (HHV, also called the heat of combustion) assumes the water is fully condensed back to liquid and that latent heat is recovered.
The gap between the two is exactly the latent heat needed to vaporize however much water the reaction actually produces, scaled by how many moles of fuel that water came from: Hc = LHV + Hv × (nH2O/nFuel), where Hv is water's heat of vaporization per unit mass and nH2O/nFuel is the stoichiometric moles of water produced per mole of fuel burned, straight from the fuel's balanced combustion equation.
This distinction matters commercially, not just academically: a standard car engine's exhaust is hot enough that combustion water leaves as vapor, so LHV is the practically relevant figure for engine efficiency calculations, and it's the number most commonly quoted for automotive and gas-turbine fuels. A condensing furnace or boiler, by contrast, is specifically engineered to cool exhaust gases enough to condense that water and reclaim its latent heat, which is exactly why condensing appliances are rated against HHV and can report efficiencies that would look impossibly high if measured against LHV instead.
- Enter the fuel's lower heating value, LHV, in MJ/kg.
- Enter water's heat of vaporization, Hv, in MJ/kg (2.257 MJ/kg is the standard value at 100°C).
- Enter the moles of water formed and the moles of fuel combusted per the fuel's balanced combustion equation.
- Read the calculated higher heat of combustion, Hc, in MJ/kg.
Worked example — converting a lower heating value to HHV
Take a fuel with a lower heating value of 50 MJ/kg — in the same range as methane's real LHV — and a combustion reaction that, for the sake of this worked calculation, produces 5 moles of water for every 2 moles of fuel burned. With water's heat of vaporization at the standard 2.257 MJ/kg reference value: Hc = 50 + 2.257 × (5/2) = 50 + 5.6425 = 55.6425 MJ/kg. The gap between LHV and HHV here, about 5.6 MJ/kg, is entirely the latent heat tied up in vaporizing the water that combustion produces. (Methane's own real balanced equation gives a different water-to-fuel ratio, 2:1 rather than 5:2 — see the FAQ below for how that real ratio is worked out.)
Compare that to hydrogen combustion (2H2 + O2 → 2H2O), where essentially the entire product stream is water: with an LHV around 120 MJ/kg and a 2H2O-to-2H2 ratio (1:1 by mole), Hc = 120 + 2.257 × 1 = 122.257 MJ/kg. Hydrogen's exceptionally high heating value, on either basis, is exactly why it's attractive as a high-energy-density fuel even though storing and handling it presents its own well-known engineering challenges.
Questions
What is the difference between LHV and HHV?
Lower heating value (LHV) assumes water produced by combustion leaves as vapor, so the energy needed to vaporize it is counted as lost. Higher heating value (HHV), also called heat of combustion, assumes that water fully condenses back to liquid and its latent heat of vaporization is recovered as usable energy. HHV is always the larger number, by exactly the latent heat tied up in vaporizing however much water the reaction produces.
Which value should I use — LHV or HHV?
It depends on whether your application actually recovers the water's latent heat. Standard combustion engines, gas turbines, and most industrial furnaces exhaust water as vapor and never recover that energy, so LHV is the practically relevant efficiency figure for them. Condensing boilers and condensing furnaces are specifically designed to cool exhaust enough to condense the water and reclaim that heat, so HHV is the appropriate basis for rating their efficiency.
Why does hydrogen have such a high heat of combustion?
Hydrogen combustion produces almost nothing but water — there's no carbon to form CO2, so essentially all the fuel's mass converts into the exceptionally energy-dense O-H bonds of water, and hydrogen's low atomic mass means a given mass of fuel represents an unusually large number of moles reacting. Both effects push hydrogen's heat of combustion, on a per-kilogram basis, well above that of hydrocarbon fuels like methane or propane, even though hydrocarbons release more total energy per mole burned.
Where does the stoichiometric ratio of water to fuel come from?
It comes directly from the fuel's balanced combustion equation — the mole ratio between water (as a product) and fuel (as a reactant) once the equation is balanced for both mass and charge. For methane (CH4 + 2O2 → CO2 + 2H2O), that ratio is 2 moles of water per mole of fuel; for propane (C3H8 + 5O2 → 3CO2 + 4H2O), it's 4 moles of water per mole of fuel. Getting this ratio right requires correctly balancing the specific fuel's combustion equation first.
Is 2.257 MJ/kg always the right value for water's heat of vaporization?
That figure is water's heat of vaporization at 100°C and standard atmospheric pressure, which is the standard reference value used in most heat-of-combustion calculations. Water's heat of vaporization does shift somewhat at other temperatures and pressures, but 2.257 MJ/kg is the conventional figure used across LHV-to-HHV conversions unless a specific application calls for a temperature-adjusted value.