The calorific value of gas, also known as its combustion heat, is the energy released in the form of heat when the gas is burned.
Why is this important?
Unlike water, not every cubic metre of gas contains the same amount of energy. To ensure fair billing, your consumption is not based on the raw volume (m³) but on the actual energy supplied (kWh).
The link between the volume recorded by your meter and the energy billed is established using a conversion factor. This factor takes into account the calorific value of the gas, as well as the delivery pressure and temperature specific to your area.
What is its value?
The calorific value is expressed in kilowatt-hours (kWh) or megajoules (MJ) per unit of weight or volume.
Following the switch from "lean" to "rich" gas, Belgium is now exclusively supplied with rich gas. The gross calorific value (GCV) of this gas in Brussels averages between 11.3 and 11.7 kWh/m³.
By comparison, the GCV of the old lean gas fluctuated between 9.5 and 10.8 kWh/m³.
Since gas is a naturally occurring product, these figures are approximate values. The composition of gas varies depending on its source, much like water, iron ore, or coal.
Net vs gross value
The calorific value of a fuel is expressed using two figures: the gross calorific value and the net calorific value.
- Gross Calorific Value (GCV) takes into account the total energy released during combustion.
- Net Calorific Value (NCV) is limited to the heat produced directly by the flame.
To understand the difference, we have to look back at our science lessons: every fuel produces water vapour during combustion.
- This vapour contains energy that can be recovered when it condenses back into water. This is exactly what happens in a condensing boiler.
- In a traditional boiler, however, this energy is lost and escapes through the flue with the combustion gases.
Good to know
Your bill is always calculated based on the Gross Calorific Value (GCV). This is a standardised way to convert the energy content of gas for billing purposes. Consequently, with a condensing boiler, you can recover more heat for the same billed price.
What about other fuels?
In principle, any fuel that can be burned has a calorific value. This value varies significantly per type of fuel. For comparison, it takes approximately 3 kg of dry wood to replace 1 litre of heating oil or 1 m³ of natural gas. When comparing fuel prices, it is therefore well worth taking the calorific value into account.
A few examples
| Energy | Net Calorific Value (NCV) | Gross Calorific Value (GCV) |
|---|---|---|
| Lean natural gas | 9.16 kWh/m³ | 10.17 kWh/m³ |
| Rich natural gas | 10.12 kWh/m³ | 11.46 kWh/m³ |
| Butane | 12.66 kWh/kg | 13.7 kWh/kg |
| Propane | 12.78 kWh/kg | 13.89 kWh/kg |
| Heating oil | 9.96 kWh/l | 10.70 kWh/l |
| Coal (e.g. anthracite 10/20) | 8.72 kWh/kg | 9.27 kWh/kg |
| Wood (20% moisture content) | 3.82 kWh/kg | 4.14 kWh/kg |
Extra insight
Although calorific value is an intrinsic physical property, the figures in this table are averages that may vary in practice depending on the quality of the energy source supplied:
- Natural Gas: The chemical composition varies according to geographical origin (the ratio of methane, ethane, or nitrogen). The network operator therefore adjusts monthly the conversion factor to bill for the energy actually consumed rather than just the volume in m³.
- Solid Fuels (wood, pellets): The calorific value depends on the species of wood, but primarily on the moisture content. Wood that has not been dried properly can lose more than 20% of its effective energy value, as part of the heat is wasted evaporating the water present.
- Liquid Fuels (heating oil, petroleum): These are the most stable, although additives or minor variations in the refining process can slightly affect energy density.