The calorific value of gas is the energy that is released in the form of heat when it is burned. It is also sometimes called the heat of combustion.

The calorific value is expressed in kilowatt-hours (kWh) or megajoules (MJ) per unit of weight or volume.

Wondering what relevance this has for you?  It's simple: this is used to calculate your natural gas bill. In fact, one cubic metre of gas is not always the same. This is why the quantity of heat produced per cubic metre (m3) of gas is taken into account on your energy bill.

What is the calorific value of gas in Brussels?

Since September 2022 only "rich" natural gas, which has a higher calorific value (HCV) between 11.3 and 11.7 kWh/m³, has been distributed in the Brussels-Capital Region. It has replaced what is called "lean" gas, which was previously distributed. This gas comes from the Netherlands, where its extraction is being progressively phased out. Its HCV was between 9.53 and 10.75 kWh/m³.

Why are these values only approximate? It is simply because gas is a natural product. Therefore, its composition varies depending on its origin, as is the case for water, iron ore or coal.

Lower calorific value and higher calorific value: what's the difference?

Two values are used to express a fuel's calorific value: the higher calorific value and the lower calorific value. 

To understand the difference, think back to your science lessons at school: all combustion produces water vapour. This steam contains energy that can be recovered when the steam condenses in the form of water. This is what happens in a condensing boiler. In a classic boiler, however, the energy in the steam is lost and escapes through the flue with the combustion fumes.

• The higher calorific value takes into account the total energy released by combustion, including this latent heat in the steam. This is used to calculate your energy bill (which is why you should have a condensing boiler to make use of all the energy created).
   
• The lower calorific value is limited to the heat released directly by the flame.

What about other fuels?

In principle, anything that burns has a calorific value. This varies significantly from one fuel to another. For example, 1 kg of oil has a much higher calorific value than 1 kg of wood.

When you compare the price of fuels per kilo, it is therefore useful to take into account this calorific value. In fact, as the prices of certain fuels are linked to the international markets, they are not necessarily proportional to their calorific value. In other words, suppose that two quantities of fuel can produce the same quantity of heat, but not necessarily at the same price.

Here are some examples: