Why low moisture content in woodfuel is essential for efficiency

For wood-burning boilers and stoves to operate at their most efficient, the right fuel must be used at all times. Helen Bentley-Fox from Woodsure, looks at the moisture content in different types of woodfuel and what this means for different appliances.
In order for the fuel that is being burned to be as efficient and environmentally friendly as possible, it must be high quality wood with a low moisture content.
Moisture content is one of the key parameters used to classify woodfuel because it directly influences the calorific value, or energy, in the wood. The moisture content of a piece of wood is a measure of the weight of water relative to the weight of solid wood. Water does not burn, so the more water within wood the more difficult it is to burn.
How does moisture content affect burning and efficiency?
Any water in woodfuel has to evaporate before the wood will burn, reducing the net energy released as useful heat (see table below). Any appliance not designed to burn wetter fuel may take longer to heat up and may never reach optimum efficiency.
moisture table
While wood with high moisture content will release a lot of steam, this is not heat energy that will be felt in the property being heated. Logs that aren’t dry when burned will also cause a fire that smoulders and creates lots of tars and smoke. If left unattended, this can cause chimney fires and so the chimney will have to be swept more regularly.
Woodfuel with less than 20% moisture content is over two times more efficient than fuel with over 45% moisture content. As the moisture decreases, the energy within the wood increases. Therefore, fuel with less water embedded can burn for longer and more efficiently, meaning less fuel is required to produce the same amount of heat.
What is the ideal moisture content for an appliance?
The ideal level of moisture content varies depending on what appliance is being used. Moisture content is likely to vary between logs with different size, species, cracks and where they have been stored. Moisture content from freshly felled timber is between 45% and 60% but by naturally seasoning the timber this can be brought down to 25% over a period of 6 to 18 months – dependent on the location within the UK and the storage conditions.
Different appliances can operate with a different level of moisture content dependent on its use – non-domestic boilers like Binder can burn woodfuel with up to 45% moisture, while for wood burning stoves and boilers used in the everyday home, the moisture content of the fuel should be below 20%.

  • Cut firewood to the right length for the stove or boiler before seasoning.
  • To dry logs, store them in a sunny, well-aired space for one or two summers, ensuring fuel is not exposed to rain or snow.
  • Stack wood in early spring for use the following winter.
  • Radial cracks and bark that comes off easily are signs that the wood has been well-seasoned.