Moisture content is a critical parameter for woodfuel. It is vital to the correct operation of boilers and stoves, and for the assessment of calorific value of a load of fuel.

Most woodfuel boilers are designed to operate with fuel of moisture content of a limited range. If you try to use fuel that is outside this range the boiler will either operate inefficiently, leading to increased emissions, or trip out the control system and fail to function at all. It is very important to find out from your boiler or stove manufacturer or installer what is the correct moisture content range you should be obtaining.

Also, if you buy woodfuel by weight, then moisture content is critical to knowing how much you are paying for your energy. This is far less significant if you buy by volume as the water content does not add appreciably to the volume. You still need to be sure that it is compatible with your boiler, however!

• Calorific value as a function of moisture content

This is an Excel spreadsheet tool giving the net calorific value (LHV) of wood (in GJ/t in sheet 1, MWh/t in sheet 2)as a function of moisture content, together with some representative values of density for solid wood, and bulk density for various forms of woodfuel, for typical hardwoods and softwoods.

Please note that values of wood density vary considerably between species, and even between different samples of the same species depending on growth conditions, etc. You can substitute values for specific species from the spreadsheet below, or average values from combinations of woods, into cells E4 (hardwood) and F4 (softwood) of sheet 1 (net CV in GJ) and the appropriate values will be substituted elsewhere in the spreadsheet as required.

Also includes conversion of moisture content quoted on dry basis to wet basis.