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Technical Information

This information has been taken from the HETAS web site

Selecting Wood For Burning

Why Use Wood?

Wood from sustainable sources is a renewable, environmentally friendly energy resource. It is considered carbon neutral, in that the CO2 released during combustion balances the CO2 that was absorbed during its growth. As conventional energy prices continue to increase, the use of wood becomes more and more attractive.

What Types of Wood are Available to Buy?

Wood can be divided into two major classes, either hardwood or softwood. Hardwoods are typically slow growing deciduous broadleafed trees such as Beech, Ash and Oak. They have typically tightly packed annual growth rings reflecting the fact that they are slow growing. Softwoods are typified as being fast growing evergreens or coniferous species such as Pine, Spruce and Fir. Their annual growth rings are usually bigger indicating faster growth.

Factors to Consider

Measured by weight, hardwoods and softwoods have similar energy contents of around 5.3 kWh/kg (oven dry). However, hardwoods are typically twice as dense as softwoods and so, on a volume basis, you would require half the amount of hardwood to provide the same heat output as softwood. For this reason, hardwoods are preferred for burning. Softwoods are usually easier to light than hardwoods and are often used as kindling. Beech, Ash, Hornbeam and Cherry are considered to be the best hardwoods for stoves with Beech producing arguably the best flame pattern.

The major concern in terms of combustion is however the wood’s moisture content. Freshly harvested wood can have water contents typically greater than 60% depending on the species and the time of year the tree was felled. Burning logs with this amount of water is not good for several reasons. As wood moisture content increases, so its useful energy content decreases, since energy is used up to drive off the excess moisture. At 60% moisture, wood can have an energy content of typically 1.7 kWh/kg, but at 25% moisture this can increase to 4 kWh/kg.

Trying to burn wet wood not only produces excess steam but can also contribute to excessive smoke caused by incomplete combustion. Unburned fuel can result in tar like deposits on the lining of the chimney so contributing to the risk of chimney fires.

Prior to use on an appliance, it is therefore necessary to reduce the wood’s moisture content to acceptable levels using a process known as seasoning. The most effective way to do this is to cut the “as felled” trees into the required lengths for the stove and split these into logs typically 40 to 150mm diameter. These split logs should be stored for a period of time until their moisture content reduces to less than 25%. For natural seasoning, it is preferable to stack the logs under cover in an open sided store to allow a natural ventilation flow across them. The seasoning process can take up to two years or more depending on the tree species, when it was felled, and the drying conditions. It is also possible to buy logs that have had accelerated seasoning by being force dried in a kiln to the required moisture content.

You should note that if you are in a smoke control area, it is illegal to burn wood except on a DEFRA exempted appliance. These appliances are marked † in the HETAS Guide.

Selecting the Right Firewood Supplier

There are various factors which should influence the choice of supplier:

• From an environmental standpoint it is preferable, where possible, to select a local supplier, as this reduces the energy required to transport the fuel.
• Does the wood supply come from a sustainable source?
• Has the wood been seasoned to a low moisture content?
• What type of wood is being supplied – hardwood, softwood or a mixture of both?
• What is the general size of the logs being supplied? – Will they suit your appliance?
• It is preferable to buy logs by volume rather than weight as the influence of moisture content is removed.

The following should be avoided as combustion of these materials can give off unpleasant odours, and can also generate emissions that damage the environment and be harmful to health.

• varnished or plastic-coated wood,
• wood treated with wood preservatives,
• household waste

Call 01759369894 for more information

This information was posted in January 2012