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Burning wood

Burning wood is one of the most environmentally friendly ways of heating individual rooms, or indeed your whole house. Because timber is a carbon neutral fuel, it absorbs as much carbon whilst growing as it releases when burnt. Leaving wood to rot naturally releases as much carbon dioxide as it does when used for burning.

As a fuel source wood comes into the overall classification of 'biomass' - sometimes also referred to as a bio fuel, which basically means its source is organic material. Biomass is further sub-divided into woody biomass and non-woody biomass which include things like animal waste, high energy crops (sugar cane, rape & maize) and some biodegradable bi-products from food processing. Domestic biomass applications are usually woody biomass and range from traditional open fires to ultra-modern pellet-burning boilers and room heaters.

What kind of wood to burn.

In terms of the species of wood to use there is a common misconception that hardwoods i.e., Oak, Ash, Beech etc., are better firewood than softwoods such as Scots Pine or Conifer. In fact, weight for weight, the energy contained in the wood is very similar. The only difference being is that because softwoods are less dense, a greater volume of wood is needed. However this is a relatively small difference and softwoods can in fact be easier to use simply because they dry more quickly than hardwoods.

Some trees make better firewood than others. Broadleaf trees are denser than conifers so provide more heat energy volume for volume.

Properly seasoned ash, oak, beech, birch, sycamore and hornbeam are all good firewood.

Conifers are prone to throw sparks as are sweet chestnut and turkey oak although they can be used very dry in a closed wood-burning stove or boiler.

Alder, willows and poplars are poor firewood due to their high moisture content although they are suitable for biomass production for burning in commercial boilers.

Log size

Provided they are of suitable size to fit in the firebox, feed hopper or grate, log sizing is relatively straightforward. If larger than 10-15 cm diameter they should be split.

Open fires

Apart from the suns rays, open fires are probably the oldest form of heating known to man and are a real pleasure on cold winter evenings. They can make a very attractive focal point to a room, but are the least energy efficient form of wood-based heating. They are not always that practical and need care to ensure they do not present a fire risk. Although attractive, open fires may waste up to 85% of the heat.

Wood-burners can be a great alternative being 3 or 4 times as efficient. They come in a wide range of different shapes, sizes and heat outputs (usually rated in terms of kW).

Wood burners are typically made from either cast iron or steel; cast iron allows more ornate or intricate designs and finishes, but is less tolerant of overheating (or over-firing). By contrast, steel is much more tolerant of the extremes of heat that wood burners can generate which is likely to translate to a longer lifespan, and modern technology can make it an even more efficient heat conductor. Wood burners operate at around 75% efficiency meaning that 75% of the heat generated by burning the wood is radiated out into the room and only 25% is lost up the chimney. By comparison an open fire is likely to operate at around 15-20% efficiency - so to get the same heating effect an open fire will need 4 logs for every one put into a wood burner! A further advantage of wood burning stoves is the less frequent need to have your chimney swept.

Moisture content.

Firewood should always be seasoned before use, usually before supply, preferably for 1-2 years, depending on when it was cut. If a tree is felled in spring or summer it will have a much higher moisture content than if it is felled in autumn or winter.

Small round wood (SRW) should be dried by stacking on bearers in a sunny place, exposed to the prevailing wind. Most moisture loss is through the ends of logs, and these should be open and exposed, but air flow through the stack should be unhindered. Bark may be scored or partially or fully removed to assist drying, and the stack should be covered to prevent direct re-wetting from rain, with either a roof, or waterproof or semi-permeable sheeting.

Logs that are well seasoned and fully air dry (typically around 25% moisture content) have cracks and splits across the grain, and the bark comes off easily.

Firewood is environmentally friendly because using it results in virtually no 'fossil' carbon dioxide being added to our present environment and thus helps minimise the effects of climate change as compared to using gas, oil or coal.

Seasoning and storing logs

Because trees contain a lot of water, freshly cut logs will contain around 50% water and are difficult to burn without some drying or seasoning taking place. Wood felled during one winter should be seasoned over the following summer and burnt the next winter when they should have around 33% moisture. If possible, two years seasoning is best to bring it to 25% moisture content. Some trees contain naturally less water than others. Freshly felled ash for example contains only a 33% moisture content while fresh poplar has a moisture content of sixty six percent.

Coppicing.

Cutting firewood through thinning and coppicing can re-establish traditional woodland management. It is a decline in this type of practice that has led to the loss or decline of some of our most attractive woodland wildlife.

What is coppicing?

Coppicing involves cutting trees down to near ground level. Although this may appear drastic, it has the effect of causing the tree to sprout vigorously the following spring as well as increasing light levels into the woodland. This improves the diversity of the ground flora and creates a habitat that is favourable to many species of butterfly, birds and bats. In prehistoric times, people noticed that certain trees grew again after they had been felled. The new growth consisted of many thin poles, useful because they were easy to harvest and suitable for firewood, woven fencing and house building.

When people began to develop a settled way of life and to keep domestic animals, they were able to keep an area of woodland regularly cut over. This practice of cutting trees to a cycle is known as coppicing. The length of the cycle depends on the species being cut and the size of poles that are required.

coppicing is done on rotation: small areas of a woodland are cut each year in sequence leaving the areas not being cut to grow on for between 15 and 20 years for chestnut, and about 7 years for hazel. When an area of coppice is cut, it is all cut down, and creates a clearing. This periodic coppicing encourages the individual trees to live for up to hundreds of years. If the coppice cycle is managed correctly it can increase biodiversity in the woodland because of the beneficial effects of varying light levels reaching the woodland floor, and the range of different aged trees and stools in the woodland.

The best time to coppice chestnut is well after the autumn leaf fall when the sap has gone down, and certainly well before the sap rises in the spring.

Later cultures had a need for much larger timbers, so within areas of coppice, some trees were left to grow straight, tall and thick- these are called standards. Time-consuming to fell and process, standard trees were nevertheless extremely valuable for use in shipbuilding and the construction of important buildings like churches and barns.

How coppicing declined

When fuels like coal and oil came to be widely used, firewood was no longer valuable. There was less demand for, and less money in all the products of coppicing and it fell into a general decline throughout the country. Many woodlands were left without their traditional management, and the wildlife that thrived with the coppice cycle began also to decline. In recent years, ecologists and conservationists came to see how the loss of coppicing was having a detrimental effect on the diversity of Britain's wildlife, and gradually the practice and the skills have been re-discovered.

Links

www.forestry.gov.uk - www.nationaltrust.co.uk - www.rspb.org - www.woodlandtrust.co.uk - www.walksinkent.co.uk - www.walkinginsussex.co.uk