Wood as a fuel

Wood Burning Stoves: Wood as a Fuel

During its life, a tree takes up moisture from the ground, together with various mineral salts essential for its growth, and this is generally known as sap. During the summer months an oak will lose about 99% of its daily uptake by evaporation through its leaves, and may take up more than 100 gallons of water a day through its roots. Contrary to popular thought however, the bulk of the solid material of a tree is not formed from substances derived from the ground: hard as it is to believe, all those tons of wood are formed primarily “out of thin air” -- or, to be more precise, from carbon dioxide in the air.

Using energy derived from sunlight via the process of photosynthesis, carbon dioxide combines in the presence of chlorophyll with water to form sugars. This food from the leaves then flows in solution through the inner bark, or phloem, and is used for the generation of new growth. New wood or “sapwood” grows in annual rings around the perimeter of the tree, whilst the older wood nearer the centre dies and forms the stronger heartwood.

The amount of sap present very much depends on the time of year. During the spring and summer, the sap rises and the moisture content of the wood is considerably higher than during the dormant winter months. After felling, this moisture within the wood finds it difficult to evaporate away unless the bark is removed. During the life of the tree, the bark acts as a very efficient barrier, not only keeping disease organisms and rainwater out, but conserving moisture by preventing evaporation: and unless the bark is removed after felling, it will continue to slow down the now-desirable loss of moisture.

Different species of tree naturally have different levels of moisture content -- Ash, for instance, has one of the lowest natural levels, whilst species such as Willow and Poplar, both of which thrive in wet ground next to streams and springs, have a high moisture content.

Most firewood offered for sale originates as a by-product of some other operation -- forestry thinnings and “lop and top”, road widening schemes, building site clearance, and some from private gardens. Because of the state of the ground in winter and also because of nesting birds in the spring, most felling takes place during the summer and autumn periods, i.e. after the sap has risen.

Typically, such newly-felled timber will have 50%-70% moisture content by weight. The aim is to reduce that level of moisture content down to the level found in the atmosphere, about 15%-20% during the summer months. Without putting the timber in a drying kiln, it is not possible to maintain a lower content than this since if the wood is drier than the atmosphere, it will simply absorb moisture until equilibrium is reached.

This process of reducing the moisture content of wood is termed “seasoning”: and the best method to accomplish this is to facilitate as much air circulation and warmth to reach the woodstack as possible. For this reason, wood to be seasoned should not be stacked or thrown into the garage or a closed shed: all that will happen is that large growths of mould and fungi will thrive on the damp, decaying wood in the cool, still conditions, and after several months the wood will still be too damp to burn. Similarly, wood stacked or thrown in a heap next to the shaded and cool north side of a building, or hidden out of sight under the rhododendrons where the sun rarely reaches, the rain drips and the humidity levels are high after every downpour, is very unlikely to produce the desired result.

The best place for your woodstack is, so far as is possible, right out in the open. A southerly aspect is best, perhaps along a paddock fenceline, or stacked against an outside wall. A simple lean-to type of roof structure could be put up with little effort or expense to keep the worst of the rain from soaking down through the top of the stack, or a tarpaulin sheet could be stretched and weighted down along the top. But remember that rain water is not the main problem, at least during the summer, since that will dry off with a day or two of hot weather. Rather, it is the sap level within the wood itself which is the problem and which must be reduced to an acceptable level.

Thus it becomes evident that successful wood burning depends to a large extent on a regular cycle being established. Wood needs to be felled, or bought in, by June at the very latest for use the following winter: and this should really be regarded as an absolute minimum. Many species, such as oak, beech and elm have a dense grain, and unless cut and split into small pieces, will have scarcely seasoned sufficiently by October / November when the cold weather sets in. A better cycle to aim at -- and it may take a year or two before you can get organised enough to store the extra volume of timber -- is to allow two summers for seasoning, laying in a fresh stock of wood for use 18 months later during the following winter. You will certainly notice the difference both in ease of lighting and in the amount of useful heat which is produced by your stove.

Wood should ideally be stacked with a lot of air spaces between the logs. If you have acquired cordwood (long lengths of timber such as branches and thinner trunks), it should be cut up into stove-length pieces before being stacked for seasoning. Any logs more than about 4 inches (100mm) in diameter should be split in half lengthwise to expose the grain and allow the wood to dry off over a large surface area. Larger “rounds” of wood should similarly be split into a number of smaller chunks. As explained above, the bark is very efficient at preventing moisture loss both from the living tree and the felled timber: an unsplit log can therefore only dry out from its exposed end grain, and will consequently take much longer to lose its moisture if stacked unsplit.

For those who feel at home with percentages, it may be helpful to consider that a given piece of wood, once properly seasoned, will yield approximately 300% (i.e. three times) more useful heat into your home compared to when the tree was just felled. With such a “return” available on your “investment” in your woodpile, it certainly makes economic sense to try to get into an 18-month cycle as soon as possible. Conversely, it is really very uneconomical to struggle with unseasoned wood, quite apart from all the possible complications which can result from the formation of tar and creosote which will probably result from burning wood with a high moisture content.

Let us suppose for a moment that you try to burn some logs which are either improperly seasoned, or are perhaps soaked with rainwater. Before you can get much useful heat from burning such fuel, all the moisture in the wood must be literally boiled out and turned into steam. And, just like the kettle or cooking pot boiling on the hob, that requires a huge input of energy. Each pound or kilogram of wood only has a finite quantity of latent energy which is released during the combustion process: and a large proportion of that energy is now being literally wasted in converting water (sap) into steam. So if you ever hear a fire hissing and spitting away, and can see the whitish sap boiling out of the ends of the logs, you will now know what is going on!

Because of this tremendous variability in the potential heat available from different species and samples of timber, it might appear logical that output ratings which are claimed for different stoves and boilers by their respective manufacturers should all be based on the same agreed species, moisture content and rate of wood consumption in kg/hour. Unfortunately no such agreed standard exists, and different manufacturers use their own criteria as a basis for their claimed outputs. We would thus recommend that the maxim “let the buyer beware” (or perhaps better rephrased, “let the buyer educate himself and think logically about claims which he may read”) should be applied.

A well-seasoned log has certain characteristics to look out for. First, it should be noticeably lighter in weight than freshly cut timber. There should be no visible moisture oozing out of the ends of the log, and indeed there should be deep cracks radiating out from the centre caused by the loss of moisture tearing the wood fibres apart. And when two seasoned logs are struck together, a sharp ring should be heard rather than a dull thud which is typical of wood with a high moisture content.

The surface area of a log also comes into the equation. A large log can only ignite on the outside, so it will take longer to burn up but will release heat at a slower rate. Thus larger logs are best when you want a longer but slower burn, but are not much use if you want to get a lot of heat quickly! For that you need smaller pieces of wood with a lot of surface area exposed to the flames. Thus smaller, thinner pieces are best to get the stove up to temperature quickly from cold, whereas larger, denser logs are fine once the room is warm enough and you just need to maintain the temperature. You will also find that in general, softwoods such as pine are better for releasing lots of heat quickly (ie. they act as a “sprint fuel”), whereas dense hardwoods such as oak are best for long but rather slow and unenthusiastic burns.

If you are at all dubious about the condition of your logs, or if you would like to have some point of reference to know what your stove is capable of, try burning a couple of dry used pallets, some old floorboards, or a bag of reconstituted wood-waste fuel which is obtainable from our showroom. You will then have a yardstick by which you can gauge your stove’s performance using your normal wood.