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As part of The Carbon Cycle, wood only emits as much carbon to the atmosphere as the tree took in when growing, so wood is considered the carbon neutral fuel. But wood needs considerable care in preparation and use for it to burn satisfactorily.
When wood is cut down its cells are full of water. Burning such wet or 'green' wood wastes heat in making steam and produces flammable, acidic tars which will cling to, and rapidly damage, your stove and chimney. See Forestry Commission Information Sheet
SOFTWOOD & HARDWOOD
Hardwoods are typically slow growing deciduous broadleafed trees such as Beech, Ash and Oak. They have tightly packed growth rings showing the fact they are slow growing.
Softwoods are typically fast growing evergreens or coniferous species such as Pine, Spruce and Fir. Their annual growth rings are usually bigger.
There is very little difference between different woods in the amount of heat energy they contain per kilogram, but they vary a lot in their water content and how dense they are. Hardwoods are more dense than softwoods so on a volume basis you need about half the amount of hardwood to produce the same heat output as softwood. (See: Fuel Properties)
When wood is cut down its cells are full of water, typically about 60% of the wood weight is water. A 1kg log of fresh wood will contain about a tea mug's worth of water.
Such wet or 'green' wood does not burn well, but worse, it wastes most of the heat it produces in simply making steam. Worse still, the steam dissolves-out flammable, acidic tars which will cling to, block up, and can very rapidly damage, a stove and chimney.
Almost all problems associated with burning wood are caused by damp fuel. To burn effectively, wood needs to be dried out, 'seasoned', to a maximum of 15 to 20% water.
To burn effectively, wood needs to be dried out, 'seasoned', to a maximum of 15 to 20% water.
The difference is huge. A fresh 1kg log with 60% moisture may be able to give out just under 2kW of heat energy, a 1kg log dried to only 25% roughly doubles the heat per kW to about 4kW.
SEASONING WOOD FOR BURNING
Preparing fuel wood usually follows four phases:
1 FELLING: Timber for fuel should be felled at the point when it contains the least sap. This will normally be in April, or earlier for broadleaved trees.
2 FOREST DRY: Logs are stacked in the forest, in the round, on bearers to prevent contact with the ground, in a sunny and windy place, for the whole summer.
3 SPLIT AND STACK: In autumn, the logs are cut to length, split to about 100mm across and stacked under cover in an airy location for a full year.
4 INDOOR DRY: The next autumn, nearly two years after felling, logs are taken indoors to briefly surface dry before burning
BUYING WOOD FUEL
Because wood contains moisture (bad), there is no point buying it by weight - wet, poor quality, fuel wood weighs more.
Wood is usually sold by volume, either by the Cubic Metre, or by the 'cord', a variable North American term which is commonly 128 ft³ (= c3.6m³).
If the wood is sold as 'seasoned', as being suitable for burning immediately, it should be split and have an average moisture content of no more than 25%.
The cool damp climate of the British Isles means that naturally seasoned wood is not always easy to prepare, artificially kiln dried logs are often a very economical choice.
GOOD WOOD and BAD WOOD
A good, seasoned, wood log:
Will feel dry
Will have a hollow sound when tapped
Will be free from bark, or have bark which falls away easily
Often has cracks in the end where it has dried out
Will usually have been split, not left 'in the round'
Will burn easily and cleanly
An unseasoned wood log:
May feel damp and dense
Will have a dull sound when tapped
May have green or white mould on the surface
May often be round
May have bark firmly attached
May be solid and free from radial cracks
May have leaves attached
Will hiss, spit and smoke when burned
The moisture content of a log can be roughly estimated using a device which measures the electrical conductivity between two prongs pushed into the log. The damper the log, the more readily it conducts electricity.
However, there is little value in measuring the surface of the log. It may be dry on the outside yet very damp inside, or an otherwise perfectly dry log may have gained a little surface moisture from rain etc. A more reasonable indication of the moisture content of a batch of logs is made by cutting across several logs and measuring the moisture in the centre.
Burning with little smoke will be burning with high efficiency.
Light using only firelighters or screwed-up paper with a small quantity of very thin sticks of very dry wood, and the appliance set to supply maximum air to the fuel. Wait until this kindling is burning very vigorously before adding fuel.
Good performance will be achieved by:
Using only absolutely dry wood which has been split lengthways into pieces no more than about 100mm across.
Filling 'little and often'. Most hand-fired wood stoves will perform best if 'fed' with a log about once per hour or two. Filling the firebox very full and setting the air controls very low in order to get a long burn-time will usually lead to the formation of tars and smoke.
Filling with logs criss-cross, so air can circulate between them.
Always having the airwash (overfire) control at least a little open. On many stoves smoke emission will be lowest when the primary (underfire) air control is completely closed.
If the fire has all but died down, prevent smouldering and smoke by reviving it with a few small sticks before adding extra fuel.
Adjusting the air controls to give the laziest possible flame is likely to give the very highest efficiency. A fire blazing away with vigorous yellow flames is likely to be wasting much of its heat.
Keep checking your chimney to see what is happening, and so learn which are the best settings for your fire.
The fine white residue produced when wood partly burns is not ash, but the remains of cell walls which can burn if kept hot enough, so don't de-ash a wood fire until absolutely necessary. Instead, allow a bed of cinders and ash to build up.
Wood is not a smokeless fuel, but it can burn with very little Smoke on some specially-designed appliances.
Very dry joinery waste can be a perfectly acceptable fuel. But it is often made from softwoods, the open texture of which means that they tend to rather bulky and very quickly re-absorb water if stored in damp conditions.
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