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One of the questions we get asked most frequently is: "How much fuel will a wood burning boiler, cooker or stove use?"
This is one of the hardest questions to answer, as there are so many variables. To ensure that you get the most out of your wood and your appliance we have put together a handy guide on using wood as a fuel.
Each species of tree burns differently, both in terms of speed and flame height. You always want to choose a species which burns well, however the amount of heat produced is almost entirely determined by the moisture content. It may come as a surprise but 1 KG of almost any wood at 20% moisture will produce within spitting distance of 4KW of heat.
Like any living organism a tree contains a large amount of water when it's alive. Each species and variety of tree has a different standing moisture content. Length of trunk and diameter also alter the green moisture content. Larger trunked trees normally have a higher level of water than smaller trees of the same species and variety.
It would be easy to assume that Willow had the highest moisture content, but at 50% it is much less than Popular which is 66%. Of the common trees Ash has the lowest green moisture level of around 30% (large trees up to 40%). As hardwoods are denser than softwoods they generally have lower green moisture levels.
So why does this matter, after all it's only water? To burn wood should be below 20% moisture, if the wood is above 20% the fire must dry the wood before it can be burnt. This uses up a lot of the heat energy in the wood and hence means you get less warmth where you want it. In addition to the reduced heat, the fire will create more soot and other deposits and result in more chimney cleaning with a shorter life for any flue components. You will notice the glass sooting up quickly as the first sign.
As an example a freshly cut Popular log will produce less than 35% of the heat of a well seasoned Popular log. In other words if your wood is damp you could be wasting 65% of your money, time and wood! Patience is very much a virtue when it comes to logs.
The process you follow and how you store wood will have a large impact on how quickly the wood is ready to be burnt. It is no good felling a tree and then cutting it up on the day you want to burn it, as the bark of the tree is its water proof barrier to the world and it will still be to damp. It normally takes a year for every 2.5 cm of diameter of a just felled trunk to dry, therefore it's important to process the wood to speed up seasoning. To season well and reduce time you will need to expose, within reason as much of the wood core / grain to the air as you can.
You must take appropriate safety measures when performing any of the steps below. The list below is purely a guide to the steps, not what actions and safety precautions you will need to take for each step.
- Fell the tree
- Cut trunk and boughs (minimum diameter of 10 cm) into burnable lengths or rounds
- Split rounds length ways, rough guide:
- 10 cm diameter log into 2
- 15 - 20 cm diameter log into 420 -30 cm diameter log into 6
- Decide where to store:
- The logs must be off the ground, or on a dry floor, such as concrete.
- Covered from rain
- Have good airflow
- Ideally be warmed by the sun
- Stack logs in the store, all in the same direction
To check the logs are seasoned enough take one from the center of the stack. Split it in half and ideally check it with a moisture meter to see moisture is below 20%. In the UK it is hard to air dry wood to much below 18% moisture, with normal weather. If you don't have a meter, a good guide is if the wood feels cold to the touch, then it is probably still too damp. Many people only test or sample the outside of a log, but if you check the centre, you can be sure you'll get the most out of your wood. You should leave almost any logs to season for six months, ideally over the summer, however some types need more time. Even Ash, which has a low green moisture content, will benefit massively from a few months to dry out.
As we mentioned earlier in the guide the heat output of wood at 20% moisture is 4KW per KG for almost any species of wood. However when you buy wood you will nearly always buy it by volume, so the species is important. You should ideally be buying hard wood, Ash, Beech, Birch, Oak etc are examples which will give you the largest amount of KW per volume, as hardwoods are much denser than softwood. Hornbeam is the hardwood which has the highest KW / volume of 3221KW per cubic metre. So if you are buying a cubic metre of Hornbeam for the same price as a cubic metre of Oak you will get more heat per £.
You want to make sure the wood has been well seasoned, i.e. it is dry. It may be worth checking the moisture of your wood when it's delivered. There are only a few hardwoods to avoid, the most notable one is Chestnut. It is a wood which burns slowly, with a small flame and hence will not produce the amount of heat you need quickly enough.
Which type of wood burns the best? Ash is the top of most lists. This is because it's a good all round wood, however most manufacturers of stoves use beech for testing when they send the product to a lab.
Recommended: Apple, Ash, Beech, Birch, Hawthorn, Holly, Hornbeam, Oak, Yew
You could also use: Cherry, Douglas Fir, Elm, Larch, Walnut
Try to avoid: Chestnut, Leylandii, Poplar, Willow
Exotics such as the Sequoias and Monkey Puzzle are generally not worth the effort. Some woods are too light, and some are too resinous to burn cleanly.
Below is a table which has the heat output of most common species of wood in the UK, with some interesting ones added in. This is based on wood at 20% moisture, the moisture content is key to all values.
|Tree||LB / cord||MBTU / cord||KG/ cord||KG / M3||KW / cord||KW / KG||KW / CU M|
The concise Piping Hot Guide to getting the best out of your wood fuel:
- Make sure your wood is truly dry
- Try and burn Hardwood
- Be patient or...
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