Wood Heat - an Essential Guide to Burning Wood
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Wood Heat - an Essential Guide to Burning Wood

The heat energy that is available from wood varies with the type of wood and how well it is seasoned. Hard cash can be saved by choosing the correct wood.

Wood Heat - Burning wood is virtually a carbon neutral form of heating. This is because the incineration of a tree liberates the same amount of carbon dioxide as is absorbed by a growing tree. There is a small carbon imbalance due to haulage and cutting processes.

Depending on the local landscape and meteorological conditions, wood heat can produce air pollution, but modern, ecologic, wood heat burners can significantly reduce particulate production.

The majority of people in modern society are inclined to think of firewood as an unpretentious substance used for chucking on fires. Even those who have wood burners tend to purchase and burn what they are offered. This was not true for our forefathers who had to collect their own wood for fuel and culinary purposes. To them each type of wood had individual burning characteristics, and it was crucial to understand what they were.

Wood that burned quickly needed too much tending, whereas wood that contributed more heat for its weight meant a reduced amount of chopping and hauling. It is extraordinary how timber can vary in these regards. Poplar, for instance, contributes little more than half the quantity of wood heat as the same mass of oak. By being selective in the type of wood you purchase, you can acquire a lot more energy for your money.

Wood needs to be stored before it is ready for burning. Recently cut timber has too much residual moisture for effective combustion. Up to 60% of the mass of a tree can be water. Most of this must be dried out before trying to use it on an open fire or wood burner. When sufficient moisture is removed it is termed seasoned wood. Wood with high residual moisture content is termed green wood.

Wood Heat - The Outcome of Burning Green Wood

There are adverse consequences from burning wood that is not dried to below 25% moisture content.

  • The available quantity of heat energy is less because there are fewer wood fibers per unit weight of wood. Green wood yields approximately 10 MJ/kg of heat energy, whereas seasoned wood yields approximately 16 MJ/kg of heat energy. The figures vary, of course, with the type of wood.
  • A percentage of heat energy must be used to evaporate the water before the wood fibers can start to ignite.
  • The incidence of excessive moisture is inclined to have an extinguishing effect. The resulting incomplete combustion creates creosote and other pollutants.

Wood Heat - How to Recognize Seasoned Wood

  • Wood that is correctly seasoned commonly has blackened ends.
  • Seasoned wood makes a clunk when two portions are rapped with each other.
  • The bark usually strips off without difficulty from seasoned wood.
  • Seasoned wood with a moisture content of less than 25% is easier to ignite, yields more heat, and burns cleanly.

The only way of making sure you have suitably seasoned wood is to purchase the timber well in advance of when you expect to use it. Store the logs in a dry, well ventilated place. An interesting point to note is that once wood is properly seasoned, it will ordinarily not be affected by small amounts of rain.

Wood Heat - Combustion characteristics of some woods

  • Almond is a good burner with high heat output and a nutty smell.
  • Apple is a first-rate burning wood but is a poor starter. It gives off a pleasant smell and is slow to burn, although it must be properly seasoned.
  • Ash is perhaps the best wood for a fire. It has low residual moisture content and can be burnt green if absolutely necessary. However, it gets better when seasoned. It is best after at least 6 months in storage. It is only fair at starting.
  • Beech is almost as good as Ash, but does not burn as well when green. It is a poor starter.
  • Birch provides an agreeable smell and a respectable amount of heat. It burns cleanly although quite rapidly and is good at starting.
  • Blackthorn burns quite slowly but produces a lot of heat without much smoke.
  • Cherry burns quite slowly but is hot. It also gives off a pleasing smell but is a poor starter.
  • Douglas-Fir burns with a trivial flame and low heat.
  • Hawthorn gives off quite a lot of heat without much smoke.
  • Hazel burns fairly cleanly with plenty of heat.
  • Hickory is a reasonable burning wood with a pleasant smell. It tends to sparkle rather than spit and is a fairly good starter.
  • Holly is a good burning wood.
  • Horse Chestnut emits good heat but can spit, especially if it is not well seasoned.
  • Larch has a decent smell with good heat if well-seasoned.
  • Lime is a mediocre burning wood. The high moisture content means it needs to be well seasoned.
  • Maple is a decent burning wood but is a poor starter.
  • Mesquite gives a lot of heat and has a reasonable smell.
  • Oak gives out a lot of total heat but doesn’t give much flame and is a poor starter. It needs a long seasoning period, but can be one of the best woods. The smoke is slightly bitter.
  • Olive burns well with good flames, has good heat content, and smells quite pleasant.
  • Pear is a good burning wood. It gives off a reasonable scent and is slow to burn, but must be properly seasoned.
  • Pine is likely to spit, but burns with a lively flame due to resin high content. Not the best wood to choose for a fire but is an excellent starter. Not a good wood to choose.
  • Poplar creates an acrid smoke and spits a lot.
  • Spruce burns rather swiftly and tends to spits. Not a good wood to choose.
  • Sycamore gives a decent blaze with reasonable heat output.
  • Walnut tends to crackle as it burns. It has huge flames that give out a lot of heat.
  • Willow is not a good burning wood, but is reasonable provided it is seasoned for a long time.

Wood Heat – Conclusion

The energy yield from wood heat varies with the variety of wood and residual moisture content. Wood that is not properly seasoned is difficult to ignite, produces less heat, and more pollutants.

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