Oct 242012
 
 

Using wet or fresh wood for a fire

In simple terms, the word ‘seasoned’ means ‘dry’ and the term ‘green’ means ‘freshly cut from living tree’.

 

Why is it not a good idea to burn green wood on a fire?

Green wood carries about a third of its weight as water, which on the fire evaporates and helps to carry the heat away up the chimney.  You can often hear a log sizzle or hiss in the fire if it is too green or damp.  

 

How do I know if wood has been seasoned?

It’s essential to know that the wood you are buying is properly seasoned and not still full of moisture. Here are some quick checks to help you tell the difference:

  • Seasoned wood weighs much less than green wood.
  • Seasoned wood looks darker, or grey when compared to freshly cut or “green” wood which is usually more yellow or cream coloured.
  •  Seasoned wood will have drying cracks showing at the flat, cut end.
  • Seasoned wood will have loose bark compared to the bark of green wood which will be tightly attached.
  • When two pieces of seasoned wood are knocked together they make a high, hollow sound whereas green wood makes a more dull sound.
  • Seasoned wood should not hiss when burnt, this is the water in the wood boiling and being forced out as steam. 
 

What to do if you buy wood that has not been seasoned

If you buy logs which have not been seasoned, you should cut them to length (300mm or 10″ to 18″), split to size and stacked. Tree branches and trunks contain thousands of microscopic tubes which carry water from the roots to the leaves, and these tubes can stay full of water for years after the tree has been felled (or pruned). Cutting the wood to shorter lengths opens these tubes to the atmosphere which increases evaporation.

The stack should be completely covered on the top to prevent rain wetting the wood but air must be allowed to reach the sides of the stack. Leave to season for at least 1 year (more if possible). Some logs may take 3 to 4 years to fully season. Bring the fuel into the house a few days before you want to use it to get it as dry as possible. 

All wood will, when supported by lots of kindling and/or a good ember base, will burn unseasoned. However, you may find some woods are not worth using in this state due to the length of ‘hissing’ time (as the water boils from within the wood) before they can burn.

 

Moisture content of firewood

  • by volume wet wood has about 85% of the energy of oven-dry wood
  • by weight wet wood has less than half –  42% – of the energy of oven-dry wood
Whilst seasoning it should preferably be stored under cover in an airy place such as an open sided lean-to. The only way of knowing if your firewood is ready to burn is by knowing the moisture content. Wood should be burned when the moisture content is below 25% – ‘air-dry’. You can tell if a log is dry because the bark will come away easily in the hand and the log will have splits across the grain.  
 
You can also purchase a damp or moisture meter from any hardware store that is suitable for use on wood.   If you buy your own moisture meter you will be sure that your logs are dry enough to burn. Owning a moisture meter will also reduce any ‘misunderstandings’ on the suitability of the logs you buy in: pick a random log, split it and use the moisture meter to measure the moisture content of the wood across the grain.
 
It is important the logs are dry and well seasoned. Burning wet or unseasoned wood is less efficient and can cause harmful build up of deposits in the chimney over a very short time. Thick coatings of creosote or resinous material can cause chimney fires, or prevent the chimney functioning properly. This can allow harmful fumes to escape into the dwelling.
 
 

 Heat values from wood, moisture content of wood, and how long to season firewood

Wood from different trees has different heat values The table below provides a useful comparison. 
               Weight per m3 in kg Gross heat value kW/kg (btu/lb)  
 
Ash        674   4.1   (6,350)   35% moisture green  1 summer season needed
  Beech 690   4.3   (6,700)   45% moisture green   1-2 summers season needed
  Birch   662   4.1   (6,350)   45%  moisture green  1 summer season needed
  Elm      540   3.6   (5,600)   60%  moisture green 2-3 summers season needed
  Oak       770  4.5   (7,000)   50%  moisture green2-3 summers season needed
  Poplar 465   2.6  (4,100)    65%   moisture green 1 summer season needed
Softwoods Pine 410 2.6 (4,100) 60% moisture green 1 summer season needed
 /Fir
 October 24, 2012  post archive, tree Tagged with: , ,
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