When choosing wood for burning there are two significant factors that have an effect on the amount of available heat generated by each log. These two factors are moisture content and wood density.
The most important factor is the moisture content of wood. Wood density has only a comparatively minor impact and for domestic use can be largely discounted. Any water in the timber has to boil away before the wood will burn, and this will reduce the net energy released as useful heat (as opposed to steam up the chimney). If you can get them to light at all, logs that aren’t dry will result in a fire that smoulders and creates lots of tars and smoke. These tars can be corrosive, potentially damaging the lining of the flue and increasing the danger of a chimney fire. Wet logs will tend to blacken glass in stoves even if the stove is designed to keep the glass clean. As a rough guide well seasoned logs generate approximately twice the heat of the equivalent green logs.
Conventionally firewood is felled in the winter with a moisture content of around 50% and must be seasoned down to between 20% and 30% to make it suitable for use as fuel in smaller combustion systems like wood burning stoves. Anything over 30% moisture content is unsuitable for domestic fires in our view.
How to season logs
After felling, timber should ideally be left in an exposed site outside the woods where it can be stacked off the ground on bearers, facing the prevailing wind. Dense hardwoods with smaller cells like oak, beech, sycamore and hornbeam need to be seasoned for two summers and a winter. Conifers and fast growing broadleaves like ash, birch and poplar, with larger cells, can often be seasoned in one spring and a summer depending on the weather.
Stacked logs would benefit from covering during the winter to prevent reabsorption of moisture. Anecdotal evidence suggests that it is particularly important to prevent snow from settling on seasoning timber.
Seasoning for the log market can be speeded up by logging, splitting and covering soon after felling. Storage in a well-ventilated area under cover is ideal for logs.
Are some species better than others?
Some species are better than others. When deciding which wood is best for domestic fires we find the following old poem helpful:-
Logs to burn; logs to burn;
Logs to save the coal a turn.
Here’s a word to make you wise
When you hear the woodman’s cries;
Never heed his usual tale
That he’s splendid logs for sale
But read these lines and really learn
The proper kind of logs to burn.
Oak logs will warm you well,
If they’re old and dry.
Larch logs of pinewoods smell
But the sparks will fly.
Beech logs for Christmas time;
Yew logs heat well;
‘Scotch’ logs it is a crime
For anyone to sell.
Birch logs will burn too fast;
Chestnut scarce at all;
Hawthorn logs are good to last
If cut in the fall.
Holly logs will burn like wax,
You should burn them green;
Elm logs like smouldering flax,
No flame to be seen.
Pear logs and apple logs,
They will scent your room;
Cherry logs across the dogs
Smell like flowers in bloom,
But ash logs all smooth and grey
Burn them green or old,
Buy up all that come your way
They’re worth their weight in gold.
Honor Goodhart, 1926
How to tell a well seasoned log.
There are a number of tell tale signs to look for:
- Radial cracks and bark that comes off easily suggests well-seasoned wood
- Well seasoned wood is much lighter in weight than green wood
- If you own a moisture metre you will find that 15-20% moisture content is optimal, anything approaching 30% or over is too wet to burn efficiently in domestic use.
Acknowledgements
Phil Potter Woodfuel East http://bit.ly/1ExGpgC
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