When choosing wood for burning, there are two significant factors which have an effect on the amount of heat produced (calorific value or CV) for any volume of fuel.
1. Moisture Content
The moisture content of wood has by far the greatest effect on CV.
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 (also known as green logs) will result in a fire that smoulders, creating large amounts of tar and smoke.
These tars can be corrosive – potentially damaging the the lining of the flue and increasing the chance of a chimney fire.
Wet logs will also tend to blacken the glass in stoves, even those designed to remain clear.
Well-seasoned dry logs have approximately twice the CV of green logs.
You should always take care to burn only dry, seasoned wood, either by buying it dry or by seasoning your own logs.
You can tell well-seasoned dry logs by the radial cracks, easily removed bark and resonance when struck – a properly seasoned log will almost “ring” when you hit it with a hammer!
2. Wood Density
When you buy logs, the seller will often describe them as softwood, hardwood or mixed logs.
Hardwoods, from deciduous broad-leaved species, tend be denser than softwoods from evergreen, coniferous trees and so a tonne of hardwood logs will take up less space than a tonne of softwood logs.
Dense logs will tend to burn longer and mean fewer “top ups” of your wood burning stove.
If you buy wood by volume, you will receive more kilowatt hours (kWh) of heat from a cubic metre of hardwood logs than the same volume of softwood logs (at the same moisture content).