When choosing wood for burning there are two significant factors which have an effect on the calorific value (CV) or the amount of available heat per unit (volume) of fuel:
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 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. Well seasoned logs can have approximately twice the CV of green logs.
You should always take care to burn only dried (seasoned) wood, either by buying it dry, or by buying green logs and drying them yourself. Radial cracks and bark that comes off easily suggests well-seasoned wood.
The moisture content of a piece of wood is a measure of the relative weight of water and weight of solid wood. The effect of drying on Calorific Value
A fresh green log of about average moisture content has only around half the energy content of an equivalent, well seasoned log. While the type of tree the log comes from can have some impact on the calorific value, it is usually extremely small. Most of the variation in calorific value between species is due to natural differences between moisture content in fresh cut logs (rhose.g. Ash has a particularly low moisture content when green).
There are a myriad of wood types to choose from, all of which have their own burning qualities and properties and although there are references to burning green wood in this guide, we would stress that for the most efficient and effective burn in your wood burning stove only very dry wood should be used. We have listed below a brief but by no means comprehensive guide.
In addition there are of course the compressed reclaimed 'eco' type of logs and briquettes. Theses tend to burn well and for a decent length of time because they are dense and very dry, however try to choose a product that does not break apart too easily.