Moisture content and the density of the wood are the two main things to bear in mind when choosing logs for a wood burning stove. Not only will they affect the amount of heat you get per kilo of fuel, but they will also make a difference to how well the wood burns - whether you get nice flames or tar and smoke.

Moisture content has the most effect. For the wood to burn, any water in it must evaporate. This evaporation process - ie steam going up the chimney - means that less of the energy released will be used as useful heat.

If your logs aren't dry, then the fire will smoulder and create tar and smoke. This in turn can damage the lining of the chimney (or gunk it up), and blacken the glass of the the stove.

So, to maximise the heat you get from the wood, either buy it well seasoned, or make sure that you dry it yourself before burning. Cracks in the wood, and bark that comes off easily, are both ways of telling if it is well-seasoned. Also you can tell by the weight of the log, or by using a moisture meter.

Fresh green logs with a moisture content of around 60% will provide 2,000 kWh of heat per tonne. Air dry logs with moisture content of c.25% will provide twice as much heat.

The other key issue is the density of the wood. Hardwood (which comes from deciduous, broadleaf trees, such as oak or ash) tends to be heavier than softwood (mainly conifers). This means a tonne of hardwood will need less storage space than a tonne of softwood. Hardwood also burns for longer than softwood, so you won't have to top up the stove so often.

However, softwoods are cheaper to buy, and easier to light.

This article is based on information provided by the Biomass Energy Centre

Photo by Scott Campbell

If you have a question about anything in the above blog, please ask it in the comments section below.