What is the best moisture content for logs?
There are three factors which determine how much heat you can get from a log when you burn it. The variety of the wood (and how 'dense' it is) and the efficiency of the stove are two, but these are fairly insignificant when compared to the importance of the third: moisture content. Depending on the variety, a log from a recently felled tree can have as much moisture as 60% – something like a pint of water for every kilo of log. A seasoned log will produce twice as much heat as a freshly cut (aka 'wet' or 'green') log because this water must be boiled away first before the log gives up any real heat. This makes unseasoned logs an incredibly expensive heat source, not to mention the damage that burning them does to our air quality, so it is vital that your logs are always 'dry' and ready to burn.
Virtually every wood burning stove manufacturer recommends only burning wood logs with a moisture content of less than 20%. Somewhere between 10% and 20% is ideal. Once the moisture content gets less than 10% then the wood quickly burns away, sucking in too much air and cooling the flue gases as it does this, which in turn increases unwanted emissions. A little moisture is good for moderating the burn rate.
In 2017, Woodsure, supported by Defra, Hetas and the Stove Industry Alliance, introduced their 'Ready to Burn' quality assurance scheme which guarantees a moisture content of less than 20% for firewood when it carries the 'Ready to Burn' label. Of course you can easily season your own wood, which can take up to two years, but this will save you considerable money on your heating bills. To find out more the Forestry Commission has produced a very informative Guide to Choosing and Drying Logs. Finally, it pays to make the comparatively small investment in a Log Moisture Meter to ensure that you always get it right.
The Stove Yard highly recommend the use of a moisture meter to determine the moisture content of wood before burning.
They can be found on our website here.
Please be aware that when using the moisture meter it is important to force the prongs as deep as possible into the wood, otherwise only an inaccurate surface reading will be obtained.