Many people aren’t sure of the best type of firewood to use for their wood-burning or multi-fuel stoves. With so many options to choose from, so many differing points of view and so little information available it can certainly be a confusing landscape.
So, what are the main types of firewood that people use? A recent study conducted by the consumer champions at Which? asked owners of wood burning or multi-fuel stoves what type of fuel they primarily use. The most popular fuel types were:
- Seasoned logs – 69%
- Kiln dried firewood – 14%
- Free wood, collected from their local area – 13%
So, are these consumers using the best and most efficient fuel? Interestingly, the simple answer is ‘no’. Ideally, you should only burn wood with a moisture content of 20% or less. Here’s why:
- It is more efficient, and so the heat output is higher
- Fewer harmful particulates and air pollutants are released than when burning wet wood
- Sooty deposits and smoke are minimised. This is not only important for air pollution but also because the sooty debris that collects in your chimney can actually pose a fire hazard.
Now let’s take a look at how this relates to the most commonly burnt types of firewood:
These are logs that you may have partially dried out yourself or have been sold pre-dried by the manufacturer or supplier. Seasoned logs need to be stored correctly to aid the drying process and typically contain a moisture content of between 25% – 40%. Because of this, they have a heat output of around 3kWh per kg.
Kiln Dried Logs
The kiln effectively bakes the firewood, forcing out the moisture that is naturally present. When sold, this type of firewood contains less than 20% moisture making it efficient to burn as well as minimising the emission of smoke, debris and other pollutants that can arise from wood with a higher moisture content. Kiln dried logs typically produce a heat output in excess of 4.5kWh per kg.
This is usually wood that has been collected in the local area; from back gardens or foraged from open spaces. It could have as much as 90% moisture in it – not to mention a whole host of unwanted creepy crawlies! The resulting heat output is very low, unlikely to be more than 1kWh per kg.
Making the Decision
Undried wood, of any type, does not burn efficiently. Not only will it generate a lower heat output than kiln dried or well-seasoned logs but its combustion produces a range of by-products that can clog your chimney or flue. Burning unseasoned wood causes ‘spitting’ as well as generating large quantities of smoke and soot that will leave a debris behind that can pose a potential fire hazard. In many cases burning this type of fuel can invalidate the warranty or product guarantee on your stove. In addition, burning unseasoned wood creates pollutants that compromise air quality.
Many people choose to burn unseasoned wood or to store seasoned logs rather than purchase premium kiln dried logs as they expect to make some significant cost savings. However, this is not necessarily the case.
As you might expect, it is true to say that wood with a higher moisture content is cheaper than kiln dried firewood. As an average, pre-seasoned logs cost around £125 per cubic metre whereas kiln dried logs cost around about £140 per cubic metre. However, given their greater efficiency and improved heat output kiln dried logs will burn for longer and give out more heat making them overall more economical. Using cheaper seasoned or unseasoned wood is often a false economy.
In summary, our advice when using a log burner, woodburning or multi-fuel stove is always to use kiln dried logs. When compared with seasoned logs and free wood, kiln dried logs offer the following benefits:
- Generates greater heat output – Kiln dried logs typically produce a heat output in excess of 4.5kWh per kg. This is significantly higher than that of seasoned logs (typically around 3kWh per kg), or unseasoned wood (typically less than 1kWh per kg)
- Improved fuel efficiency – In simple terms, kiln dried logs burn hotter and longer than seasoned logs or unseasoned logs. With less moisture contained in the kiln dried wood, less energy is required to burn off residual moisture and therefore more energy converts into heat.
- More economical – Just because wood with a higher moisture content is cheaper than kiln dried firewood doesn’t mean it is easier on the pocket overall. As an average, kiln dried logs cost around about £140 per cubic metre whereas seasoned logs cost around £125 per cubic metre. However, given the greater efficiency and heat output of kiln dried logs it is true to say that they will burn for longer and give out more heat making them overall more economical.
- Reduces stove and flue problems – the reduced moisture content found within kiln dried logs means that they burn easily and evenly. The drying process ensures that the sap will not spit when the logs burn. Black smoke, pollutants, soot and other unpleasant deposits are therefore minimised and they will not cause stove and flue problems. For this reason, many stove manufacturers recommend the use of kiln dried logs and the burning of unseasoned wood can even invalidate your warranty or product guarantee.
- Easy to transport and store – since the weight of a log drops by as much as half when it is properly dried, kiln dried logs are easily portable and will not require a huge amount of strength to load into the fireplace. Kiln dried logs are dry and often supplied in nets, no complicated or messy transportation or storage is needed.
- No insects or fungi – the kiln drying process kills insects and fungi, so there is no risk of introducing unwanted parasites into your home.