Now that is a very good question and certainly one that many people who think they know what they are talking about will quickly say “far too quickly and faster than damper logs”!
What are kiln dried logs?
Firstly, lets define what exactly are ‘kiln dried log? These are logs, normally 23-25cm in length, with a diameter of 5-15cm, and split into wedge shapes and then force dried from a moisture content of around 60% down to 10-20% over a period of 3-5 days normally, although here are Certainly Wood, with our unique kilns, we dry them down to 10-15% in about 40-50 hours. Kiln drying is just a much faster version of natural air drying, which can take anything from 18 months to 3 years to achieve the same result.
Does the term ‘kiln dried logs’ guarantee their quality?
Absolutely not. Since the government introduced its new Ready to Burn legislation in May 2021, it became illegal to sell firewood in volumes less than 2m3 that is not Ready to Burn approved and dried to below 20% moisture content*. As a consequence, more and more logs suppliers are drying their logs with ‘kilns’ but there are different types and also different results! Your only guarantee of quality is to buy from an approved Woodsure Ready to Burn supplier.
How long do kiln dried logs burn on different appliances?
This is where there will be significant differences which is why it really is difficult to answer the key question here – How long do kiln dried logs burn for?
If you are burning the kiln dried firewood on an open fire, which will include outdoor appliances such as firepits, chimeneas, pizza ovens and campfires, where there is no control over the airflow, then the burn time will be faster than less seasoned wood which might have a moisture content of perhaps 25-35%. In this case, “the drier the wood, the faster it burns” would be a fair statement. However, if you are burning on a wood burning stove, and more importantly on a modern Ecodesign stove, then the speed of burn is significantly slower, where the burn efficiency is so much better.
As a comparison of the two different types above, the number of kiln dried logs you will require to produce 4kW of heat over a five-hour period will vary hugely. You will require approx. 16 logs for an open fire compared to 5 logs for a modern Ecodesign stove – That is a massive difference.
How do softwood and hardwood kiln dried logs compare?
Whilst there is no issue with using softwood logs (e.g. pine, spruce, douglas fir) on your appliance the logs are far less dense than hardwood logs. The fact that they reach maturity much faster than hardwood (e.g. oak, ash, beech) already suggests they will be less dense. As a consequence, you will need about twice the amount of softwood kiln dried logs to provide the same amount of energy as hardwood kiln dried logs. To take this even further, and further confuse the issue, some hardwoods will burn slower than others. Silver Birch for example, which is still a hardwood and quite common for imported kiln dried logs, will burn about 20-25% faster than the likes of oak or beech kiln dried firewood.
Finally, to add a further twist, thinly split kiln dried logs will burn much faster than large chunky kiln dried logs, hence they are used for different applications. We would always recommend the thinly split ‘Grill and Chill’ logs for cooking, in particular in pizza ovens as they provide a quick heat, which enables the fire to managed more effectively. Alternatively, once you have a good base to any fire, placing a large chunky log will give a good long burn.
What are the benefits of kiln dried logs?
So, having given several reasons why there is no simple answer to the question – How long do kiln dried logs burn for? there is a very simple answer. Kiln dried logs burn much slower than higher moisture content logs when burnt on a modern Ecodesign stove, and to take that down to a more practical level, my modern Ecodesign stove which is a Charnwood Aire 5, uses about 5-6 standard logs per evening when I use the fire from about 6 p.m to 10 pm. What really amazed me, when I upgraded my stove about 2 years ago from my old stove which was about 12 years old, was the difference it made to the amount of wood I burn. I believe I use about 20% less fuel now than I did before – now that really makes a massive difference on the cost of fuel.
The other major benefits are probably all too well known now, but just to reiterate – Kiln dried logs provide a greater heat output and a cleaner burn. This ensures the glass does not go black and there will be far less build up of soot and ultimately tar in the chimney or flue. You also need to buy less logs for the same heat output.
In summary, these days, you must really only ever consider burning kiln dried logs or firewood that is approved under the Woodsure Ready to Burn accreditation scheme. Not only is it far better for the environment due to lower emissions, but it is just so much more cost effective.
* Presently this only applies to England, but undoubtedly will be the same for Scotland and Wales very soon.