Freshly cut timber is classified as green timber, which doesn't mean it’s green in colour, it means it has a moisture content greater than 60%. All timbers are hygroscopic which means they have the ability to absorb and release water like a sponge. The amount of water timber can hold varies, but it can be as much as 200%.

This moisture is not bad, during life it’s what allows the tree to take up nutrients from the ground. High moisture levels can also make timber more flexible and easier to work with. But from a firewood point of view, high moisture levels pose several environmental and health difficulties.

Why burning wet wood is bad

Burning wood with a high moisture level will produce heavy black smoke which contains coal tar (creosote). This residue builds up in the chimney and flue increasing the risk of a chimney fire. If left unchecked it can also lead to structural damage to the chimney.

Burning wet wood is also much less efficient than dry wood, so you will need more wood to heat your home. And because green timber doesn't burn at such a high temperature it produces more particulate matter. These tiny soot particles eventually find their way into the atmosphere where they contribute to global warming.

So before firewood can be burned it must be dried to remove moisture to a level below 20%. There are several methods of doing this, but kiln drying is by far the fastest and most convenient. But what exactly is kiln-drying and how does it compare with other timber drying methods such as air-drying?

What is kiln-dried wood?

Kiln-dried timber is wood that has been heated in a purpose-built kiln. This heats the timber without burning away fibres, thus persevering more fuel for burning. As the timber heats the excess moisture held within the wood evaporates, thus drying out the wood from the inside.

Kiln drying is typically used to bring the wood's moisture content down to less than 20%, making it suitable for burning. The kiln drying process also helps to remove impurities taken up by the wood during the growth phase, thereby further improving burning efficiency.

Kiln-drying vs air-drying

We have known for centuries that wood needs to be dried before burning, but we have not always used kilns. The traditional method for drying wood is to stack it in the open where air can circulate to draw out moisture. However, this process is incredibly slow, it can take anything from 6-months to a year before the wood can be used.

Kiln drying on the other hand only takes 4-6 days to do the same thing. Kiln drying also helps to remove impurities in the wood. making it even more efficient to burn. The heating process also removes any insects that took up residence in the timber in the forest.

How kiln drying works

After the timber is harvested they are delimbed and taken to the sawmill where they are sorted by species. The trunks are then debarked and grouped by size and moisture level. This grouping process is important because the kiln drying process works more efficiently if all the timber is of equal size and contains similar moisture levels.

Once debarked and grouped the logs are ready for kiln drying. Because the kiln drying process takes between 4-6 days, the prepared logs are usually stored until space is available in the kiln. The timber is then passed through the kiln in batches, the size of the batches depends on the type and size of the kiln.

Several types of kiln are used to dry timber for firewood, but the two most common types are conventional and dehumidification kilns.

Conventional kilns

Conventional kilns consist of a thermally efficient chamber that is heated using a gas-fired boiler. The heat is then fed across the timber using circulating fans where it draws out moisture (a process called evaporation). The excess water vapour is then discharged through a chimney.

This type of kiln is very inefficient however because most of the heat generated is discharged through the chimney. For this reason, most conventional kilns are being phased out and replaced with more efficient dehumidification kilns that recycle heat energy.

Dehumidification kilns

Dehumidification kilns use heat pumps to generate and recycle heat. The process starts by heating the air to a temperature of around 29ºC. This warm air is then fed into the chamber where it is forced over the timber using circulating fans. This causes the water held in the timber to evaporate much like in a conventional kiln.

However, unlike a conventional kiln, this vapour is not discharged, it’s fed into an evaporator coil which cools it down to a temperature of about 15ºC, thus condensing the vapour back into a liquid. Meanwhile, the heat removed during the condensation process is recycled to reheat the water.

The main advantage of this process is that the water gets hotter with each cycle through the pump. The water starts at around 29ºC but can reach as high as 72ºC after a few cycles. This makes dehumidifier kilns much more efficient and environmentally friendly than conventional kilns.

As a result, even though dehumidification kilns are powered by electricity, which is more expensive than gas, they are still cheaper to operate than conventional kilns. When properly sized they can also dry timber as fast as a conventional kiln. As a result, most new kilns installed today are of the dehumidifier variety.

Kiln-dried firewood at CoziLogs

CoziLogs offer premium firewood dried using our own specially designed carbon-neutral dehumidification kiln. This ensures that all firewood meets a minimum of 20% moisture to ensure the most efficient burn rate. And we guarantee that all firewood is sourced from sustainably managed forests.

All CoziLogs firewood is Ash based, which is one of the best timbers for burning. Ash has a naturally low moisture content which makes it more efficient to dry than other woods. While its dense structure ensures a steady burn rate and high burning temperature. Check out our range of kiln-dried Ash firewood.