Wood fuel moisture content

Thursday 4th May 2017

Moisture content has the most significant effect on how efficiently the wood will burn and how much heat is produced, as well as for how long heat will be generated. Avoid burning wet wood or green logs as it is by far less efficient and wet logs are difficult to light, because any excess water will need to be boiled off first before any heat is generated. Burning wet wood produces a fire that smoulders and creates a lot of smoke and tars, which can be corrosive and potentially damaging to the lining of the flue and can increase the risk of a chimney fire. As a consequence, chimneys will need to be swept more frequently. Smoke and residue from burning wet wood may also blacken glass in stoves even if the stove is designed to keep the glass clean.

For these reasons, many stove manufacturers often specify 20% moisture content or less. Fresh green logs will have water content of 50%+ depending on the species (where water content is defined as the quantity of water present in relation to the entire mass of fresh wood or biomass t) while well-seasoned logs will have moisture content of less than 25% (usually between 15% and 20%). Here moisture content is defined as the quantity of water present in relation to the mass of oven-dry or well-seasoned wood. The value describes the ratio of water mass to dry mass of dried wood and is commonly used in the wood industry. You can either buy kiln-dried wood or briquettes or season green logs in right conditions for 18+ months. The least moisture content is found in heat logs where it can be as low as 4-5%, making these by far the most efficient fuel in relation to the space and weight, and often, cost per kWh generated (depending on the availability of firewood in your local area). Heat logs can give 3 to 4 times the heating value of green logs and produce heat for a longer period of time. If you do choose to use seasoned wood, please pay attention - radial cracks in logs and bark that comes off easily points to well-seasoned wood.

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