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The moisture content of the wood affects the rate at which it burns and the efficiency of
combustion. When trees are cut, the wood moisture content ranges between 35 and 60 per
cent by weight*. If you attempt to burn wood this wet it will be hard to ignite, slow to
burn and will hiss and sizzle in the firebox. So much energy will be consumed in boiling
off the excess water that the efficiency of combustion and the heat to your home will be
low,
condensation and corrosion may be occurring in the flue and smoke may be causing
problems to your neighbours. Properly seasoned wood ignites readily and burns
efficiently.
Firewood should be cut and split in the early spring and stacked under cover, with
good ventilation, to be ready for burning when required.
Look for checks or cracks in the end grain as a sign of dry wood. The stacks of firewood
should be in an open area so that air can circulate through them. During the summer, as
warm breezes flow through the stacks, carrying away the evaporating water, the moisture
content of the wood will fall to around 20 per cent. At this moisture content the wood is
ready for burning.
Although the energy content of dry wood per kilogram is almost the same regardless of
species, softwoods and hardwoods burn differently because of differences in density.
Softwoods, such as pine, are less dense than hardwoods like gums, Manuka or Black
Wattle. A denser wood will produce a longer-lasting coal bed, while a less dense wood
will bring a fire to an optimum burning temperature more quickly.
The size of the firewood pieces affects the rate of combustion. Larger pieces ignite and
release their energy more slowly than small pieces. Smaller pieces are better for short, hot
fires and larger pieces are preferable for extended firing cycles. In general, commercial
firewood dealers supply firewood in thicker pieces than modern wood-burning heaters
can handle. It is often necessary to split some of the wood again before using it. The
thickest piece size for high-efficiency heaters should not exceed about 150 mm (6 in.)
across the largest dimension and a range of smaller pieces will be needed for effective
stoking. Maximum log length varies according to model.
*CANTERBURY RESIDENTS PLEASE NOTE: WOOD WITH A MOISTURE
CONTENT OF 25% OR LESS IS THE ONLY APPROVED FUEL FOR USE IN
THE CANTERBURY CLEAN AIR ZONE.
1-05-06
