8
ABOUT BURNING WOOD
Before starting the boiler, check the following:
• Heating network and boiler are filled with water, pressure at least 0.5 bar
• If there is a smoke pipe damper, make sure that it is open
• e heat supply pump is switched on
• e accumulator or network valves are open
• e combustion air outlet is open
• e relief valve is in unhindered contact with the boiler and is in working order.
Firewood
Fresh firewood contains 80–90% water during the growing season and 70% in winter. Wood
must be allowed to dry before it can be combusted. It is best to fell the trees in winter when
their wood contains less water. It is important to cut firewood to the appropriate length for the
boiler’s combustion chamber. Good firewood should contain no more than 15-25% water. Raw
wood must be cut and split to a suitable size to allow it to dry well and fit in the firebox. If the
wood is from a small tree and not split, strips of bark should be removed lengthways. Large,
damp pieces of wood burn poorly and carbonise slowly. is causes a lot of tar and soot to form
in the boiler and burns inefficiently. You can check the water content of wood by weighing it.
Cut a test piece from freshly-cut wood. It is fit for combustion when its weight has decreased by
about 60%. If the fresh piece of wood weighs 1,000 g, it is fit for combustion when it weighs 400
g.
Burning
Wood is an organic material. Its combustible constituents are carbon and hydrogen. When
wood heats up, these constituents are released and gasify. If the burning process is efficient (with
sufficient primary and secondary air available), the combustion temperature rises sufficiently
high to cause clean burning. Wood also contains some minerals, which result in impurities. In
order to achieve good burning performance and energy production and as few polluting and
sooty emissions as possible, bear the following in mind: Dry firewood. e water must evapo-
rate before the wood is fit for combustion. If the water content is too high, the combustion is
incomplete.
A sufficient supply of air and a sufficiently high temperature in the combustion chamber are
needed. Appropriate air flow is important. If the wood is damp, a lot of heat goes to waste.
Damp weather, a cold chimney pipe and insufficient cross-sectional flue surface area have a ne-
gative impact on the draft. A good supply of oxygen is required in the ignition stage to achieve a
good layer of charcoal and efficient combustion.
