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Heating value
11 Heating value
Split wood will dry out in just one summer
The moisture content of split logs intended for a log
boiler should be below 20% (air dry).
Damp wood with a moisture content of above 20%
creates a moist climate in the fuel chamber. Wood that
is not sufficiently dry can cause the boiler's fuel
chamber to rust through.
One summer is sufficient to dry out split firewood
Unsplit logs stacked as metre-long pieces in the forest
will generally take two summers to dry.
However, it can be done faster. If the timber is felled in
winter (before the end of January) and split immediate-
ly, a moisture content of 15% can be reached by
September through air-drying. Split logs also burn far
better than unsplit logs.
Unsplit logs take two months longer, and thus a
second summer, to dry.
The best approach is to split the wood in lengths of 1
m immediately after felling: split in half for a log
diameter of up to 15 cm, into quarters for a diameter up
to 20 cm, into sixths for a diameter up to 25 cm, and
into eighths for a diameter up to 30 cm.
Store the split wood in a windy place on a dry surface;
ideally stack crosswise on two logs or concrete pipes
and cover only the top. Of course a sunny location is
preferable, but it is more important to find a dry, windy
place. Leave a gap between stacks as well. If you
stack wood along the wall of a house, leave an air gap
of at least 10 cm between the wall and the stack of
wood.
Wood stored outside in winter will absorb moisture
from precipitation. For this reason either move the
wood to a covered location in September or at least
temporarily store it in a warm room for a week before
burning.
During storage the heating value is reduced by
processes similar to decay; depending on how dry
storage conditions are, the loss can be between 1%
and 3% per year. For this reason, only store firewood
for longer than 3 years in exceptional circumstances.
Split the wood and store it in a dry, windy place,
one percent moisture more or less will not make a
big difference
To sum up: the heating value changes by only 2%
between 20% and 10% moisture content. It is
easy to achieve less than 20% – even if you cut the
wood in summer – if you split the wood and store it
outdoors with protection against the rain for a year.
Fig. 11-1: Source: "Rationelle Scheitholzbereitsstellungsver-
fahren" (economical methods for preparing split logs), Report
11, TFZ Straubing, with heating values added by ETA.
Heating value of wood
Split wood is typically sold in stacks by the stere. The
energy content does not only depend on the kind of
wood. The amount of heat contained in a stere also
Summary of Contents for TWIN 20 kW
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