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Many of the problems relating to the build up of soot and “tar” in a chimney can be laid at the door of incorrect
wood-burning. Wood has been the prime source of energy provision for many thousands of years and over
this period, the most effective and economical way of burning it has been learnt and passed down through
generations. Countries which do not benefit from reserves of oil, coal or natural gas have traditionally relied
on wood for fuel and to some considerable extent, still do.
It is these countries who have made the most advances with combustion equipment dedicated to the use of
wood, - Japan, Sweden, Canada, Norway, Italy, Denmark - to mention just a few. However, with the
advent of other more easily used fuels, the techniques for correctly burning wood has, in many countries
including significant parts of Europe, been overlooked or forgotten.
Against this background it might be useful to explain what happens when wood is burned and why it is so
important to correctly prepare the wood before it is used. The table overleaf illustrates the normal moisture
content of freshly cut wood by species, which will vary depending on whether the wood is soft or hard. It
should be borne in mind that timber from heavy oily woods will possess additional and unique compounds.
The most noticeable content is water and it is this constituent which creates most problems when the wood is
burned. When water is heated to boiling point, circa 100°C, it changes state to a vapour - watch the spout of a
boiling kettle. In order for the water to turn into water vapour or steam, (and you can't actually see steam), it
has to absorb a very large amount of heat. When the vapour condenses back into water, which it does at
about 60°C, it releases that heat. Most freshly cut timber will contain at least 60% of its weight as water and if
the wood is burned immediately, much of the heat energy content of the timber will be used by the combustion
process to turn that water into vapour or steam, and that goes straight up and out of the chimney! So not only
is most of the heat being wasted, when the vapour cools down to 60°C it condenses, and if it does that on the
flue/chimney walls, it creates a sticky mess, generically called wood tar. The heat released by the
condensing process is also vented up and out of the chimney, so even more heat is wasted. The actual
chemistry is a little more complex than described, as other acidic constituents will also “evaporate” during the
combustion process. These turn to vapour and also condense back to aggressive chemical compounds, but
at different temperatures to those of water and also contribute to the wood tars. Those same wood tars can
also spontaneously ignite within the flue under the right conditions, creating chimney fires of considerable
ferocity. Temperatures in excess of 1500°C have been recorded in such conditions, and considerable
damage can be sustained by the chimney materials; irrespective of what it is constructed from.
Hopefully from this it can be seen that the most important requirement when burning wood is to make sure that
it is as seasoned and dry as possible. Because wood-burning is in many cases a “fashionable” past time, it is
not unusual for a wood burning Stove to be installed on one day, half the local copse or forest to be subjected
to a chain saw massacre the next and then the timber burned on the third day! Admittedly these time scales
have been exaggerated, but hopefully you get the drift.
Problems with the flue, let alone the appliance, are inevitable given this scenario, which is why it is so
important that wood is correctly stacked, stored and seasoned before it is burned. The seasoning period will
depend on the wood. Normally, it should never be less than two years and be preferably at least three. Even
then it will be impossible to get the water content much below 18% by weight, but that is a significant
improvement on the original 60%.
8. Wood as a Fuel
Under these and even correct combustion conditions, it is not uncommon for a chimney
serving a wood-burning appliance to require thorough cleaning as often as 4 or 5 times a
year.
Incidentally, burning seawater driftwood is not a good idea, particularly if burned on an
appliance served with a prefabricated metal chimney or flue liner. Moisture content aside,
the salt decomposes to hydrochloric acid during the combustion process and is very
aggressive to stainless steel.
Hopefully this information will have explained why it is so important that wood is as dry as
possible when it used as a heating fuel. Well seasoned timber will not only provide a much
more efficient heat source, it will create less problems for both the heating appliance and
chimney.
Use of the
SmartBurn Moisture Meter
will hopefully enable you to utilise the most well
seasoned timber for your heating appliance, saving you energy as well as contributing to
less wear and tear on the appliance and chimney.
