8 | Fuel material/-quantity
EN
Operating manual Heidi Back 2.0
20
8
Fuel material/-quantity
8.1
Fuel
Wood
Many of our native types of wood can, after being appropriately dried, be burnt CO2-neutral in heat-
producing appliances. Wood stands out because it grows again, can be sustainably harvested, re-
quires little or no extra external energy to be invested in the fuel, and has a short journey from the pro-
ducer to the consumer, which provides an optimal climate balance.
The most common types of wood to use in the stove are beech, yoke elm, birch, larch, spruce and
pine. The most important criterion for burning in a heat-producing appliance is that the fuel has been
sufficiently dried through. For this purpose the prepared pieces of log should be stored for at least two
years. The legislator stipulates a maximum water content of 20%. That is still however a lot of water sit-
ting in the capillaries of the wood. A piece of firewood only becomes good when its water content
comes to a maximum of 12 to 13%. The differences in combustion behaviour are striking and clearly
detectable even by the layman.
Types of wood other than the ones referred to above are also suitable for burning. However each heat-
producing appliance should be checked for how their use affects combustion behaviour. Oak for ex-
ample burns with a rather shorter flame and more embers. Oakwood catches fire with greater diffi-
culty, and due to hardness of the wood it is not split as well (small) as other types of wood, which again
has a negative effect on combustion behaviour. It is however a good energy source, and, correctly pro-
cessed, also suitable as firewood.
Softwoods (fir, pine, spruce) should if possible be mixed with hardwood. Softwoods have a higher pro-
portion of tannin, which leads to more deposits in the heat-producing appliance, the connecting pipes
and the chimney. If only softwood is burnt, this can lead more quickly to a build-up of shining soot.
Mixing with hardwood (for example, beechwood) reduces this effect.
The size of your heat-producing appliance's combustion chamber will dictate whether your firewood is
25cm, 33cm or 50cm in length. 50cm pieces of log should preferably dry for somewhat longer (lain
down for three years or more), because wood is dried out mainly by the leakage of water from the ca-
pillaries - and that simply takes time. And the path from inside to outside in a 50cm-long piece of log is
twice as long as in a 25cm-long piece of log. The important thing is that the wood is well split. This
means that the circumference of a piece of log (once all sides are measured around the cutting point)
must be a maximum of 15-25cm. Pieces of wood cut to this size dry more easily - above all, however,
they give the fire (the temperature) a greater attack area and thus make it easier to evolve gas and thus
to burn. Also the quantity of fuel can be better measured out. Thanks to their outstanding levels of effi-
ciency, modern heat-producing appliances require just a fraction of the amount of wood of earlier
stoves. Here smaller pieces of log can be placed according to the manufacturer’s specifications - large
pieces of log often exceed the maximum specified quantity with just a single piece.
Not everyone has a hygrometer at home for determining the water content in wood, particularly when
good reliable units cost several hundred euros. However you can make a good estimate of how suit-
able your fuel is by carefully observing combustion behaviour. A “good” wood fire will only smoke
briefly in the ignition phase, hardly at all after that. It will burn with a light (yellow to orange-coloured)
long flame. Little residue will occur in the combustion chamber, most of it disappearing again when
the fire is fully developed. Another good criterion for assessing whether a piece of log is suitable for
combustion is its weight. Most manufacturers also specify the amount to be laid in kg/weight. Ex-
ample: a piece of beechwood 33cm in length with a circumference of approx. 20cm weighs approx.
1kg when it is suitable (dry enough) for burning. Therefore 25cm in length gives approx. 750
grammes, and 50 cm in length approx. 1.5kg.
