oil- or gas-fired heating systems, a wood boiler does not produce
its nominal heat output constantly 24 hours a day. Depending on the
type of wood, one loading suffices for 3 to 4 hours maximum output.
However, in practice, one does not heat continuously and at night
or during periods of absence the fire might even be allowed to go
Buffer storage tank capacity
these down-times, heat is supplied by a buffer storage tank. However,
it is essential that the buffer storage tank is recharged whenever
the wood boiler is on. This means that, in addition to providing
the heat for the house, the wood boiler has to recharge the buffer
storage tank. The boiler must therefore be designed to provide 30
% to 50 % more heat than the heat requirement of the house. The
heat requirement of a house is calculated on the basis that there
are maybe 10 extremely cold days each winter when one would have
to heat the house more consciously and for longer than normal. A
house with a heat requirement of 18 kW should therefore be fitted
with a 25 kW boiler unit
following diagram shows non-binding standard values for a detached
house with a room height of approx. 2.5 m.
law (BlmSchV) requires a minimum capacity of 25 litres per kW of
boiler output. However, this is not enough for a pure wood-fired
heating system. We recommend a buffer storage tank capacity of 50
to 70 litres per kW boiler output if the system is based purely
on wood and not combined with an oil- or gas-fired heating.
energy stored in the buffer storage tank not only depends on the
volume, but also on the utilisable temperature difference. In general,
one can assume a maximum buffer temperature of 85°C. A floor heating
system requiring a flow temperature of 35°C therefore produces a
utilisable temperature difference of 50°C. Most radiators require
a higher flow temperature, with the result that the utilisable temperature
difference tends to be lower. Assuming a heat requirement during
8 night hours of 50%, the necessary buffer storage tank energy for
an 18 kW house can be calculated as follows:
kW x 50% x 8 hours = 72 kWh per night
litre of water stores 0.86 kW per degree centigrade. On the basis
of a utilisable temperature difference during night reduction of
50°C, the required buffer capacity can be calculated as follows:
72 kWh : (0.86 kW/°C x 50°C) x 1000 = 1675 litres
loading the fire box before retiring it is possible to reduce the
night phase for example to 6 hours. This would reduce the capacity
requirement to 1250 litres.
storage tank gives added comfort!
A few words about firewood...
is solar energy in solid form. Through the process of burning, wood
is converted back into heat. Wood from different types of trees varies
in calorific value. Hardwoods such as oak or beech have the highest
calorific value by volume. Softwoods like pine or fir have the highest
calorific value by weight.
space required to store softwoods for heating is approx. 25% larger
than the space required for hardwood. When planning the storage
space, it must be taken into account that freshly cut wood has to
dry for 2 years before use.
....and its storage
needs time to dry. Depending on the location, it is air-dry
(20% residual moisture content) after 1 to 2 years. Never store
fresh wood in the cellar, because due to inadequate ventilation
it will not dry properly. Only wood which is already dry may be
stored in a ventilated cellar.
Firewood should be stored ready cut, because
smaller logs dry faster than large trunks. Logs should be stacked
on 2 or 3 cross-timbers allowing ventilation and drying from underneath.
Firewood should be stacked in a well aired, sunny place, protected
against rain (for example under the eaves against a south-facing