Selecting the right boiler
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Unlike
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
out altogether.
During
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
The 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: 18 kW x 50% x 8 hours = 72 kWh per night One 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 By 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. A larger buffer storage tank gives added comfort! |
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Wood
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.
The 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.
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