Selecting the right boiler

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Boiler performance

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 following diagram shows non-binding standard values for a detached house with a room height of approx. 2.5 m.

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Buffer storage tank capacity

German 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.

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 85C. A floor heating system requiring a flow temperature of 35C therefore produces a utilisable temperature difference of 50C. 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 50C, the required buffer capacity can be calculated as follows: 72 kWh : (0.86 kW/C x 50C) 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!

A few words about firewood...

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.

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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.

....and its storage

  • Wood 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 wall)

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