To calculate the output requirement at a certain outdoor temperature is easiest if you know the energy needs of over one year, for example, via a utility bill or oil consumption, etc. This is a good aid when this task includes the property's actual consumption that includes both housing needs as well as the family's hot water habits, ventilation and the like. Important to remember when doing these calculations is that there is a net energy can be calculated. It means that you also have to take into account the existing plant efficiency in case it started from a known consumption. An old and poor oil boiler can indicate an excessive power demand if you just take into account oil consumption.
If one assumes that a villa with an oil furnace that is 20 to 25 years can be expected that the system effectively garden is rarely better than 65 - 70%. One cubic meter of oil contains about 10 000 kWh. This means that oil consumption of 2.6 m3, representing a net energy of 26 000 kWh x 0.70 = 18,200 kWh / year.
On an electricity consumption, it is common that even household electricity included. For a normal family usually calculate household electricity between 5-6000 kWh / year. This must therefore be subtracted from the total. If you make the 25 000 kWh, the net demand for heating hot water 25 000 kWh - 5000 kWh = 20,000 kWh / year. This applies if you have direct electric heating. If you have a water boiler and radiators can instead assume that the system efficiency over the year is around 95%, resulting in a net requirement of 20,000 kWh x 0.95 = 18 000 kWh / year.
A good wood quality contains about 1.4 kWh per liter trotting dimensions. A reasonable system efficiency for a modern and efficient wood heating systems without culverts can be up to 80%. This means that 0.80 x 1.4 kWh = 1.12 kWh / liter firewood useful done to the building. The energy needs of 18 000 kWh will thus need about 18 / 1.12 = 16 cubic meters of stacked wood on an annual basis. So more than 1 KWh per liter of firewood!
The choice of the boiler, insulation and the installation method is crucial for the result can be illustrated by the calculation example to the left of the two seemingly equivalent systems with an environmentally certified wood boiler and 1500 liter accumulator. In case A has chosen a good natural draft boiler with 75% boiler efficiency and 3 500 liter pre-insulated storage tanks with 50 mm mineral wool. In case B has chosen a modern high-efficiency fan controlled wood burner and 2 x 750 liter storage tanks insulated with 70mm polyurethane.
When you add up the losses and calculate the system efficiency drops from 77% in case B to 56% in the case A. If it turns out that in one year's wood consumption lowers the modern heating system wood consumption by 27%. This applies not just wood boilers, but these are the losses that determine the thermal efficiency of the system regardless of the fuel. Therefore, there is every reason to review its heating system. Even if the boiler still works it can be costly in the long run not to switch to a modern and efficient heating system.