We try to answer that question shortly.
Because we do not need the entire energy which the boiler will produce during the combustion process of one feed of the boiler chamber!!!
and now more precisely:
The accumulator tank should be selected according to three principles:
- Area of the building heated.
- Capacity of the loading chamber
- Boiler nominal power
Assuming the boiler with the chamber capacity of 100 litres, we can load approx. 50kg of wood (depending on the wood type and weight) into it.
One kg of wood has a calorific value from 3 to 4.4 KW/h, which means that if we assume 3.5 kW/h on average, we have approx. 175 kW of energy included in wood in the boiler chamber.
Assuming the building insulated as standard with the heated area of 100 m2, it needs approx. 100 W per m2 of area at outdoor temperature of -5ºC (it is very realistic). Therefore, the building needs 10000W (10kW) per hour.
If the wood boiler has the nominal power of 25 kW, it generates much more energy than we can use at the given moment. The boiler, working without the heat receiver, achieves quickly the nominal temperature and switches off.
The wood boiler has high efficiency only at full load, while at partial load, the boiler sooting process is started in the loading chamber and the boiler heat exchanger that is a change of unburnt wood gas into soot.
The wood boilers burn one chamber fully loaded within from 4 to 6 hours (depending on the manufacturer and the wood species used).
Assuming 6 hours for our example, our building needs only 60 kW during those eight hours? no !!! even less. The building walls are also the heat accumulator, therefore soon after the boiler ignition, the building absorbs energy for 1-2 hours and during the following hours it only needs heat to make up for losses connected with ventilation and thermal insulation and we can assume that it is approx. 40 - 45 kW.
175 KW - 45 KW = 130 KW So where is that energy? It may escape through the chimney?
That energy must be therefore kept and the helpful solution is the accumulator tank that can store the excessive energy to return it to the building when the boiler operation is finished.
It is easy to answer the question how big the accumulator tank should be
Because we know that the tank with the capacity of 1000 l stores approx. 1kW per 1ºC of temperature difference. If we ignite the boiler when the accumulator tank temperature is still 40ºC and we heat it up to 90ºC, it is able to store 50kW.
Going back to our example, we can say that the suitable tank will be the accumulator tank with capacity of 2000 l.
When the boiler is selected, attention should be paid to the boiler type (its power, loading chamber capacity etc.), taking into account losses in the installation and accumulator tank in the boiler room and the fact that moisture of the wood used for burning will be in practice about 25%. In practice the boiler capacity that was calculated in theoretical considerations must be reduced by approx. 30%, but not less than 50-75 l per each 1 kW of the boiler nominal power.