The formula for calculating the combustion of solid fuel boilers KOLVI

Формула расчета горения твердотопливных котлов КОЛВИ

In the context of annual increases in prices for gas heating, the popularity of solid fuel boilers is growing. They are especially relevant for rural areas, where it is more profitable to heat a room with firewood than to pay exorbitant gas bills.


Produce industrial and household solid fuel boilers. In industrial fuel supply occurs automatically, in household it is done manually.


The choice of heating equipment is approached systematically, three groups of technical factors are taken into account to create the most comfortable atmosphere in the house:

  • The first group is the characteristics of heating boilers, which determine the efficiency of combustion of a certain type of fuel (coal, wood, briquettes, various waste), that is, the efficiency of the boiler, on which the volume of fuel consumption depends.
  • The second group is the boiler design parameters, that is, the quality of the metal or cast iron from which the boiler body and heat exchange surfaces are made (quality of welds, quality of casting of cast iron sections), as well as the presence of certain safety elements (automation, cooling coil).
  • The third group of characteristics of solid fuel boilers is operational parameters, that is, comfort and convenience for the user.

The operating parameters determine how easy it is to set up and control the boiler, to clean it and maintain it. The third group includes the most important factor for household solid fuel boilers: the burning time of the fuel insert, how often it will need to be supplied.


Ideal boilers, that is, having all the above advantages (high efficiency, high-quality layout and ease of use), are very insignificantly represented on the heating market, besides, their cost is often not affordable for most consumers. Therefore, in the process of choosing the required model of a solid fuel boiler, a compromise option is most often chosen, in which the emphasis is precisely on the comfort of operation.


The burning time of a solid fuel boiler is affected by:

  1. Combustion chamber volume. It is quite a logical parameter, since the larger the volume of the furnace, the correspondingly larger the volume of the contained fuel. In addition, the geometrical dimensions of the combustion chamber affect the comfort of use, especially for wood combustion. It is much more practical and faster to serve large logs rather than small pieces of wood. The type of combustion chamber itself is also important: upper, lower or volumetric combustion.
  2. Fuel type and quality. These include: calorific value, humidity, bulk density of each type of fuel. In addition, for coal, the size of the fraction is taken into account, and for firewood, the size of the logs.

Coal fuel has a high calorific value (on average 5500-6000 kcal per kg) and bulk density (800 kg 1 cubic meter). Wood, in turn, has a lower level of heat generation (on average 3500 kcal per 1 kg) and density (550-600 kg per 1 cubic meter). Obviously, the same loading volume holds more coal than firewood, and such coal will burn longer than a similar laying of a tree.


Actually, the duration of fuel burning is calculated simply:

  1. The useful thermal power of the boiler is determined (passport data, taking into account efficiency).
  2. The volume of the loading chamber is determined (a figure from the passport data or the product of the height, width and depth of the furnace).
  3. The type of fuel used is determined and, accordingly, the amount of fuel contained in the combustion chamber (taking into account the fuel density and the fuel stacking factor)
  4. The amount of heat energy that can be “extracted” from the loaded fuel is calculated.
  5. The calculated amount of heat energy from the tab is divided by the rated power of the boiler. The resulting figure is the burning time of the bookmark at rated power.

Example. Solid fuel boiler Kolvi Eurotherm KST-12. Thermal power 12 kW. The type of fuel is dry oak firewood. The volume of the combustion chamber is 33 liters (0.033 cubic meters). The efficiency of this wood boiler is 70%.


The density of dry oak is 650 kg per 1 cbm. Accordingly, with a furnace volume of 0.033 cubic meters, the theoretically possible capacity of the furnace is 650×0.033 = 21 kg of firewood. However, we understand that the actual filling volume will be an order of magnitude less, since the loaded firewood will have a certain amount of gaps between itself and the walls of the firebox. Therefore, for wood, an indicator such as a stacking coefficient is used. This parameter is rather arbitrary and can vary within 0.35-0.5. For this example, let’s take an average of 0.4.

Thus, taking into account the stacking factor, the amount of oak firewood: 21×0.4 = 8 kg, which is a very real indicator.


Next, we calculate the calorific value of the bookmark: 8×4200 kcal = 33600 kcal, or 39 kW. Taking into account the boiler efficiency, this tab will give 39×0.7 = 27 kW.


Now we calculate the burning time of the bookmark: 27 kW/12 kW = 2.25 hours.


All factors cannot be taken into account with absolute precision. This calculation gives approximate values, the formula indicates the nominal power of the boiler with the maximum heat loss.


The heating power level depends on the outside temperature. If you reduce the intensity, the burning time of the fuel tab will increase. It is not recommended to reduce the boiler output by less than half. During smoldering, heat loss occurs, since the fuel does not burn out, condensate forms and the boiler furnace becomes clogged.


The Kolvi Eurotherm KST-12 boiler, depending on the selected power, can operate on dry oak wood for 2 – 4 hours. The operating time of other solid fuel boilers is calculated according to the same principle.


Useful articles

твердотопливные котлы Колви
Alternative to solid fuel boilers

The constant increase in the cost of energy carriers and expensive gas connection requires looking for other types of heating devices for the heating system. An electric or gas heat source is an alternative to a solid fuel boiler. The article introduces the types and characteristics of solid fuel equipment, its pros and cons, methods […]

термомодернизация в Украине
Features of insulation of Khrushchev houses according to Zelensky’s “Great Thermal Modernization”

The house insulation program in Ukraine has not started its operation, but it raises many related questions. There are many factors that make the use of allocated funds ineffective.   Literally on December 1, in a message to the Verkhovna Rada, the President of Ukraine announced that the Great Thermal Modernization project would soon be […]

развития восстановительной энергетики
The main tasks for 2022 in Ukraine for renewable energy

Among the arguments responsible for the development of energy sources are political, economic and environmental ones. And, if an exhaustive amount of information has been written about environmental issues, today these arguments are the key factors for the development of renewable energy. There are few of them to support the improvement of the presented sector […]

Contact with us


You choose us, so we will create all conditions for comfortable and mutually beneficial cooperation

Address Ukraine, Kyiv oblast,
Sofievskaya Borshagovka, st. Gorky, 1
Opening hours: Mon-Fri 9:00-18:00

    Please prove you are human by selecting the Cup.