These boilers use wood, brown or black coal, peat, pellets, etc. as fuel. Most solid fuel boilers on the market are designed to work on one fuel type. The use of another type of fuel leads to a decrease of heating system efficiency and a reduction of boiler equipment life. Cheap solid fuel boilers on the market allow to regulate power in a small range (from 100% to 80% of nominal capacity), which causes difficulties in system operation. For example, at ambient temperature of + 5°C, to ensure comfort,  boiler should operate at capacity of 15% of rated value, however cheap models of solid fuel boilers can provide power reduction only up to 80%. There is a need for incomplete loading of furnace, which leads to difficulties in automating the heating system and maintaining optimum temperature of the coolant. It should be noted that solid fuel boilers require correct flue organization to divert the combustion products. Thrust depends on height of chimney. More modern type of solid fuel boilers are pyrolysis boilers. Principal difference between solid-fuel pyrolysis and solid-fuel classic is as follows: in pyrolysis boiler, the combustion phases are separated: in the first phase, wood is not burned, but gas is released from wood under influence of high temperature and lack of oxygen, and in the second phase –  fixed residual burning. The advantage of pyrolysis boilers is high efficiency for this type of boiler, reaching 85%, virtually no ash and soot. The advantage of more expensive models of solid fuel boilers, such as pellet and pyrolysis, is the ability to adjust the power in range of 100% -30% of rated output. However, the cost of pellet and pyrolysis boilers is much higher than the cost of the simplest types of solid fuel boilers. The boiler is the main element of modern heating system, designed to produce heat energy for heating and hot water supply of buildings, structures, premises. Conditionally, all boilers, depending on fuel type, can be divided into four types: