Why does tar form?


What is tar and how does it form?

During the heating season, people often suffer from tarring in the boiler, which is one of the consequences of imperfect combustion.

Tar as a greasy, sticky and flammable substance is a product of incomplete combustion, low flue gas temperature and high fuel humidity. Depending on the perfection of the wood gasification process, solid dust particles (soot and ash) are formed, which are the basic component of tar.
The problem arises when the flue gases come into contact with the walls of the boiler body at a lower temperature, where the formed tar settles. There are several reasons for the formation of tar, such as insufficient maintenance and cleaning of the boiler and an oversized boiler for the given object. If they are not removed, the tar will seriously damage the boiler and clog the flue gas passages of the boiler. In the worst case, tar settled on the walls of the chimney can cause a fire.
In this article, we will look in detail at the possible causes of tar formation and how to prevent them, so that the boiler and chimney achieve proper combustion and a long service life.

What does a boiler damaged by tar look like?

A boiler sealed with tar cannot be overlooked. Tar can damage essential parts of the boiler, such as: exchanger, turbulators and fan impeller. Tar removal is very
Several factors or a combination of factors can contribute to tar formation. Let’s explain in detail the possible causes of tar formation.

Incorrect moisture content of the wood

One of the very common mistakes is too high humidity of the wood. Fresh wood after purchase for heating has a moisture content of around 40-50%. Wood with such moisture cannot be burned in the boiler. Such wood must be stored for at least a year or two before the heating season.
Recommendations wood moisture is a minimum of 12% and a maximum of 20%. Wood with a moisture content of less than 12% is dried wood in the degradation process, which does not have the correct structure and properties. Wood with a moisture content of more than 20% creates tar when burned.

Rozkurenie kotla na drevo vlhkost

Correct humidity of wood 12-20%


Chimney connected to boiler

Incorrect draught and height of chimney

Incorrect height and diameter of the chimney causes incorrect draught behind the boiler, incorrect combustion in the boiler, when the flue gases do not reach the required temperature. In this case, incomplete combustion occurs, which causes excessive formation of tar on the walls of the boiler and the chimney.
For the correct installation of the boiler, it is necessary to comply with the prescribed parameters of the chimney – height, diameter and draught, which are listed in the Operating Instructions. The design of the chimney is regulated by the relevant standards and directives, which set the conditions for the chimney. Design and the corectness of the chimney must be consulted with a chimney sweep or designer, who directly addresses it in the heating project documentation.

Not installed Buffer tank without OVENTROP mixing device

During normal operation, the boiler reaches the optimal operating temperature of the flue gases and the highest efficiency.
Without a buffer tank, when the boiler does not have the possibility to further dissipate excess heat, it switches to the damping mode, in which the fan is turned off, in order to avoid overheating and subsequent damage to the boiler. During this process, the draught in the boiler chamber will decrease, which results in a decrease in the temperature of the flue gas and an increase in the formation of tar.

Buffer tank – significantly prevents the formation of tar and serves to remove excess heat, when the boiler can continue to work at optimal combustion parameters, regardless of the current need for heat for heating.
The volume of the storage tank is determined by the designer depending on the boiler output and the size of the heating system.
Depending on the insulation of the house and the need for heat for central heating and hot water, energy can be pumped from the storage tank for several days.
ATTACK Oventrop – This device significantly extends the life of the boiler. It prevents the boiler from undercooling, thus reducing the formation of tar.


ATTACK DPX With buffer tank and ATTACK Oventrop mixing device


Ilustračné zobrazenie prehriatého vykurovacieho okruhu

Overheating of the heating circuit

With proper and foresightful operation of the wood gasification boiler, there is no unnecessary overheating of the buffer tank. During burnout, the boiler still transfers a certain amount of heat to the buffer tank, which must be taken into account during the next loading.

Primary and secondary valve

In order for the gasification of the boiler to proceed correctly, the air supply is very important. In ATTACK boilers, we set the primary and secondary air supply. The correct air setting can be found in the boiler’s operating instructions. The correct adjustment of the air flaps should be performed by a service technician using a flue gas analyzer.


Primárny a sekundárny prívod vzduchu

Protective elements in ATTACK boilers

The ATTACK SLX is equipped with a dry coats that serves as a high and cold temperature compensator, the dry coats creates a temperature barrier between high and low temperatures in the loading chamber, resulting in lower deposition of precipitated tar on the walls of the combustion chamber.

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