The Chimney

The chimney is probably the single most important part when you are installing a new boiler.  It is the chimney that will ‘drive’ the boiler, i.e. the chimney shall carry away waste gas and lead fresh air into the combustion chamber. Therefore chimney determines how good or bad it is for the combustion process, as well as how much heating efficiency you can get from boiler. The high temperature heats up smoke gas, and fireball is rising up during combustion process just as hot air balloon. In other words, the hotter the gas, the quicker for the smoke gas flows up. High temperature flue gas is able to drive boiler even in a tight chimney. That’s why old boilers can burn in an unsuitable firewood burning chimney. You do not want to have high temperature smoke gas and heat loss, because you are supposed to heat up your boiler and house instead of your bird neighbor’s nest on your roof.

A modern boiler with combustion efficiency requires a negative pressure at 25 Pa (2.5 mm WG) for getting 30 kW. This is equivalent to 1 whole x ½ stone masonry chimney (12.5 x 25 cm) and optional tube at diameter 160 with 6-8 meters length. A masonry chimney is called ”heavy chimney” that means it takes longer time to heat up the chimney to drive boiler. Steel chimney is called “light” because it quickly gets hot and drives well. Before you change boiler, contact to your chimney sweep if you feel unsure. It may also be a good idea to test your chimney before replacing boiler. Old boilers have flue gas temperature at 300-500 °C, and the size of the chimney does not matter. While modern and efficient boilers have flue gas temperature at only 150-200 °C, and it places much higher demands on chimney. If you do not have these prerequisites you can have Extuna’s flue gas fan fitted your chimney then you are able to obtain an even better effect than with a self-drawing-in-air chimney.

Air for combustion

The boiler needs to get air in to burn. Fresh air inlet to the boiler should be of at least the same size as the chimney area and be positioned as low as possible inside of boiler burning room. Placing fresh air inlet highly should install a ventilation duct which leads down fresh air as low as possible. Otherwise heat from the boiler room will get out from boiler burning room and at the same time the boiler desperately tries to draw in fresh air.


If firewood consists of excessive moisture, it is burned badly. Therefore only dry firewood can be burned.

If you have damp wood you might have trouble to get the fire started, and also lots of energy included in firewood is consumed to get the wood itself dried, and until then it starts to heat up to the wood boiler and house. Burning damp wood means that you get an unnecessarily large wood consumption. Dry wood contains about 20% moisture. The wood shall be felled, sawed and cleaved one year before it fired, as well as stacked under cover so that sun and wind can help to dry the wood.

NOTE!!  The wood must never be covered with a tarpaulin, because it uses up moisture instead of drying it. There always must be an air gap between the roof and the woodpile! Pile up firewood with the cleaved part up towards the roof so that the wood dries faster. If you are able to bring the firewood inside of your boiler room 2-4 weeks before it is burned, then they have absolutely best conditions to burn. Waste/debris contains minimal amount of gas and produces more smoke and pitch than the combustion gas, therefore they shall not be burned.


Extuna has its own production of ceramics to obtain full control of all the important elements which are required (material quality, casting, reinforcement, curing, drying as well as furnace burning) to produce a product with high strength and long service life. Today we have one of the longest service life of wood boilers ceramic for houses. The Ceramic in V 202 K UB is very easy to change.

Security System

All heating systems must include an expansion tank to absorb excess water caused by thermal expansion, which means that expansion tank is used to absorb the increased volume of water created when water heated. There are two types of expansion tanks, open and closed, see the list of sets. In an open expansion system, water volume in expansion tank shall reach 5% of the total water volume from the whole heating system; water volume shall be reach 12 % when closed expansion system.
Installer shall carry out the installation under warm and hot water Instructions, in such a way that boiler/complete heating system’s temperature does not exceed 100° C. If the temperature is higher than 100° C, the solution could be temperature limiter and cooling coil. Otherwise, Installer can look for other solutions.
Domestic water installation shall be equipped with safety valve, check valve and shut-off valve. Installation shall also be fitted with a mixing valve so that the domestic water temperature does not exceed 65 ° C.
Preset limits of safety valve pressure shall not exceed 1.5 bar.

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