How to choose a chimney for a boiler

Recommendation points

• What kind of fuel is used
• Chimney assembly diagram
• Subtleties exhaust duct configuration
• Boost section parameters
• Types of chimney ducts and fittings

Today there is no longer any need to assemble a chimney from what comes to hand. There are a lot of modern chimney systems on the market, but another problem arises – how to navigate their assortment without making a mistake. We will help you make the right choice – we will tell you about the types of chimneys and their purpose.

What kind of fuel is used

The quality of the exhaust gases is of decisive importance for the durability of the chimney. First of all, their temperature is important: if gas boilers can be connected to the booster section through a flexible aluminum corrugation, then solid fuel units require the use of heat-resistant materials, at least in the immediate vicinity of the outlet of combustion products. The presence of unburned particles in the smoke has a particularly negative effect on the state of the chimney, because of them one has to fight for the smoothness of the inner surfaces of the channel.

The chemical composition of the combustion products is also extremely important. When natural gas is burned, it forms virtually nothing but water, carbon dioxide and a small amount of soot. But boilers for liquid fuel, and even more so for wood and coal, carry hundreds of different substances in their smoke, many of which, in contact with water vapor, form quite corrosive compounds. Thus, the internal surface of the pipes can be required for low adhesion and chemical inertness towards acids..

The third aspect is the presence of large quantities of water vapor in the smoke. In winter, a section will inevitably form on the pipe body, where temperature and humidity contribute to water condensation with the formation of black tar condensate. Therefore, for the pipe of the accelerating section, there may be an additional requirement for the presence of insulation, the material of which is able to withstand a fairly severe thermal regime..

Chimney assembly diagram

Chimney insulation may be absent if there is a condensate drain system. In this case, it is customary to place the booster section entirely outside the heated building and connect the chimney with an external sump. According to the general technology, a collecting cup with a drain device must be equipped at the bottom of each vertical section. So the boiler chimney system can be arranged somewhat more complicated than just a pipe with access to the street..

Chimneys of complex configuration are assembled from separate parts: straight girders, swivel bends, tees, couplings. A well thought out connection system must be used. So, if condensation is allowed in the chimney, the joints between the segments must be tight. It would be too difficult to use for this connection on flanges and clamps, therefore pipes are usually connected “in a glass” or “in a sleeve”. That is, the open edge of the segment has either a constant diameter or a flaring, due to which one segment can be tightly inserted into another. For conventional steel pipes, it is possible to use instead of expanding a sleeve of a larger diameter welded to the body of the segment.

You need to learn how to use the assembly diagram shown correctly. If the smoke flows in a segment from the funnel to the thin edge, it is called “smoke-collected”. If the smoke passes towards the funnel, the segments are collected “by condensate”, which will not protrude outward or into the insulation, flowing down the inner walls of the channel. Sometimes a more complex technical requirement is put forward – that the system is assembled both by smoke and by condensate, for example, with a direct upper connection of the booster section to the boiler. In such cases, the system is collected by condensate, while the joints between the segments are sealed with a heat-resistant sealant.

The indicated features explain that in the end any chimney can be represented as a finite number of segments of different configurations. This design approach is considered the most correct: it is obvious that we choose not a specific exhaust system, but a set of its components.

Subtleties exhaust duct configuration

It is possible that in the process of building a chimney, you will refuse to use one type of channels for the sake of their combination. This is usually done, because, as a rule, some materials are better suited for creating straight sections, others for joining. Using products and materials for their intended purpose, you can not only ensure their long-term operation, but also save well.

You can start selecting parts already when two key reference points have been identified – the boiler and the location of the booster riser – the last vertical section. Further, a connecting channel is stretched between them – it is advisable to start with the boiler and the smoke outlet it has. The main difficulty here is the need to switch from one conditional section to another.

So, for example, many solid fuel boilers have a rectangular neck, while the main channel is usually round. Well, the misalignment of the channel sizes is quite a common practice, so you should be prepared for the need to manufacture an atypical gearbox for this transition. The main rule for such products is that the nominal bore should not be underestimated, including protruding parts of the connection system.

Most boilers are installed in the immediate vicinity of the chimney well; here it is enough to simply make a tie-in at the desired height. In gas boilers, the combustion products are practically cold, so they are discharged without any special tricks through a corrugated sleeve or a segment box, depending on the shape of the outlet section. In solid fuel boilers, the smoke is hot, so the length of the connecting insert is increased and it is supplied with various kinds of economizers, for example, radiator fins. Please note that the minimum length of the connecting channel is regulated by the manufacturer and fire regulations.

It is more difficult if the boiler flue outlet is located a few meters from the booster. At such distances, laying a connection to the chimney horizontally is simply pointless: at 1–1.5 m from the boiler, the smoke is no longer hot enough to overcome the aerodynamic resistance of the pipe. This section must necessarily be inclined, preferably with a gradual rise as you move away. Since the corrugation over a long length does not have sufficient rigidity, for gas boilers, it is recommended to use stainless steel segment chimneys. They can also be used with solid fuel boilers, but it will be better if the channel is made with a thick-walled steel pipe at a length of 1.5-2 m from the beginning..

Boost section parameters

It is generally accepted that the driving force of the chimney is created together with the draft in all vertical sections. However, in practice, it is much more convenient to calculate the thrust only along the final section of the channel – the direct acceleration section. It is understood that no thrust is created in small vertical and inclined channels; on the contrary, they resist the flow..

The correct arrangement of the booster section is the key to ensuring high traction performance without blowing and exhaust fans. There are several basic requirements for the channel:

1. Corresponding length and diameter.
2. Absence of elements protruding into the lumen.
3. Low tendency to narrow the bore over time.

The vertical booster channel is an area with an increased risk of condensation, so its material should not only be immune to corrosive chemicals, but also resist the settling of particles on the walls. These qualities are best combined in polished stainless steel. A pipe of this type, in the presence of insulation, practically does not need to be cleaned. However, when burning with raw solid fuel, even such a chimney will “cry” and gradually become overgrown with soot, so here it is better to use asbestos cement, ceramics or thick-walled steel – any materials that are not afraid of a wire brush.

The assembly of segmental chimneys on sockets is very versatile, with its help most tasks are solved. However, if a large outlet height into the windy zone above the roof is required, the intrinsic rigidity of such chimneys may not be enough. But the riser from a thick-walled pipe will be quite stable, even if it is assembled from separate parts on flange connections. Of the best options for the device, you can also call a pipe enclosed in a brick well – this is both insulation and reliable fixation.

Types of chimney ducts and fittings

Taking into account the listed features, you should make a choice in favor of one or at most two chimney systems. Preferably, these are modern factory-made products. Practice shows that using branded products is more profitable and more convenient, especially if the installation is done by hand. The main benefit of using smoke extraction systems is that they include all the necessary fittings: from dampers to roof cuts.

The most popular today are the stainless steel sandwich pipe systems. Due to the insulation and sufficient spatial rigidity, it is quite easy to arrange a chimney of any type and configuration. At a cost, this option will be somewhat more expensive than simple metal pipes. The investment pays off, however, with easier maintenance and elimination of the risk of condensation..

The second, slightly less popular chimney system is based on ceramic pipes inside thermal insulation blocks. Products of such a plan are extremely useful when constructing wells under booster sections, including when connecting a boiler directly. It is also convenient to lay out the blocks from the blocks themselves, the accelerating sections, attached to the outer wall of the building..

Another type of modern system has plastic channels. Their area of ​​application is exhaust ducts with a pre-installed economizer or recuperator, that is, smoke cooling devices. And although plastic cannot boast of heat resistance, its strengths are chemical inertness and a low tendency to stick to carbon deposits. The most obvious is the efficiency of using such chimneys in conjunction with condensing gas boilers..