Chimney for a gas boiler in a private house: requirements, materials

When installing gas boilers, it is necessary to strictly observe the norms. Also, strictly according to the standards, it is necessary to mount a chimney for a gas boiler. Although the temperature of the flue gases is low and has no color, it will cause no less harm, but more – because leaks are poorly detected. Therefore, you must immediately do everything diligently, paying special attention to the tightness of the joints. 

Requirements for chimneys for gas boilers

All requirements for smoke channels are prescribed in regulatory documents – SNiP 2.04.05-91 and DBN V.2.5-20-2001. Their implementation is mandatory. To summarize, everything can be reduced to several points:

• The cross section (diameter) of the chimney cannot be smaller than the outlet pipe on the boiler. That is, if the outlet of the gas boiler is 150 mm, then the chimney must have an internal section of at least 150 mm. More is possible, less is not. In extreme cases, they can close their eyes to a difference of a few millimeters.
• The chimney must run vertically upwards. It is desirable to develop a design so that there are no inclined sections. In extreme cases, a slope of 30 ° is allowed. The length of the inclined section is not more than the height of the room.
• Throughout the chimney, it should not have curvature and narrowing.
  

• The chimney must be made of gas-tight materials.
• The joints must be carefully insulated – they must be airtight (they must not let in gaseous substances and must not let in moisture).
• Since the flue gases at the outlet of modern gas boilers have a low temperature, the likelihood of condensate formation is high. Therefore, when installing a chimney in its lower part, it is necessary to provide a condensate trap. This is a removable cup made of chemical resistant materials. The best option is a stainless steel condensate trap, the cheaper one is plastic. Galvanized steel is a cheaper option but breaks down quickly.
• The chimney for a gas boiler in a private house should have a height that will provide good draft. To do this, it must rise 50 cm above the ridge of the roof, if it is displayed in the immediate vicinity of it.
• At the top of the pipe, it is desirable to install a protective visor – an umbrella. It protects the pipe from clogging and rainfall.

These are the basic requirements. It is imperative to fulfill them. They provide the required degree of security. After all, the fact that the exhaust of a gas boiler has no color does not mean that it is harmless. Therefore, all aspects of security must be given maximum attention.

Boiler designs and chimney construction methods

There are two types of burners for gas boilers:

• Open. In them, oxygen for combustion is taken from the room in which the boiler is installed. Therefore, this type of burners or trapping is also called “atmospheric”. In such gas boilers, the removal of flue gases requires a pipe with an outlet to the atmosphere and good draft.
  

• Closed-type combustion chambers (turbocharged boilers). They work in tandem with coaxial type chimneys – from two pipes inserted into each other. Combustion products are discharged through one pipe, air is supplied through the other to maintain combustion. The coaxial pipe is usually brought out through the wall to the street. Therefore, they say that such gas boilers do not have a chimney. But, if desired, it is possible to bring the chimney into an atmospheric chimney. In this case, normal traction is needed (when it is brought into the wall, this does not matter, since the movement of gases is provided by the turbine).

It is clear that the choice of the type of chimney primarily depends on the type of combustion chamber. In one case, it should be a coaxial pipe, in the other, a regular one. But besides this, there are many more nuances in terms of design.

What material

A chimney for a gas boiler in a private house can be made from different materials. The main requirement is resistance to chemically aggressive substances, inability to pass gases. Traditionally, several materials are used. Let’s talk about the advantages and disadvantages, the features of the assembly of each of them in more detail.

brick chimney

Today it is no longer the most popular type of chimney. It turns out to be heavy, at high altitude it requires a foundation. In addition, laying a brick chimney takes a lot of time.

However, this type of chimney has a number of negative qualities. The first one is inits inner walls are not smooth, which contributes to the accumulation of soot, impairs traction. The second is tobrick is hygroscopic. Therefore, the condensate flowing down the walls is absorbed, which contributes to rapid destruction.

To solve these problems, a smooth pipe of suitable diameter is inserted inside the brick chimney. Usually it is a pipe made of stainless steel or asbestos. When building such a combined chimney, attention should be paid to the following things:

• The joints of the liner pipe must be sealed. If these are ordinary or stainless steel sandwich pipes, everything happens as standard – we collect the chimney through condensate. If the liner is made of asbestos-cement pipes, you will have to take care of the tightness of the joints. And to cover the joint with cement is not an option. Such a connection is in no way airtight – condensate will be absorbed. You will have to come up with airtight clamps, use hydrophobic (water-repellent) compounds. Moreover, they must also be chemically resistant. As an option, lubrication of the joints with heat-resistant sealants with an operating temperature of about 200 ° C can be considered.
• In order for condensate to form as little as possible, it is better to insulate pipes (even inside a brick casing). To do this, it is desirable to use a heater that is not afraid of getting wet.
• At the bottom, a condensate collector must be attached to the liner pipe. Access to it should be free.

If you make a chimney for a gas boiler according to these rules, then even with abundant condensate release, it will be easy to deal with it.

Stainless steel – single wall pipes and sandwich

Modern gas boilers are designed so that the temperature of the flue gases at the outlet is not very high. This is why condensation always forms. With good traction, most of it flies into the pipe, with good insulation, the rest evaporates. So it turns out that liquid is not always present in the condensate collector. But the condensate itself is formed during the operation of the gas boiler all the time. Sometimes more, sometimes less. In this regard, the requirements for stainless steel for the chimney are high: it must withstand prolonged contact with caustic substances. These requirements are met mainly by food grade stainless steel. Yes, it costs a lot, but only it will serve for years.

Now about how to make a chimney for a gas boiler from a single-wall pipe or from sandwich pipes. In order for condensation to form in minimal quantities, it is desirable that the chimney does not cool down. That is, it must be insulated. And although the sandwich chimney has a gasket made of insulation, with an external gasket (on the street) it is also better to insulate it – it will last longer, traction will be better. But in this embodiment, less insulation is required – one layer, while an ordinary pipe may have to be wrapped in two or even three layers. So the cost of arranging a chimney from a single-wall stainless pipe and sandwiches will be comparable. Just in the first case, you will have to use a larger amount of insulation, in the second, less.

If we talk about reliability, then sandwich chimneys are more reliable, if only because they consist of two layers of metal. By the way, if you insulate the chimney, the outer pipes can be made of galvanized steel – they do not come into contact with condensate, the temperatures are low, and the appearance is unimportant, since everything will be wrapped with insulation.

Ceramic chimneys

Ceramic chimneys are good for everyone: they are durable, reliable, well tolerate contact with aggressive substances. But they have two significant drawbacks. First, they are expensive. The second one is heavy, so when installing a high chimney, they require a foundation. And this is an additional cost to the already considerable amount. But the life of such a chimney is calculated in decades.

Asbestos-cement pipes

Once it was the most popular type of material in the construction of chimneys for a gas boiler in a private house. The material, of course, is porous, has rough walls, and its cross section is not ideal (not round, but rather oval). But this is probably the cheapest option.

When using asbestos-cement pipes for the chimney of a gas boiler, it is necessary:

• Make it as straight as possible, trying to make the joints even.
• Sealing joints. As already mentioned, just covering it with cement is not an option. A sealed connection is required. Several solutions to the problem are the use of hydrophobic additives in the mortar, coating the dried cement mortar with sealant, and the use of airtight clamps.
• To reduce the amount of condensate, make the pipe high and insulate well.

In general, nothing new, all the same rules as for the materials described above, but the hassle with joints is added. So as a result, the price of a chimney made of asbestos pipes is almost the same as that of stainless steel.

For boilers with an open combustion chamber

For gas boilers with an atmospheric burner, a smoke channel is required that provides good traction – the removal of combustion products occurs due to the movement of air through the pipe. Therefore, it is made as straight as possible, preferably with smooth walls. There are two versions:

• Bring horizontally through the wall, then, along the outer wall, up to the required height. This option is also called an external chimney.
  

• Lead the pipe up from the boiler, go through all the ceilings, the roof, lead it over the roofing material. If necessary, you can take the pipe away from the wall by making two 45 ° elbows. Bends at 90 ° with this design is best not to do.

Which option is better? An external chimney is easier to implement – with a conclusion through the wall. It is only important to pass through the wall correctly (observe the fire gap if the walls are combustible). But this option requires good insulation, high-quality fastening to the walls. And even under such conditions, there is usually a lot of condensate. Therefore, the installation of a tee and a condensate collector at the outlet is mandatory.

In the case of the exit of the chimney through the roof, there are at least two difficult moments – the passage through the floor of the first floor and through the roof. In these places, special passage nodes are installed. They provide an adequate degree of fire safety.

Read more about how to run sandwich pipes through a wall or ceiling here. 

Features of the assembly of metal pipes

If they use sandwich pipes or single-walled metal pipes, then the external chimney for a gas boiler is assembled “by condensate”. That is, inserting the top tube inside the bottom one. This is possible due to the presence of a corrugated edge on one side.

When assembling the chimney inside the building, the structure is assembled “by smoke”. In this case, it is more important that gases do not enter the room. Therefore, the pipes are unfolded so that the upper element is put on the already installed one.

There is a third option – to assemble two circuits in different ways: the outer one for smoke, the inner one for condensate. For such an assembly, it is imperative to use sandwiches, since only they have two circuits. This is the best option, as the protection is complete, but the assembly is difficult.

Chimneys in the mine (box)

So that communications do not spoil the interior, they are often “packed” into a shaft – a specially built box. Inside there are, as a rule, a chimney (or chimneys, if there are several devices), ventilation ducts, there may be water supply, heating, and sewage risers. In any case, it is better to close the chimney pipe with thermal insulation. If the heater can still not be used in a heated room, then in the attic (especially if it is cold), insulation should be mandatory. Use basalt wool with a temperature regime of operation of at least 300 ° C.

Insulation will positively affect the temperature inside the chimneys, which will increase traction and reduce the amount of condensate. But do not forget that we are talking about gas boilers, and they have a low temperature of combustion products.

For closed combustion chambers

A coaxial chimney looks like a pipe in a pipe. The design is delivered ready, assembled quickly and without problems. You only need to know the diameter of the outlet pipe and the parameters – height, length.

The coaxial chimney device is the simplest. The pipe rises above the boiler and turns 90°. From it to the ceiling should be at least 20 cm. Further, it is carried out through a hole in the wall, from the outside it should end at least 30 cm from the wall.

The height relative to ground level is also normalized – the pipe outlet must be at least 20 cm above the ground, and the distance to the nearest wall – from the end of the pipe to the wall must be at least 60 cm.