Thinking about building a chimney in your home? Before getting down to business, let’s figure out how to correctly design and mount it.
All about chimneys
- Brick chimney
- Steel chimney
- Ceramic chimneys
Chimneys and fireplaces
Is it possible to connect two pipes to the chimney
How to improve traction
In which pipes does condensate form?
General requirements for chimneys
Fireplace inserts can be connected to an existing chimney, but in this case the choice of firebox power is dictated by the cross-sectional area of the smoke channel. General requirements for chimneys are contained in SNiP 41–01-2003 “Heating, ventilation and air conditioning”. This regulatory document allows the use of chimneys made of double-walled steel pipes with thermal insulation made of non-combustible material (at a flue gas temperature of not more than 500 ° C) and the installation of umbrellas, deflectors and other nozzles on chimneys, if these devices do not prevent the free exit of smoke (in this its difference from SNiP 2.04.05–91).
The height of the chimneys from the mouth to the grate must be at least 5 m. The height of the parts of the pipes rising above the roof is at least 0.5 m above the flat roof, as well as above the ridge or parapet at a distance of no more than 1.5 m; not lower than the ridge or parapet at a distance of 1.5–3 m to them; not lower than a line drawn from the ridge down at an angle of 10 to the horizon, with a distance of more than 3 m between the pipe and the ridge.
Pipe bends are allowed at an angle of up to 30 to the vertical at a distance of no more than 1 m. Pipes on buildings with roofs made of combustible materials should be equipped with spark arresters made of metal mesh with holes no larger than 5 g 5 mm. The distance from the outer surfaces of brick or concrete chimneys to rafters, battens and other roof parts made of combustible materials must be at least 130 mm; from ceramic pipes without insulation — 250 mm, with insulation — 130 mm. As for walls and floor elements made of combustible materials, the distance from the inner wall of the smoke channel is normalized here: 500 mm to unprotected structures and 380 mm to protected ones.
However, the appendix to SNiP operates with the term “cutting”, that is, we are talking about a brick pipe. There are no clear rules for modern modular systems, and developers usually follow the manufacturer’s instructions.
Types of chimneys
Until recently, brick chimneys, both in urban and rural construction, were practically uncontested. Being a universal structural material, brick allows you to vary the number of chimney channels and wall thickness (you can make the necessary thickenings in the places where floors, roofs pass, as well as during the construction of the outdoor part of the chimney).
With the observance of building technologies, a brick chimney is very durable. However, it also has disadvantages. Due to the significant mass (a pipe with a cross section of 260 130 mm and a height of 5 m, laid out in half a brick, weighs about 1.5 tons), it is necessary to arrange a foundation. And for the construction of all this structure will require a lot of time and effort. The cross section of the channel (rectangular or square) is not optimal for traction. In addition, with periodic use in the cold season, the service life of a brick pipe is greatly reduced due to the aggressive effects of condensate.
For the device of a brick chimney, a very high qualification of builders is necessary.
Common building mistakes
- The choice of low-quality or unsuitable bricks (weakly burned partition or wall).
- The thickness of masonry joints is more than 5 mm.
- Laying on edge; the use of stepped (“toothed”) masonry on inclined sections.
- Incorrect preparation of the solution (for example, if the ratio of parts of clay and sand is chosen without taking into account the fat content of clay), inaccurate splitting or cutting of bricks.
- Inattentive filling and dressing of masonry joints (the presence of voids and double vertical joints).
Laying pipes close to structures made of combustible materials. The condition of the brick pipe requires constant monitoring. Previously, it was certainly whitewashed, since it is easier to notice soot on a white surface, indicating the presence of cracks.
Stainless steel pipes can be safely attributed to the most widely used type of chimney today. Steel modular systems have a number of undeniable advantages. The main ones are low weight, ease of installation, a wide selection of pipes of different diameters and lengths, as well as shaped elements. Steel chimneys are made in two versions — single- and double-circuit (the latter is in the form of a “sandwich” of two coaxial pipes with a layer of non-combustible thermal insulation).
The first ones are designed for installation in heated rooms, connecting a fireplace to an existing chimney, as well as sanitizing old brick pipes. The latter are a ready-made design solution that is equally suitable for installing a chimney both inside the building and outside. A special type of smoke channels made of stainless steel are flexible single- and double-walled (without thermal insulation) corrugated sleeves.
For the production of single-circuit chimneys and inner pipes of “sandwich” type chimneys, alloyed heat- and acid-resistant sheet steel (usually 0.5–0.6 mm thick) is used. Single-circuit chimneys made of carbon steel, coated on the outside and inside with a special black enamel, even surpass stainless steel pipes in heat resistance; they are also not afraid of condensate, but only if the coating is intact, which is easy to damage (say, when cleaning the chimney). The service life of uncoated pipes made of “black” steel with a thickness of 1 mm does not exceed 5 years.
The casing (shell) of “sandwich” pipes, as a rule, is made of ordinary (non-heat-resistant) stainless steel, which is electrochemically polished to a mirror finish, and some manufacturers offer enamel painting in any color according to the RAL scale. The use of a galvanized steel casing is justified only when installing a chimney inside the building. Outside, such a pipe, if the chimney is actively used, will not last long: due to periodic heating, corrosion intensifies.
The layer of thermal insulation in “sandwich” pipes solves three problems at once: it prevents overcooling of flue gases that negatively affects draft, does not allow the temperature of the inner walls of the chimney to drop to the dew point, and, finally, provides a fireproof temperature of the outer walls.
The choice of insulating materials is small: usually it is wool — basalt (or organosilicon, perlite sand (but it can only be covered during the installation of the chimney).
Such a very important characteristic of the chimney as gas tightness depends on the design of the pipe joints, so each manufacturer strives to bring it to perfection. Thus, the sealing of some chimneys is provided by centering couplings; a double annular protrusion formed at the junction is crimped by the clamps included in the delivery of each module. In other chimneys, a cone-shaped connection is provided in combination with an annular ledge.
The vast majority of stainless steel chimneys are mounted in the traditional way, and much depends on the quality of the parts. Usually, the upper module is put on the lower one, however, single-circuit ones, and for external laying, the double-circuit modules should be joined, inserting the upper one into the lower one, which will avoid condensate leakage through the joints.
Ceramic chimneys are the same “sandwiches”, but “cooked” according to a completely different recipe. The inner tube is a pottery made of fireclay mass, the middle layer is unchanged basalt wool, the outer one is sections of lightweight concrete or mirror stainless steel.
Ceramic chimneys are resistant to high temperatures (up to 1000 °C), condensate and at the same time have the main advantage of modular systems — they can be quickly and easily assembled.
There are ceramic systems and their disadvantages. Chimneys with a casing made of concrete have a significant mass (1 linear meter weighs from 80 kg), can only be used as indigenous (free-standing), do not allow to bypass obstacles. The “weak link” of such chimneys is the connection node. Manufacturers provide for the use of a metal module (modules), which has a shorter service life and therefore will require replacement in the future, which must be taken into account when building a fireplace.
And finally, the metal does not go well with ceramics, since it has a high coefficient of thermal expansion: along the perimeter of the steel pipe, where it enters the ceramic, it is necessary to leave a rather large (about 10 mm) gap, which is filled with asbestos cord or heat-resistant sealant.
However, the high reliability and durability of ceramic chimneys (the factory warranty is 30 years, and the actual service life, according to manufacturers, is more than 100 years) make it possible to turn a blind eye to the listed shortcomings.
What pipes are suitable for fireplaces
The degree of reliability and efficiency of chimneys largely depends on the heating devices connected to them, and vice versa. Therefore, for each type of fireplaces, there is an optimal chimney option.
The use of thermally insulated modules along the entire length of the chimney (starting from the firebox) is justified in those rooms where the fireplace performs mainly decorative functions. Usually, in order to use the heat of the exhaust gases, the first 1.5–2 m of the chimney is collected from single-wall pipes in compliance with the necessary fire safety measures.
The following requirements are imposed on the chimney of a classic masonry fireplace with an open firebox: its channel can only be straight-through, with a cross section of at least 1/10 of the cross section of the furnace opening. “Island” fireplace), they are equipped with a smoke hood that passes into the chimney. The latter should have an increased cross section, and quite often it is also equipped with an exhaust device (smoke exhauster).
Closed hearth fireplaces are the most common today. Their fundamental difference from the classical ones is that the air necessary for combustion does not enter through the furnace opening, but through the blower, and by manipulating the dampers, you regulate the intensity of combustion. The vast majority of fireplace inserts can be equipped with a stainless steel modular chimney.
And finally, the last type is the fireplace stove. The main distinguishing feature of such devices, which makes them look like a real stove, is the presence of a built-in smoke channel, passing through which the flue gases are cooled to a fairly low temperature. In this regard, there is a need for a massive masonry or well-insulated modular chimney.
Exhaust gas temperature, °C
With open hearth
Air access is not limited
Up to 600*
Brick, heat-resistant concrete
With closed firebox
Air access may be limited
Brick, from heat-resistant concrete, modular insulated from stainless steel or ceramic, within the heated premises — single-circuit steel enameled
Air access is limited, gases are cooled passing through integrated channels
Up to 85
In addition to those listed above: from talcomagnesite or talcochlorite — massive or with an inner pipe (steel, ceramic)
* — when using hardwood, coal as fuel, as well as with excessive traction, the temperature may exceed the specified value;
** — for fireplace stoves made of talcomagnesite; for metal — up to 400 °C
Is it possible to connect two pipes to the chimney
The question of the possibility of connecting two fireplaces to one chimney is debatable. According to the requirements of SNiP 41–01-2003, “as a rule, a separate chimney or channel should be provided for each furnace. It is allowed to connect two stoves located in the same apartment on the same floor to one chimney. When connecting chimneys, cuts should be provided in them (middle walls dividing the chimney into two channels. — Ed.) At least 1 m high from the bottom of the pipe connection.
As for the cut, it can only be done in a brick chimney. If the chimney is modular, it is enough to connect the pipe of the second furnace to the pipe of the first one using a tee (if the smoke channels have different diameters, then the smaller one is cut into the larger one), after which it is necessary to increase the cross section of the channel. How much? Some experts believe that if the simultaneous operation of furnaces is planned, then the cross-sectional area is determined by simple summation. Others believe that it is enough to “throw” 30–50%, since two fireboxes will warm up the common pipe better and the draft will increase, but this only applies to chimneys more than 6 m high.
When connecting two stoves located on different floors to the same chimney, everything is much more complicated. Practice shows that such systems work, but only with careful calculation and numerous additional conditions (increasing the height of the chimney, installing gates after the lower furnace and on the inlet pipe of the upper one, observing the kindling sequence or completely eliminating simultaneous operation, etc.).
We draw your attention to the fact that everything said in this section applies only to fireplaces with a closed firebox. An open firebox is more fire hazardous and demanding on draft, therefore it does not allow any “liberties” and requires the construction of a separate chimney.
How to improve chimney draft
Poor draft is usually due to errors in the design of the chimney. The desire to explain it by adverse weather conditions (changes in atmospheric pressure and air temperature) is unreasonable, since these factors are also taken into account with a competent decision.
Causes of poor traction
- Insufficient height of the chimney as a whole or that part of it that rises above the roof.
- Incorrectly selected channel section: too narrow a pipe cannot ensure the exit of the entire mass of gases formed; too wide warms up worse, swirls of gas flow are possible in it, and cold street air can form reverse flows.
- Poor pipe insulation.
- Too long non-vertical sections, especially at the top of the chimney.
- Lack of combustion air in the room (it was necessary to provide an additional supply channel in the design of the chimney).
The overturning of the draft by the wind occurs due to the insufficient height of the pipe above the roof. Roof ridges create airflow turbulences; on the lee slope, it is directed downward and is able to blow the flue gases back into the stack.
It is much more difficult to determine the cause in each specific case, since several factors often act at once, none of which plays an independent role. To improve draft, it is necessary to change the design of the chimney, sometimes not too significantly (for example, increase the thickness of the thermal insulation on the last one and a half to two meters of the pipe). There is also such a problem as excessive traction. You can deal with it with the help of a gate. It is only necessary to provide for its installation before starting the installation of the chimney.
In which pipes does condensate form?
The main gaseous combustion products of carbonaceous fuels are carbon dioxide and water vapour. In addition, during combustion, the moisture present in the fuel itself (wood) evaporates. As a result of the interaction of water vapor with sulfur and nitrogen oxides, acid vapors of low concentration are formed, which condense on the inner surface of the chimney when they are cooled to a temperature below the critical one (when wood is burned — about 50 ° C).
The amount of condensate is directly dependent on the area of this surface and inversely on its temperature. If you heat a fireplace with an external non-insulated metal chimney in the cold season, the amount of condensate can be measured in liters per day.
A brick pipe is able to accumulate heat, therefore it behaves differently: condensate forms only at the stage of heating the pipe (although this is a rather long period of time). In addition, the material partially absorbs condensate, so the latter is not too noticeable, which, however, does not prevent it from exerting a destructive effect on the masonry. If the intensity of combustion is low and the ambient temperature is low, the brick may cool down and condensation will begin to form again.
With an insufficient thickness of the insulation and a low temperature of the exhaust gases (the furnace is adjusted for long-term combustion), condensate can also appear in the sandwich-type modular chimney. One way or another, it is impossible to completely get rid of condensate, it is only necessary to reduce its amount to a minimum (the main means for this is the use of more effective thermal insulation) and prevent leaks.
We have touched on only a small part of the problems associated with the coexistence of a pipe and smoke. Trying to answer all the questions that the owners of fireplaces have in one article is an impossible task. Often an individual approach is required, and, as experts note, sometimes only experience and professional intuition can suggest the right decision.