How to independently calculate the diameter and height of the chimney, making the stove safe. Independent calculation of the chimney height with formulas and examples How to calculate the chimney for a boiler

The stove or fireplace is called “the heart of the house” for a reason. But taming a fire inside a residential building involves a number of steps and a long set of rules. After all, any mistakes in the design of a chimney are too expensive, from suffocating smoke inside a building to a fire. And most often it all starts with a violation of the draft and destruction of the walls of the chimney, and then the adjacent building structures ignite.

Today calculations of the height of the chimney are often carried out through special programs, although experienced specialists always check the obtained values manually, using formulas that it makes sense to get acquainted with for the sake of peace of mind.

They are not difficult, for their understanding it is enough school knowledge of geometry and the ability to substitute values in the right place. And we, in turn, will try to explain to you why each indicator is so important for determining the height of the chimney, and how exactly it affects it.

According to all building codes and regulations, the chimney must rise above the roof at a certain distance. This is necessary so that the air on the protruding parts of the roof does not cause back draft due to turbulence.

Reverse thrust can be seen with your own eyes in the form of smoke that pours from the fireplace directly into the room. But the extra height of the chimney is also not needed, otherwise the draft will become too strong and you cannot wait for heat from such a fireplace: the wood will burn to ashes, like a match, without having time to give heat.

If the pipe is located too close to dense trees or a high wall, it must be extended with an asbestos-cement or steel pipe.

In this video, you will also find valuable tips for chimney construction and solutions to problems with its height:

Traction: How To Achieve Perfect Fuel Combustion

The traction force itself is influenced by several important factors at once:

chimney material;
foundation height above sea level;
flue gas temperature at the outlet of the furnace;
cross-sectional shape of the chimney;
smoothness or roughness of the inner surface;
violation of the internal tightness of the chimney;
temperature and humidity of the outside air;
ventilation of a room with a boiler or stove;
completeness of fuel combustion;
the degree of contamination of the boiler (or stove) and chimney;
type of burner used (modulating or discrete).

First of all, you need to determine the value of the static draft of the chimney, and it is measured in the value of ∆p [Pa]. Here is the formula to calculate:

h [m] = (∆p · Tp · Tn) / (3459 · (Tp-1,1 · Tn))

Tr Is the average temperature in the pipe, and Tn- outside temperature. It is measured by default in degrees on the Kelvin scale, but you can specify it in Celsius by adding +273.

Calculating the average temperature is not difficult. Usually it is reported in the technical data for the boiler, but it is also important to take into account the cooling. This is 1 degree for each meter of a brick pipe, 2 degrees for a meter of insulated steel pipe and 5 degrees for an uninsulated pipe.

At the same time, it is advisable to take the value of the outside temperature that is typical for summer as the most problematic time for traction:

Make an aerodynamic calculation and find out the exact required height and diameter of the chimney. By itself, the magnitude of thrust means the difference in density of air and flue gas, multiplied by the height of the house. It is 5 meters of the chimney that provide vacuum and draft for smoke.

But what if the height of the pipe cannot be set higher, and the thrust is still insufficient for certain reasons? This often happens when the flue gases cool down too quickly, especially during the cold season. Then, to restore traction, the desired section of the pipe is simply insulated.

Also remember that the real thrust is always less than the static thrust due to the resistance of the gas movement inside the pipe walls. The narrower the flow area of the chimney, and the more bends, horizontal sections and the like in it, the worse the draft will be, because the draft is affected by the pressure loss along the entire length of the pipe.

Another problem with chimney height is the cold air from the fireplace. So, when he is not working, cold air from the street is released from him. This happens when the chimney head is below the end of the ventilation hood, or when the attic is too large and poorly insulated.

Chimney design depending on the heating configuration

And first of all, when designing a chimney, its minimum throughput rates are calculated. If you make mistakes here, flue gases will accumulate inside the pipe and cause a lot of problems.

The general layout of the chimney looks like this:

If the temperature of the flue gases is low, as in modern low-temperature boilers, then so-called electric smoke exhausters are installed in the upper part of the chimney.

They are a small fan with blades. Such a device forcibly removes the combustion products from the pipe, thereby increasing the traction force. And then the thrust force no longer directly affects the height of the chimney, because it is achieved in a different way, and not by "catching the wind".

If there is no additional device, then you still have to catch the wind. And in this case, you need to build on the available power of the boiler, stove or fireplace, which can be found in the technical documentation. It is expressed in the amount of fuel that is burned in one hour of operation.

If the amount of fuel volume is known, then the volume of gases is calculated using the following formula:

Vg = B ∙ V ∙ (1 + t / 273) / 3600

The result will be in m 3 / s. This is the speed of movement of gases in the pipe. We calculate the section of the pipe using the following formula:

F = π ∙ d² / 4

And the resulting value is determined in m 2. This is the cross-sectional area of the chimney, and the diameter is calculated by the formula:

dt = √4 ∙ B ∙ V ∙ (1 + t / 273) / π ∙ ω ∙ 3600

The rest of the characteristics are almost the same for most heating devices. So, the rate of exit of gases in the chimney is usually not less than 2 meters per second, and the temperature of the gases at the entrance to the pipes is from 150 to 200 degrees.

Also, the standard pressure of gases per 1 meter is not less than 0.4 mm H 2 O, or 4 Pa:

Therefore, according to SNiP, the height of the chimney from the grate must be at least 5 meters.

Roof flammability: when a spark leads to trouble

The roofing material itself is also important, namely, its flammability. So, with a fireproof coating, the height of the pipe should be increased by 1-1.5 meters so that the sparks do not reach the roof:

Dependence of the height of the chimney on other elements of the roof

Of great importance is also how close the chimney itself is to the ridge of the roof, the parapet or its other elements:

So, with regard to the elevation of chimneys above the roof, there are the following rules:

Not less than 1.2 meters above a flat roof.
Not less than 50 cm above the roof ridge if the pipe is located up to 1.5 meters from the ridge.
Not below the level of the ridge if the pipe is located 1.5 to 3 m from the ridge.
Not below a line that can be drawn from the ridge down to the horizon at an angle of 10 degrees if the pipe is located more than 3 meters from the ridge.

In this case, the smoke channel should be located from other elements of the structure at a certain distance, at least:

150 mm for pipes with insulation;
500 mm for pipes without insulation.

The minimum allowed pipe height is 50 cm. But these are too low pipes that are allowed to be installed only on flat roofs without any protrusions. If the roof is with a more complex configuration, you will have to tinker and take into account all the protruding parts.

So, if all these parts are located at a distance of 1.5 meters from the pipe itself, then the pipe just needs to be higher than all these elements. If they are closer than 1.5 meters, then the chimney must exceed their height by at least 59 cm:

It is not difficult to maintain a chimney height of 5 meters from the grate of the furnace to the upper cut of the chimney if a one- or two-story house was built. But problems arise if the fireplace was installed on the upper attic floor - there is not enough ceiling and attic height.

Environmental considerations for industrial buildings

In short, the throughput of the chimney must ensure the unimpeded passage of smoke and its release into the atmosphere. Moreover, the ecological point is also important here, namely, whether the products of fuel combustion are properly dispersed in the atmosphere.

So, during the construction of commercial and factory enterprises, certain sanitary standards are taken into account. And they depend on the weather conditions of the area, the typical flow rate of air masses, the relief of the landscape and many other factors.

So, what value did you get and is it exactly 5 meters?

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How to calculate a chimney - 4 important points to consider when installing a chimney

For heating private houses in the cold season, most often they use either ordinary brick stoves and fireplaces, or household heating boilers for solid, liquid or gaseous fuels. An indispensable condition for the normal operation of such heating devices is the free flow of a sufficient amount of fresh air into the combustion zone of the flame, and the rapid removal of spent fuel combustion products into the atmosphere. To ensure that these conditions are met, before installing the stove chimney, it is very important to perform a competent calculation of the chimney with natural draft, since this will affect not only the efficiency of heating devices, but also the safety of residents of a private house.

Due to which natural draft is formed in the oven

Most heating and cooking stoves and autonomous heating boilers are not equipped with a system of forced supply of fresh air and removal of exhaust flue gases, therefore the process of fuel combustion in them directly depends on the presence of natural draft in the chimney pipe.

In theory, the chimney calculation method is quite simple. To make it clear to the reader where the natural thrust comes from, then I will try to briefly explain the physics of thermal and gas-dynamic processes that occur in a furnace during fuel combustion.

The chimney is always installed vertically (with the exception of certain horizontal or inclined sections). Its channel begins in the upper part of the furnace vault, and ends in the street, at a certain elevation above the roof of the house;

Hot flue gases in the fuel combustion zone have a very high temperature (up to 1000 ° C), therefore, according to the laws of physics, they quickly rush up;
Rising up the chimney at a speed of about two meters per second, the flue gases create a zone of reduced pressure in the furnace;
Due to the natural rarefaction in the firebox, the inflow of fresh air through the blower and grate into the combustion zone is ensured;
Thus, it is easy to understand that for the formation of good natural traction, several conditions must be met at once:

The chimney must be located strictly vertically... In addition, the ode should be of sufficient height and the most straightforward configuration, without unnecessary bends and turns at an angle of more than 45 °.

The internal section of the flue duct must be designed in such a way that it allows the entire volume of flue gases that are formed during the combustion of fuel to freely pass into the atmosphere;
In order not to create significant aerodynamic resistance to the movement of smoke, the inner walls of the pipe must have the most even and smooth surface with a minimum number of transitions and joints;
As they move along the pipe, the flue gases gradually cool down, which leads to an increase in their density, and a tendency to form. To prevent this from happening the chimney must have good thermal insulation.

Outdoor wind has a significant positive effect on natural draft. This is due to the fact that a continuous flow of air directed perpendicular to the axis of the chimney creates a reduced pressure in it. Therefore, in windy weather, there is always good draft in the stove.

Moment 1. The choice of material and design of the chimney

The regulatory and technical construction documentation does not stipulate any stringent requirements for the arrangement of stove chimneys, therefore, each homeowner makes a chimney at his own discretion. At the same time, I must say that all types of chimneys differ not only in design and appearance, but also in heat engineering, weight and gas-dynamic characteristics.

The chimney made of masonry is characterized by high strength and durability, withstands long exposure to high temperatures well, but poorly resists the effects of aggressive smoke condensate. Thanks to its massive brick walls, it has a high heat capacity and satisfactory thermal insulation properties. As for the issue of condensation of water vapor and gas dynamics of a brick chimney, not everything is so good here.

Massive brick chimney is heavy, therefore, to install it, you need your own foundation, which in turn also requires separate calculations;

The rectangular or square shape of the cross-section of the flue ducts, in combination with uneven and rough inner walls, create significant resistance to the movement of flue gases, therefore, the cross-section of such chimneys should be selected with a small margin;
Lack of additional thermal insulation can lead to the formation of condensation inside the chimney, therefore, its walls must be of sufficient thickness so that the temperature of the flue gases inside does not fall below the dew point.

Asbestos-cement and ceramic pipes are sold ready-made, and are easily assembled by hand, so they are often used in the construction of private houses to connect gas or solid fuel boilers. Many homeowners are attracted by their not very low price, but I want to remind you that when installing a chimney from asbestos-cement pipes, you need to take into account the following points:

Asbestos-cement pipes have high thermal conductivity and poorly retain the heat of flue gases, due to which condensation can form inside, which will quickly lead to the destruction of the walls;
To prevent this from happening, when installing an asbestos-cement chimney, it is important to choose the right heat-insulating material and calculate its thickness in such a way that the temperature of the flue gases at the outlet does not fall below 110 ° C;
At temperatures above 350 ° C, asbestos cement can crack and collapse, therefore, between the inlet of the chimney and the outlet of the boiler, I advise you to install a spacer made of an insulated metal pipe;
Its length should be calculated so that the flue gas temperature at the inlet to the asbestos-cement pipe does not exceed 300-350 ° C;
Asbestos-cement pipe itself has sufficient rigidity. Despite this, for better thermal insulation and protection from mechanical damage, I recommend installing such a chimney inside a protective jacket made of half-brick masonry.

Metal sandwich pipes made of stainless steel, in my opinion, are the most successful option for a home chimney, which is equally well suited both for a massive brick slab and for a modern compact heating boiler. They are recruited from separate sections, so they allow you to make an external or internal chimney of almost any configuration with your own hands.

The inner sleeve made of heat-resistant stainless steel has a perfectly smooth surface and a circular cross-section, therefore it creates a minimum aerodynamic resistance to the flow of flue gases. For this reason, the inner diameter of the flue duct must correspond to the minimum value of the design characteristics;

Insulated metal sandwich pipe has good thermal insulation properties, and does not need additional insulation, therefore, heat engineering calculations, in this case, are not necessary;
When installing and assembling the chimney, each section must be mounted in such a way that it is attached to the inner wall or facade of the building at least two points. In this case, the distance between the mounting brackets should be no more than 1200 mm.

Prefabricated insulated ceramic chimneys have similar characteristics, and can also be used with almost no restrictions, in combination with any type of stove, fireplace or domestic heating boiler.

They are designed and manufactured at the factory, in compliance with all the necessary heat engineering calculations and the requirements of fire safety rules;
This makes it possible to mount them in the form in which they are, without thinking about their own additional calculations;
Despite this, I want to remind you that such a sandwich made of expanded clay concrete blocks, mineral wool insulation and an insert from a ceramic pipe, assembled can be heavy, therefore it is also necessary to calculate and make a separate foundation for it.

Recently, a relatively new type of polymer chimneys, better known under the trade name "Furan Flex", began to appear on the building materials market. It is a flexible armored hose that is installed in an existing flue duct and then filled with hot steam at high pressure. Under the influence of pressure and high temperature, the sleeve straightens and polymerizes, as a result of which it completely fills the lumen of the smoke channel and strengthens the walls of the pipe from the inside.

Installation of such a polymer insert requires the use of special equipment. and strict adherence to technological modes, therefore it can be performed exclusively by qualified specialists;
Based on this, in this case, I do not recommend bothering myself with complex formulas, and entrust the execution of all calculations to the engineers of the contractor that will carry out the installation.

Asbestos-cement pipe has a rough inner surface, which contributes to the rapid adhesion of soot and soot. Over time, the growing layer of soot reduces the internal cross-sectional area and increases the aerodynamic resistance of the smoke duct, so I do not recommend using such pipes for stoves and boilers for solid and liquid fuels.

Moment 2. Calculation of the inner diameter of the chimney for solid fuel stoves and fireplaces

To perform the correct calculation of the chimney draft, it is necessary, first of all, to determine the required internal cross-sectional area. In this section, I will tell you how this is done, using an example of calculating the cross-section of a chimney for heating stoves and solid fuel fireplaces.

First of all, you need to determine how much flue gas will be generated when a certain type of fuel is burned in a furnace in one hour. This calculation is performed according to the following formula:

V gas = V * V fuel * (1 + T / 273) / 3600, where

V gas- volume of flue gas that will pass through the pipe in one hour (m³ / hour);

B- the maximum mass of fuel that burns within one hour in the furnace (kg);

V topl- the coefficient of the volume of flue gases that are emitted during the combustion of a certain type of fuel (m³ / kg).

This value is determined according to special tables, and its value is: for dry firewood and lump peat - 10 m³ / kg, for briquetted brown coal - 12 m³ / kg, and for coal and anthracite - 17 m³ / kg;

T- temperature of flue gases at the outlet of the chimney (° C). With a normally insulated chimney, its value can be from 110 to 160 ° C.

Having the obtained value of the total volume of gas passing through the pipe per unit of time, it will be easy to calculate the required cross-sectional area of the chimney channel. It is defined as the ratio of the volume obtained to the speed of movement of the flue gases, and is calculated using the following formula:

S smoke = V gas / W, where

S smoke- cross-sectional area of the smoke channel (m2);
V gas- the volume of flue gases per unit of time, which we received in the previous formula (m³ / hour);
W- the reduced velocity of the ascending movement of the gas-flue flow inside the pipe (m / s). Here I must say that this value is conditionally constant, and its value is 2 m / s.

In order to understand what pipe diameter we need to make a chimney, based on the obtained value of the area of the circle, we need to determine its diameter. For this, the following formula is applied:

D = √ 4 * S smoke / π, where

D- inner diameter of a round chimney (m);
S smoke- the area of the internal section of the chimney, obtained in previous calculations (m2)

To make it clear to the reader, I propose to consider a simple example of calculating a chimney for a heater-stove, if it is known that during heating, 8 kg of dry firewood burns in it per hour, and the temperature of the flue gases at the outlet is 140 ° C.

Using the first given formula, we determine the maximum volume of smoke that can be released in one hour of burning 8 kg of dry firewood: V gas = 8 * 10 * (1 + 140/273) / 3600 = 0.033 m³ / hour;
Using the second formula, you need to calculate the required cross-sectional area of the smoke channel: S smoke = 0.034 / 2 = 0.017 m²;
The last formula allows you to determine the required pipe diameter, based on the known area of its cross-section: D = √ 4 * 0.017 / 3.14 = 0.147 m;
Thus, we have determined that for this stove, a chimney with an internal diameter of at least 150 mm will be needed in the bath.

If during the calculations you get a non-integer number, then I advise you to round it up to an integer value, however, such rounding is allowed within reasonable limits, because in this case, a very large diameter does not mean very good.

Moment 3. Calculation of the chimney pipe for household boilers

In this article, I deliberately did not give separate calculations for prefabricated household solid fuel and gas boilers, since any instruction on the use of boiler equipment already contains all the necessary technical information.

Knowing the passport thermal power of your gas boiler, the diameter of the chimney is easy to select, in accordance with the previously calculated parameters.

For small heating boilers with a maximum heat output of no more than 3.5 kW, a pipe with an inner diameter of 140-150 mm will be sufficient;

For household boiler equipment of average power (from 3.5 to 5 kW), chimneys with a diameter of 140 to 200 mm will be required;
If the power of the heating boiler is from 5 to 10 kW, then pipes with a diameter of 200 to 300 mm will need to be used for it.

If the gas boiler is equipped with a built-in turbine to create forced draft, then the diameter of the exhaust pipe may be much smaller than the values given above. In this case, the recommended pipe size must be indicated in the product data sheet.

Moment 4. Determination of the height of the pipe and location on the roof

The force of natural draft largely depends on the height difference between the level of the grate of the firebox in the lower part of the stove, and the windproof deflector or the mouth of the smoke channel in the upper part of the chimney.

In order for the heated flue gases to use their energy as efficiently as possible to create natural draft, it is very important to correctly calculate the height of the chimney relative to the grate and relative to the ridge of the roof.

The relative height of the chimney, from the grate level to the chimney mouth, must be at least 5000 mm;

On residential buildings with an exploited flat roof, the chimney mouth must be located at least 500 mm higher than the maximum height of the side parapet or roof railing;
On houses with a gable or hip sloped roof, the chimney mouth should be located at least 500 mm from the level of the roof ridge;
If on a sloped roof the chimney is located on one of the slopes, at a distance of no more than 1500 mm from the ridge of the roof, then it must also rise 500 mm above the level of the ridge;

In the event that this distance is from 1500 to 3000 mm, the fume's windproof deflector can be located at the level of the roof ridge;
On gently sloping roofs with a small slope angle, the chimney can be located at a distance of more than 3000 mm from the ridge. In this case, its optimal height is calculated in accordance with the diagram shown in the figure below.

The wrong choice of the height of the pipe or its location relative to the ridge of the roof, with an unfavorable wind direction, can cause the formation of reverse thrust. This phenomenon is very dangerous, as it can lead to the release of burning embers and poisonous carbon monoxide from the ashtray or firebox into the living space.

Conclusion

Summing up, I want to note that when choosing materials, sizes and configuration of the chimney, first of all, one should proceed from the maximum heat output of the heater. In this case, you also need to take into account your financial capabilities, and what types of fuel your stove or heating boiler is designed for.

More details about all the described types of chimneys can be found in the attached video in this article, and if you have any questions or comments, I suggest discussing them in the comment form.

September 14, 2016

If you want to express gratitude, add clarification or objection, ask the author something - add a comment or say thank you!

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Much attention is paid to the chimney duct in the manufacture of a fireplace or stove, since it depends on this element how efficiently the entire system will function. In this case, one of the main tasks is to perform calculations and its cross-section. These parameters determine the conditions for optimal draft, depending on the type of heater and other parameters. Today we will talk about how such calculations are performed. At the same time, we do not urge you to perform them yourself, since without practice no theory can guarantee a high-quality result - however, new knowledge will not harm anyone.

There are several reasons why we calculate the height of the chimney.

Sandwich Chimney Prices

Sandwich Chimney

How to calculate the pipe height yourself

Next, a self-calculation methodology will be given - it is focused on SNiP 41-01-2003 "Heating, ventilation and air conditioning"... According to this document, we have the following information:

the minimum height of the chimney, which is considered from the head to the grate, is 5 m;
the optimal height should be 6 m.

However, these data absolutely do not say anything about which parameter to choose in a particular case, for a specific equipment. Therefore, experts use the following formula.

Based on the formula presented, we will derive the main parameters that are necessary for accurate calculations.

A- meteorological conditions of the surrounding area. This refers to a certain coefficient that has already been calculated by professionals and presented in descriptive documents. For example, for the northern regions, this parameter is set to 160.
Mi Is the mass of substances passing through a certain unit of time. This parameter can be found in the documentation supplied with the heater.
F- the sedimentation rate of particles that are formed during fuel combustion. This indicator can be found in the regulatory documents for the type of fuel used. Take wood and an electric heater as an example. In the first case, the calculated value is 25 units, and in the second - 1.
(Spdki and Sphi)- the concentration of various substances in the gas to be removed. Both indicators are also taken from the instructions supplied with the heating equipment.
V- the volume of the evacuated gas.
T- temperature difference between incoming air and gas at the outlet of the pipe.

The height of the chimney above the roof is also a very important parameter. It is determined based on the shape of the roof - all data are also taken from the above SNiP.

If the roof of the building is flat, then the height of the pipe is determined as follows.

Table. Flat roof pipe height.

If your roof is pitched, then the height of the chimney will be influenced by its location relative to the roof ridge - we mean the distance (distance between them).

Table. The height of the pitched roof pipe.

Also, the height of the chimney above the ridge can be affected by third-party factors, such as buildings located in the immediate vicinity and tall trees. The presence of such obstacles forms a wind support zone. In this zone, it is almost impossible to equip a good draft, which will be enough for the normal operation of the heater. To get out of this wind zone, it is necessary to increase the height of the chimney by at least 50 cm.

A similar situation will be if the heating equipment is located in a low utility room, attached to the house or located in the immediate vicinity. Both options are shown in the diagram above.

I greet my dear reader and bring to your attention an article on how to correctly calculate the chimney in your home.

The main component of comfort in a home is warmth. The owner of the house has to solve the problems of heating a private house. For heating, which can be assembled independently (except for gas), including chimneys.

In order to ensure complete removal of flue gases from the room, good draft and prevent the draft from overturning, it is necessary to comply with the requirements of regulatory documents.

It is the chimney that ensures the safety of residents. An improperly designed chimney is unable to completely remove toxic combustion products (carbon dioxide and carbon monoxide, sulfur oxides), smoke and soot. Even a fire is possible if hot smoke enters the room. Particularly dangerous is the overturning of the draft - the rotation of the smoke movement in the opposite direction.

Therefore, when installing a chimney, it is necessary to correctly calculate its parameters - it is they that ensure the creation of an ideal draft and the complete removal of harmful substances.

For what parameters do you need a calculation

The main parameters of any vertical chimney are the length and internal (working) section of the flue duct. All the information below is equally true for heaters using any type of fuel, but the parameters of chimneys for gas boilers are calculated by specialists of a licensed design organization.

Length

The chimney length is determined by SP 7.13130.2013. Its length (or height) is measured from the head at the top of the pipe to the grate of the heating boiler. The standard provides for the minimum height of the chimney and the height of the head relative to the ridge of the roof.

It should be borne in mind that the height determined by the calculations is nominal. In fact, the chimney of a modern boiler is located on the side, the boiler outlet is located in the upper part of the unit at a certain height relative to the bottom of the furnace. Therefore, the actual length of the vertical part of the chimney will be slightly less than the calculated data - by the distance from the bottom of the furnace to the outlet. This distance can be determined from the unit drawing or measured.

Do not think that lengthening the chimney is good. The draft is based on the upward rise of hot exhaust gases, and if the pipe length is too long, the gases are cooled and the draft is reduced. For the same reason, the chimney must be insulated. Even traditional red brick chimneys need to be insulated. The draft situation is aggravated by the fact that the exhaust gases from modern boilers with high efficiency have a low temperature.

With a moderately elongated chimney, the draft increases, while more heat is emitted into the atmosphere. Excessive gas flow will extinguish gas heating units. ...

The installation of a low chimney in a residential building is completely unacceptable - there is no sufficient vacuum in it, the gases rise at a slower speed, due to which the draft decreases. It is allowed to install low chimneys on freestanding stoves, for example, in a garden for a barbecue, etc.).

Chimney diameter

The diameter of the chimney is no less important for the creation of draft. When installing modern heating units, the minimum diameter is determined simply - according to the unit's passport (and the diameter of the outlet pipe). In addition, the regulatory documents indicate the minimum dimensions of the chimney cross-section corresponding to a unit of a certain power. These data are given in the unit's passport.

If the internal section of the chimney is chosen square for some reason, then its area should be no less than a round structure.

When using an existing traditional stove, the chimney is left the same. When installing a brick oven and chimney with your own hands, you need to use the project (preferably not downloaded on the Internet!) And lay out the pipe in exact accordance with the drawing. The same applies to free-standing brick structures.

Calculation methods

Professionals use several methods for calculating the parameters of the chimney. When installing large industrial pipes, designers are entrusted with very complex calculations. They take into account:

quantity and composition of flue gases;
concentration of harmful substances in combustion products;
pipe material;
roughness of the inner surface of the hog;
meteorological situation in the region.

Calculation of large pipes often serves as a topic for graduate work at the institute. Fortunately, the size of chimneys in a private house is smaller, and the power of heating units is also inferior to industrial facilities.

When designing a small pipe for a private house, simpler, albeit fairly accurate, calculations are used. The parameters of the chimney obtained as a result of calculations ensure complete evacuation of combustion products from the furnace and ensure the safety of residents. Such calculations allow avoiding errors in the choice of parameters and during the installation of the chimney, to avoid subsequent alteration of structures. Such alterations in an already built and finished house are much more difficult than in the construction process.

When calculating the parameters of the chimneys of a private house, they use accurate and approximate (Swedish) calculation methods.

Exact method

For heating, ready-made heating or heating and cooking units of industrial production with a certain diameter of the chimney are often installed. With sauna stoves or oil stoves for a garage, ready-made sandwich chimneys of a small diameter (100 mm) are usually also mounted.

For more accurate calculations, it is necessary to know the characteristics of the fuel and the heating boiler, the rate of rise of flue gases, the amount of fuel burned, the shape of the chimney (cross-sectional shape, number of bends). The calculations are quite complicated and require the use of some special data, therefore, we will consider the calculation for a heating unit (fireplace) on wood - they are often made original, relying only on the experience of the stove-maker, therefore, the calculation of the cross-section is necessary.

Let the chimney be of a circular cross-section, without turns. Calculation data:

the average speed of movement of flue gases along the hog v is taken equal to 2 m / s;

the mass of firewood that burns within an hour in the furnace - B = 5 kg / h (humidity 25%);

temperature of outgoing gases - t = 160 ° С.

Formula for determining the volume of outgoing gases:

where V is the volume of air required for complete combustion of 1 kg of fuel. For firewood with a moisture content of 25%, for combustion of 1 kg, 10 m3 of air is required.

The calculated value is rounded up to the nearest round pipe size (for example, 125 mm). A square or rectangular brick chimney is not made less than 140 × 140 mm. The section of the pipe for a traditional stove should be one and a half times larger than the size of the blower.

The volume of combustion products under normal conditions and the temperature of the outgoing smoke are normative data. See table:

Swedish calculation method

There is a so-called Swedish calculation method based on the choice of the height of the chimney depending on the ratio of the area of the hearth of the furnace and the cross-section of the pipe. Usually the method is used for wood-burning fireplaces. Calculate the ratio:

then the ratio is converted into percentages and according to the graph, depending on the resulting size and the shape of the hog, the height of the chimney is chosen.

When using the Swedish method of calculation, it is possible to calculate the cross-sectional area of the boron channel based on the area of the hearth of the furnace and the height of the pipe, determined in accordance with SP 7.13130.2013, depending on the height of the roof ridge and the location of the chimney relative to the ridge (see below).

How to correctly calculate the parameters of the chimney

After calculations, the height of the pipe is checked against the requirements of the regulatory document - it must be no lower than that required by SP 7.13130.2013. In any case, a larger length value is taken.

The best chimney diameter - corresponding to the passport of the heating unit.

Determination of the chimney height

The height of the chimney is determined even at the design stage of the house, depending on the location of the chimney on the roof, the distance to the ridge or to neighboring tall objects (houses, large trees).

Why is this parameter needed

The draft depends on the height of the chimney, which means the reliability and safety of the heating unit and the safety of the residents of the house. In case of insufficient removal of flue gases, the efficiency of the boiler, stove or fireplace decreases, there is a danger of poisoning the residents with carbon monoxide or carbon dioxide and the danger of fire.

SNiP requirements

In accordance with SP 7.13130.2013, the minimum height of the chimney from the head to the grate of the heating unit must be at least 5 m.When the pipe comes out onto a flat roof, its height must be at least 0.5 m. The maximum length of the horizontal section must not exceed 1m.

To create the required vacuum when the chimney leaves the roof, the height of the pipe head above the roof is also specified (see the table below).

There is also the concept of wind back. If the chimney is close to a tall building, tree, or a low freestanding chimney is close to a house, then these objects can block the chimney from the wind and reduce draft. To determine if the pipe does not fall into the zone of wind support, it is necessary to draw an inclined line at an angle of 45 ° to the horizontal through the top point of the house or tree. The pipe head must be above this line.

The larger of the two heights is chosen. If, in order to comply with regulatory requirements, it is necessary to build a very high chimney (much higher than 5 m), then you will have to install a smoke exhauster that provides sufficient traction for any length of the chimney.

Chimney height table above the ridge

The height of the chimney above the ridge must comply with SP 7.13130.2013. Information is given in the table:

The distance from the ridge to the pipe is measured strictly horizontally.

Calculation of the section of the smoke channel

SP 7.13130.2013 determines the minimum cross-sections of the chimney channel of a brick or concrete chimney, depending on the power of the heating unit:

up to 3.5 kW - 140 × 140 mm (Ø not less than 158 mm);
3.5-5.2 kW - 140 × 200 mm (Ø not less than 189 mm);
5.2-7.0 kW - 140 × 270 mm (Ø not less than 219 mm).

When modernizing furnaces with an unknown power, either the chimney of the existing section is left, or a round stainless steel insert of a suitable diameter is inserted inside. If modern ceramic or sandwich pipes are installed to the old furnace, then the inner diameter of the modules must be no less than the diameter of the liner that would fit into the old pipe when installed. Theoretically, you can look for an old furnace design, but in most cases this is not possible.

Most often, a stove-maker was invited, and he put together stoves and chimneys of 2-3 mastered standard sizes without any projects and calculations. It is impossible to really determine all the features of a brick stove by its appearance, and the only guideline when choosing a cross-section of a liner or a new chimney is the section of a hog of an already existing chimney (provided that it worked well). If a chimney, then when replacing or modernizing it, it is better to invite a heating engineer and entrust him with the appropriate calculations.

The diameter of pipes with an inner surface made of smooth ceramics or stainless steel can be slightly smaller (by 30%). In the valves of solid fuel stoves, there must be an unblocked opening with a cross section of at least 15 × 15 mm.

Internal section of the chimney: which is better?

In a square or rectangular chimney, there will be swirls of smoke in the corners, slightly inhibiting the flow of gases and contributing to the settling of condensate and soot. For the same reason, the corners of the pipe practically do not participate in the flow of gases. In addition, square and rectangular hogs have a larger wall surface area than round hogs - this increases the friction of the rising gases against the walls and slows them down more.

Therefore, it is advisable to choose a round-shaped chimney, and modern pipes made of sandwich or ceramics are produced precisely of a circular cross-section, like pipes made of stainless steel and asbestos cement. The square or rectangular shape is found in old brick chimneys. But for modern boilers, stoves and fireplaces, a round stainless steel insert must be inserted into the brick box.

What to do if the chimney cross-section is square

But it is also possible to use a square-section chimney - a square or rectangular shape does not significantly affect the draft, and a complete alteration of the pipe is difficult and costly. Brick chimneys of traditional stoves from time immemorial had a square or rectangular shape, if necessary, you can use the old chimney now.

It should be borne in mind that with a modern heating unit, a brick pipe without an insert will not work for a long time - it will be destroyed by chemically aggressive condensate.

The brick pipe should have a cross-section of approximately one and a half times that of a round chimney with smooth steel walls. Therefore, the installation of the liner does not impair the smoke extraction capacity of the square brick pipe.

What should be the diameter of the chimney for the stove

Ideally, the diameter of the chimney should be equal to the diameter of the boiler outlet pipe or slightly larger (if the pipe has a non-standard section).

The diameter of the chimney for the stove is determined from the cross-sectional area according to the eternal school formula: s = πd2 / 4. The cross-sectional area must be equal to or slightly less than the value of the cross-sectional area of a square hog, determined according to regulatory documents, depending on the power of the boiler.

How the diameter of the chimney affects its height

When installing a chimney, there is an immutable relationship: the narrower the section of the hog, the higher the pipe should be. The taller chimney increases the draft and the rate of rise of the flue gases. But an excessively high one begins to slow down the traction - the hot gases are cooled, they are also slowed down by friction against the walls of the hog, especially of a rough brick one.

Calculation of the optimal thrust indicator

Draft - reducing the pressure in the chimney. The draft calculation is needed to check whether the height and section of the chimney hog are correctly determined.

where ∆P is the pressure difference at the inlet and outlet of the pipe, Pa;

a - atmospheric pressure, Pa;

h - pipe height, m;

Ti is the average indoor temperature, K;

To - external temperature, K.

For a layman, the draft indicator does not provide sufficient information, therefore, the amount of exhaust smoke that is provided by the presence of draft in the chimney is usually calculated. It is the volume of waste gases that shows how the pipe ensures the complete removal of waste gases with toxic substances and the safety of residents.

The formula is accurate only for air, for smoke the formula is approximate. But even by an approximate value, it can be estimated whether the chimney ensures the safety of people. The calculated amount of exhaust gases must be greater than or equal to the calculated amount of exhaust gases (see above).

where Vcalc is the flow of removed gases, m / s3;

S is the cross-sectional area of the hog, m2;

С - coefficient taking into account friction losses; usually taken equal to 0.65-0.7;

g - acceleration due to gravity, 9.807 m / s2;

h - pipe height, m;

Ti is the average indoor temperature, K (the temperature in Kelvin is 273 units higher than the temperature in ° C);

To - external temperature, K.

Due to which natural draft is formed in the oven

The natural draft in the chimney arises from the rise of hot and less dense smoke upward. Dense cold air is sucked into the firebox from below, oxygen reacts with fuel, air with smoke heats up in the firebox, rises, and so on.

Calculation example

As an example, I will show the calculation of the volume of exhaust air for a pipe with an internal diameter of 0.125 m, an area of 0.012266 m2 and a height, for example, 6 m, an internal temperature of 20 ° C and a minimum external temperature of -20 ° C.

Conclusion: such a pipe ensures complete removal of flue gases from the furnace. Vcalc noticeably exceeds Vinit, therefore, to reduce the cost of the chimney, it is worth calculating Vcalculated for a shorter or narrower pipe and considering the possibility of installing them.

When choosing a chimney with a smaller section and height, it is imperative to calculate the draft at 10 ° C - it will be less than at low temperatures, but Vout should be equal or greater than Vcalc and at positive temperatures, otherwise the complete evacuation of carbon monoxide and carbon dioxide from the room will not be ensured in warm weather.

In this case, Vcalc is significantly greater than Vin, since in the calculations we took a structure 6 m long (and not the optimal 5 m) and rounded up the calculated diameter of 0.118 m upward.

Online calculators

The calculations of the parameters of the chimney are greatly facilitated by ready-made online calculators.

Conclusion

I say goodbye to my dear reader. When building a new cottage or renovating an old house, pay special attention to chimney design - this is the guarantee of the safety of your family. Get new articles from the site, bring friends to the site, share interesting and useful information with subscribers in social networks.

For any boiler rooms - industrial and domestic, as a rule, one chimney, common to all boilers, is designed. The most important part of the project is the aerodynamic design of the chimney.

The material for it can be brick, reinforced concrete, fiberglass. The use of steel analogs with a diameter of more than 1m is permissible only if the technical and economic benefits of such a choice are made.

The main types of calculations for industrial chimneys

Calculation of tube aerodynamics

Note!

This part of the design is needed to determine the minimum throughput of the structure.

It must be sufficient to ensure the trouble-free passage and further removal of fuel combustion products into the atmosphere, when the boiler is operating at maximum load.

It should be noted that the incorrectly calculated pipe throughput can cause the accumulation of gases in the duct or boiler.

A competent aerodynamic calculation makes it possible to objectively assess the performance of the blast and traction systems, as well as the pressure drop in the air and gas paths of the boiler room.

The result of aerodynamic calculations is the recommendations of specialists on the height and diameter of the chimney and optimization of sections and elements of the gas-air path.

Determination of the height of the structure

The next point of the project is the environmental justification of the size of the pipe, based on calculations of the dispersion of harmful products of fuel combustion in the atmosphere.

The chimney height is calculated based on the dispersion conditions for the emission of harmful substances.

At the same time, all sanitary standards for commercial and industrial enterprises must be observed, as well as the background concentration of these substances must be taken into account.

The last characteristic is dependent on:

the meteorological regime of the atmosphere in a given area;
air mass flow rate;
terrain;
the temperature of the discharged gases and other factors.

During this design stage, it is determined:

optimal pipe height;
the maximum permitted volume of emission of harmful substances into the atmosphere.

Durability and stability of the pipe

These calculations are performed to determine the ability of the selected structure to withstand the effects of external factors:

seismic activity;
soil behavior;
wind and snow loads.

Operational factors are also taken into account:

pipe weight;
dynamic vibrations of equipment;
thermal expansion.

Strength calculations make it possible to choose not only the structure and shape of the structure's trunk. They allow, and calculate the foundation for the chimney: determine its structure, depth, foot area, etc.

Thermal calculation

Heat engineering calculation is required:

to find the thermal expansion of the pipe material;
determining the temperature of its outer casing;
choice of type and thickness.

Calculation of the parameters of the chimney in a private house

What you need to know when calculating

To determine the parameters of the chimney of a household boiler room, there is no need to make serious calculations. It is enough to use a simplified calculation scheme.

Note!

To make such a calculation, you need to know the power (heat transfer) of the boiler or furnace, in other words: the amount of fuel burned per hour. This figure is easy to find out by looking in the equipment passport.

The rest of the parameters for all household structures are approximately the same:

temperature of gases at the inlet to the pipe - 150/200º;
their speed in the chimney is not less than 2 m / s;
the height of the household chimney, according to SNiP, must be at least 5 m from the grate;
natural gas pressure per 1m - not less than 4 Pa (or 0.4mmH2O)

To find out the amount of gravity, it is worth considering what it is: the difference in density that air and flue gas have, multiplied by the height of the structure.

In other words: the calculation of the chimney diameter depends on the amount of fuel burned per hour.

Suppose that you already know the amount of fuel burned, then the volume of gases at the inlet to the pipe, at a certain temperature t, is as follows:

Vg = B ∙ V ∙ (1 + t / 273) / 3600, in m³ / sec.
Knowing the speed at which the gases must move in the pipe, you can calculate the area (F) of its cross section:

F = π ∙ d² / 4, in m²

And, based on the formula for determining the area of a circle, you can calculate the diameter (d) of a round pipe:

dt = √4 ∙ B ∙ V ∙ (1 + t / 273) / π ∙ ω ∙ 3600, in meters.
An example of calculating a pipe, we find the required diameter

Let's give a specific example of how the calculation of household chimneys is made.

Let it be a metal insulated pipe.

Let us assume that 10 kg of firewood per hour with a moisture content of 25% is burned on the grate of the furnace.
Then the volume of gases (V) under normal conditions (taking into account the excess air ratio) required for combustion is 10 m³ / kg.
The temperature at the inlet to the pipe is 150º.
Therefore, Vg = (10 ∙ 10 ∙ 1.55) / 3600. After making calculations, we get the volume of gases, 0.043 m³ / sec.
Taking the gas velocity as 2m / s, we calculate the diameter:
d² = (4 ∙ 0.043) / 3.14 ∙ 2, we get the value 0.027.
We substitute all the numbers in the formula dt = √4 ∙ 0.34 ∙ 0.043 ∙ (1 + 150/273) /3.14∙10∙3600. Having made the calculations, we get the required diameter of 0.165 m.

Determination of gravity

Let us determine how the gas is cooled per 1 m of the structure. Knowing that 10 kg of firewood is burned per hour, we calculate the power: Q = 10 ∙ 3300 ∙ 1.16, we get the figure 38.28 kW.
The thermal coefficient for our pipe is 0.34, which means that the losses per meter will be: 0.34: 0.196 = 1.73º.
Therefore, at the exit from the 3 meter trunk (from the total 5 m we subtract 2 m of the furnace)
gas temperature: 150- (1.73 ∙ 3) = 144.8º.
Self-draft value when determining the air density, under normal conditions
at 0º = 1.2932, at 144.8º = 0.8452. We make calculations: 3 ∙ (1.2932-0.8452). We obtain the value of the natural gas pressure equal to 1.34 mmH2O. This wave is sufficient for normal pipe operation.

As you can see, the calculation of a chimney for domestic use is not as difficult as it might seem.