Chimney calculation: dimensions, height above the roof

Mistakes made during the construction of a chimney in a house or in a bathhouse can eventually lead to serious consequences. Therefore, it is so important to comply with all requirements and norms during construction.

Incorrect design can cause serious financial losses caused by the need for rework. This may cause a fire or serious poisoning by combustion products. Therefore, the first stage of construction is the calculation of the chimney.

Chimney - fundamental design

Today we will talk about how to properly calculate the chimney.

Designing and building a chimney for a house or a bath, it is important to choose the right material from which it will be built (see Chimney pipes). It is necessary to know exactly what fuel will be used in heating appliances. After all, chimneys designed to work with one type of fuel are completely unsuitable when working with another.

An example is a brick chimney. Brick pipes work great with wood fuel, but are completely unsuitable for working with gas-fired heaters.

It is important to choose the correct height and diameter of the chimney, its cross section. The wrong choice of one of these parameters will at least affect the quality of traction and the efficiency of the heating system, and at the maximum it can lead to sad consequences. Let's figure out how to calculate the height of the chimney.
If one heater is used with the chimney, the issue can be resolved by simply viewing the technical documentation provided by the manufacturer. But when several different thermal devices are connected to one smoke exhaust system at the same time, then in order to calculate the chimney, solid knowledge of thermodynamics, material properties and much more is needed. That is, a professional engineering calculation will be required, even if a chimney is being built not for residential premises, but for a bathhouse.

Therefore, amateur performance in such a matter can cost a negligent "designer" dearly.

Universal chimney - reality and myth

As mentioned earlier, the basis of any chimney is the material from which it is made. Many manufacturers of modern chimney systems often advertise their chimney pipes as universal, capable of operating under any conditions, with any type of fuel. We can say for sure that such systems do not exist.

There are, of course, chimney systems that work well in various connection options and on various fuels. But it is worthwhile to understand that working well does not mean that they meet all the parameters that distinguish a high-quality chimney from just a chimney.

The internal section of the chimney - which is better

The optimal shape of the chimney is a cylinder. During the operation of the chimney, the heating of the walls does not occur quite evenly. Because of this, the combustion products move upward in the chimney with twisting along the central axis.

Naturally, a cylinder is the most acceptable form for this if we want to get strong traction.

If rectangular pipes are used, then in the corners, turbulences are formed that prevent normal traction. But let's make a reservation, turbulences are formed, first of all, from the very speed of thrust. Thus, the greater the thrust speed, the greater the turbulence and the greater the resistance to the normal process of gas movement in a rectangular pipe.
From this it becomes clear that a rectangular chimney can be successfully used with thermal appliances that do not require high traction. On the operation of fireplaces and wood-burning stoves, this only affects positively; this option is also suitable for the construction of a wood-burning bath.
For example, if the smoke circulation in the stove or fireplace is not properly arranged, a rectangular chimney will significantly save heat, unlike a chimney with a round cross section, from which it, as they say, “fly out into the chimney”.
As for the new generation boilers, it is better to use cylindrical pipes. Typically, such boilers operate on the principle: stop-start.
The main savings when using these boilers depends on the heating system heating rate. The faster the system warms up to the desired temperature, the faster the boiler will turn off and go into standby mode and, accordingly, economy.
In order for the boiler to warm up faster, in addition to fuel, it needs a good supply of fresh air. The air flow is created due to draft, respectively, the greater the draft, the better the boiler works and the heating system warms up faster (see How to improve draft).

Accordingly, the best option for such boilers would be a cylindrical chimney.

What should be the inner diameter of the chimney?

Our advice: the calculation of the diameter of the chimney for a fireplace with an open firebox or for a wood-burning sauna stove is done as follows. The ratio is drawn up 1:10 , in relation to the firebox. This applies to cylindrical type pipes.

The calculation of the cross section of a square type chimney is determined in proportion to the size of the furnace 1:1,5 . The diameter of the pipe for the chimney of the furnace should not be less than the diameter of the blower.

If the heat transfer is less than 300 kcal/hour, the cross section should be 140×140 mm, no less. The correct choice of the diameter of the chimney is the key to the successful operation of the heating system as a whole.

Chimney height - calculation and reality

But if we figured out the section of the chimney, the next question arises - what should be its height?

There is no single answer here either. To understand how to calculate the correct height of the chimney, many factors must be taken into account.

Warm air tends to rise. As it leaves the chimney, it cools down. Accordingly, the longer the air is in the chimney, the higher its temperature. This means that more air will tend to rise, creating thrust.

Therefore, the larger the volume of the chimney, the more warm air tending upwards and the greater the amount of draft. But after all, the volume of the chimney increases not only due to its height, but also due to the size of the internal section.

Suppose there is a cylindrical chimney 5 m high in the sauna room. In order to obtain maximum draft values, we increased its inner diameter. According to the above, the volume of warm air in the chimney has increased and the draft should increase.

And here it is not. Due to the large internal diameter, the combustion products cool rapidly.

It's like pouring hot tea from a cup into a saucer. Condensation forms on the walls of the chimney, preventing normal smoke outflow. As a result - loss of traction.

Let's reduce the inner diameter of the chimney and increase its height. Logically, the volume of warm air increases, it cools more slowly, and the draft should be good.

Indeed, traction will be good. And even too much. Strong traction will literally throw your money away. A strong influx of fresh, cold air into the boiler furnace will reduce the efficiency of the heater, it will take more time to warm up the heating system, which means that the boiler will consume more fuel. As a result, savings can only be dreamed of.

If the height of the chimney is too high and the inner diameter is too small, the draft may deteriorate. This happens due to aerodynamic resistance in the chimney pipe. As a result, carbon monoxide gases will enter the room.

Here is an example of an incorrect calculation in the design and construction of chimneys.

The scheme for calculating the height of the chimney is also interpreted by some rules and technical characteristics of the materials and the building itself.
You can calculate the height of the chimney pipe using the available special programs. If the chimney pipe is lower or at the level of the ridge, a swirl will form at the outlet, which is bad.
The height of the chimney above the roof must be in strictly established parameters.
The height of the chimney above the ridge directly depends on the distance at which the chimney is located relative to the vertical line of the ridge. If the chimney is at a distance of less than 1.5 m, the minimum height of the chimney above the ridge must be 500 mm.
Accordingly, if it is further from the vertical line of the skate, its height is adjusted. At a distance of 1.5-3 m, the chimney can be flush with the ridge.

reference- in any case, the minimum height of the chimney above the roof slope of the bathhouse or other room should not be less than 500 mm.

The height of the chimney pipe above the roof is calculated individually. The calculation of the height of the chimney is made taking into account various parameters:

roof slope angle;
roof thickness;
distance from the vertical axis of the ridge.

Reference: calculation of the height of the chimney does not take into account the height of the umbrella covered by the chimney.

Collective chimney: correct connection

The dimensions of the chimney used simultaneously for several heaters are a separate topic for conversation.

If it is necessary to remove smoke from a number of heating devices, then the calculation of the chimney is made taking into account all the heating devices used in the system, their type, power, fuel consumption.

For example, in a house with a single chimney, several heating appliances, a boiler of a common heating system and a fireplace are installed.
It immediately becomes clear that we have completely different systems. The diameter of the fireplace chimney does not match the diameter of the boiler chimney at all.
As a rule, fireplaces run on wood fuel, and the boilers of the home heating system run on natural gas.

Is it possible to combine these 2 completely different systems? Can. Moreover, with the correct placement of heating devices, they will not only not create problems, but will also complement each other.

How does this happen?

As a result:

improper operation of heating devices;
the ingress of carbon monoxide into the room.

It's already life-threatening. Carbon monoxide poisoning is often fatal.

Two-way chimney - the solution to the problem

How to solve this issue?

Our advice is to use a two-way chimney.

The advantage of a two-way chimney:

the possibility of using various thermal appliances simultaneously and separately in one chimney;
stable operation of each device;
savings in construction and installation;
space saving.

When installing such a chimney, it is necessary to comply with all conditions for the correct operation of each device. If the fireplace can function perfectly with a brick chimney, then a sleeve must be made for the boiler. This will protect the brick from the alkaline effects of condensate.

Before undertaking the installation and installation of a chimney for several systems, familiarize yourself with the operating parameters of each heater. The best solution would be to purchase a special industrial-made two-way chimney. So you save yourself from possible design errors. Even for an experienced specialist, connecting different types of heating devices to one chimney is a difficult task.

Serious engineering solution

The calculation of chimneys for a house or a bath is a complex engineering task. If you are not an expert and you have a question about how to calculate a chimney for your home, use special programs in which it is easier for you to make the correct calculation - whether it is the height, cross section or length of the chimney.

In order to make correct calculations, it is necessary to know the structure and physical properties of materials, thermodynamics, aerodynamics.

If you do not plan to further connect your life with the construction and design of chimneys, contact the specialists. After all, the comfort of your home, safety and, most importantly, health, and sometimes life - yours and your loved ones - depend on the proper operation of the chimney.