Chimney sandwich pipes: technical parameters and installation rules

A fireplace stove in a private house allows you not only to spend time comfortably, but also to rationally spend money on heating. Correct installation is essential for safe operation. A few decades ago, the installation of the furnace was possible on a specially poured foundation, and the chimney was laid out of brick. The development of technology has made it possible not to pour a heavy foundation, but to arrange smoke removal with light systems. These include smoke sandwich chimney designs.

Design features

The name comes from the English word «sandwich» or «multi-layered». The system has gained popularity due to ease of installation and low weight. Developers got the opportunity to assemble with their own hands. You can figure out how to do it right by studying the recommendations.

The design includes two pipes of large and small diameters, between which a heater is distributed to isolate from the effects of mechanical damage and temperatures. Both external and internal are assembled from stainless steel sheets, joined in an argon atmosphere by plasma welding. This method produces welds that are close in structural characteristics to the properties of steel. In the middle they lay a heater, non-combustible stone wool.

The thickness of the heat insulator is influenced by several factors:

• the difference between the outer and inner diameters;
• temperature range during operation;
• the size of the outlet of the heating furnace.

Sandwiches are made from different types of steel: galvanized and stainless steel. They can differ in the diameter of the outer pipe and the thickness of the thermal insulation. It is impossible to use pipes of the same characteristics in different heating furnaces. Therefore, the choice is made depending on the characteristics of power and temperature.

Sandwich assembly can be done in many ways:

• Mounting with corrugated edges. The advantage is ease of assembly, but in this case it is necessary to seal the seams, which significantly increases the cost.
• Connection with sockets. The main advantage is the high level of tightness achieved during installation. Minus — this is a difficult installation due to the high accuracy of fitting parts.
• Mounting with corrugated edges. The advantage is ease of assembly, and the disadvantage is the increase in cost due to sealing the seams with expensive sealant.
• Connection with sockets. The main advantage is the high level of tightness achieved during installation. The downside is the difficult installation due to the high accuracy of fitting parts.

The minimum service life of a properly installed chimney is about 15 years, but with regular cleaning and the absence of overloads at extreme temperatures, operation can last much longer.

Pros and cons of using chimneys

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Ease of construction in light weight allows you to install smoke extraction systems in any home. Since the choice of a chimney is a very important part of the development, you need to carefully study all sides before it is completed. Sandwich pipes used in a chimney installation have pros and cons:

• In the process of smoke removal, there is a significant heating of only the inner part. The outer pipe remains cold or moderately warm due to the stone wool insulation. This reduces the possibility of burns and fires.
• Rapid heating of the insulated pipe increases traction.
• On a smooth steel surface, soot settles less often than on brick or ceramic. The ease of smoke removal does not contribute to the accumulation of condensate, combustion products do not react with water.
• Layering provides protective functions. The outer layer protects the inner tube from mechanical damage, and is insulated from high temperatures.
• Slight condensation may form on the outer pipe.
• The weight of the chimney is much lighter than that of a brick chimney, installation without an additional foundation is possible.

The disadvantage is the possibility of partial depressurization of the seams after about 10 years of operation.

Calculation procedure

To determine the diameter of the sandwich pipe, it is necessary to know in advance the conditions under which the operation will be carried out. According to the size of the area of ​​\u200b\u200bthe heated room, a furnace suitable for productivity is selected. To heat 10 sq.m. you need 1 kW of power, respectively, for a room of 50 square meters. meters, a 5 kW oven is suitable.

The temperature of the outgoing smoke depends on the type of heating boiler and the fuel used. The more degrees in the chimney, the more it heats up. The ratio of the temperature height and the type of heater is clearly shown in the table.

Smoke temperature in different boilers:

Type of heating boiler	Temperature smoke, C
fireplace stove	350-650
Sauna oven	400-700
pyrolysis boiler	160-250
pellet boiler	120-250
Gas boiler	120-200
diesel boiler	150-250
Boiler on coal	500-700

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In a sandwich chimney, the inner pipe heats up the most, so first of all choose its diameter. The more powerful the stove, the wider it should be. As the power increases, the inner hole must also be increased. The power calculation can be seen in the table:

Average values ​​​​of the ratio of boiler power and chimney cross-section:

Boiler power, kW	Chimney section, mm	S outlet diameter, sq. mm	Pipe diameter
less than 3.5	140 x 140	19 600	158
3.5 — 5.2	140 x 200	28 000	189
5.2 — 7	140 x 270	37 800	220

For open fireboxes, the smoke channel must exceed 1/100 of the boiler volume. In closed boilers, where air enters the furnace from the lower blower, the internal area of ​​the outlet section of the chimney must exceed the dimensions of the supply opening. If the diameter exceeds the calculated one and a half times or more, then the traction force drops, and turbulences form inside the chimney.

The arrangement of layers in sandwiches allows isolating the outer pipe from exposure to high temperatures, the technical characteristics prevent the formation of condensate. Stone wool is non-combustible, its thickness depends on the temperature of the outgoing fumes from 25 mm at 60 C to 100 mm at 600 C.

Chimney Height Calculation

For safe operation and removal of combustion products outside the residential area, the height of the pipe must exceed 5 meters. Otherwise, operation becomes unsafe due to a violation of traction and smoke in the residential area. The most common height is 5-10 meters.

On pitched roofs, the dimensions of the chimney sandwich pipe exceeding the ridge should rise by at least 0.5 m if the axis of the pipe is located at a distance of one and a half meters or less from the highest point. If the distance is 1.5 — 3 meters, then the top point of the pipe should be flush with the ridge. If the axis of the pipe is located more than three meters, then the top point of the pipe should be lowered from the ridge by 10 degrees.

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For installation on flat roofs, a total minimum sandwich pipe dimension of 500cm is taken into account. The location of the upper point of the chimney should be 0.5 — 1.5 m. It is allowed to make chimneys with an excess of the specified height by 10-30 cm, subject to the mandatory installation of fixing braces.

Criterias of choice

For different heating boilers, it is necessary to individually select the type of high-alloy stainless steel sandwich chimney. Combustion products accumulating in the pipe may contain chemically active substances:

• sulfur;
• sulphuric acid;
• hydrochloric acid.

Exhaust flue temperatures can vary considerably between different types of fuel. All indicators differ in the level of impact on different steel grades. The most common ones are:

• AISI High alloy steel alloy of titanium stabilized stainless steel. The limiting temperature of use is 800 C. In an environment containing sulfur, it corrodes.
• AISI High-alloy stainless steel alloy, molybdenum stabilized. Differs in resistance to acid, does not corrode, is steady against influences of aggressive substances.
• AISI High-temperature steel alloy with a high content of chromium and nickel. It is used at elevated temperatures up to 1000C. Use at 60°C adversely affects the structure and leads to brittleness.
• AISI 409, AISI Poor high temperature resistance and poor acid resistance. This brand is most often used in the manufacture of budget models.

A non-specialist does not have the ability to determine the steel grade visually. The only possible way to check would be to use a magnet. A low quality alloy will attract it, but a high alloy will not.

Good insulation provides insulation from high temperatures and fire, so you should choose trusted manufacturers. These are Paroc, Isproc and Rockwool. Stone wool is used at temperatures up to 600C. The thickness of the insulation is 25-50 mm and depends on the temperature of the rising smoke, the indicators of which differ depending on the type of fuel used. In chimneys brought outside, the insulation layer should be maximum.

Assembling a sandwich chimney with your own hands

The chimney can be installed without having experience in construction. Before starting the installation, you must select the type of assembly:

• By smoke, when the lower sections rise inside the upper ones. For light stoves and fireplaces made of cast iron.
• By condensate, where the one going above is inserted into the lower element. Used in gas boilers.

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Installation through the roof begins with the heater. The walls behind the chimney are insulated with calcium silicate or mineralite. The assembly order is as follows:

1. The lower element, due to exposure to high temperatures, is made of galvanized steel and, after being connected to the boiler, is closed with a clamp to prevent heating.
2. The links are assembled, starting with the one that has a smaller diameter. At this step, it is important to check that there are no obstacles for the condensate to drain. Pipes inside the room are assembled with the top pipe outward, and from the side of the street, the assembly takes place, mounting the top pipe inside.
3. The outer and inner pipes are connected. The joint is strengthened by winding with a zinc clamp and tightened.
4. The finished chimney is sealed with a compound that can withstand 1000C.

When passing floors, a special role is given to fire safety. A 70 x 70 hole should be prepared for the pipe and filled with non-combustible basalt wool. When passing through the roof, special elements such as master flash seal the joints with the chimney.

When assembling along the wall of the house, a hole is made and strengthened with wooden bars at the point of transition to the street. First, the bracket is mounted, and after it the tee. The wall is covered first with plywood, then with asbestos and a metal sheet, which is varnished to protect against rust.

Tools and inventory

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To assemble a high-quality chimney, you need to prepare the necessary tools and materials:

• a set of pipes for assembly with accessories for the roof;
• protective umbrella from precipitation for installation on the roof;
• starting pipe made of stainless steel;
• heat-resistant silicone sealant;
• support bracket;
• clamps;
• stone wool reinforced with foil;
• mineralite or calcium silicate for wall insulation;
• Metal sheet;
• Bulgarian saw;
• drill and dowels;
• screwdriver;
• a hammer;
• protective equipment.

Recommendations for the operation of chimneys from sandwich pipes

After completing the assembly work, it is necessary to melt the furnace. This is necessary to check the overall tightness of the structure and the absence of overheating of individual elements.

During the initial operation of the system, smoke and odors may occur. When the remnants of oil, dirt and silicone sealants on the pipes burn out, these phenomena will disappear.

During operation, soot accumulates inside the pipes. Over time, this leads to a decrease in the diameter of the chimney. To prevent a fire, it is necessary to clean the pipes twice during the heating season. It is forbidden to clean the chimney using household chemicals.

Assembling a sandwich chimney can be done even by non-professionals. The result depends on the quality of the material and compliance with the instructions. A chimney, properly assembled with your own hands from sandwich pipes, will last for many years.