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The Chimney

During the combustion process in heating systems, smoke and exhaust gases are generated, which must be vented to the outside.
 

This is done through an appropriate flue system or exhaust system.

A chimney (or flue) refers to soot-resistant exhaust systems made of stainless steel, ceramic, or fireclay, which are suitable for all types of fuel, including solid fuels such as firewood, pellets, wood chips, and charcoal.

For heating systems using liquid or gaseous fuels, an exhaust duct can alternatively be used. Due to the lower exhaust gas temperatures, the use of PP pipes along with appropriate sealing materials such as silicone or EPDM is often possible, making the exhaust duct suitable for overpressure, depending on the requirements of the heating appliance.

Jeremias offers a wide range of exhaust systems to perfectly match the chimney or exhaust duct to the connected heating appliance, the type of fresh air supply, the fuels used, and the installation location.

Functionality

Functionality of a Chimney

All chimneys operate according to the same basic principle, known as the Bernoulli Effect (chimney effect).

During the combustion process, heating appliances create a natural draft that automatically directs the exhaust gases out of the chimney.

This is made possible by the fact that heated air and hot exhaust gases have a lower density than the surrounding air, causing them to rise (thermal convection). The resulting negative pressure in the chimney creates a flow of cold fresh air into the heating appliance, ensuring the continuous maintenance of the combustion process.

The strength of this effect depends on various factors in practice, such as chimney height, chimney dimensions, exhaust gas velocity, or the difference in density and temperature between the exhaust gases and the supplied combustion air.

For the perfect interaction between the heating appliance and the chimney, an individual calculation of the appropriate chimney cross-section is essential. Only in this way can sufficient draft be ensured, safely directing the exhaust gases outdoors.

Basics for Chimney Selection

Before Purchasing a Chimney – What to Consider

Heating systems and exhaust systems form a unified system and must be perfectly coordinated for efficient and safe operation. Before choosing the appropriate chimney or exhaust pipe, several factors need to be considered.

 

Fuel

A key criterion when selecting the appropriate exhaust system is the type of fuel used for heating. Fuels can be classified according to their origin.

Fossil fuels are derived from plants and animals that died millions of years ago, primarily consisting of carbon and hydrogen. They represent a finite energy source and release large amounts of stored CO2 when burned. Fossil fuels include coal, oil, and natural gas.

Biomass, on the other hand, refers to renewable fuels that have actively participated in the carbon cycle before use, which is why they are considered "CO2-neutral." Common examples of biomass include wood (logs, pellets, wood chips), specially cultivated plants such as corn, grains, and elephant grass, as well as all kinds of organic waste.

In 2021, nearly 50% of heating in Germany was powered by gas, followed by heating oil at 25%. Gas and oil thus account for about three-quarters of heating energy, while solid fuels like logs, wood pellets, or coal contribute just over 6%.

 

Exhaust Temperature

The choice of fuel has a significant impact on exhaust temperature, which in turn influences the selection of the appropriate exhaust system.

Gas or oil heating systems operate with much lower exhaust temperatures compared to combustion systems using solid fuels such as wood or coal. This is especially true for gas/oil condensing units, which use energy very efficiently, resulting in even lower exhaust temperatures.

Masonry chimneys or traditional concrete chimneys require a high exhaust temperature of 180°C – 200°C at the chimney inlet to prevent condensation inside the chimney, which could cause moisture buildup and lead to long-term issues such as creosote buildup. These are therefore only suitable for dry operation, i.e., for solid fuels with sufficiently high exhaust temperatures.

Jeremias stainless steel chimneys are also suitable for high exhaust temperatures, and thanks to moisture-resistant materials, they can also be used with modern condensing units, where exhaust temperatures often do not exceed 80°C and condensation in the chimney is even desirable (moist or condensing operation). The same applies to our plastic exhaust systems.

 

Pressure Tightness

The low exhaust temperatures of modern condensing heating units can sometimes hinder natural chimney draft, as cooler air rises more slowly than the warm exhaust gases from solid fuel heating appliances.

To ensure the proper expulsion of exhaust gases outdoors and maintain smooth combustion, most modern condensing units are equipped with an integrated fan. This supports the chimney effect and creates positive pressure in the exhaust pipe, necessitating the use of pressure-tight exhaust systems.

 

Soot Fire Resistance

Burning solid fuels produces soot, which settles in the chimney. When dry firewood is used, the soot deposited in the exhaust pipe can be easily removed by a chimney sweep.

However, using too damp or unsuitable fuel results in wet soot accumulating on the chimney walls. This does not pose an immediate danger due to its moisture content but is often not fully removable with standard cleaning tools. As the soot hardens with increasing exhaust temperature, it becomes highly flammable. Normal sparks from the combustion process can then ignite a soot fire, causing flames at the top of the chimney and temperatures exceeding 1000°C.

Chimneys designed for dry operation with solid fuels must withstand such soot fires without damage, such as cracks, otherwise, they would become unusable.

Information about soot fire resistance is provided by the classification of the exhaust system.

All Jeremias systems that are approved for dry operation are either soot fire-resistant (marked "Gxx") or moisture-resistant (marked "Oxx") if used in condensing operation.

Non-soot fire-resistant exhaust systems, such as plastic systems, may only be connected to devices that use liquid or gaseous fuels in wet operation.

A special feature of our condensate-acid-resistant SILVER systems is that they can continue to operate in a condensing mode even after a soot fire, making them both moisture-resistant and soot fire-resistant.

 

Exhaust System Diameter

The cooler exhaust temperatures today would not be sufficient to safely expel exhaust gases outdoors in the traditionally large domestic chimneys, potentially leading to incomplete combustion, smoke leakage into the building, or risk of chimney fouling.

To maintain proper chimney draft, modern heating appliances with lower exhaust temperatures require smaller chimney diameters. These should be determined by a professional through a cross-sectional calculation tailored to the specific heating unit.

 

Chimney Height

Chimney height also significantly affects chimney draft. Generally, the taller the chimney, the better the draft.

Minimum height requirements for chimneys are defined in regional regulations for oil and gas heating appliances and nationwide for solid fuels in the Federal Emission Protection Ordinance (BImSchV).

With the new flue gas discharge conditions effective from January 1, 2022, under §19 (1) of the 1st BImSchV, the chimney outlet of newly installed small and medium solid fuel appliances with less than 1 MW output must be close to the roof ridge, improving air quality in residential areas, particularly reducing fine dust, odors, and smoke.

The minimum chimney height according to §19 (1) BImSchV must take into account factors such as the roof shape and slope, the distance from neighboring buildings, and the appliance's output. In general, the chimney must be higher the further it is from the ridge (maximum ¼ of the building width).

Alternatively, the required chimney height can be calculated using VDI 3781 Sheet 4.

All information on the topic can be found in our
special feature „New Discharge Conditions“

 

Room Air-Dependent or Room Air-Independent Operation

Heating appliances can draw the necessary combustion air either from the living space (room air-dependent) or from an external air supply duct (room air-independent).

The advantage of room air-independent operation is that the usual requirements for the minimum air volume of the room can be omitted, and simultaneous operation of the heating appliance and ventilation or exhaust systems can proceed without restrictions.

Fresh air can be supplied either via a separate intake system or through a concentric exhaust system (also known as a LAS chimney), where the intake air is directed to the appliance through the annular gap between the exhaust pipe and outer pipe.

 

Acceptance

When installing a heating system with an exhaust system, several important regulations must be observed. For example, the minimum distances to combustible materials must be clearly defined and adhered to. Additionally, it must be ensured that the exhaust system is compatible with the heating system, no exhaust gases can enter the living space, oxygen supply is ensured, chimney placement and height requirements are met, and many more.

Therefore, it is advisable to consult the responsible district chimney sweep during the planning phase. They must approve the entire system before the first operation and can offer valuable installation tips. The installation itself should ideally be carried out by a qualified specialist.

Types of Chimneys

Stainless Steel Chimney – The Chimney of the Present and Future

The masonry chimney is not very flexible due to its design. The location within the building cannot be changed afterward, and therefore, the connection points are fixed. This leads to limited options for positioning a new stove or other heating appliance. In such cases, the stainless steel chimney has established itself as a flexible and retrofittable exhaust system. The double-walled stainless steel chimney by Jeremias stands out in this regard. Insulation between the inner gas-carrying pipe and the outer pipe prevents excessive cooling of the exhaust gases, ensuring the continuity of the chimney draft, especially when used as an external chimney. By using the highest quality materials and the latest production methods, a chimney with premium quality is produced. The DW-ECO 2.0, DW-FU, and DW-VISION series are among the most popular chimney systems in Germany.

 

Single-Walled Exhaust System for Chimney Renovation

An additional use case is the renovation of existing masonry chimneys that may become unusable due to long-term use or other factors such as soot formation. These chimneys can be renovated with a single-walled stainless steel chimney. The stainless steel pipe is simply inserted into the masonry chimney, creating a new exhaust system and saving the existing chimney for many more years.

 

Lightweight Chimney F90 – The Chimney for Interiors

The lightweight chimney completes the range of domestic exhaust systems. These special fireproof F90 shafts are installed within the building. The low weight and manageable size of the components make installation much easier. Jeremias' lightweight chimneys truly live up to their name and provide an affordable and safe alternative to masonry chimneys and ceramic chimneys. Combined with various inner pipe solutions and double-walled stainless steel pipe variants for roof installations, the possibilities are endless.

 

Stove Pipes, Chimney Caps, Exhaust Silencers – All-around Solutions with Jeremias

In addition to the aforementioned exhaust systems, Jeremias also offers an extensive range of products related to exhaust technology. The range includes various supply systems, such as the FERRO-LUX series stove pipe, as well as exhaust silencers for both private and professional use.

The stove pipes are coated with a special Senotherm coating and fired at the factory. This ensures that the stove pipe will not produce any odor when used for the first time. A wide range of inner diameters is offered, so all types of stoves can be connected. The stove pipe assortment consists of around 100 individual parts, which means that each unique connection situation can be considered, minimizing the construction effort.

Chimney caps, such as the well-known Napoleon hood or the Meidinger disc, are made from high-quality stainless steel and protect the chimney head of masonry chimneys. Jeremias offers many different models, all of which are custom-made to meet customer requirements. This ensures optimal fit and protection. As is typical for Jeremias, only corrosion-resistant stainless steel is used for these components, which, combined with high-quality processing, results in an extremely durable product.

 

Steel Chimney – From Vision to Customized Solution

In addition to stainless steel chimneys, Jeremias is also a manufacturer of steel chimneys with decades of experience. After over 20 years in the industrial sector, we are able to provide various complex solutions for customers worldwide. Our experts provide advice from chimney design and planning to production and installation. Jeremias develops the optimal solution for every chimney project.

Our service range for industrial chimneys includes complete project management, consulting and planning, construction, production, transport, installation, and maintenance with condition monitoring.

As a member of the IVS (Industrial Chimney Association), Jeremias has the opportunity to incorporate suggestions for improvement and customer needs into standardization while staying up-to-date and often ahead of the curve.

Advantages of Modular Stainless Steel Chimneys

Advantages of Modular Stainless Steel Chimneys

The modular exhaust systems from Jeremias offer many advantages over welded alternatives and chimneys from many other manufacturers.

With over 100 CE and UL certified systems, complemented by building code approvals, country-specific certifications, and special approvals (e.g., for the maritime sector), Jeremias offers probably the largest selection on the market.

Jeremias modular chimneys are easy to assemble due to the lightweight individual chimney elements and precise plug connections. Installation does not require welding on-site, and in most cases, heavy lifting equipment can be omitted.

Modular chimneys can be expanded or extended at a later time. In addition to standard components, custom-made parts are available whenever necessary.

By exclusively using corrosion-resistant stainless steel alloys, Jeremias provides the highest level of protection against damage and related functional impairments of the chimney, which is why we offer a 25-year warranty on the corrosion resistance of our stainless steel products.

In our double-walled chimneys, rigid and high-quality mineral wool insulation, available in different thicknesses, prevents the formation of thermal bridges between the inner and outer casing and allows for installation in close proximity to combustible materials.

TIG-welded and passivated longitudinal seams guarantee condensate and gas tightness of our exhaust pipes. The use of high-quality sealing materials also ensures a long lifespan for our overpressure systems.

Thanks to our own test laboratories for thermal, static, and acoustic (exhaust silencers) testing, we are able to quickly bring innovations to market with certified safety and ensure consistently high product quality.

An extensive range of accessories complements the Jeremias portfolio of chimney elements.

History of the Chimney

A Brief Overview of the History of Chimney Development

The development of smoke venting through chimneys runs parallel to the historical use of fire for heating purposes.

In ancient times, until the 14th century, smoke typically had to escape through door and window openings or through a hole in the ceiling or roof. However, as early as the Imperial Palace heating system in Goslar (1000 AD), vertical shafts were built to carry away fire and smoke gases.

With the increasing use of stone as a building material instead of the traditional timber, the fireplace was moved from the center of the room to the wall, which served as a fire wall. The smoke was caught by a protective roof and directed outward through the roof beams. For fire safety reasons, fire walls were soon built as double-walled structures in the area of the fireplaces, becoming smoke ducts (referred to as Scharren), which safely carried the smoke gases outside. Initially, these were called smoke tubes, and it was only in documents from the 15th century that the term "chimney" appeared in its modern meaning.

Craftsmen (e.g., bakers, blacksmiths) quickly realized that a masonry vertical flue provided the fire with more combustion air and expelled the smoke more efficiently. In the Middle Ages, such flue shafts were often built into the supporting walls of castles and palaces.
 

With the advent of the first iron stoves in the 15th century, there was better heat utilization, but also a significant risk due to the hotter flue gases and the chimneys, which were still largely made of flammable materials (wooden beams, wooden boards coated with clay).

In Germany, a royal instruction in 1822 introduced more detailed specifications regarding the minimum permissible width, thickness of chimneys, and the necessary cleaning. This marked the first time that structural strength and fire safety requirements were established. At that time, the tops of chimneys were mostly adapted to the style of the building, but did not always meet functional requirements.

In the 20th century, fuel use shifted from wood and coal to heating oil and natural gas. This change, along with the technological development and modernization of heating systems from individual stoves to central heating, also required changes in chimney technology.

For a long time, the most common form was the single-walled masonry chimney. This simple system, made either from a brick bond or stacked form stones without additional insulation, was sufficient for a long time, though never ideal, because the risk of cracking due to temperature stress was always present. For easier and faster installation, this construction method was replaced by the use of single-walled, fully-walled components or single-walled chimneys made of cellular components.

The first significant change occurred at the end of the 1950s and early 1960s when more and more heating systems switched from solid fuels to oil.
 

The result: The temperature in the chimney and the flue gas mass flow decreased noticeably. Especially in the roof and roof covering areas of the chimney, the dew point temperature was dropped, leading to the accumulation of moisture (condensation). Water mixed with aggressive combustion residues settled on the inner walls of the chimney, slowly but steadily causing the construction to become damp. This led to significant soot formation.

The situation became even more problematic at the end of the 1970s and early 1980s when the first major energy crisis caused heating costs to rise sharply.

At the same time, there was a fundamental shift in the use of energy sources. The share of gas-fired heating systems steadily increased. Growing environmental awareness also played a significant role. It became clear that the CO2 released during the combustion of fossil fuels was a major contributor to the so-called greenhouse effect.

From an environmental protection standpoint, this led to the necessity of significantly reducing energy consumption for building heating. The industry responded by bringing a new, far more economical generation of boilers to the market in a short period of time.

Since the flue gas temperatures of the new heating boilers were considerably lower than those of previous models, the stress on the flue and chimney systems increased further.
 

Another complicating factor was the higher water vapor content in the exhaust gases. When oil was predominantly used as a fuel, the exhaust gases contained about 7% water vapor, whereas with gas fuel, the water vapor content is approximately 14%.

Due to the changing heating technology and the resulting increase in chimney damage, various chimney repair methods were developed to address the new conditions. The most popular repair method today is the installation of a stainless-steel pipe into an existing chimney, starting from the chimney top.

The development also led to the creation of three-layered domestic chimneys. In these systems, an inner pipe made of stainless steel or fireclay was placed concentrically into a brick shaft or casing piece, and the remaining space between the masonry and inner pipe was filled with insulation material.

This measure helped reduce temperature stresses (overheating of the chimney's outer walls).

With the increasing use of oil and gas, as well as new heating technologies, the previously "dry" chimney, which handled hot, dry flue gases, became a "wet" chimney. In addition to the requirements for stability and fire safety, new demands for acid resistance, condensate tightness, and thermal insulation had to be met. This led to the development of multi-layered chimney systems, consisting of an outer shell, insulation, and flue pipe, where the stress is distributed across the different components. Each component serves a specific purpose:

 

  • The inner shell (the flue pipe) is acid-resistant and condensate-tight, and, if necessary, also pressure-tight if the flue system operates under pressure.
  • The thermal insulation ensures minimal cooling of the flue gases and prevents or reduces condensate formation. In the case of pressure systems, the insulation is replaced by continuous ventilation.
  • The outer shell (the cladding or casing stone) handles the structural requirements and, together with the insulation, provides fire protection and soundproofing.

These double-walled chimneys could no longer be built arbitrarily from the available building materials, but became approved flue gas systems, where the individual components must be compatible. Only after thorough testing and proof of their functionality do these systems receive their approval or CE certification.

The increasing use of low-temperature heating boilers and condensing devices with integrated heat exchangers led to the development of special flue gas systems.
 

The most common two-layer flue gas system is the flue gas pipe for connecting a condensing appliance or as a so-called LAS system (Air-Flue Chimney) for connecting multiple wall-mounted gas devices in a room-independent operation (e.g., the concentric Jeremias TWIN-P and TWIN-PL systems).

These flue gas systems consist only of an outer shell and an inner pipe, which is sized according to the flue gas temperature, chimney height, and the number of connections. Today, pipes made of stainless steel and plastic are used. It is important for the flue gas system to conform to the fire protection regulations of the country. The flue pipe must be surrounded by air on all sides. The flue gas systems are operated in counterflow or parallel flow depending on the location of the boilers. The vertical part can be operated under negative or positive pressure, and this must be determined for the planning of the connection technology and pipe diameter.

Increasingly, gas engines are also being used in the smaller and medium power range, typically micro gas turbines that produce both heat and electricity. In these cases, the flue gases are discharged with high pressure, so special flue gas systems (e.g., Jeremias EW-KL, DW-KL, or DW-POWER) are required.