Coaxial vs. twin pipe flue gas exhaust system – which is better
Coflow vs. twin pipe flue system – which is better for your condensing boiler?
When a customer decides on a condensing boiler, most attention is directed at performance, brand, and price. The flue system is usually chosen later – and it is precisely here that the most uncertainties arise. The two basic concepts, the coaxial and twin pipe system, are not just technical details. They influence installation, costs, boiler performance, safety, and long-term operation. This article will guide you through both systems in detail and help you make a decision – not based on marketing phrases, but based on real technical facts and practical experience.
What is a coaxial system and how it works
The coaxial system – in practice you will also find it under the names "turbine", "pipe in a pipe", or briefly "coax" – works on the principle of two coaxial pipes, where the inner one carries the flue gases out and the outer one supplies fresh air to the burner. The entire device forms a single compact unit that passes through the wall or roof via a single opening.
The air coming from outside moves between the walls of the outer and inner pipe towards the boiler. In the process, it is preheated by the heat of the flue gases passing through the inner pipe – which is a small but measurable bonus for combustion efficiency. The inner pipe (usually 60 mm in diameter) carries the flue gases at a temperature of 40–70 °C typical for a condensing boiler, while the outer pipe (100 mm) supplies air.
Standard dimensions of a coaxial system for residential condensing boilers are 60/100 mm and for larger outputs 80/125 mm. For Protherm boilers directly, you can find on our e-shop, for example, Protherm adapter for flue pipes 60/100 mm A1KP or Protherm adapter for flue pipes 80/125 mm A25KP, which serve as a transition between the boiler and the flue system of the given diameter.
What is a twin pipe system and how it works
The twin pipe system (in English "twin pipe", sometimes also called "separate" or "bifilar" in our country) uses two physically separated pipes: one carries the flue gases, the other supplies air. Both pipes must be run separately from the boiler to the outlet on the external wall or roof. The diameters are usually the same for both pipes – typically 2× 80 mm, or 2× 60 mm for shorter runs and smaller outputs.
The key advantage of the twin pipe system is that air and flue gases travel completely separate paths, which allows for significantly greater flexibility in route design. The air pipe can be shorter or longer than the flue gas pipe without causing hydraulic imbalance – unlike the coaxial system, where both functions share the same physical space.
A typical example from practice: in a detached house with a boiler room in the basement, where the boiler is 8–12 m from the external wall, the coaxial system is at the limit of its capabilities (limits of hydraulic resistance). A twin pipe system, in such a case, easily handles routes of 20–30 m with proper dimensioning.
When a coaxial system is suitable and when it is not
The coaxial system is an ideal solution for apartments and smaller detached houses, where the boiler is installed near an external wall – typically in a kitchen, in an entrance hall, or in a small technical room on the ground floor. The installation is quick, only one wall penetration is needed (a single round hole is sufficient), and the overall cost of the system including labor is usually lower than with a twin pipe solution.
Examples where the coaxial system works without problems:
- Panel apartment, boiler on the wall – exhaust directly through the façade, route 0.5–1.5 m
- Detached house, boiler in the kitchen adjacent to the external wall – horizontal route up to 3–4 m
- Apartment renovation, where the original installation had a coaxial system and the new boiler also supports it
- Installation in a limited space, where there is no room for two parallel pipes
On the other hand, the coaxial system is not suitable in these situations:
- Long horizontal routes (over 5–6 m for 60/100 mm, over 8–10 m for 80/125 mm) – hydraulic resistance increases, the boiler may report an error or shut down
- Large number of elbows (each 90° elbow corresponds to the equivalent length of 1–2 m of pipe)
- Installations where air and flue gases must be routed in different directions (e.g., flue gases up the chimney, air from the roof)
- Cascade connection of multiple boilers
- Areas with extremely low temperatures, where condensate freezing in the outer part is a risk (for long extensions beyond the façade)
Maximum equivalent lengths (so-called "equivalent length") for coaxial systems are clearly defined in the technical documentation of most manufacturers (Protherm, Vaillant, Buderus, Bosch). For 60/100 mm it is usually 5–8 m, for 80/125 mm 10–15 m – but it depends on the specific boiler and its fan. Never exceed these values, otherwise there is a risk of insufficient supply of combustion air and dangerous operation.
When is a two-pipe system a better choice
A two-pipe system is more powerful and flexible – and in practice, we choose it whenever a coaxial system is not sufficient. Its biggest advantage is that each pipe (flue gases and air) has its own hydraulic resistance, which is dimensioned separately. This opens up much more room for route design.
Typical scenarios where a two-pipe system wins:
- Boiler room in the basement or technical room far from the exterior wall – routes of 10–25 m are common practice
- Vertical exhaust of flue gases through a chimney and air supplied separately through a pipe from the façade – typical in renovations where a masonry chimney opening exists
- Cascade connection of 2–4 boilers – each boiler has its own flue gases, but air can be shared from one supply (with proper dimensioning)
- Installation into an existing chimney using an insert Ø 80 mm – flue gases go up the chimney, air is supplied via the façade or shaft
- Houses with a complicated layout, where the route has to bypass load-bearing structures, pass through multiple rooms or change direction several times
For branching into multiple branches in a two-pipe system (e.g., in a boiler cascade), components such as Protherm Splitter Ø 2 x 80 mm R2KP are used, which allow to combine flue gases from two boilers into one collective pipe or vice versa – to split the air supply into two branches.
Important: in a two-pipe system, the air pipe must be properly insulated or placed indoors to prevent unintended heating of cold outside air – this would reduce the air density and thus the combustion efficiency. The flue gas pipe must be sloped at least 3° (about 5 cm/m) towards the boiler to allow condensate to drain back into the boiler, where it is collected by the condensate drain.
Technical differences in detail: condensate, insulation, pressure
Condensate and its drainage
A condensing boiler produces condensate not only inside the heat exchanger, but also in the flue pipe itself. In a coaxial system, condensate forms on the inner wall of the outer pipe (cooled by contact with outside air) and flows back to the boiler – which is correct. Problems arise with incorrect slope (insufficient gradient) or when the outer part of the coaxial pipe is too long and the condensate freezes in winter. Therefore, coaxial systems have a limit for extending beyond the façade (typically max. 15–20 cm beyond the wall surface).
In a two-pipe system, the situation is simpler – the flue gas pipe is routed with a slope towards the boiler (3–5°) and the condensate drains by gravity. The air pipe does not produce condensate (cold air from the exterior does not condense during flow). Condensate from the boiler must be connected to the sewer via a neutralizing insert – this applies to both systems.
Thermal insulation of pipes
A coaxial pipe is naturally "self-insulating" – air in the annular space acts as a thermal buffer, so the outer surface of the coaxial pipe remains relatively cool. This is an advantage when running through unheated areas or when in contact with flammable structures (of course, always maintaining minimum distances according to standards).
The flue gas pipe in a two-pipe system must be insulated everywhere it passes through unheated areas – otherwise the flue gases would cool too quickly and condensate would form in inappropriate places. Proper insulation also prevents heat losses and reduces the risk of surface condensation on the outer wall of the pipe in warm heated rooms (on the contrary – a cold pipe in a warm room "sweats").
Pressure balance and wind influence
This is one of the less discussed, but practically important aspects: a coaxial system is pressure balanced. Because air is supplied from the same place where flue gases are exhausted, wind acts on both pipes simultaneously and equally. If wind increases resistance at the flue gas outlet, it simultaneously increases pressure at the air inlet – the effects partially compensate each other and the boiler operates stably.
In a two-pipe system, the outlets are in different locations, so pressure imbalance can occur in strong winds. Modern condensing boilers with electronically controlled fans can cope to some extent (they adjust the speed), but under extreme conditions (gale winds over 15 m/s), a two-pipe system may show combustion instability. Therefore, it is important to position the air and flue gas outlets so that they are in the same pressure zone – ideally on the same side of the building or at least in adjacent areas of the façade.
Installation and economic comparison
| Criterion | Coaxial system | Two-pipe system |
|---|---|---|
| Number of openings in wall/roof | 1 opening | 2 openings |
| Max. length of route (approx.) | 5–15 m | 20–40+ m |
| Material cost (comparable route) | Lower | Higher (2 pipes) |
| Installation complexity | Simpler | More complex |
| Route flexibility | Limited | High |
| Pressure balance (wind) | Natural | Requires careful design |
| Suitability for cascade connection | Limited | Ideal |
| Condensate solution | Automatically back to boiler | Flue gas pipe slope |
Practical scenarios from real customer cases
Scenario 1: Panel apartment, 3rd floor, boiler in the kitchen
The customer wanted to replace the old boiler in the kitchen of a panel apartment with a condensing one. The boiler remained on the same wall where the original exhaust was – the distance to the façade was 35 cm. Solution: clearly a coaxial assembly 60/100 mm with horizontal discharge, specifically Protherm horizontal assembly 60/100 mm – 0.8 m, S1KP. The installation took 3 hours, one hole in the wall, minimal structural interventions. An economically and technically optimal solution.
Scenario 2: Family house, boiler room in the basement, 11 m from the external wall
Here, a coaxial system was excluded from the beginning – the total equivalent length (11 m straight sections + 2× 45° elbow = additional approx. 2 m equivalent) would exceed the maximum allowed values for any coaxial system. Proposed solution: a double-pipe system 2× 80 mm, exhaust upwards through the masonry into the original chimney opening (insert DN80), air supplied via a separate pipe (Ø 80 mm) through another wall of the boiler room. It should be emphasized: in a double-pipe system, the air intake must be placed so that it draws fresh air from the exterior, not from the technical room (otherwise the boiler would be "stealing" hot air from itself and efficiency would drop).
Scenario 3: Boiler room with two boilers in cascade (apartment building, 12 apartments)
Two condensing boilers 2× 24 kW in cascade. Each boiler has its own flue pipe Ø 80 mm, both are raised into a common collector (manifold), and from there one pipe Ø 100 mm goes vertically up the chimney. Air for both boilers is supplied via a common pipe Ø 100 mm from the façade, distributed to both boilers via a distributor. This type of connection uses a component such as Protherm Splitting branch Ø 2 x 80 mm R2KP. Such a solution could not be implemented with a coaxial system.
Scenario 4: Cottage, boiler near the balcony doors
The customer wanted to save money and proposed to route the coaxial pipe through the balcony door frame. This is absolutely unacceptable – apart from safety risks (exhaust gases in the interior in case of any leakage), it also violates the standard STN EN 13384 and manufacturer regulations. Correct solution: a hole drilled through the masonry next to the door, a short coaxial assembly with a façade head.
Standards, safety and approval of installation
The installation of exhaust for a condensing boiler in Slovakia is mainly governed by standards STN EN 13384-1 (static calculation of flues), STN EN 15287 (design, installation and commissioning) and binding manufacturer instructions. Each boiler manufacturer (Protherm, Vaillant, Buderus, Viessmann and others) issues its own installation documentation, in which the following are precisely defined:
- Maximum equivalent lengths for each type of exhaust
- Permitted types and materials of flue (usually plastic PP for condensing boilers, not enamelled sheet metal)
- Minimum distances from windows, doors, building corners and ventilation
- Requirements for tightness of joints (tightness class T400 N1 D 3 G for most condensing applications)
- Conditions for combined operation of multiple boilers
From a safety perspective, it is absolutely crucial to properly seal every joint in both systems – even the slightest leak of exhaust gases into the interior can cause carbon monoxide (CO) poisoning, which is odorless and colorless. Therefore, a pressure test of the entire system is required after installation. More on the topic of leaks and common errors can be read in the article Common faults and exhaust leaks in flues of condensing boilers in the same Knowledge Centre.
The flue must always be compatible with the specific boiler – manufacturers do not accept warranties when non-compatible components are used. More on the compatibility of the flue with the boiler can be read in the article Compatibility of flue with boiler: Protherm, Vaillant and other brands.
Material of pipes: plastic PP vs. stainless steel vs. aluminum
Condensing boilers operate with exhaust gas temperatures typically 40–80 °C (less in condensing mode), which allows the use of plastic (polypropylene, PP) components. Plastic is cheaper, lighter, resistant to corrosion and resistant to condensate with low pH (typically 3.5–5.5). For both systems – coaxial and double pipe – PP is today considered the standard solution for condensing boilers.
Stainless steel flues are used for classic (non-condensing) boilers or when inserting into existing masonry chimneys, where resistance class T200 or higher is required. When a condensing boiler is routed into a chimney, however, a special tight condensing-resistant stainless steel (class W2 or V2) must be used, not regular boiler stainless steel.
Aluminum pipes (usually only the outer shell of the coaxial) are commonly used as a compromise – lighter than stainless steel, stiffer than plastic, but more prone to corrosion damage from prolonged contact with acidic condensate. Therefore, PP is now preferred for the inner pipe in purely condensing applications.
What Influences the Choice of Diameter: 60/100 or 80/125 mm?
The choice of diameter is not just a matter of preference – it is a calculated matter. Basic rules:
- Coaxial 60/100 mm: boilers up to approx. 24 kW, runs up to 5–6 m equivalent length, max. 2 elbows of 90°
- Coaxial 80/125 mm: boilers 24–35 kW or runs 6–15 m, more elbows
- Double pipe 2× 80 mm: practically all outputs up to 45 kW of a single boiler, runs up to 30–40 m
- Double pipe 2× 100 mm: higher outputs or cascade connections with long runs
For more detailed information on sizing diameters, you can read the article What flue diameter do I need: 60/100 or 80/125 mm. If you need to switch from one coaxial system diameter to another, or connect an existing pipe of a different diameter, use reducing adapters – for example, Protherm adapter for flues 80/125 mm A25KP.
Combined Solutions: When a Hybrid Approach is Used
In practice, we sometimes encounter so-called hybrid or combined solutions – the boiler is connected via a coaxial adapter, but the system is interrupted behind (or in front of) it, and each pipe continues separately. This approach is used when the route near the boiler allows for coaxial (e.g., a passage through a technical partition), but it is more advantageous to lead the pipes separately further on (e.g., each through a different shaft).
Such a hybrid system requires a thorough hydraulic calculation and must be approved by the boiler manufacturer – not all allow it. With Protherm boilers, this procedure is possible provided that the total resistance does not exceed the limits defined in the technical documentation of the boiler. Special transition elements are used for the correct connection between the boiler's coaxial output and the double pipe system.
Most Frequently Asked Questions (FAQ)
Is a coaxial system safer than a double pipe system?
Both systems are equally safe if they are properly designed, installed, and maintained. A coaxial system has a certain natural advantage in that air and exhaust gases are physically separated in one body, which reduces the risk of exhaust gas leakage into the space (only one body passes through the wall). A double pipe system, on the other hand, contains more joints, each of which is a potential leak point – so the quality of the installation is even more important. In practice, the decisive factor is the correctness of the installation, not the type of system.
Can I change a coaxial system to a double pipe system without replacing the boiler?
Yes, it is possible, provided the boiler supports both types of exhaust (most modern condensing boilers do). For the change, appropriate adapters are needed – for example, an adapter that separates both pipes from the boiler's coaxial output and allows them to be led separately. The boiler manufacturer must approve such a solution, and we recommend checking with an authorized service technician. In some cases, it may also require changing the boiler's control unit settings (air intake).
What happens if the flue is too long for a coaxial system?
Too long or too many elbows in a coaxial system will cause increased hydraulic resistance. The boiler typically detects this as insufficient air flow – modern boilers with IoT and diagnostics will display it as an error code (e.g., F.33 for Protherm, F62 for Vaillant). In the worst case (with an older boiler without adequate protection), it can lead to incomplete combustion and increased CO formation. Therefore, you must never exceed the maximum allowed lengths stated in the boiler's documentation.
Can I use one shared double pipe system for two boilers side by side?
Yes, but only with so-called shared (common) exhaust, which must be designed from the start for multiple appliances. Each boiler must have its own flue branch connected to a common collector with sufficient cross-section. The air supply can be shared, but it must be sufficiently dimensioned for the simultaneous operation of all boilers. This system is subject to special standards (STN EN 15287-2 for shared flues) and must be verified by calculation. Connecting a second boiler to an existing exhaust of the first boiler without proper planning is dangerous and unacceptable.
How can I find out what exhaust system I currently have installed?
The easiest way is to look at the boiler's output and the passage through the wall or ceiling. If you see one pipe (a circle in a circle or a double-walled pipe) passing through one opening – it is a coaxial system. If you see two separate pipes (of the same or different diameters) passing through two different openings – it is a double pipe system. In case of doubts, check the boiler's technical documentation or call a service technician.
Does the type of exhaust influence gas consumption and boiler efficiency?
Directly, no – with proper dimensioning, both systems allow full condensing operation with the same efficiency. Indirectly, yes: if the coaxial system is too long and increases resistance, the boiler works with a higher fan power (higher electricity consumption) and may have problems achieving optimal lambda. Likewise, with a double pipe system, poor pressure balance in the wind can temporarily worsen combustion quality. The key is proper oversizing and quality installation.
Conclusion: Which system is better, then?
The answer is the classic technical one: it depends on the specific conditions. A coaxial system is simpler, cheaper, and excellent for short runs in apartments and smaller homes. A double pipe system is more flexible, more powerful for long runs, and necessary for cascade connections or complicated layouts.
If you are unsure which system is right for your specific case, start with a project or consultation with an expert. It is always important to comply with the maximum equivalent lengths defined by the manufacturer, correctly dimension the pipe diameter, and use only components compatible with your boiler. Information on choosing the right exhaust can also be found in the article How to choose an exhaust for a condensing boiler, and on the installation process in the article How to install a condensing boiler flue step by step.
The full range of components for both types of systems – from adapters, through assemblies, elbows, couplings, to façade heads – can be found in the category flue gas exhausts and flues on Atria.sk.
Do you have a question about this topic?
Not sure how to decide or dealing with a specific situation in your home? Write to us – we are happy to help.