Frequently Asked Questions about Flue Gas Extraction and Flue Systems of Condensing Boilers

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Everything you were afraid to ask about flue gas exhaust systems for condensing boilers

Anyone who has undertaken the replacement of an old boiler with a modern condensing one will sooner or later run into the same problem: flue gas exhaust. This seemingly simple issue raises a surprisingly large number of questions – even among people who have worked in heating for years. In practice, we encounter dozens of similar situations: a customer buys a condensing boiler, then finds out that the old chimney is not sufficient, or misjudges the pipe diameter, or assembles the whole system in a way that causes insufficient draught or waterlogging from condensate. This article gathers the most common questions from practice and answers them comprehensively and clearly, without oversimplifying.

If you're looking for specific products, you'll find an overview of systems and components directly in the category flue gas exhausts, flue pipes. Throughout the text we also refer to related articles in our Knowledge Center – if you want to dive deeper into a particular topic, continue reading there.

Why are flue gas exhausts for condensing boilers different from classic ones?

This is the very first and most important thing to understand. A condensing boiler operates on completely different physical principles than a standard boiler that vents through chimney draught. Flue gases leave a condensing boiler at a temperature of typically only 40–60 °C (compared to 150–200 °C for a classic boiler). This has two fundamental consequences:

Chimney draught is weak – cold flue gases rise much more slowly. That's why condensing boilers have a built-in emergency fan (exhaust) that actively pushes the flue gases out.
Water condenses in the flue pipe – when hot flue gases touch the cold wall of the pipe, the vapor in them condenses. The condensate must drain back into the boiler (or into a siphon), otherwise the system clogs or gets damaged.

It follows that a condensing boiler's flue pipe must be:

Gas-tight – a pressurized system. Flue gases are pushed out under slight overpressure, so any leaky joint can leak into the room.
Resistant to condensate – acidic condensate (pH 3–5) corrodes ordinary steel pipes or classic brick chimneys. The correct material is plastic (PP, PPs) or stainless steel certified for condensate.
Sloped in the right direction – horizontal sections must have a slope of at least 3° (5 cm per meter) towards the condensate outlet, not the other way round.
Compatible with the specific boiler – every manufacturer defines the allowed types and dimensions of the flue pipe. An incompatible system leads to malfunction or even a dangerous situation.

Coaxial or twin-pipe system – what does it actually mean and which one should you choose?

This is one of the most frequently asked questions, and yet the answer isn't always simple. In short:

Coaxial system (also "pipe within a pipe") is a solution where the inner pipe carries flue gases out and the outer annulus draws combustion air in from outside. Both functions are performed by one compact component, for example Protherm horizontal set 60/100 mm - 0.8 m, S1KP. With this solution, the boiler is completely separated from the air in the room – it draws air directly from outside. This is referred to as turbo boilers or boilers with a sealed combustion chamber (type C).

Twin-pipe system (separate pipes) is used when air is not brought in coaxially, but instead via an existing chimney or a separate pipe. It is typically used for longer runs, cascade installations of multiple boilers, or renovations where a coaxial pipe cannot reach the outer wall. The 2× 80 mm dimension is commonly used for this – for example the Protherm splitter piece Ø 2 x 80 mm R2KP.

A more detailed comparison of both systems, including their pros and cons, can be found in the article Coaxial vs. twin-pipe flue exhaust system – which is better in our Knowledge Center.

What pipe diameter do I need – 60/100 mm or 80/125 mm?

This is one of the most common misconceptions in practice. Many customers think a larger diameter is always better. It isn't. The boiler's output determines the correct diameter. Simplified, the following applies:

60/100 mm (coaxial) – for boilers up to approx. 35 kW, for short runs (up to 4–5 m total equivalent length including elbows)
80/125 mm (coaxial) – for boilers from approx. 25 to 100 kW and/or longer runs, across multiple floors
2× 80 mm (twin-pipe) – for renovations, cascades, or long runs over 10–12 m

The documentation for each boiler contains the so-called equivalent length – the maximum permissible exhaust length. Each 90° elbow counts as 1–1.5 m of straight pipe. So if you have a 3 m run with two 90° elbows, the total equivalent length is 3 + 2×1.5 = 6 m.

When you need to connect a boiler with a spigot of a different diameter than the standard flue pipe, reducers and adapters can help. For example, the Protherm adapter for flue pipes 60/100 mm A1KP allows connection of the boiler to a 60/100 mm system, while the Protherm adapter for flue pipes 80/125 mm A25KP is used for 80/125 mm systems. These adapters are a key component in renovations, when the old flue pipe doesn't match the spigot of the new boiler. We cover the topic of dimensions in more detail in the article What flue pipe diameter do I need: 60/100 or 80/125 mm.

Can I connect a condensing boiler to an old brick chimney?

This is probably the most common dilemma during renovations. The answer depends on several factors and is usually more of a compromise than the customer would like:

Technically speaking – not directly. An old brick chimney is not airtight, is not resistant to acidic condensate, and does not have the correct diameter. Flue gases from a condensing boiler would cool down in the damp chimney, the condensate would damage the masonry, and with incorrect draught they could leak into the building.

Solution: chimney lining. A plastic (PP or PPs) or stainless steel flexible pipe certified for condensing boilers (marked "Wet – condensate") is inserted into the existing chimney flue. This inner liner takes over the function of the flue pipe, while the chimney shell serves only as a protective casing. Such a solution is common and fully functional when done correctly.

Before lining, you need to verify:

The clear dimension of the flue – whether the liner will even fit
The height of the chimney – with very short chimneys there may be a problem with flue gas dispersion
The condition of the masonry – heavily cracked or reconstructed chimneys can cause problems
The liner's certification for the boiler type and fuels (gas, propane)

Horizontal or vertical exhaust – what matters?

In practice we encounter both directions, often in combination. Horizontal exhaust (directly through the wall) is simpler and cheaper to install – no chimney work is needed, just a wall duct. Vertical exhaust (through the roof or above the roof) is used when flue gases cannot be routed through a wall (e.g. the boiler is indoors far from an outer wall, or local regulations prohibit exhaust on the façade).

From a physical standpoint, both directions are equivalent in a correctly designed system with forced exhaust – the fan in the boiler ensures sufficient overpressure in both cases. However, with horizontal exhaust it is critically important to correctly slope the horizontal sections – flue gases must not condense while stagnant, water must drain back toward the siphon.

A common mistake in practice: an installer creates a horizontal exhaust without a slope, or with a slope in the wrong direction (towards outside). Result: condensate accumulates in the pipe, the boiler reports a fault or "gurgles" during operation. We see this several times a year in service calls. The solution is simple – check the slope with a spirit level and re-support or redo the pipe.

A more detailed look at this topic is provided in the article Horizontal vs. vertical flue exhaust – what to choose during a renovation in our Knowledge Center.

What is a wall/roof duct and when do I need one?

A duct (or roof/wall bushing) is a sealed passage through which the flue pipe passes through a wall or roof. It's not just a cosmetic element – it has a functional and safety significance. A proper duct:

Seals the gap between the pipe and the building structure – prevents flue gases from entering the wall and moisture from entering through rain
Compensates for pipe expansion – plastic flue pipes expand and contract with temperature changes; a rigid anchoring without expansion allowance can lead to cracking
Prevents mechanical damage to the pipe at the passage point

For wall passages with horizontal exhaust, masonry flanges with seals designed specifically for flue pipes are used. Roof ducts must be waterproof and meet thermal-technical requirements (no freezing). More on this topic in the article Roof or wall duct: differences, types, and installation.

Compatibility – do I have to use original flue pipes from the boiler brand?

This is a sensitive topic. Boiler manufacturers (Protherm, Vaillant, Viessmann, Buderus...) supply their own certified flue pipes and adapters – for good reason: they take exact responsibility for tightness and compatibility with the given boiler. Warranties are conditioned on the use of approved components.

In practice, however, universal systems from manufacturers such as Almeva, Schiedel, Brilon, and others are also very commonly used – as long as they are certified for the given boiler type and fuels. What matters is that the system has EN 14471 certification (plastic flue pipes) or EN 1856 (metal), is dimensioned for the operating conditions (flue gas temperature, condensate, overpressure), and that the installer can document that the system is approved for the given boiler by the manufacturer or a certification authority.

If you want to avoid the risk, stick with original components or consult directly with the supplier. An overview of the compatibility of various systems and brands can be found in the article Flue pipe compatibility with boilers: Protherm, Vaillant, and other brands.

How are boilers connected in a cascade – what does that mean for the flue pipe?

A cascade (several boilers connected in parallel) is a typical solution for larger buildings – apartment blocks, office buildings, industrial premises. In terms of the flue pipe, this brings specific requirements:

Each boiler must have guaranteed flue gas exhaust unaffected by the others
Twin-pipe systems with a collector are commonly used in cascades – one common exhaust duct for flue gases and one intake duct for air
Special pieces are used to split a coaxial spigot into two separate pipes, such as the Protherm splitter piece Ø 2 x 80 mm R2KP
Sizing is more complex – the equivalent length calculation is done for the whole system, not for an individual boiler

Cascade installations always require a design by a professionally qualified person. Self-installation without calculation can lead to flue gases leaking back between boilers.

What are PP, PPs, and stainless steel in flue pipes?

The flue pipe material is not a secondary matter. Conditions in a condensing flue pipe are demanding: flue gas temperature of 40–120 °C at startup, condensate with pH 3–5 (carbonic acid, and to a lesser extent nitric and sulfuric acid), slight overpressure. The following materials are used:

PP (polypropylene) – the most widespread plastic flue pipe. Usable up to 120 °C, fully resistant to condensate. Lightweight, inexpensive, easy to install. Suitable for most gas condensing boilers.
PPs (polypropylene with additives increasing temperature resistance) – for higher temperature loads (up to 160 °C) or for oil-fired boilers, where flue gases can be hotter.
Stainless steel (AISI 316L or equivalent) – metal flue pipe for more demanding applications. More expensive, but mechanically stronger. Used for chimney lining or higher temperatures. For condensing boilers it must be marked as a "wet system".
Aluminum – used only in the outer parts of some coaxial sets. It is not used inside or for condensate due to corrosion.

Can flue gases leak into the room? How do I recognize it?

Yes, it can – and it's a serious situation. Condensing boilers operate under overpressure, which means that even a small leak causes flue gas leakage (containing CO₂ and potentially CO). The most common causes of leaks:

Incorrectly seated seals at flue pipe joints
Cracked or damaged pipe (mechanical damage, plastic aging)
Incomplete insertion of joints – pipes not inserted deep enough
Use of incorrect components (e.g. waste PVC pipe instead of a certified flue pipe)
Damaged wall duct

Signs of flue gas leakage: smell of combustion gases in the utility room or in adjacent spaces, condensate on walls, black discoloration around joints, recurring boiler error codes (e.g. gas pressure or flue gas temperature faults). The topic is covered in more detail in the article Common faults and leaks in condensing boiler flue pipes.

How long do flue pipes last – when do they need replacing?

A quality PP plastic flue pipe should last 20–30 years with correct installation and normal operation. In practice, however, there are factors that shorten its lifespan:

Operating the boiler with excessive flue gas temperature (operation outside condensing mode, too high return water temperature)
Mechanical damage (e.g. during construction work)
Unprofessional installation – too tight bends, incorrect expansion compensation
Contamination by condensate containing residues from oil combustion or from burning other fuels

A visual inspection of the flue pipe should be part of the boiler's annual service. If cracks, discoloration (browning, blackening) are visible on the pipes, or if condensate drips onto the floor from the joints, it's time to call a professional. Practical maintenance tips can be found in the article Cleaning and maintenance of a condensing boiler flue pipe.

How does installation in an apartment building differ from a family house?

In family houses, the situation is relatively simple: the boiler stands in a boiler room, utility room, or kitchen, and the flue pipe leads either through the wall outside or up through the chimney. The customer has full control over the route.

In an apartment building, it's more complicated. You need to resolve:

Where to vent the flue gases – in apartment blocks, it is not always permitted to vent the coaxial pipe on the façade (aesthetics, protection of neighbors). Building management or the building authority may set conditions.
Shared flue system (LAS) – when replacing multiple boilers in an apartment building at once, a so-called LAS (Luft-Abgas-System) is sometimes built: one central duct into which each apartment connects separately. This is technically advantageous but requires a design and the consent of all parties involved.
Permits – in an apartment building, a building permit or at least a notification to the building authority and the consent of the building administrator is usually required.
Routing pipes through another's property – the flue pipe must not pass through the premises of another apartment without an easement.

Why does the condensing boiler "whistle" or "gurgle" – is it related to the flue pipe?

Customers report these sounds fairly commonly. The causes can vary:

Gurgling at startup or shutdown – typically condensate in an incorrectly sloped horizontal section, which the fan has to "work through" at startup. Solution: fix the slope.
Hissing at joints – flue gas leakage through a loose joint. Call service immediately.
Loud fan noise – could be excessive flue pipe resistance (too long a run, too many elbows, wrong diameter). The boiler operates at higher fan speeds and may report a pressure or flue gas temperature fault.
Plastic cracking sounds – expansion of pipes when heating up. Normal on first startup, but if the cracking persists, the pipes are poorly anchored (missing sliding brackets).

What is a condensate siphon and why is it important?

A condensing boiler produces condensate – at 24 kW output this can be 1–3 liters per hour depending on the operating mode. This condensate must drain into the sewer through a siphon. The siphon serves two functions:

Prevents flue gases from entering the sewer – the water seal in the siphon prevents gases from flowing in the opposite direction
Prevents sewer odor from entering the boiler and the room

A common mistake: the siphon has "dried out" (the boiler hasn't been running for a long time or there's too little condensate). Result: odor or pressure problems. Solution: top up water in the siphon, or install a siphon with automatic water refill. Never connect condensate directly to the sewer without a siphon – there is a risk of sewage backflow and boiler corrosion.

What to watch out for when choosing and buying flue pipe components?

From practical experience, here are the most common purchasing mistakes:

Wrong diameter – the customer buys 60/100 mm and finds out at home that the boiler's spigot is 80/125 mm. Always check in the boiler's documentation what system the manufacturer recommends before buying. Using adapters (e.g. Protherm adapter for flue pipes 60/100 mm A1KP or Protherm adapter for flue pipes 80/125 mm A25KP), different systems can be connected, but you need to know which direction of transition is technically permissible.
Insufficient set length – the customer buys one set for 0.8 m and finds out at home that at least 1.5 m is needed. Always measure the route and calculate the equivalent length before buying.
Forgetting sliding brackets and supports – the flue pipe must be secured at regular intervals (usually every 50–100 cm), with some brackets needing to allow axial movement (expansion).
Wrong material for the boiler type – an oil-fired boiler requires PPs or stainless steel, not ordinary PP.

If you're also handling the primary medium for Protherm boilers, don't forget the hydraulic connection either – the connection manifold for Protherm boilers for copper pipes significantly simplifies installation and ensures the tightness of hydraulic connections when installing the boiler.

Does the installation have to be done by a professional, or can I do it myself?

Slovak law (Decree of the Ministry of Transport, Construction and Regional Development of the SR No. 508/2009 Coll. on the safety of technical equipment, Act No. 309/2009 Coll. on renewable energy sources, and the relevant STN standards) requires that the installation of gas appliances and flue gas exhausts be carried out by a person with the appropriate professional qualification. Therefore: the installation itself and the first commissioning of the condensing boiler, including the flue pipe, must be carried out by a professionally qualified person – otherwise the customer will not have a warranty relationship and may have problems with insurance.

However, the customer can:

Pre-design the flue pipe route and purchase the components
Do the construction work (drill an opening in the wall)
Check that the supplied components are correct

The advantage is that this way the customer can save on materials by purchasing them themselves and paying the professional only for the labor. If you're not sure how to design the route, read the article How to choose a flue gas exhaust for a condensing boiler or How to install a condensing boiler flue pipe step by step in our Knowledge Center.

Frequently Asked Questions (FAQ)

Can I use ordinary PVC waste pipes instead of certified PP flue pipes?

Absolutely not. PVC pipes are not certified for contact with flue gases, cannot withstand the temperatures of a condensing boiler's flue gases (can be 80–100 °C at startup), and release toxic chlorine compounds when thermally degraded. In addition, they don't have seals resistant to flue gas overpressure. Using PVC pipes in this context is dangerous and contrary to standards. Always use components certified according to EN 14471 or an equivalent standard.

How many 90° elbows can I have in a flue pipe without losing boiler performance?

It depends on the flue pipe diameter and boiler output. Each 90° elbow counts as 1–1.5 m of equivalent straight pipe length. The maximum permissible equivalent length is stated in the boiler's documentation – for a typical 24 kW boiler with a 60/100 mm diameter, this is typically 5–8 m. If you have a 3 m straight run and 3 90° elbows, the total equivalent length is 3 + (3×1.5) = 7.5 m – which may be at the limit for some boilers. 45° elbows have less resistance (0.5–0.7 m equivalent length). If it's tight, use a larger diameter or a twin-pipe system.

What to do if condensate from the siphon overflows and can't drain fast enough?

First thing – check whether the siphon is clogged. Condensate can contain deposits (carbonates, debris from the appliance) that gradually clog the siphon. Flushing with water and possible mechanical cleaning of the siphon should be a standard part of the annual service. If too much condensate is flowing, it may signal that the boiler is operating outside the optimal condensing mode (heating water temperature too high) – in that case the hydraulic settings should be checked. With excessively acidic condensate (pH below 3), neutralization of the condensate via a neutralization tank is required before discharge into the public sewer.

Can frost damage the flue pipe or the exhaust outlet?

Yes, in extreme cases. The outer end of the flue pipe (outlet) can freeze over in frosty weather if the flue gases contain enough moisture and the temperature is well below zero. Modern boilers have protective functions (defrost mode), but with long outdoor sections or an unsuitably placed outlet (e.g. facing into a concrete shaft), this can happen. The flue pipe outlet must never be covered, even partially. In practice we have seen cases where snow piled up by a neighbor on a windowsill blocked a coaxial outlet – the boiler shut down due to an overpressure safety fault. Solution: place the outlet high enough above ground level (at least 40 cm from the ground) and protect it from mechanical obstructions.

What is the difference between type B and type C condensing boilers in terms of the flue pipe?

Type B (e.g. B23) is a boiler with an open combustion chamber that draws combustion air from the room. It vents flue gases into a chimney or flue pipe, but the air is not supplied coaxially. This type can be dangerous with insufficient ventilation of the utility room or negative pressure in the building (e.g. a kitchen extractor hood). Type C (e.g. C13, C33) is a boiler with a sealed combustion chamber that draws air directly from outside and also vents flue gases outside – the whole system is separated from the installation space. Most modern condensing boilers are type C, and for these a coaxial flue pipe or a twin-pipe system with outdoor air supply is mandatory.

How do I know if my flue pipe is okay when there are no visible problems?

The best prevention is regular servicing by a professionally qualified person, who can perform a flue pipe tightness check (a so-called pressure test) and check the flue gases with an analyzer. At home you can help yourself with a match near the joints – if the flame near a joint moves, it suggests air flow, which could indicate a leak. However, this is not a foolproof method. CO (carbon monoxide) detectors in the utility room are an absolute necessity – they are cheap and can save a life. At any hint of a flue gas smell in the room, ventilate immediately, turn off the boiler, and call a professional.

Conclusion: the flue pipe is not a minor detail

After reading this article, it's clear that the flue gas exhaust system is just as important as the condensing boiler itself. An incorrectly designed, sized, or installed flue pipe can cause boiler malfunctions, shorten its lifespan, increase energy consumption – and in extreme cases endanger the health or lives of people in the building.

If you're planning an installation or system replacement, we recommend starting from the basics: first read the article How to choose a flue gas exhaust for a condensing boiler, find out what diameter you need (article What flue pipe diameter do I need: 60/100 or 80/125 mm), decide between coaxi

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