Flue Gas Discharge and Air Supply for Condensing Boilers

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Flue Gas Discharge and Air Supply for Condensing Boilers – A Complete Guide

When a customer decides on a condensing boiler, most of the attention goes to output, efficiency or price. The question of how and where the air and flue gases will go usually ends up at the bottom of the list. Yet this very topic can complicate the whole installation, make it more expensive, or in the worse case cause safety and functionality problems. We have seen dozens of cases where the boiler was correctly selected, correctly connected to the heating system, but the flue gas discharge system was underestimated – resulting in a boiler that did not work reliably, or an installation that failed inspection.

This article focuses on this topic in depth: what options exist for discharging flue gases, what the different discharge classes mean, how the coaxial system works, what the sizing rules are, where mistakes tend to occur, and what you need to know before every installation.

Why flue gas discharge for a condensing boiler is different from a classic boiler

A classic atmospheric boiler operates with flue gas temperatures typically of 160–220 °C. Such hot flue gases have natural draught – they rise up the chimney even without the help of a fan. A condensing boiler is a completely different animal in this respect. Precisely because it utilizes maximum heat from the flue gases, including the latent heat of water vapor, the flue gases leave the boiler at a temperature of only 50–70 °C, sometimes even less. At such a temperature, natural draught is weak or practically zero. That is why every condensing boiler contains a built-in fan that actively pushes (or draws) flue gases out of the boiler while simultaneously drawing in combustion air.

This principle is called pressurized or negative-pressure discharge (depending on the fan design). Most modern condensing boilers work with a pressurized fan – flue gases are actively pushed out. This has a fundamental impact on which chimney system you can use and how it must be sized.

Another specific feature: flue gases from a condensing boiler are moist and mildly acidic (the pH of the condensate is around 3–5). This means the entire discharge system must be resistant to moisture condensation and to an acidic environment. An ordinary brick chimney is not sufficient for this.

Types of flue gas discharge systems – overview and labeling

In Europe, flue gas discharge systems are labeled according to the EN 15502 standard and corresponding national regulations (in Slovakia STN EN). The basic classes you will encounter with condensing boilers are:

Class B – air drawn from the room, discharge into a chimney

A Class B boiler draws combustion air directly from the room in which it is installed, and discharges flue gases into a shared chimney (or a separate discharge duct). For condensing boilers, this configuration is used today only in exceptional cases – it requires an open boiler (which is rare for condensing units), sufficient fresh air in the room, and does not rule out the risk of flue gas backflow during strong wind or a low-pressure situation. If someone proposes installing a Class B condensing boiler, that is at least a reason to ask further questions.

Class C – closed system, air and flue gases via the facade or roof

This is the standard system for condensing boilers. The boiler is hermetically sealed – combustion air intake and flue gas discharge both take place through the same or separate pipes led outside the building. The boiler has no contact with the room air (in terms of the combustion process). Class C is further divided according to how the pipes are routed:

C13 – coaxial system (air in and flue gas out through one double-pipe duct), connected through the exterior wall. The most common case for residential condensing boilers.
C33 – coaxial system routed through the roof.
C43 – separate pipes, air and flue gas routed differently, e.g. air from outside through a separate pipe, flue gas elsewhere.
C53 – separate pipes, air and flue gas each routed to a different location (e.g. air from the facade, flue gas through an existing chimney fitted with a liner).
C63 – boiler is designed to be connected to standardized discharge systems (e.g. systems for multiple boilers).
C83 – air is drawn from a shared air duct (e.g. a shaft in an apartment building), flue gas goes into a shared discharge duct.

In practice, for family houses and apartments, C13 (coaxial through the facade) and C33 (coaxial through the roof) dominate. For renovations where a chimney already exists, C53 with a stainless steel liner comes into consideration.

Coaxial system – how it works and its advantages

A coaxial pipe (also called an "air-flue" system or LAS – Luft-Abgas-System) is a solution in which two concentric pipes are combined into one assembly. The inner pipe discharges the flue gas, while the outer annular gap supplies fresh air from outside. Both functions are thus solved through a single penetration of the wall or roof.

Advantages of the coaxial system:

Preheating of combustion air: As air flows toward the boiler, it passes around the outer wall of the warm flue gas pipe and absorbs some of the heat. This slightly improves boiler efficiency and reduces the risk of condensation on the cold side.
Simple installation: One opening in the wall, one penetration, one outdoor terminal. Less space, less work.
Safety: The boiler is hermetically separated from indoor air – there is no risk of flue gas backflow into the room.
Fan draught control: Since both air and flue gas are routed through fixed pipes with a defined flow rate, the boiler's fan can precisely control the combustion process.

A typical coaxial system for an average residential condensing boiler has diameters of 60/100 mm (inner/outer pipe) or 80/125 mm for more powerful boilers (25–35 kW and above). Some manufacturers also offer 80/80 mm systems with separate parallel pipes (the so-called "twin pipe" or "PP" system) – in this case it is a C43 or C53 type, where air and flue gas are routed separately.

Important note from practice: the coaxial terminal on the facade must be positioned so that flue gases do not enter windows, ventilation openings or neighboring spaces. Standards and manufacturers specify minimum distances (usually 300–500 mm from windows, 300 mm from a building corner, 200 mm above ground level, etc.). These distances are not recommendations – they are mandatory.

Stainless steel chimney liner – when and how to use it

In renovations of older houses, we commonly encounter the following situation: an existing brick chimney, originally used for solid fuel or an old gas boiler. Can a condensing boiler use such a chimney? Not directly. A brick or concrete chimney is not resistant to the moisture and acidity of the condensate – it would gradually deteriorate from the inside.

The solution is lining the chimney with a stainless steel flexible or rigid liner. This is a pipe made of acid-resistant steel (grade 1.4404 or similar) that is inserted into the existing chimney. The liner must be correctly sized and must not be too long – otherwise the condensate will freeze in winter or accumulate without being able to drain away.

When using a stainless steel liner in an existing chimney, the following applies:

The liner diameter must correspond to the boiler manufacturer's recommendation (typically DN 80 to DN 130, depending on output and duct length).
The liner must have a condensate collector with drainage at the bottom.
The chimney space around the liner must be thermally insulated to prevent excessive cooling of the flue gases and excessive condensation.
In this case, combustion air is usually sourced differently – either from the room (Type B, which for a condensing boiler requires meeting the room ventilation conditions), or through a separate pipe from outside (C53).

From practice: lining a chimney for a condensing boiler is common and works well if done correctly. Problems arise when a cheap uninsulated liner is used, or when condensate is allowed to drip directly onto the chimney masonry – this causes moisture damage, mold, and in extreme cases the disintegration of the masonry.

Sizing the discharge system – specific numbers and rules

Sizing a chimney or discharge system is not just about "something" fitting. Incorrect sizing causes either insufficient draught (the boiler reports an error on startup, unstable combustion), or excessive flue gas cooling and excessive condensation in the pipe. In practice we follow these rules:

Maximum length of coaxial pipe

Each boiler manufacturer specifies the so-called equivalent length of the coaxial system. For example, for a 60/100 mm system this is typically 3–5 m (total equivalent length), for 80/125 mm it can be 10–15 m for more powerful boilers. Each pipe elbow (90°) is added as equivalent length (usually 1–1.5 m for a 45° elbow and 1.5–2 m for a 90° elbow). You will find these values in the installation manual for the specific boiler – and they must be followed, not bypassed.

From practice: a very common mistake is that the installer extends the route, for example to go around a ceiling beam, and thus exceeds the allowed limit. The boiler then works, but at low outdoor temperatures the fan may fail to push the flue gases through sufficiently and the boiler triggers an error. Solution: for longer routes, switch to a larger diameter or a different type of system.

Pipe slope towards the outside

The coaxial pipe should have a slope of at least 3% (3 cm per 1 meter) towards the outside – i.e. the end at the terminal must be lower than the start at the boiler. The reason is simple: condensate that forms in the pipe (and with a condensing boiler there always is some) must flow back into the boiler, where it is collected and drained. If the pipe had a slope in the opposite direction or was horizontal, the condensate would accumulate and block the flow or freeze.

Chimney height when lining

For vertical routing (stainless steel liner, system through the roof), the minimum height of the chimney outlet above the roof must be at least 0.5 m above the parapet of a flat roof or 0.65 m above the ridge of a pitched roof (according to STN EN 15502 and relevant regulations). For a condensing boiler with low-temperature discharge, these requirements are in some cases less strict, but they should never be underestimated – the inspection technician will check them.

Shared discharge systems in apartment buildings

In apartment buildings the situation becomes more complicated – you cannot have every apartment with its own terminal on the facade (at least not always). The solution is shared discharge shafts (C83 system or LAS shafts). Principle: a shared vertical shaft contains one large duct for air supply and one for flue gas discharge from all apartments. Each boiler is connected to the shaft via a branch fitted with a non-return valve.

These systems are technically well solved, but when renovating existing apartment buildings, their implementation requires coordination of the whole building, a building permit, and usually also technical supervision. It is not something a single tenant can solve on their own.

An alternative in apartment buildings is an individual coaxial discharge through the facade for each apartment – if the facade allows it and the building authority approves it. In panel buildings and older apartment blocks, complications often arise regarding the placement of the terminal (local regulations, aesthetic requirements of the building administrator, proximity to neighbors' windows). This needs to be addressed in advance – before purchasing the boiler.

Air supply – why it matters more than it seems

A Class C condensing boiler supplies its own air via an external pipe – that much is clear. But what if we want a Class B boiler (or a boiler that draws air from the room for another reason)? Then we need to actively address the air supply to the utility room.

Burning gas consumes air. For a 24 kW boiler, this is on the order of 4–6 m³ of air per hour during operation. If the room is small and airtight (modern houses with excellent window sealing), a negative pressure can occur that affects the boiler's draught or causes flue gas backflow. In practice this is rare with Class C boilers, where the air comes from outside. But for Class B boilers (or in combination with another air-consuming appliance such as a kitchen range hood) this needs to be addressed.

Ventilation requirements for a room with a Class B boiler:

The air supply must be permanent (a grille, a vent), not just a window.
The opening cross-section is calculated according to the boiler's output – roughly 6 cm² per each 1 kW of boiler output.
The room must not be under negative pressure (e.g. due to a range hood in an adjacent kitchen).
If there is more than one air-consuming appliance in the room, the calculation must account for their sum.

These are requirements of the standard and of inspections – failure to comply can directly affect safety. Proper installation of a condensing boiler must always comply with applicable regulations and should be carried out by a certified installer with a gas qualification. More detailed requirements for the entire installation can be found in the article Installing a Condensing Boiler – What the Installation Must Meet.

Condensate in the discharge pipe – not just a matter for the boiler

Condensation of water from the flue gas occurs not only in the boiler's heat exchanger but also in the discharge pipe – especially on longer routes or in winter. Any point where the flue gas is sufficiently cooled produces condensate. Therefore:

Coaxial pipes must have a slope towards the boiler (as mentioned above).
In chimney liners, condensate is collected in a collector at the bottom of the liner and drained through a drainage pipe into the sewer.
The condensate is mildly acidic – its pH is around 3–5 depending on the type of fuel (with natural gas it is less acidic than with propane). Before discharging it into the sewer, it is good practice to neutralize it, especially if the quantity is larger. This topic is covered in a separate article, Boiler Condensate – How to Drain and Neutralize It Correctly.

In practice we encounter cases where a chimney liner was installed without a condensate collector – the installer either "forgot" it or considered it unimportant. The result: condensate runs down the masonry, causing moisture stains on walls and eventually damage to the plaster or masonry. The solution is simple – a condensate collector is a cheap component, but it must be present.

Safety aspects and legal requirements

Flue gas discharge is not just a technical but also a legal matter. According to applicable Slovak regulations (the Air Protection Act, the Ministry of Environment decree, STN EN standards, and gas distributor regulations), every installation of a gas appliance, including a condensing boiler, must:

Be carried out by an authorized person (a certified gas installer).
Be approved by an inspection technician (commissioning inspection).
Meet the requirements for the chimney system (material, dimensions, routing).
Be equipped with safety features (safety valves, shutdown in the event of discharge failure – most modern boilers have a flue gas detector or shutdown on discharge error).

A condensing boiler with an active fan usually has a built-in pressure switch or flow sensor that monitors whether air and flue gas are actually flowing correctly. If not – the boiler shuts down and displays an error code. This is a safety feature, not a malfunction. If this error appears repeatedly, it is a signal that something is wrong with the discharge (blocked pipe, incorrect sizing, frozen terminal). You can read more about typical faults in the article Common Condensing Boiler Faults and Their Causes.

Terminal freezing – a real problem in cold weather

The coaxial terminal on the facade is exposed to frost in winter. The flue gas leaving it is moist – and at temperatures around and below zero, condensate on the terminal can freeze. Ice or a frost layer can partially or completely block the terminal. The boiler detects this (discharge safety feature) and shuts down.

This really does happen – especially when the terminal is installed inappropriately (for example, under an overhanging drip edge, where ice forms faster), or when the pipe is too long and the flue gas is very cold by the time it exits. Solutions:

Correct terminal placement – away from areas with increased condensation, not directly under a drip edge.
Use of a terminal resistant to freezing (some manufacturers offer special terminals for cold climates).
Shortening the pipe to a minimum so that the flue gas still has sufficient temperature at the outlet.
Inspection and possible de-icing during frosty weather.

Most common mistakes in installing a discharge system

Based on experience from ordinary field practice, these are the most common mistakes we encounter during servicing or inspections:

Exceeding the maximum coaxial pipe length – the pipe is too long, the boiler operates at or beyond the limit.
Incorrect pipe slope – the pipe rises slightly outward, condensate stands in the pipe or leaks out instead of flowing inward.
Missing condensate collector when lining – condensate destroys the chimney's masonry bed.
Unsuitable liner diameter – too small for the boiler's output, or too large (the flue gas cools too much and the boiler operates inefficiently).
Terminal near a window or ventilation grille – flue gas enters back into the interior, risk of CO poisoning.
Uninsulated liner in a cold chimney – excessive condensation, increased wear of the liner.
Use of an incorrect pipe material – for example, aluminum pipe where the manufacturer requires PP or stainless steel.

Materials for discharge pipes

The following materials are used for condensing boilers:

Polypropylene (PP) – resistant to condensate and acids, lightweight, wide range of fittings. The standard solution for coaxial systems indoors and for chimney liners. Resistant up to temperatures of around 120 °C.
Acid-resistant stainless steel (1.4404 / 316L) – for outdoor or long installations, more resistant to mechanical damage. More expensive, but more durable in extreme conditions.
Aluminum – used by some manufacturers for covered outdoor sections (terminal), but not suitable for long-term use in a condensing environment – it reacts with the acidic condensate.
Ceramic or firebrick – not suitable for condensing boilers without special impregnation, ceramics absorb moisture and the condensate will damage it over time.

The rule always applies: use the material recommended by the boiler manufacturer. Some boilers are certified only for specific systems – if you use a different one, the warranty and responsibility for correct function are forfeited.

Practical example from the field: renovating a family house from a gas boiler to a condensing boiler

A customer had an old classic gas boiler in a family house with a brick chimney measuring 200 × 200 mm, 8 m high. He wanted to switch to a 24 kW condensing boiler and make the most of the existing chimney. Solution:

A DN 100 mm stainless steel flexible liner (recommended by the boiler manufacturer for outputs up to 25 kW at a length of up to 10 m) was inserted into the existing chimney.
The liner was thermally insulated with mineral wool in the gap between the liner and the masonry.
An inspection section with a condensate collector and a DN 32 drain leading into the sewer via a neutralization box was installed at the bottom of the chimney.
Combustion air was supplied via a separate pipe (DN 80 PP) routed through the facade next to the chimney – a C53 system.
The inspection was passed without objections, and the boiler works without any problems.

The total cost of the discharge system, including the liner, insulation, neutralizer and labor, was in this case around €600–900 – which is a normal range for such a renovation. This item should be factored in when planning the total investment in a condensing boiler.

Flue gas discharge and energy efficiency – a connection often overlooked

A correctly sized and insulated discharge system has a direct impact on the boiler's actual efficiency. A condensing boiler achieves a declared efficiency above 100% (relative to calorific value) precisely because the flue gas is sufficiently cooled and the water vapor in it condenses in the heat exchanger. If the discharge pipe is too short, the flue gas does not have time to release its heat and leaves warmer – condensation is lower, and efficiency drops. If the pipe is too long and uninsulated, the flue gas cools in the pipe rather than in the heat exchanger – condensate goes into the pipe instead of the boiler, and the heat exchanger operates suboptimally.

Therefore, the correct length and insulation of the discharge system is important not only from a safety standpoint but also from an economic one. You can read more about the choice and comparison of condensing and classic boilers from an economic perspective in the article Condensing vs. Classic Boiler – Is Paying More for Condensation Worth It.

Frequently Asked Questions (FAQ)

Can I connect a condensing boiler to an old brick chimney without lining it?

No. A brick or concrete chimney is not resistant to the moisture and acidity of the condensate from a condensing boiler's flue gas. Without a stainless steel or PP liner, the condensate would gradually saturate the masonry bed, causing its degradation, moisture damage to walls, and in extreme cases even structural problems with the chimney. A certified liner made of suitable material is always required.

What if I have nowhere to route the coaxial terminal to the facade – for example, in an apartment?

If a facade outlet is not permitted (building administrator, proximity to neighbors' windows, aesthetic requirements), alternatives exist: an outlet through the roof plane (C33), connection to a shared LAS shaft (C83), or lining an existing chimney. Each of these options has its own technical and spatial requirements. This needs to be addressed before purchasing the boiler, not after.

How long can a 60/100 mm coaxial pipe be?

This depends on the specific manufacturer and boiler model – always check the installation manual. As a general guide, for a 60/100 mm system the maximum equivalent length is 3–5 m, with each 90° elbow counted as 1.5–2 m of equivalent length. For longer routes, it is necessary to switch to a larger diameter (80/125 mm) or use a different type of system. Exceeding this limit causes unstable combustion or boiler error messages.

Does the coaxial pipe need a slope? In which direction?

Yes – the pipe must have a slope of at least 3% (3 cm per 1 m of length) towards the boiler, i.e. the terminal on the facade must be physically lower than the connection at the boiler. This ensures that the condensate flowing in the pipe runs towards the boiler (where it is collected and drained), and not out onto the facade. A reverse or zero slope is one of the most common installation mistakes.

The condensing boiler's terminal freezes in winter – what can be done?

Terminal freezing is a real problem at low temperatures, especially if the pipe is long and the flue gas has a low temperature at the outlet. Solutions include: correct terminal placement (not under a drip edge, not on the north side without sun), use of a frost-resistant terminal, and shortening the pipe route. In an emergency, the terminal can be de-iced manually (carefully, with lukewarm water), but recurring freezing is a sign of incorrect installation.

Can a condensing boiler operate without a chimney, just through a wall?

Yes – and this is actually one of its advantages. A condensing boiler with a coaxial system (C13) operates without any chimney at all – both air and flue gas are routed through a simple double pipe directly through the exterior wall. The terminal on the facade replaces the chimney. This is a common solution in apartments and in new-build family houses, where a chimney is often not built for this very reason.

Conclusion

Flue gas discharge and air supply are, for a condensing boiler, a topic that deserves just as much attention as choosing the boiler itself. A poor discharge solution can mean functionality problems, structural damage, safety risks, or a failed inspection. A good solution, on the other hand, guarantees that the boiler will operate reliably, economically and safely throughout its service life.

Basic rules to remember: a condensing boiler belongs to Class C (closed system), a coaxial system through the wall is the simplest and most common solution, renovations with an existing chimney require a stainless steel liner with insulating wrap, every system has its own maximum permitted length and must have the correct slope. Installation should always be handled by a certified installer, and every new installation must undergo a commissioning inspection.

If you are planning an installation and want to clarify which boiler and which type of discharge is suitable for your house, also read the article How to Choose a Condensing Boiler – What to Focus on Before Buying or What Output of Condensing Boiler Do I Need for My House – both will help you make an informed decision before making the purchase.

Do you have a question about this topic?

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