Flue gas discharge and air supply for condensing boilers: what you need to know

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Flue gas exhaust and air supply for condensing boilers: a complete technical overview

When customers decide to buy a condensing boiler, most of the attention naturally goes to performance, brand or price. The flue gas exhaust and combustion air supply system, however, belongs to the things that can make the entire installation more expensive, more complicated or, on the contrary, simplify it — to an extent that most people do not even consider before purchasing. I have seen cases where a homeowner paid 1,200 euros for a boiler, and then found out that the flue gas termination would cost another 600 euros due to an unsuitable location of the boiler room, wall thickness or a neighbor's window exactly in the direction where they wanted to route the flue. That is why I am addressing this topic in great detail — so that you know what to expect before purchasing.

This article is intended both for those who are just deciding which condensing boiler to buy, and for those who already have a boiler and want to understand why the installer suggests exactly this solution. If you are looking for a broader overview of boiler selection, I recommend reading the article How to choose a condensing boiler: what to focus on before purchasing in our Knowledge Center.

Why flue gas exhaust for a condensing boiler is different from that of old boilers

A traditional atmospheric boiler (those old white cabinets with ceramic heat exchanger) operated at high flue gas temperatures — typically 150 to 200 °C. Such flue gases had enough buoyancy to rise up the chimney on their own. A condensing boiler works fundamentally differently: it uses precisely that energy that would otherwise be lost with the hot flue gases. The flue gas temperature at a condensing boiler typically ranges between 40 and 80 °C, and in the case of full condensation (low-temperature circuit, outdoor temperature above zero), it can even be lower than 50 °C.

This has two major consequences:

Flue gases do not create natural draft — they are too cold to be drawn up the chimney on their own. A condensing boiler therefore needs its own blower (fan) to actively push the flue gases out.
Flue gases contain water vapor and condensate — when cooling in the pipe, they condense, which means that all flue gas paths must be resistant to acids and corrosion, and must have a slope towards the condensate collector.

In addition, a condensing boiler is almost always a boiler with a closed combustion chamber (type C according to EN 437 standard) — this means that the air for combustion is not taken from the room, but directly from outside via a separate pipe. This is a major safety advantage, but it adds another pipe to consider during installation.

Boiler types according to the method of flue gas exhaust: EN 437 standard and C/B designation

When you look at the technical data sheet of a boiler, you will encounter designations such as B23, C13, C33, C43, C53, C63, C83 and similar. These are not random codes — they are a European classification according to the EN 437 standard, which describes where the boiler takes air and where it exhausts the flue gases.

Type B — boiler with an open combustion chamber

Type B boiler takes combustion air directly from the room and exhausts the flue gases through a chimney. This type is used less and less for condensing boilers today, but it still exists — for example, in boilers connected to an existing ceramic chimney system. The problem is that with type B, the room must be sufficiently ventilated, there must be no other devices with an open combustion chamber in the room, and the chimney must have certification for condensing equipment (wet operation, resistance to acids — designation V2 or V3 in EN 1443).

Type C — boiler with a closed combustion chamber (turbo)

This is now the standard for wall-mounted condensing boilers. The boiler has a closed combustion chamber — air and flue gases are routed through separate pipes (or coaxial double-pipe systems) directly through the wall or roof to the outside. Type C boiler is the safest option — there is no risk of flue gas backdraft into the room, and no ventilation requirements for the boiler room are imposed.

The digits after the letter C indicate the specific arrangement:

C13 — horizontal exhaust through the wall (coaxial or double-pipe)
C33 — vertical exhaust through the roof
C43 — air from the chimney body, flue gases into the chimney body (shared shaft)
C53 — air and flue gases routed through separate pipes, each terminated at a different location
C63 — boiler without an attached chimney system, suitable for connection to a certified chimney system of the manufacturer
C83 — air from the building's chimney body, flue gases exhausted through the wall

In practice, the most common variant for single-family homes is C13 (horizontal exhaust through the wall), and during renovations with an existing chimney, the variants C33 or C43 (vertical exhaust, inserted chimney) are used.

Coflow vs. twin pipe system: what is the difference and which one to choose

This is one of the questions I address in every second project. You basically have two main options for routing air and exhaust gases between the boiler and the outside environment:

Coflow system (pipe in a pipe)

A classic solution for shorter distances. One pipe is inserted into the other — cold air flows in through the outer shell, and exhaust gases go out through the inner pipe. The advantage is that you need only one hole in the wall. The disadvantage is the limited maximum length — depending on the manufacturer and model, but usually 3 to 10 meters of equivalent length (each 90° elbow counts as 1–1.5 m of equivalent length). For greater distances between the boiler and the outside wall, a coflow system is not sufficient.

Standard diameters for wall-mounted boilers: 60/100 mm (inner/outer pipe) and 80/125 mm for more powerful boilers or longer runs.

Twin pipe system (separate pipes)

Air and exhaust gases are routed through completely separate pipes — each has its own outlet to the outside. The advantage is a much greater maximum length of the run (some boilers allow up to 40–80 meters of equivalent length at a diameter of 80 mm), the possibility to route air from a different direction than the exhaust gases, and lower flow resistance. The disadvantage is two pipes, two holes in the wall or roof, and a slightly higher material cost.

A twin pipe system is also used when connecting multiple boilers to a common exhaust system — so-called cascade, which is typical for larger apartment buildings or industrial applications. For a single-family house with one boiler, a coflow system is usually sufficient if the boiler is located near an outside wall.

Flue pipe materials: what can withstand and what cannot

Exhaust gases from a condensing boiler contain not only CO₂ and water vapor but also acids — mainly carbonic acid (from CO₂ dissolved in condensate), and at higher temperatures also sulfurous acid. Condensate typically has a pH of 3.5 to 5.5 — that is quite acidic. This places strict requirements on the material:

PP (polypropylene) — the most common material for condensing systems. Resistant to acids, lightweight, and inexpensive. Temperature resistance up to about 95 °C — more than sufficient for condensing boilers. You can recognize it by its beige or light gray color.
PPs (polypropylene with glass fiber reinforcement) — glass fiber reinforcement increases temperature resistance and stiffness. Suitable for boilers with higher output or longer runs.
Stainless steel (AISI 316L or 316Ti) — the longest lifespan, suitable for all types of boilers including low-temperature ones. Used when inserting into existing chimneys and in systems where a condensing boiler is combined with another heat source (e.g., a fireplace insert).
Galvanized sheet metal, ordinary steel, aluminum — NOT suitable for condensing boilers. Acids will quickly destroy them. This is a mistake I have seen during renovations, where someone cheaply removed an old boiler and "temporarily" used an existing pipe — after two years, the pipe was completely corroded through.

Important: pipes must be certified for condensing operation. On the market, there are systems marked with CE with a specific classification according to EN 14471 (for plastic) or EN 1856 (for stainless steel). Always request a certificate from the installer.

Slope of the flue pipe and condensate drainage: technical details

This is a detail that is easily overlooked during installation and later causes problems. Since condensate forms in the exhaust gases, the pipe route must have a slope back towards the boiler — not outwards. The water that condenses in the pipe must flow back into the boiler, where it collects in a condensate collector and is then drained into the sewer.

The minimum slope of a horizontal section is 3° (approx. 5 cm per meter of length). If the pipe runs slightly uphill towards the outside, condensate will accumulate, may freeze in winter, and in the best case, the boiler will shut down due to a fault, and in the worst case, the pipe may crack.

Condensate must be drained into the sewer. According to Slovak regulations (and most technical standards), it should be neutralized before being discharged into the sewer — a condensate neutralizer (also called a neutralizer) is filled with lime granules that increase the pH of the condensate to an acceptable level (pH > 6.5). Most boiler manufacturers sell neutralizers as accessories. The amount of condensate depends on the boiler's power and operating conditions — for a 10 kW boiler, expect 1 to 2 liters of condensate per hour under full condensation conditions.

Flue path lengths: how to calculate them correctly

Each boiler manufacturer specifies the maximum equivalent length of the flue system — this is the sum of the lengths of straight sections and the calculated "equivalent lengths" of elbows and fittings. For example, in a 60/100 mm coaxial system, it is roughly as follows:

1 meter of straight section = 1 meter of equivalent length
90° elbow = 1.0 to 1.5 meters of equivalent length
45° elbow = 0.5 to 0.8 meters of equivalent length
Wall fitting/cover = 0.5 meters

If a boiler has a maximum equivalent length of, for example, 8 meters and your route is: 2 meters of straight section + 2 elbows of 90° (2 × 1.5 m) + wall fitting (0.5 m) = 6.5 m — you are under the limit, everything is fine. If you add another elbow or extend the route, you may exceed the limit, which can cause insufficient exhaust, power drop, or a safety shutdown of the boiler.

For an 80/80 mm twin pipe system, the limits are much higher — some boilers allow up to 60–80 meters of equivalent length, which allows the boiler to be installed deep inside the building and the pipes to run through multiple rooms.

For example, the wall-mounted condensing boiler BOSCH Condens GC2300iW 22/25 C supports both coaxial and twin pipe systems, and the manufacturer specifies maximum lengths for each variant in the technical documentation — these values must always be checked before designing the installation route.

Exhausting flue gases from the building: where and how

The location of the flue gas pipe outlet on the façade or roof is not arbitrary. Technical and legislative limitations apply, based on the standard STN EN 15502 and the regulations on gas appliance safety. Here are the most important rules:

Minimum distances from openings and objects

From windows, doors and ventilation openings: at least 300 mm (600 mm or more is recommended)
From the corner of the building: at least 300 mm
From the ground (height above ground): at least 300 mm, recommended at least 500 mm due to snow and dirt
From the neighbor's property, window, or door: usually at least 600 mm, but this should be discussed with local regulations as well
Must not point directly at a sidewalk or public road below 2.1 m
Must not be covered or partially blocked (e.g. by vegetation, awnings, furniture)

Horizontal vs. vertical exhaust

Horizontal exhaust through the wall (C13) is the simplest and cheapest solution — one hole in the wall, a wall flange and an end cap. Problems arise when the wall outside is unsuitable (e.g. near a neighbor, in a courtyard with insufficient space, or when the façade is not accessible for technical reasons).

Vertical exhaust through the roof (C33) is a bit more expensive (special roof penetration, higher chimney), but is very space-efficient — it does not interfere with the façade, the draft is better, and the risk of freezing the exhaust is lower. It is the preferred solution for sloped roofs, where a horizontal pipe would have to be long and winding.

Inserted chimney: when you do not have a direct wall

In renovations in apartment blocks or in historical buildings, a horizontal outlet on the façade is often impossible. The solution is to use an existing chimney body — a stainless steel or PP insert certified for condensing operation (wet draft, class T120, P1, W, V2) is inserted into the chimney opening. Air is either supplied through a second opening of the same chimney (C43), or from the boiler room (in that case, the boiler room must have sufficient air supply).

An inserted chimney is a more expensive solution — a stainless steel insert for 8 meters in height including accessories usually costs between 600 and 1,200 euros depending on the diameter and system. But if it is the only way, it is the correct and lasting solution.

Supply of combustion air: why it is important and what happens if it is insufficient

A condensing boiler type C (closed combustion chamber) takes air directly from outside through a supply pipe — the boiler itself takes care of the air supply. This is a big advantage, because the boiler room does not need any special ventilation and the boiler is not affected by the ventilation conditions of the room.

Problems arise when the boiler is set or installed as type B (open chamber), or in the case of old atmospheric boilers. In such cases, the room must have a permanent supply of fresh air. According to the standard, at least 1.6 m³ of air per 1 kW of power is calculated — so for a 24 kW boiler, you need at least 38 m³ of air per hour. If the room is small and airtight, the boiler will "suck" air during combustion and incomplete combustion, formation of carbon monoxide or backdraft of flue gases into the room may occur.

This is not theory — every year in Slovakia, several tragedies happen precisely due to insufficient ventilation in boilers with an open combustion chamber. Therefore, for modern condensing boilers, I insist on type C wherever it is technically possible.

Condensing boiler in an apartment building: special requirements

In multi-storey apartment buildings, the situation is more complicated. Each apartment has a differently located boiler room, different distances from the façade and different chimney systems. I most often encounter these scenarios:

Scenario 1: Apartment on the top floor with original chimney openings. Ideal situation — inserting into the chimney is a short-term job, the pipe goes directly up. The roof outlet is clean, without neighbor problems.

Scenario 2: Apartment on a middle floor in an old building. The chimney opening is either not available or is occupied by another apartment. A horizontal outlet on the façade must meet the distances from the neighbors' windows. In urban buildings, this can be very limited. Sometimes the only option is a shared chimney system for the entire apartment building — this requires an agreement with the manager and other tenants/owners.

Scenario 3: New apartment building with central shafts. Here, the architect and designer usually determine the location of flue shafts and boilers in advance — each technical room has a pre-designed exhaust. In this case, it is sufficient to follow the design solution.

When planning a boiler in an apartment, I always recommend first finding out where it is actually possible to vent the exhaust — and then selecting the boiler. Not the other way around.

Concrete examples from practice: where mistakes are made

Over the years of practice, I have seen the following typical problems:

Coaxial pipe too long: A customer had a boiler inside the house in a technical room, 7 meters from the wall plus two elbows. The total equivalent length was 11 meters, while the limit was 8 meters — the boiler was blocking at higher output. Solution: switching to a double-pipe system 80/80 mm with a higher limit.

Exhaust pointing towards a window: A customer placed the exhaust directly on the façade, less than 30 cm from the living room window. In summer, when the window was open, he could smell the exhaust. He had to extend the pipe and change the direction — additional costs of about 150 EUR.

Incorrect slope: The pipe was slightly sloping upwards towards the façade. In winter, condensate froze in the pipe, and the boiler blocked. During installation, it is always necessary to check the slope with a spirit level.

Unsuitable pipe material: A builder during a renovation "helped" and used a standard galvanized pipe that he had on hand. After two heating seasons, it was corroded and started to leak. The entire route had to be replaced.

These problems are typical even for otherwise high-quality boilers — for example, the BOSCH Condens GC2300iW 24 P or BOSCH Condens GC8700iW 30 P are reliable devices that work smoothly when properly installed — but no boiler will tolerate a poorly designed flue system.

Legislation and regulations in Slovakia

The installation of a condensing boiler and flue system in Slovakia is subject to several regulations:

Act No. 657/2004 Coll. on thermal energy and related decrees
Decree of the Ministry of Transport, Construction and Regional Development No. 401/2008 Coll. on the details of the implementation of gas equipment
STN EN 15502 — safety requirements for gas heating boilers
STN EN 14471 — chimney systems with plastic linings
STN EN 1856-1/2 — metal chimney systems
Local building regulations and municipal decrees (in some cities there are additional restrictions for façade exhausts)

Installation can only be carried out by an authorized person for the installation of gas equipment (certificate according to §16 of Decree 508/2009 Coll. or equivalent). After installation, a inspection report of the gas equipment must be prepared, without which the equipment cannot be legally put into operation or a contract with the gas company can be concluded.

If you are also interested in the actual installation process of the boiler, we will cover this topic in more detail in the article How the installation of a condensing boiler proceeds and what you need to prepare.

Smart regulation and flue system: connections

At first glance, it may seem that boiler regulation and the flue system are unrelated. But this is not true. Modern smart thermostats, such as Bosch Easycontrol CT 200, allow the boiler to operate with modulated output — meaning the boiler does not always run at full power, but adapts the output to current demand. At lower output, flue gas temperatures are lower, condensation is more intense, and the demands on the flue system are effectively lower. However, during long transitional periods and at low outdoor temperatures, the pipe can freeze in winter if it is not sufficiently insulated.

Therefore, for long external sections of the flue pipe (e.g., in a vertical exhaust through a cold roof or external wall), it is advisable to consider insulation of the PP pipe — using special insulating sleeves that reduce heat loss and the risk of condensate freezing.

What to find out before buying a boiler: practical checklist

Before you order a boiler, go through this list of points. It will save you a lot of trouble:

Where is the nearest external wall from the planned location of the boiler? What is its thickness?
Are there windows, doors, or other openings on the façade in the direction of the exhaust within 60 cm?
Is a neighbor's land or window within the range of the planned exhaust?
Is a chimney opening available (and in what condition, with what diameter and height)?
What is the maximum length of the route from the boiler to the exhaust (including elbows)?
Where will the condensate be drained? Is a sewer trap available?
Is a condensate neutralizer needed?
What type of C configuration (C13, C33, C43...) is realistically feasible?

Answers to these questions will also help you choose the boiler — some models have greater flexibility in flue systems, while others are limited to one type. More about selecting a boiler according to parameters can be found in the article What power of a condensing boiler do I need for my house.

Most frequently asked questions (FAQ)

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

Usually yes, but not directly. A clay chimney must be lined with a certified lining (stainless steel or PP) for condensing operation — marked W (wet operation), corrosion class V2 or V3. A bare clay chimney is not suitable: condensate gradually damages it and there is a risk of exhaust gas leakage into the apartments. Lining the chimney is an additional investment, but it is the correct and lasting solution.

What is the maximum length of a coaxial flue system 60/100 mm?

It depends on the specific boiler — always check the technical documentation of the model. As a rough estimate, it is about 4 to 10 meters of equivalent length for most boilers. If your route with elbows exceeds this value, it is necessary to switch to a larger diameter (80/125 mm) or to a double-pipe system (80/80 mm), where the limits are significantly higher.

Do I need a condensate neutralizer, or can I drain the condensate directly into the sewer?

It depends on local regulations. Most Slovak municipalities require a neutralizer — condensate with a pH of around 4 is not suitable for the sewer in the long term. A neutralizer is a relatively inexpensive device (about 30–80 EUR), filled with lime granules that need to be replaced according to the manufacturer (usually once a year for a standard installation, more frequently for larger boilers). Some boilers have a simple siphon with a built-in neutralizer in the boiler accessories.

Can the exhaust outlet freeze in winter?

Yes, under certain conditions it can. The most risky are horizontal exhausts into a freezing environment — if the boiler is not running for a long time (e.g., a vacation home) and the outside temperature is consistently below –10 °C, residual condensate in the pipe can freeze. The solution is the correct slope of the pipe (condensate drains back into the boiler) and, in extreme conditions, insulation of the external part of the pipe. The terminal end (outlet) designed for condensing boilers has a shape that minimizes freezing.

Can I have an exhaust through a balcony or a loggia?

It is generally not recommended and in many cases it is not allowed. A closed or partially closed space of a balcony or loggia can lead to the accumulation of exhaust gases, especially in light wind. If it is the only option, it should be discussed with the designer and the gas company — sometimes it is allowed if the space is sufficiently open and the exhaust is directed directly outside.

What happens if the flue system is not properly dimensioned?

The consequences can vary — from reduced performance and efficiency of the boiler (insufficient exhaust removal = boiler reduces output), through frequent blocking by the safety thermostat, to serious safety risks (leakage of exhaust gases into the room, CO formation). Modern condensing boilers have several safety sensors that detect most problems before they become dangerous — but it is much more comfortable if the system is properly designed from the beginning. Proper dimensioning of the flue system should always be designed by a qualified installer or designer.

Conclusion: flue gas system as a project component, not an afterthought

Exhaust gas removal and air supply for a condensing boiler are not supplementary elements decided upon only during installation. They are technical components that must be planned in advance — even before selecting the boiler, because the available exhaust options determine which boiler and capacity you can afford. A good installer will conduct a site inspection before each job, assess the exhaust possibilities, and propose a specific solution including materials, lengths, and equivalent resistances. If your installer says "we'll somehow manage it" without an inspection, this is a warning sign.

If you are also interested in other aspects of selecting a condensing boiler — for example, the difference between wall-mounted and floor-standing boilers, or whether a boiler with a flow heater or with a storage tank is better — you will find these topics in other articles from our Knowledge Centre: Wall-mounted vs. floor-standing condensing boiler: which is more suitable and Condensing boiler with flow heater vs. with storage tank: the difference and selection. I also recommend reading Maintenance and servicing of a condensing boiler — because the flue gas system is also part of the regular annual inspection, during which the technician checks the tightness of connections, the condition of the condensate drain, and the pipe passability.

Do you have a question about this topic?

Are you unsure or dealing with a specific situation in your household? Write to us — we are happy to help.