What chimney diameter do I need: 60/100 or 80/125 mm

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What flue diameter do I need: 60/100 or 80/125 mm?

This is one of the most common questions we deal with when choosing accessories for condensing boilers. At first glance it seems simple – it's just a difference of a few centimetres. In practice, however, choosing the wrong flue diameter can cause insufficient flue gas draught, boiler malfunctions, increased fuel consumption, or even dangerous situations related to flue gas leaking into living spaces. That's why this topic deserves serious attention.

In this article, we'll explain what the individual dimensions mean, how coaxial flues work (where these numbers come from), what factors determine the choice of size, and we'll go through specific real-world scenarios – from a small apartment boiler to a more powerful boiler in a family house or a cascade system.

What do the numbers 60/100 and 80/125 mm mean?

Both of these designations describe a coaxial (double-walled) flue. A coaxial system consists of two concentric pipes – an inner and an outer one – each of which performs a different function. This arrangement is typical for condensing boilers with a closed combustion chamber (type C), where the boiler does not draw air from the room but from outside through the gap between the pipes.

Inner pipe – carries flue gases out (from the boiler outward)
Outer pipe (intermediate space) – brings fresh air from outside into the boiler

So the numbers mean:

60/100 mm – the inner pipe (flue gas) has a diameter of 60 mm, the outer pipe (air) has a diameter of 100 mm
80/125 mm – the inner pipe (flue gas) has a diameter of 80 mm, the outer pipe (air) has a diameter of 125 mm

This refers to the diameter of the entire coaxial pipe as a system. The larger number always denotes the total outer diameter, the smaller number the inner flue pipe. This dimension directly determines the capacity of the whole system – how much flue gas it can remove and how much air it can supply per unit of time.

The main factors that determine the choice of dimension

Choosing between 60/100 and 80/125 mm is not a matter of taste or chance. It depends on several specific technical parameters that need to be assessed comprehensively:

1. Boiler output (kW)

This is the most important factor. Higher output means more gas burned per hour, i.e. more flue gas and more combustion air needed. The basic rule of thumb is as follows:

60/100 mm – suitable for condensing boilers with an output up to approx. 24–28 kW (some manufacturers state up to 35 kW for short runs)
80/125 mm – suitable for boilers with an output above 24–28 kW or for any boiler with a longer flue run

However, it is important to know that these limits are not absolute – boiler manufacturers specify them in the technical documentation for each model individually, and they may also vary depending on the length of the run. Always check the specific data in the boiler's manual.

2. Total equivalent flue length

Every metre of pipe and every elbow increases the hydraulic resistance of the system. Manufacturers define a maximum equivalent length (MEL) – the sum of straight section lengths and elbow equivalents. A 90° elbow is typically counted as 1.0–1.5 m of straight pipe, a 45° elbow as 0.5–1.0 m.

60/100 mm – maximum equivalent length typically 4–8 m depending on the manufacturer and boiler output
80/125 mm – maximum equivalent length typically 8–20 m or more

In practice, this means: if you have a boiler by an outer wall with a 60–80 cm passage through the wall, 60/100 mm will be perfectly sufficient. But if you need to route the flue across the entire layout of the flat or house – for example 3 metres horizontally, with 2 elbows and a wall sleeve – 60/100 mm starts to become tight, and 80/125 mm is clearly the right choice.

3. Slope of the horizontal section

Condensing boilers produce condensate – water that condenses out of the flue gas. This water must drain back into the boiler (or into the sewer system via a neutraliser). The horizontal section of the flue is therefore installed with a slope of 3–5° towards the pipe. The longer the horizontal section, the greater the hydraulic resistance and the more condensate needs to be accounted for. A larger diameter mitigates these problems.

4. Cascade connection of multiple boilers

For a cascade of two or more boilers, a special solution is used – either each boiler has its own flue, or the flue gases are combined into a common collecting pipe. Larger dimensions and special fittings are essential for cascade connections. For example, the Protherm Splitting Element Ø 2 x 80 mm R2KP is used precisely to route the common flue gas discharge to two boilers with 80 mm outlets – in this application, the 80 mm size and the corresponding 80/125 mm coaxial pipe is a standard, not an exception.

5. Boiler manufacturer requirements

Every manufacturer has a certified flue system and specific accessories for its boilers. Using an incompatible or incorrectly sized flue can result in the warranty being voided, and in the event of a fault, insurance claims may also be rejected. Protherm, Vaillant, Baxi, Buderus, Bosch – all of them have their own ranges of adapters and sets. For the Protherm brand, the following are directly available:

Protherm Adapter for 60/100 mm Flues A1KP – a basic adapter for the 60/100 standard, suitable for Protherm boilers with this outlet size
Protherm Adapter for 80/125 mm Flues A25KP – the equivalent for more powerful boilers or longer runs in the 80/125 dimension

Practical scenarios from everyday installation experience

Theory is one thing, practice another. Let's look at specific situations we encounter most often:

Scenario A: New apartment condensing boiler in a panel building, output 18–24 kW

This is probably the most common situation in Slovakia. The boiler hangs on the kitchen or bathroom wall, and the outer wall is 30–80 cm away. The flue runs horizontally outside, and the total equivalent length including elbows does not exceed 2–4 m. The boiler output is typically 18 kW, maximum 24 kW.

Recommendation: 60/100 mm. This is the standard solution for this type of application. For example, the Protherm Horizontal Set 60/100 mm – 0.8 m S1KP is designed specifically for this typical installation – it contains everything needed for a short horizontal discharge through the wall in one package.

Scenario B: Family house, boiler 28–35 kW, flue routed through a hallway with 2 elbows

The boiler is located in a utility room in the middle of the layout. The horizontal section of the flue measures 4 metres, plus 2 90° elbows (each = 1.5 m equivalent). Total equivalent length: 4 + 3 = 7 m. Boiler output: 30 kW.

Recommendation: 80/125 mm. With 60/100 mm, you would reach or exceed the maximum equivalent length limit, which would cause draught problems. 80/125 mm provides sufficient margin. You would use the Protherm Adapter for 80/125 mm Flues A25KP for the transition from the boiler to the pipe.

Scenario C: Renovation – existing chimney, existing shaft

During renovations, we often necessarily have to adapt to existing shafts or chimney dimensions. If you have a 130 × 130 mm shaft, 80/125 mm will still fit (outer diameter 125 mm plus a small clearance). 60/100 mm (outer diameter 100 mm) will of course fit as well. If the shaft is even tighter – for example 110 × 110 mm – you will be limited to 60/100 mm. This is a case where the shaft dimension may dictate the decision regardless of boiler output.

Scenario D: Cascade of two boilers in an apartment building boiler room

Two Protherm 24 kW boilers in a cascade connection, with a common flue running vertically to the roof, height 6 metres. Here the solution is clear: each boiler has an 80 mm flue gas outlet, which are joined via a splitter and lead into a vertical pipe of 130 mm or larger. A splitting fitting such as the Protherm Splitting Element Ø 2 x 80 mm R2KP is a key fitting for such a connection.

Scenario E: Replacing an old boiler while keeping the existing 60/100 mm flue

A customer has an existing 60/100 mm flue and wants to replace an old 18 kW boiler with a new 28 kW one. Question: can the old flue be kept? The answer depends on the length of the run and the technical parameters of the new boiler. If the run is short (up to 3 m equivalent length) and the new boiler's manufacturer still accepts 60/100 mm at this output, it is potentially possible. If not – the size needs to be upgraded to 80/125 mm. In that case there is no way around it, and investing in a new flue is unavoidable.

Hydraulic resistance: why the dimension matters more than you think

A condensing boiler has a built-in fan that actively drives flue gases through the flue. However, this fan has a limited pressure capacity – it is not infinitely powerful. When the hydraulic resistance of the flue exceeds the fan's capability, the boiler either triggers a fault (typical error code "insufficient flue gas draught"), or – worse – starts operating under conditions where flue gases are not being sufficiently removed, which can lead to their back-diffusion.

Hydraulic resistance depends on:

Pipe length – a linear relationship, each metre adds resistance
Number and type of elbows – elbows correspond to equivalent lengths
Pipe diameter – resistance increases with the fourth power of the decreasing radius (Poiseuille's law), meaning a small difference in diameter has a large impact
Flow velocity – higher output = higher flow rate = higher resistance at the same diameter

From a physical standpoint: moving from a 60 mm inner pipe to an 80 mm inner pipe increases the cross-section by 78% (area of a 60 mm circle = 2827 mm², area of an 80 mm circle = 5027 mm²). This means that with the same amount of flue gas, the flow velocity in the 80 mm pipe is almost half as low, and the hydraulic resistance is several times lower. This is the technical basis for why 80/125 mm "draws" much more easily on long runs.

Coaxial vs. twin-pipe system: how does it relate to diameter selection?

So far we have talked about the coaxial system (flue gas + air in one coaxial pipe). There is also an alternative – the twin-pipe system (LAS/PP), where flue gas and air are routed through separate pipes. In this case, a different diameter designation is used – typically 80 mm for flue gas and 80 mm for air, each separately.

A twin-pipe system has lower hydraulic resistance at the same nominal diameter, because each pipe serves only one medium. It is suitable for longer runs and higher outputs. If you're interested in this topic in more depth, see the article Coaxial vs. twin-pipe flue gas discharge system – which is better in our Knowledge Centre.

Materials and compatibility: not all pipes are the same

Flues for condensing boilers are made from various materials, and not all of them are compatible with each other:

PP (polypropylene) – the most common material for condensing boilers, resistant to acids (condensate is acidic), lightweight, easy to install. Tolerates temperatures up to approx. 80–90 °C.
Stainless steel – suitable for boilers with higher flue gas temperatures or for runs in chimney shafts, where it is combined with thermal insulation.
Aluminium / aluminium coaxial – used by some manufacturers for outdoor runs or special applications.

Fundamental rule: coaxial flues from different manufacturers are NOT compatible with each other, even if they have the same diameter. Seals, couplings and the geometry of threaded joints are designed by the manufacturer specifically for its system. Mixing brands is, in practice, a source of flue gas leaks and problems. The topic of compatibility is covered in more detail in the article Flue Compatibility with Boilers: Protherm, Vaillant and Other Brands in our Knowledge Centre.

Installation and common mistakes when choosing the dimension

From an installation practice point of view, we see these recurring mistakes:

Mistake #1: Underestimating the length of the run

The customer measures the wall and says "it's only 80 cm". However, they forget about the elbow at the boiler outlet, the elbow at the wall passage, and the flange adapter. The actual equivalent length is 2–3 metres. This is usually still fine with 60/100 mm, but if there is one extra elbow on top of that, problems can arise.

Mistake #2: Using 60/100 mm for a more powerful boiler "because it's cheaper"

This is a classic saving that isn't actually a saving. The boiler runs with insufficient draught, the fan operates at maximum, the boiler faults, a service technician arrives, finds the wrong flue size, and the warranty is voided. The cost of replacing the flue and the service call far exceeds the original saving.

Mistake #3: Incorrect slope of the horizontal section

The horizontal section must have a slope of at least 3° towards the boiler so that condensate flows back into the boiler and from there into the sewer system. If the slope is reversed or zero, condensate accumulates in the flue, causing corrosion and blocking the draught. This is not directly a matter of dimension, but combined with too small a diameter, the problems are multiplied.

Mistake #4: Missing seals or improperly screwed joints

Flue joints must be airtight. Any leaking joint means flue gas escaping into the interior, which is a safety hazard. With the correct dimension, the pressure in the pipe is lower, which slightly reduces the risk of leakage, but this is not a substitute for proper installation. More on this topic can be found in the article Common Faults and Flue Gas Leaks in Condensing Boiler Flues.

Sizing step by step: a practical procedure

If you want to correctly choose the flue diameter for a specific situation, we recommend the following procedure:

Determine the boiler output – from the technical data sheet or the boiler's rating plate (nominal thermal output in kW)
Plan the flue route – sketch it out and measure all straight sections in metres
Count the elbows – each 90° elbow = approx. 1.0–1.5 m equivalent length, a 45° elbow = 0.5–0.8 m (exact values are in the flue manufacturer's documentation)
Calculate the total equivalent length – sum of straight sections + sum of elbow equivalents
Compare with the boiler manufacturer's table – for the given output and diameter, check whether you are below the maximum permitted equivalent length
If you are not below the limit for 60/100 mm, choose 80/125 mm
Check compatibility – make sure all fittings and adapters are from the same manufacturer or are explicitly certified as compatible

This procedure will save you time, money, and complications during final inspection and warranty repairs. If you are unsure, don't hesitate to consult an expert – a mistake in flue sizing is much more expensive than a consultation before installation. A detailed guide to the entire installation can be found in the article How to Install a Condensing Boiler Flue Step by Step.

Special situations and exceptions

Reducer and diameter reduction at the boiler outlet

Some boilers have a flue gas outlet with a diameter that is neither exactly 60 mm nor 80 mm – for example 55 mm or 75 mm. In such cases, reducing adapters are used. It is very important that the adapter is certified by the boiler manufacturer, because the seal at the transition point is critical. As an example: for Protherm boilers, the Protherm Adapter for 60/100 mm Flues A1KP serves as a certified transition element between the boiler outlet and the flue system – it is not a generic reducer, but part of a certified system.

External routing along the façade

If the flue runs along the outer façade of the building, it must be thermally insulated to prevent excessive condensation or freezing of the condensate in winter. A larger diameter (80/125 mm) is more resilient in this respect, because the flow velocity is lower and the flue gases retain their temperature longer. For outdoor runs, this is another argument in favour of a larger diameter.

Vertical discharge through the entire height of a building

In vertical discharge across multiple floors, chimney draught also plays a role – hotter flue gases have lower density and naturally rise. At a height difference of 5 m or more, this effect becomes helpful (though not sufficient on its own without a fan). On the other hand, excessive chimney draught in low-temperature boilers (which, however, mostly do not use a coaxial system) can cause the flue gases to cool and lead to excessive condensation. For condensing boilers with a closed chamber, the active fan is always decisive, not passive draught. More on this topic can be found in the article Horizontal vs. Vertical Flue Gas Discharge – What to Choose During Renovation.

Combination with a wall/roof sleeve

Every passage through a wall or roof requires a special sleeve – a shaped element that ensures the seal of the passage and also serves as a structural element (fire safety, waterproofing). Sleeves are dimensionally tied to the flue diameter, so when changing the dimension, the sleeve must also be changed. More on this topic can be found in the article Roof or Wall Sleeve: Differences, Types and Installation.

Comparison table: 60/100 vs. 80/125 mm

Parameter	60/100 mm	80/125 mm
Outer diameter	100 mm	125 mm
Flue gas diameter (inner pipe)	60 mm	80 mm
Typical boiler output	up to 24–28 kW	24–50+ kW
Max. equivalent length (approx.)	4–8 m	8–20+ m
Typical use	Apartment, shorter run	House, long run, cascade
Cost of accessories	Lower	Higher
Space requirements	Smaller	Larger
Cross-sectional flue capacity (ratio)	100% (base)	~178%
Suitability for cascade	No	Yes

Frequently Asked Questions (FAQ)

Can I use an 80/125 mm flue for a weaker 18 kW boiler?

Yes, from a safety and functional point of view this is not a problem – the system will work well, and the hydraulic resistance will be even lower. The only downside is the unnecessarily higher cost of accessories and the larger diameter, which may complicate the wall sleeve or the placement in a shaft. For a typical apartment boiler of 18 kW with a short run, 60/100 mm is a fully sufficient and more practical solution.

What happens if I use too small a flue diameter?

The most common consequence is that the boiler repeatedly faults with an insufficient flue gas draught error code (typically E11, F11, or a similar code depending on the manufacturer). The fan has to run at maximum output, which shortens its lifespan. In extreme cases – if the boiler is older and the fan weakens – flue gases may leak back into the room. Therefore, oversizing is less risky than undersizing.

I have 60/100 mm and want to extend the run. Can I extend it as long as the diameter is sufficient?

Yes, but always with a calculation of the equivalent length. Every boiler manufacturer specifies a maximum equivalent length for a given model and diameter. You will find it in the boiler's technical documentation. If extending the run exceeds this limit – for example from 3 m to 6 m for a 24 kW boiler – you must switch the entire run, including the adapter on the boiler, to 80/125 mm. It's not enough to just change one section.

Are 60/100 mm fittings from different manufacturers interchangeable?

No, and this is one of the most important practical points. Even though the diameters are the same, the couplings, seals, and socket geometry differ from manufacturer to manufacturer. For example, Protherm and Vaillant fittings have the same nominal size of 60/100, but they are not directly compatible without a certified transition adapter. Always use accessories from your boiler's manufacturer, or adapters explicitly approved by the boiler manufacturer.

Why is the flue on a condensing boiler always double-walled (coaxial) and not a single pipe?

Condensing boilers with a closed combustion chamber (type C) must draw air from outside the building – they must not draw it from the living space. The coaxial system elegantly solves both functions in one pipe: flue gases exit through the inner pipe, and fresh air enters through the outer space. The air is preheated by the heat of the flue gases, which increases overall efficiency. A single-pipe discharge would only be possible with boilers with an open combustion chamber (type B), which are practically not permitted in modern flats and houses.

How do I find out what flue diameter my specific boiler requires?

The most reliable way is the boiler's technical documentation – the installation manual, where the manufacturer states the permitted diameters and maximum equivalent lengths for each output in a table. Another option is to contact the manufacturer's authorised service centre or distributor. Never rely solely on the diameter of the outlet socket on the boiler – this tells you what adapter you need, but does not always clearly determine the size of the whole system over a longer run.

Conclusion: clear criteria for the right decision

Choosing between the 60/100 mm and 80/125 mm flue systems is not a matter of chance or personal preference – it is a technical decision with a direct impact on the safety, efficiency, and lifespan of the entire heating system. Summarised in a few points:

For boilers up to 24 kW with a short run of up to 4–5 m, 60/100 mm is both the standard and the optimum
For boilers over 24 kW, long runs, or cascades, 80/125 mm is a necessity
Always check the specific limit in your boiler manufacturer's documentation
Never mix fittings from different manufacturers without a certified adapter
When in doubt, choose the larger diameter – oversizing does no harm, undersizing does

If you're planning the entire installation, also check out the articles How to Choose Flue Gas Discharge for a Condensing Boiler and How to Install a Condensing Boiler Flue Step by Step in our Knowledge Centre – you'll find a complete guide there, from selection to commissioning. You can find all the necessary components, including adapters, sets, and fittings, directly in the flue gas discharge and flues category at atria.sk.

Do you have a question about this topic?

Can't decide, or are you dealing with a specific situation in your home? Write to us – we'll be happy to help.