Horizontal vs. Vertical Flue Gas Discharge – What to Choose During a Renovation

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Horizontal vs. Vertical Flue Gas Discharge – Which to Choose During Renovation?

When you set out to renovate your heating system and replace an old boiler with a modern condensing one, sooner or later you'll run into a question that many homeowners underestimate: in which direction should the flue pipe go? A seemingly technical detail actually affects the overall installation design, the cost of the work, the accessibility of the boiler room, and, not least, whether the building authority will approve the installation without objections. Horizontal discharge through a wall and vertical discharge through the roof – each of these concepts has its strengths and its limitations, and in practice the decision can never be made across the board. This article will help you understand the physics of both solutions, walk you through typical scenarios from real-world installation practice, and show you what to really watch out for during renovation.

Why the Direction of Discharge Matters More Than You Think

A condensing boiler operates with a much lower flue gas temperature than a classic atmospheric boiler – typically 50 to 80 °C instead of 150–200 °C. This is why condensing boilers can use plastic flue pipes made of PP or stainless steel ones with a special surface treatment at all. However, low flue gas temperature has a downside: the natural draught, which worked reliably in old boilers, is weaker in condensing boilers and is not enough to overcome long runs or large height differences.

Modern condensing boilers are therefore equipped with fans that push or draw the flue gases through the entire flue pipe run. The boiler itself thus creates overpressure or underpressure in the system – and this is precisely what turns the choice of discharge direction into not just an architectural, but a purely technical issue. The maximum equivalent flue pipe length that the boiler's fan can handle is a specific parameter of every boiler, and horizontal sections, elbows, and branches count toward this length with a higher coefficient than a purely vertical run.

Basic Physics: Draught, Resistance, Condensate

Before we dive into practical scenarios, it's good to understand three physical phenomena that behave differently in each solution.

Aerodynamic Resistance of the Route

Every meter of pipe, every elbow, and every reducer adds aerodynamic resistance. A horizontal section has comparable resistance to a vertical section of the same length, but elbows change the direction of flow, and a 90° elbow usually corresponds to an equivalent length of 1 to 2 m of straight pipe (depending on diameter and manufacturer). With vertical routing, elbows can in most cases be avoided entirely or limited to a minimum at the boiler connection. Conversely, horizontal routing through a wall can be very short – ideally under 1 meter – in which case the aerodynamic load is minimal.

Natural Draught and Its Role in Condensing Boilers

For condensing boilers with their own fan, natural draught is not essential, but it can either help or hinder. A long vertical pipe of warm flue gases does create a certain chimney effect, but at the low temperatures of a condensing boiler, this effect is marginal. What's more important is that the boiler's fan has enough reserve capacity for the entire route. Every manufacturer states a maximum equivalent length – for example, for a coaxial 60/100 mm system it's around 4–8 m, for an 80/125 mm system 10–15 m, and for a twin-pipe PP 80 mm system 30–50 m depending on the boiler's output.

Condensate and Its Drainage

Condensate forms from the condensation of water vapor in the flue gases and is acidic (pH 3–5). With horizontal discharge, the pipe must slope slightly (at least 3%, recommended 5%) toward the boiler, where the condensate is drained through a siphon. If the flue pipe lacks the proper slope or is sloped away from the boiler instead, condensate accumulates in the pipe and can cause corrosion, noise from pooling condensate, or even blockage of the flue gas outlet. With vertical discharge, condensate naturally flows downward – back into the boiler – making drainage simpler.

Horizontal Discharge Through a Wall – When Is It the Right Choice

Horizontal discharge means that the flue gases leave the boiler through a horizontal (or slightly inclined) pipe and pass directly through the building's exterior wall. Outside, it terminates with a special wall sleeve or grille that prevents rain and rodents from entering, while still allowing free outflow of flue gases and air intake (for coaxial systems).

Ideal Conditions for Horizontal Discharge

Boiler room by an exterior wall – if the boiler is located directly next to an outer wall, the horizontal run can be shorter than 1 meter and aerodynamic resistance is minimal. This is the most common scenario in apartment core installations and basement boiler rooms of family houses.
No roof access possible – for example, in apartment buildings where the roof belongs to the homeowners' association and any intervention would require the consent of the whole association and a building permit.
Renovation without demolishing shafts – if there is no usable chimney shaft, or it is full of other ducting, horizontal discharge through a wall may be the only realistic option without major construction work.
Low-output boiler up to 24 kW – these boilers commonly manage with a coaxial 60/100 mm system, which has sufficient reserve for horizontal runs shorter than 5 m.

Limitations and Risks of Horizontal Discharge

Horizontal solutions come with several limitations that can't be ignored in practice:

The wall sleeve must be properly sealed and thermally insulated, otherwise there is a risk of condensation in the masonry and dampness on the facade.
The flue outlet must not be closer than 30–50 cm from windows, doors, and ventilation openings (exact distances according to EN 13384 and the boiler manufacturer's instructions).
In densely built-up areas, placement of the outlet may be problematic due to neighboring buildings or vegetation.
Longer horizontal runs with multiple elbows quickly use up the maximum equivalent flue pipe length.
The outer end of the flue pipe can create back-draught in strong wind – special anti-backdraught flaps or wind caps are recommended.

Vertical Discharge Through the Roof or an Existing Chimney – When Is It the Better Choice

Vertical flue pipe routing directs flue gases upward – either directly through the roof structure, or through an existing chimney shaft. In the case of renovation from an old boiler to a condensing one, using the existing chimney shaft is a very attractive solution: there's no need for a passage through the wall or facade, and if the shaft is sufficiently sized, it's enough to insert a new plastic or stainless-steel liner into it.

When Vertical Discharge Clearly Wins

Existing chimney with a suitable cross-section – a flexible PP or stainless-steel pipe is inserted into the shaft, the boiler is connected via an adapter, and the system is functional. Ceilings and facade remain untouched.
Boiler room farther from the exterior wall – if the boiler is located in the middle of the apartment or house layout, a horizontal run to the outer wall would be disproportionately long and subject to elbows. Vertical discharge through an existing chimney is a more elegant solution.
Higher-output boilers above 35 kW – at higher outputs, an 80/125 mm system or a twin-pipe 2×80 mm system is needed, where vertical routing is more advantageous in terms of the hydraulics of the whole run.
Cascade connection of multiple boilers – in a cascade of two or more condensing boilers, flue gases are usually routed into a common vertical manifold that exits above the roof.
Architectural or regulatory restrictions – in heritage protection zones or with special facade requirements, a horizontal wall sleeve may not be feasible.

Complications of Vertical Routing During Renovation

The vertical solution is not without its pitfalls. If you want to use an old masonry chimney, you must check several things:

Shaft cross-section – a chimney designed for an old boiler with a draught of 150–180 mm internal diameter may be oversized for a condensing boiler, where an 80 mm PP pipe is sufficient. An oversized cross-section is not automatically an advantage – it increases heat losses and can cause condensation in the shaft masonry.
Condition of the shaft – an old masonry chimney may be cracked, clogged with soot from the old boiler, or damp. Before lining, the shaft should be inspected with a camera.
Need for lining – most old shafts require a new pipe to be inserted (so-called lining), which brings additional costs. Flexible PP or stainless-steel pipes are usually used.
Condensate on long vertical runs – at a height of 6–10 m, several liters of condensate can flow down overnight. The siphon and drain must be properly sized.

Equivalent Flue Pipe Length – A Key Design Parameter

The concept of equivalent flue pipe length appears in every technical manual for a condensing boiler, but many installers fail to check it thoroughly enough during renovations. Equivalent length is the sum of the actual length of the run and the values assigned to individual fittings – elbows, reducers, T-pieces. Every boiler manufacturer sets a maximum permitted equivalent length for a given type of flue pipe.

Practical example: a Protherm Gepard 23 MKV boiler with a coaxial 60/100 mm system has a maximum equivalent length of 4 m. One 90° elbow counts as 1 m, a 45° elbow as 0.5 m. If you have a 2 m horizontal run + one 90° elbow at the boiler + one 90° elbow before the wall sleeve, the total equivalent length is 2 + 1 + 1 = 4 m – exactly at the limit. Adding another elbow or extending the run would require switching to a larger diameter or a different type of flue pipe.

For such cases, there is, for example, the Protherm adapter for 80/125 mm flue pipes A25KP, which allows a transition to a larger flue pipe diameter and thereby substantially increases the maximum permissible equivalent length of the run – typically to 8–14 m depending on the specific boiler. It is especially suitable when the route is not straight, or when the boiler room is located far from the exterior wall or chimney shaft.

Coaxial vs. Twin-Pipe System and Its Influence on Direction Choice

The type of flue pipe is closely related to the question of routing direction. A coaxial system (flue pipe inside an air jacket) is suitable mainly for shorter horizontal runs, where air and flue gases travel side by side through one compact pipe. A twin-pipe system (separate flue gas and air ducts) offers greater flexibility for long vertical runs, because air can be supplied locally (e.g., through a ventilation grille in the boiler room) and flue gases are routed through just one pipe over a long distance.

If you're dealing with a renovation and the boiler is equipped with an inlet for an 80 mm PP (twin-pipe) system, you gain access to significantly greater maximum lengths – sometimes up to 30–50 m equivalent length. This is a value that opens the door to practically any vertical run through an entire apartment building. You can learn more about the differences between these systems in the article Coaxial vs. twin-pipe flue gas discharge system – which is better right here in the Knowledge Center.

For a cascade connection of two boilers, the Protherm splitting element Ø 2 x 80 mm R2KP is suitable, allowing the flue gases of two boilers to be combined into one common vertical manifold. This solution is typical for apartment buildings or larger family houses with a cascade, where the flue gases of both boilers are routed upward through one chimney cross-section.

Typical Scenarios from Renovation Practice

Scenario 1: Apartment building, replacing a gas heater in the apartment core

In an apartment building on a housing estate, a gas instantaneous water heater in the bathroom core is being replaced with a compact condensing boiler up to 24 kW. The boiler stands next to the exterior wall. The height from floor to ceiling is 2.6 m, wall thickness 40 cm. The building's outer shell is insulated with an ETICS facade system 12 cm thick. Solution: horizontal coaxial discharge 60/100 mm, total run shorter than 1 m. The Protherm horizontal set 60/100 mm – 0.8 m, S1KP is used – a complete kit for horizontal discharge including a wall sleeve with all necessary accessories. This is the simplest and most cost-effective installation.

Scenario 2: Family house with a fireplace, boiler room in the middle of the layout

A family house has the boiler room in the middle of the ground floor. The straight-line distance to the exterior wall is 5 m, and the route would require three elbows and passage through two rooms. An existing masonry chimney from a decommissioned fireplace is free and runs through the central part of the house. Solution: vertical discharge through the chimney shaft using PP lining. The old chimney with a cross-section of approx. 200 × 200 mm is lined with a flexible PP pipe Ø 80 mm, and a separate air duct is brought into the boiler room. The transition from the boiler to the PP flue pipe is provided by the Protherm adapter for 60/100 mm flue pipes A1KP, in case the boiler has a coaxial outlet and the flue pipe is designed separately.

Scenario 3: New building, boiler room in a utility room by the garage wall

In a new building, the utility room is located next to the garage wall. A 35 kW boiler will run on natural gas. The garage wall borders a public space, so the horizontal sleeve poses no issues regarding neighbors. The straight-line run to the wall is 1.5 m. Solution: coaxial 80/125 mm system with horizontal discharge, total equivalent length around 3 m – well below the limit. Advantage: simple installation, no roof work.

Scenario 4: Apartment building, shared basement boiler room with a cascade of two boilers

A shared boiler room in the basement of an apartment building is being converted from one large gas boiler to a cascade of two condensing boilers of 50 kW each. The existing chimney passes through the basement ceiling and rises through the entire building. Solution: vertical discharge through the existing chimney, lining with stainless-steel pipe of 150 mm diameter, connecting both boilers via a splitting element. For correct hydraulic connection of the boilers, the Protherm connection manifold for copper pipes is used, ensuring correct hydraulic balancing of the water circulation in the cascade.

Materials and Standards – What a Flue Pipe Must Meet During Renovation

During renovations, it is required that every new flue pipe complies with the EN 1856-1 standard (metal flue pipes) or EN 14471 (plastic flue pipes). For condensing boilers, the key requirements are resistance to condensate (class W), pressure design (class P), and resistance to soot fire (class G or O). The STN EN 13384 standard defines the calculation procedure and dimensioning of the flue pipe.

Plastic PP flue pipes are permitted only for flue gas temperatures up to 120 °C (class T120), which is a safe range for condensing boilers with flue gas temperatures of 50–80 °C. Stainless-steel flue pipes are more resistant (class T200 or higher) and are used for longer runs, for renovations into old shafts where there is a risk of higher temperatures in the event of a boiler malfunction, or when a longer service life without material degradation is desired.

Proper sealing of joints is also important – every joint must be sealed with the manufacturer's original gasket, never with silicone or other unapproved materials. Joints must not be plastered over or embedded in masonry – every joint must remain accessible for inspection, or a permanent non-detachable connection approved by the manufacturer must be used. At least one inspection opening must be provided on the flue pipe for inspection and cleaning.

Most Common Mistakes When Choosing the Discharge Direction

Over years of practice and dozens of jobs, the same mistakes keep recurring, leading installers and customers into trouble:

Underestimating the equivalent length – the installer measures the physical pipe length but doesn't account for elbows. The boiler then fails to properly discharge flue gases, flue gas temperature rises, the fan motor runs at 100% and wears out prematurely.
Incorrect slope of horizontal routing – the pipe is horizontal instead of sloped toward the boiler. Condensate accumulates, penetrates into the combustion chamber, and damages the heat exchanger.
Unsuitable wall sleeve – an ordinary masonry sleeve without thermal insulation is used. The wall around the flue pipe becomes damp, freezes in winter, and the plaster cracks.
Flue pipe outlet facing an enclosed space – the customer inadvertently blocks the flue outlet, for example with overgrown shrubs or tarpaulins on scaffolding during facade renovation. The result is CO poisoning.
Ignoring distance requirements from openings – the flue outlet is too close to a kitchen window. The customer complains about the smell of flue gases indoors.
Lining an unsuitable shaft – a chimney from an old solid-fuel boiler was designed for natural draught and has a larger cross-section. After lining with a small PP pipe, a large air gap remains in the shaft, through which cold air enters in winter and condenses on the outer surface of the PP liner.

Mistakes and their diagnosis are covered in detail in the article Common Faults and Flue Gas Leaks in Condensing Boiler Flue Pipes in this Knowledge Center.

Administrative and Safety Aspects of Renovation

Renovating a flue pipe is not only a technical matter – it also has a legal dimension. Under Slovak building law, replacing a boiler and flue pipe is considered a change to the building, which may require notification to the building authority or a building permit, depending on the nature of the change and local regulations. In apartment buildings, any intervention in common areas (including chimneys) requires the consent of the homeowners' association.

After installation is completed, the new flue pipe must be inspected by an authorized chimney sweep, who issues an inspection report. Without this report, the installation is considered invalid, and the insurance company may refuse to pay out in the event of a claim. The inspection report is also a prerequisite for the boiler to be commissioned by the gas utility company.

Comparison Table: Horizontal vs. Vertical Discharge

Criterion	Horizontal Discharge	Vertical Discharge
Suitability for boiler near the wall	✔ Ideal	○ Less practical
Use of existing chimney	✗ Not usable	✔ Ideal
Construction complexity	Low (1 opening)	Medium–high
Max. equivalent length (60/100)	4–8 m	4–8 m (same)
Condensate drainage	Requires slope	Gravity flow ↓
Effect of wind on the outlet	Higher (headwind)	Lower (above roof)
Requirement for chimney sweep/inspection	Yes	Yes
Suitability for boiler cascade	Limited	✔ Significantly better
Installation cost (relative)	Lower	Higher (lining)

How to Choose the Right Solution – Step-by-Step Decision Process

If you're facing a renovation and can't decide, I recommend the following procedure, which in practice reduces the risk of a faulty design:

1. Determine the boiler's position relative to the exterior walls. If the boiler is within 2 m of the outer wall and the route doesn't require more than two elbows, horizontal discharge is the first choice.

2. Check the availability of an existing chimney shaft. If a free, functional shaft exists, vertical discharge with lining is an elegant solution without interfering with the facade.

3. Calculate the equivalent length of the proposed route for both variants and compare it with the limit in the boiler manual. Leave a reserve of at least 15–20% below the maximum.

4. Check the boiler's output range. For outputs above 35 kW, an 80/125 mm system or a twin-pipe PP system will likely be required, which may change considerations about the routing direction.

5. Consult a chimney sweep company about the condition of the existing chimney (if lining is planned) and a designer or the building authority about the need for notification or a permit.

6. Only then select specific components. Flue gas discharge systems from boiler manufacturers are mostly compatible only with original or certified adapters. More on compatibility in the article Flue Pipe Compatibility with the Boiler: Protherm, Vaillant and Other Brands.

Frequently Asked Questions (FAQ)

Can I use an old brick chimney for a condensing boiler without lining?

No. An old masonry chimney is neither waterproof nor airtight, condensate would penetrate the masonry, and the chimney cross-section is not properly sized for a condensing boiler. A new pipe – a PP or stainless-steel liner of appropriate diameter – must always be inserted into the existing shaft. Besides technical reasons, this is also required by the standard, and a chimney sweep will only issue an inspection report for a system with a proper liner.

What if my horizontal flue outlet faces a courtyard where people move around?

The flue outlet must be positioned so that flue gases cannot reach areas where people move, open windows, or entrance doors. Minimum distances are defined by the boiler manufacturer (usually 30–50 cm from windows and doors, 50–100 cm from building corners) and also by the EN 15450 standard. If the distances are insufficient, the outlet position must be changed or a switch to vertical discharge is needed.

How many liters of condensate are produced daily by a condensing boiler?

At full boiler output, approximately 1–3 liters of condensate are produced per hour of operation (depending on output and return water temperature). Over a heating season, this amounts to hundreds of liters. Condensate has an acidic pH (3–5) and must not be discharged directly into the sewer without neutralization in some municipalities – check local requirements. With vertical discharge, this condensate flows into the boiler's collection tray; with horizontal discharge, the pipe must have the correct slope so that condensate drains away and doesn't accumulate.

Is horizontal discharge safer in terms of CO leakage than vertical discharge?

Both systems are equally safe when properly installed. The risk of CO leakage is always associated with leaking joints or damaged piping, not with the routing direction. Horizontal routing is shorter and easier to inspect, but the outlet is on the facade, where it is more easily physically damaged. Vertical routing is longer, but most of the run is inside a protected chimney. The key is that all joints are properly sealed and regularly inspected. A separate article, Cleaning and Maintenance of a Condensing Boiler Flue Pipe, is dedicated to inspection and cleaning in this Knowledge Center.

Can I later easily extend or convert a horizontal discharge to a vertical one?

It depends on the specific installation. If the boiler was installed with a coaxial 60/100 mm system and you want to extend the run, you must check whether the new equivalent length exceeds the limit. If it does, it is possible to switch to a larger diameter using an adapter – for example, the Protherm adapter for 60/100 mm flue pipes A1KP. Converting from horizontal to vertical discharge in most cases requires a new opening through the ceiling or wall, which is a construction intervention – but it is technically feasible and, where a shaft is available, also affordable.

What is the difference between a 60/100 mm and an 80/125 mm diameter for horizontal discharge?

The 80/125 mm system has a larger cross-section for both the flue gas channel and the air gap, allowing for longer runs and higher boiler output. For a typical residential condensing boiler up to 24–28 kW, 60/100 mm is sufficient; for boilers of 30–45 kW, the 80/125 mm system is recommended. An adapter for changing the diameter, such as the Protherm adapter for 80/125 mm flue pipes A25KP, allows connecting a boiler with a 60/100 mm outlet to an 80/125 mm flue pipe – this is done when the route is longer or has more elbows and the capacity of the smaller diameter is insufficient. More on sizing can be found in the article What Flue Pipe Diameter Do I Need: 60/100 or 80/125 mm.

Conclusion: There Is No Universal Answer, But There Is a Correct Procedure

The question "horizontal or vertical discharge" doesn't have a single correct answer valid for all situations. In practice, the decision is determined by a combination of the boiler's position in the building, the availability of existing shafts, the boiler's output, architectural constraints, and of course the budget. Horizontal discharge through a wall is fast, cheap, and extremely effective for short runs – ideal for apartments and smaller family houses with the boiler near the exterior wall. Vertical discharge through a chimney shaft is more advantageous for boilers located in the middle of the layout, for higher outputs, for cascades, and when trying to avoid interfering with the facade.

More important than the choice of direction itself is thoroughness in calculating the equivalent length, selecting the right components compatible with the boiler, maintaining the slope for horizontal routing, and ensuring inspection by a chimney sweep after installation is complete. Each of these steps, if neglected, will sooner or later show up in the performance, reliability, or safety of the entire system. In the entire flue gas discharge and flue pipes category, you will find the components needed for both types of installations – from complete sets for horizontal discharge to adapters, splitters, and lining pipes for vertical solutions.

Do You Have a Question About This Topic?

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