Installing a Condensing Boiler – Procedure, Space Requirements and Flue Gas Discharge

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Installation of a condensing boiler – a comprehensive guide for plumbers and investors

A condensing boiler is not just another heating source that you hang on the wall and connect. It is a precise technical system whose performance, lifespan, and safety are directly dependent on the quality of the installation. From practice we know that most complaints and malfunctions do not arise from the boiler itself, but from incorrect placement, underestimated exhaust of flue gases, or a poorly dimensioned expansion tank. This article will take you through the entire installation process – from the first inspection of the space to the start-up and the first heating season.

If you are still considering which boiler to purchase, we recommend first reading the article How to choose a condensing boiler – selection criteria for a family house and an apartment or What power of a condensing boiler do I need – calculation according to area and insulation. Here we assume that you have already selected the boiler or are at least in the final decision-making stage and are dealing with a specific installation.

Who is allowed to install a condensing boiler?

This is the first and fundamental question. In Slovakia, the installation of a gas appliance (and a condensing boiler is in the vast majority of cases a gas appliance) may be carried out exclusively by a person with professional competence according to the decree of the Ministry of Labour, Social Affairs and Family SR No. 508/2009 Coll. in conjunction with technical standards STN EN. The plumber must have a valid authorization for the installation of gas appliances or heating systems.

The investor – i.e., the owner of the property – may and should be present during the installation, should understand what is being done, and should receive the documentation (pressure test protocol, inspection report, manufacturer's warranty card). Without these documents, the warranty conditions of most manufacturers are not valid, and insurance companies may not recognize the damage.

For condensing boilers using electrical energy (air-to-water heat pumps are sometimes classified under this category, but we are focusing on gas condensing boilers here), additional requirements apply to the electrical installation – most modern boilers operate on 230 V / 50 Hz, but require a separate circuit breaker loop.

Preliminary preparation – what needs to be resolved before the technician arrives

The most expensive work on the job is the one that has to be done twice. Therefore, preliminary preparation is key. From practice we know that unnecessary delays occur when the customer has not yet installed a gas connection of sufficient dimension, lacks an electrical socket near the boiler, or has not yet resolved the condensate drainage.

Checklist before installation

Gas connection and shut-off valve: The main gas shut-off valve must be within reach, and the pressure in the network must meet the boiler's requirements (typically 17–25 mbar for natural gas, 25–35 mbar for propane). This is verified by the gas company or an authorized technician.
Electrical connection: Minimum 230 V / 16 A, a separate circuit breaker loop with a grounding device. The socket should be within a maximum distance of 1.5 m from the boiler, ideally behind or to the side of the boiler.
Water supply and drainage: Cold water for filling the system and preparing TÚV (for combination boilers), sewer or condensate neutralizer within reach.
Space for the boiler: Dimensions according to the manufacturer's installation drawing – typically 400–600 mm width, 700–900 mm height, 250–400 mm depth. Plus at least 500–700 mm of free space in front of the boiler for service.
Flue gas pipe route: Preliminary planned route, where wall or ceiling penetrations will be. Without this, installation is not possible.
Project documentation: Project documentation is mandatory for family houses, and for replacing an old boiler with a new one in an apartment building it depends on the decision of the administrator and the building authority. More in the article Project documentation and supporting materials for the installation of a condensing boiler.

Requirements for the installation space – boiler room, bathroom, hallway, apartment

A condensing boiler is much more tolerant in terms of the installation space compared to older atmospheric boilers – precisely because it operates in a closed flue gas system (type C) and does not require air from the room for combustion. This fundamentally changes the requirements for room ventilation.

Boiler type B and type C – a fundamental difference

An older type B boiler (so-called vented) takes combustion air directly from the room where it is located. For such a boiler, strict requirements apply regarding room volume (minimum 8 m³, sometimes 15 m³ depending on the power), ventilation openings, and room accessibility. A condensing type C boiler (turbo, closed combustion chamber) takes air from outside via a double-pipe system or concentric collector and exhausts the flue gases through the same pipe outside. For such a boiler, the following applies:

There is no requirement for a minimum room volume due to combustion air.
The boiler can be located in an enclosed space (cabinet, wardrobe, technical room without a window).
The space must be dry and protected from frost (minimum temperature +5 °C).
Flammable substances, aerosols, and chlorine-based cleaning agents must not be stored there.

Chlorinated substances (chlorine-based products, solvents) near the boiler are very dangerous – even small amounts of volatile chlorides in the air can cause corrosion of the heat exchanger and flue gas tract, and the manufacturer's warranty will not be accepted.

Installation in the bathroom

Installation of the boiler in the bathroom is possible, but must comply with the standard STN 33 2000-7-701. The boiler must be located outside the protective zone 2 (minimum 60 cm from the edge of the bath or shower), and the electrical installation must be suitable for a humid environment. In practice, this solution is common in small apartments where no other space is available.

Installation in a living room

Technically possible for boiler type C, provided it is not excluded by local building regulations or the building manager's rules. For aesthetic reasons, the boiler is installed in a built-in cabinet with doors (with condensate drainage and access to flue gases). The cabinet must be made of non-combustible or difficult-to-ignite material.

Condensing boiler installation process – step by step

The description below follows the standard procedure for replacing an old boiler with a new condensing boiler in a single-family house. In a new installation, the steps are similar, with the addition of laying the piping phase.

Step 1: Removal of the old boiler and wall preparation

Before removing the old boiler, close the gas (main valve), drain the water from the boiler, and open all air vents on the system. Disconnect the electrical supply. Unfasten the water, gas, and flue connections. Check the wall – is it load-bearing? A boiler with water weighs 30–60 kg, so the wall must be masonry or concrete, not drywall without special anchoring.

If the old mounting bracket is in the correct position, you can leave it. Otherwise, install a new one according to the template provided with the boiler – manufacturers usually include a paper template with the exact location of the mounting points and penetrations.

Step 2: Hanging the boiler and rough connection

Hang the boiler on the bracket according to the installation instructions. Most manufacturers provide a system with a hanging rail (e.g., type TS or similar), where the boiler is simply clicked in and secured with a safety pin. Do not mix rails from different manufacturers – they are not interchangeable.

After hanging, check the horizontal level (spirit level) – the boiler must be perfectly vertical. Even a slight tilt can cause noise from the condensate trap or improper air venting.

Step 3: Connecting the heating system

The connection to the heating system is made via standard threaded connections G ¾" or G 1" (depending on the boiler's power). Always use new seals – old rubber seals are hardened after years of operation and may leak. Shutoff ball valves must be installed on the boiler connections to allow for disassembly without draining the entire system.

Important: when replacing a steel boiler with a condensing one, always check the condition of the existing piping. Old steel pipes may be full of deposits (magnetite, lime scale), which can fall off during new operation and clog the circulation pump or the heat exchanger of the condensing boiler. Before starting the new boiler, the system must be flushed and cleaned, ideally using chemical cleaning.

Step 4: Connecting the gas

The boiler's gas connection is standard G ½" (for lower power) or G ¾" (for power above 24 kW). The connection must be made using gas-resistant sealing compound or Teflon tape suitable for gaseous media. After connecting, a pressure tightness test is performed using soapy water or an electronic gas detector – visible bubbles = remounting is required.

Step 5: Connecting the condensate drain

This is an issue that did not exist with old boilers and is often underestimated by less experienced installers. A condensing boiler produces 1–3 liters of condensate per hour during operation (depending on power, return temperature, and combustion air humidity). Condensate has a pH of 3.5–4.5 – it is thus slightly acidic, but not dangerous.

In most towns with more than 5,000 inhabitants, condensate can be directly discharged into the sewer without neutralization – the amount of acid is so small that it dilutes before reaching the treatment plant. In more sensitive areas (sewage systems with biological treatment, water protection zones), a neutralizing insert (granulated lime stone) must be installed, which increases the pH to 6.5–7.5. Neutralizing inserts must be replaced according to the boiler's power, usually once a year to once every two years.

The condensate drain must have a trap to prevent gases from the sewer from entering the boiler. Many boilers have the trap built into the body – in this case, a direct connection to the drain is sufficient.

Step 6: Connecting the flue gas system

This is the technically most critical step. The flue gas system must be designed and installed precisely according to the manufacturer's standards and instructions. Incorrect flue gas removal is the cause of most serious incidents with gas appliances.

Step 7: Filling the system and air venting

After connecting all connections, fill the system with treated water (see requirements below). Open the filling valve and monitor the pressure on the manometer – the optimal pressure in a cold system is 1.2–1.8 bar, which will rise to 2.0–2.5 bar after heating. Vent all radiators and air valves starting from the highest point of the system downward.

Step 8: First start-up and adjustment

The first start-up should be carried out in the presence of an authorized manufacturer technician or service partner, if required by the warranty conditions. The technician will set the maximum power, modulation, temperatures, timer, and check the combustion correctness (in the case of boilers with adjustable air-fuel curve).

Flue gas removal – types, materials and dimensioning

Condensing boilers operate with much lower flue gas temperatures than old boilers (60–90 °C compared to 200–300 °C). This has two consequences: first, flue gases condense in the chimney, so the chimney must be waterproof and resistant to acids; second, the driving pressure (thermal draft) of the chimney is much lower, so boilers must have their own fan.

Types of flue gas systems for condensing boilers

Concentric pipe (60/100 or 80/125 mm): Air flows through the outer ring, flue gases exit through the inner pipe. A compact solution, ideal for a straight-through exhaust through the wall. Maximum length depends on boiler power and direction – typically 2–8 m equivalent length.
Twin-pipe system (2× ∅ 80 mm): Each pipe serves one function – air and flue gases are routed separately. Allows for much longer lengths (up to 20–30 m with elbows) and more flexible routing. Suitable for chimney installations.
Flexible insert into an existing chimney: A flexible stainless steel (AISI 316 L, smooth) or plastic (PP, for condensing boilers up to 120 °C) insert placed inside a masonry chimney. The masonry chimney must be insulated (polystyrene, mineral wool in the chimney shaft) due to condensation.
System chimney: A modular stainless steel or ceramic chimney system built from the outside or in a shaft – an ideal solution for new construction.

Dimensioning of flue gas piping

Each boiler manufacturer provides a table of maximum equivalent pipe lengths. Equivalent length combines the actual length of straight sections with the resistance of elbows and branches (e.g., 90° elbow = 1.0–2.5 m equivalent length). If the total exceeds the maximum value for a given diameter, the diameter must be increased or the route shortened.

Practical example: 24 kW boiler, concentric pipe 60/100, maximum equivalent length 5 m. Route: 1 m vertical, 90° elbow (= 1.5 m), 2 m horizontal, 45° elbow for the outlet (= 0.5 m) = total 5.0 m – exactly on the limit! Solution: either shorten the route or switch to 80/125 mm (maximum length usually doubles).

Slope of horizontal sections

Horizontal sections must have a minimum slope of 3° (5 cm per meter) away from the boiler. This slope ensures that condensate from the flue gases drains back to the boiler and does not leak onto the façade or into the wall. It is a small but critical detail – with an incorrect slope, condensate will start dripping from the outside at the outlet on the façade.

Flue gas outlet on the façade – placement

Placement of the outlet is governed by the standard STN EN 15287 and the manufacturer's instructions. Main rules:

Minimum 500 mm from windows, doors, and ventilation openings – to prevent flue gases from re-entering the building.
Minimum 200 mm above ground level (snow cover must not cover the outlet).
Minimum 300 mm from the corner of the building.
Must not point directly onto a neighbor's property or public sidewalks without a deflector.
In apartment buildings – the outlet must be high enough not to disturb residents on lower floors.

Water quality in the system – underestimated, but critical requirement

Condensing boilers have an aluminum or stainless steel heat exchanger with very thin walls and small cross-sections. Hard water (above 15 °dH) forms limescale, which thermally insulates the heat exchanger and causes overheating. Soft aggressive water (<1 °dH, pH below 6.5) corrodes metals. Manufacturers specify exact requirements for the quality of make-up water – typically pH 6.5–8.5, hardness 3–15 °dH, chloride content below 100 mg/l.

In areas with hard water (most of central and eastern Slovakia), it is recommended to:

Install a filter de-scaler at the system inlet (captures coarse impurities and magnetite).
Use a magnetic filter (captures metal particles from cast iron radiators and old piping).
Use a water softener or dosing additive (corrosion inhibitor + substance preventing scale buildup).

For a total system volume above 100 liters and water hardness above 10 °dH, chemical treatment is practically essential, otherwise you risk being charged for service under warranty with the justification "unsuitable water".

Expansion tank – dimensioning and inspection

Every closed heating system must have an expansion tank that absorbs the volume increase of water when heated. Condensing boilers have a built-in expansion tank, but its volume is only dimensioned for small systems (typically 8–12 liters for a 24 kW boiler). If the water volume in your system is larger (large family house, old cast iron system), you must connect an external expansion tank.

Approximate calculation: expansion tank volume = system volume × 0.04 × pressure correction factor. For a system with 200 liters of water and 2.5 bar pressure, it is roughly 200 × 0.04 × 1.5 = 12 liters – exactly at the limit of the built-in tank. Any larger system requires an external tank.

Before each heating season, check the nitrogen pressure in the expansion tank (must not drop below the set filling pressure of the system). This is a simple check using a standard car tire pressure gauge – more in the article Maintenance and service of a condensing boiler – what and how often to check.

Electrical system, control and thermostat

A modern condensing boiler is not a simple device – it has its own control module (processor), temperature sensors for output, return, and flue gases, an ionization flame sensor, modulating burner, electronically controlled circulation pump, and a communication interface for a thermostat or zone control.

The basis is a simple room thermostat (ON/OFF), which turns the boiler on and off based on room temperature. However, condensing boilers can fully utilize their potential only with a modulating equithermal regulator, which adjusts the output water temperature according to the outside temperature. Result? The boiler runs longer at lower power, condensation is more intense, and fuel savings can reach an additional 5–12 % compared to a simple thermostat.

Most manufacturers today also offer WiFi thermostats and boiler internet connectivity (OpenTherm, eBUS, proprietary protocols). This allows control via mobile devices, monitoring of consumption, and remote service access by the technician.

For zonal regulation (multiple temperature circuits – e.g. floor heating + radiators + DHW), a zonal controller with actuators is required, or alternatively a hydraulic distributor (distributor/collector). This sizing is part of the design documentation.

Typical installation errors from practice

Over the years of customer experience, we have seen a wide variety of installations. Here are the most common errors that keep recurring:

Incorrect flue slope: The horizontal section runs slightly upward from the boiler → condensate drips onto the facade or back into the fan.
Too long a flue path: The technician did not account for equivalent lengths, the boiler is burning in its own exhaust gases → blocking, shutdowns, error code.
Insufficient system flushing: Old magnetite from steel pipes clogs the filter and heat exchanger within 2 months.
Missing ball valve on connections: During service, the entire system must be drained instead of just closing the valve.
Undervalued expansion tank: The pressure relief valve discharges water after every heating cycle – a classic sign of undersizing.
Installation in a space with chlorides: Common in swimming pools, households where cleaning agents are stored – after a year the heat exchanger is corroded, the manufacturer does not accept warranty claims.
Missing magnetic filter: Without a filter, metal particles go directly into the circulation pump and heat exchanger.

Documentation after installation – what you must receive

The installation is not considered complete until the investor receives the following documentation:

Pressure test report of the heating system (hydrostatic test, minimum 1.5× operating pressure, minimum 6 bar).
Report of professional inspection and testing of the gas appliance signed by the inspection technician.
Manufacturer's warranty certificate filled out by the installer with the start date and stamp.
Boiler service book (many brands require it as a warranty condition).
Operating instructions in Slovak.
Training record of the operator – the investor must know how to operate the boiler, reset it, and where the main gas shut-off valve is located.

Most frequently asked questions (FAQ)

Do I need to change the chimney when replacing an old boiler with a condensing one?

In most cases, yes, at least partially. A masonry chimney without a liner is not suitable for a condensing boiler – condensate would moisten the masonry and freeze could crack it. You must insert either a stainless steel flexible liner (diameter corresponding to the flue gas sizing), or a plastic PP liner (only up to 120 °C, which a condensing boiler meets). If you install the boiler with an outlet directly through the wall to the outside, you don't need a chimney at all – a flue pipe through the building's exterior shell is sufficient.

Can I install a condensing boiler in an apartment without a chimney?

Yes, if it is possible to vent the exhaust gases through the exterior wall (outlet on the facade). This solution is common in panel apartments and is technically fully safe when the outlet is properly placed. In some apartment buildings, however, it may be regulated by the building management or the building administrator's decision – this must be resolved before installation.

What pressure should the water in the system have after filling?

In a cold system, typically 1.2–1.8 bar. After heating to operating temperature, the pressure increases by 0.3–0.8 bar (depends on the system volume and temperature). If the pressure after heating exceeds 2.5–3.0 bar or the pressure relief valve regularly discharges water, the problem is with the expansion tank (undersized or discharged). If the pressure drops on its own, there is a leak somewhere in the system.

Is an annual service inspection necessary, even if the boiler is working without problems?

Yes, for three reasons: 1) Most manufacturers require it as a warranty condition. 2) The law (for gas appliances) requires regular professional inspections. 3) From an economic perspective – a clogged heat exchanger or misadjusted combustion can increase gas consumption by up to 8–10 % annually. What and how to check during service can be found in the article Maintenance and service of a condensing boiler – what and how often to check.

Can I connect a condensing boiler to an existing steel pipe system?

Yes, but only after thorough flushing and chemical cleaning. Steel pipes produce magnetite (black sludge), which can damage the fine components of a condensing boiler. Before the first start-up, have the system flushed with a chemical cleaning agent (e.g. Sentinel X800 or Fernox F3), then rinsed with clean water, install a magnetic filter, and add a corrosion inhibitor (e.g. Sentinel X100). Without this procedure, your heat exchanger or circulation pump will be clogged in 6–12 months.

How much condensate does a boiler produce in a season?

It depends on the power, operating mode, and return temperature. As a rough estimate, count on 1–2 liters per hour of operation for a 24 kW boiler running at low output temperatures (under 55 °C). If the boiler runs for 1,500 hours in a season, that is 1,500–3,000 liters of condensate per year – therefore, the drainage must be properly designed and the neutralizing insert (if required) regularly replaced.

Conclusion – good installation pays off many times over

A condensing boiler that is properly installed achieves an efficiency of 105–109 % (related to the calorific value of the fuel) and in an average family home saves 20–35 % on heating costs annually compared to an old boiler. That is thousands of euros saved over the lifetime of the boiler at today's gas prices. On the other hand, an average installation with incorrect flue gases, hard water without treatment, and missing service can reduce the boiler's lifespan from the usual 15–20 years to 5–8 years, while the costs of warranty and post-warranty repairs will keep increasing.

If you are still in the selection stage, also read the article Condensing boilers – comparison of brands and models by efficiency and price or check out the section How to read the technical sheet and price list of a condensing boiler – what to watch out for, where we will show you which parameters are really important and which are just marketing figures. Project documentation and installation materials for specific models can be found directly in the category Guides, materials, price lists, where technical sheets, installation instructions, and other materials are collected directly from the manufacturers.

Do you have a question about this topic?

Still undecided or dealing with a specific situation in your household? Write to us – we are happy to help.