How condensing boiler installation works and what to prepare

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Installing a Condensing Boiler Step by Step: Everything You Need to Know and Prepare

You've decided on a condensing boiler – a great choice. But real energy savings and trouble-free operation don't depend on the boiler alone. They depend on how it's installed, what's connected to it, the condition of its surroundings, and how the whole system is configured. From experience, we know that installation is where mistakes are most often made and later become hard to fix – wrong position of the flue outlet, an undersized expansion vessel, neglected condensate drainage, or simply poorly set weather-compensated control. This article gives you a complete overview: what needs to be done before installation, how the installation itself proceeds, what to check during and after it, and what to watch out for at every stage.

Why installing a condensing boiler is different from a conventional boiler

A condensing boiler isn't just "a more powerful conventional boiler." It works on a different principle – it uses the latent heat of the flue gases by cooling them below the dew point and extracting heat from the condensing water vapor. This brings several specific technical consequences that directly affect the installation:

Condensate formation – the boiler produces 1 to 3 liters of condensate per hour of operation (depending on output and operating point). This condensate must be drained into the sewer system. You therefore need a functional drain near the boiler.
Different flue gas discharge – condensing boilers have a much lower flue gas temperature (55–80 °C instead of 120–200 °C for a conventional boiler), which allows the use of plastic or stainless-steel coaxial flue systems, but at the same time requires that the flue gases do not condense inside the left-over part of an old chimney.
Low return water temperatures – condensation only occurs efficiently when the return temperature drops below 55–57 °C. The system therefore needs to be designed so that this value is actually achievable during real operation.
Greater dependence on controls – weather-compensated control, or a smart thermostat, significantly improves efficiency. Their correct connection is part of the installation.

If you'd like to learn more about flue gas discharge and air supply, see the separate article Flue Gas Discharge and Air Supply for Condensing Boilers: What You Need to Know in our Knowledge Center.

Phase 1: Preparatory work before the actual installation

Before the installers arrive and start drilling and welding, several things need to be resolved. This is an area where you, as the property owner, can significantly help and save both time and money.

Choosing the installation location

A condensing boiler can be placed in a bathroom, hallway, kitchen, pantry, boiler room, or garage – it depends on the type of boiler and the conditions. For wall-mounted boilers, several basic requirements apply:

A load-bearing wall capable of supporting the boiler (15–35 kg depending on output and model)
Proximity to the gas connection (ideally within 1–2 m)
Access to the sewer system for condensate drainage (without unnecessarily long piping)
The option to route the coaxial pipe (flue gases/air) through the wall or into a chimney
A 230 V electrical outlet nearby (the boiler needs power)
Access for servicing – at least 60 cm in front of the boiler, ideally 70–80 cm

In practice, we encounter situations where the customer buys a boiler, but the wall where they want to mount it is drywall on a steel frame without a stud in the right place, or the sewer connection is a meter away and the floor needs to be broken up. All of this needs to be resolved before installation, not during it.

Checking the condition of the existing central heating system

If you're replacing an old boiler with a new condensing one, it's very important to assess the condition of the entire system. The old system may contain:

Rusty pipes with deposits that could clog the new boiler's heat exchanger
Radiators sized for high temperatures (75/65 °C), which the condensing boiler won't be able to supply adequately at low temperatures
An outdated expansion vessel (sealed tank) with insufficient volume or a damaged membrane
Poor-quality or cracked radiator valves
A poorly bled system with air pockets in the radiators

Ideally, before replacing the boiler, the system should be chemically flushed (de-scaling and cleaning), and after installing the new boiler, a corrosion inhibitor should be added to the circuit. This will extend the lifespan of the boiler and its components by years. From our experience: a boiler installed into a clogged system without flushing had a damaged plate heat exchanger after just two years, despite being a quality boiler from a reputable manufacturer.

Gas connection inspection

The gas connection must be in good condition and sized for the output of the new boiler. When replacing an old boiler with a condensing one of higher output, or if the connection has been inactive for a long time, an inspection by the gas utility is required. The gas pressure in front of the boiler (mbar) and the capacity of the connection (m³/h) must match the manufacturer's requirements. This is verified by an inspection technician.

Preparing the condensate drain

Condensate from a condensing boiler has an acidic pH – typically ranging from 3.5 to 5.5 (depending on the CO₂ content in the flue gases). Most European standards (including STN) and water utility regulations allow condensate with a pH above 6.5 to be drained directly into the sewer system. For larger boilers (over 200 kW), a neutralization tank is required. For standard residential and family-home boilers up to 50 kW, a direct connection to the sewer is sufficient – but a trap is needed to prevent odors from escaping.

Phase 2: The actual boiler installation

Installation of a condensing boiler may only be performed by a person with the appropriate qualification – a gas fitter with a valid license. For natural gas, this is a license under Decree No. 508/2009 Coll. (STN EN 1775 technician, or gas installation contractor). "Unofficial" installation not only violates the law, but also voids the manufacturer's warranty and can be dangerous.

Mounting and mechanical installation of the boiler

Wall-mounted condensing boilers are installed on a special bracket (mounting rail) that is included in the package. The bracket is anchored to a load-bearing wall – ideally with two anchors into concrete or brick, or into steel studs in drywall. The installation height of the boiler depends on the manufacturer and model, but the usual recommendation is 1.4–1.6 m from the floor to the bottom of the boiler, allowing comfortable access to service components.

Some boilers, such as the BOSCH Condens GC2300iW 24 P Gas Boiler, have compact dimensions (typically around 400 × 700 × 300 mm), which greatly facilitates placement even in a smaller room. For larger homes, the BOSCH Condens GC8700iW 30 P Gas Boiler is available, offering higher output and extended control options.

Connection to the heating system

The boiler is connected to the existing heating pipework. Standard wall-mounted boilers have G 3/4" or G 1" connections (depending on the model) for the flow, return, and domestic hot water. A set of valves (service valves) is installed in front of the boiler, allowing it to be disconnected from the system without draining the entire circuit. These valves include:

Ball valve on the heating flow
Ball valve on the heating return
Ball valve on the cold water inlet (for combi boilers with DHW heating)
Ball valve on the hot water outlet
Ball valve on the gas inlet

After the service valves, the boiler's piping is connected to the system. The installer checks the correct direction of flow (flow/return), uses suitable sealing materials (Teflon, hemp fiber with paste, or modern rubber gaskets), and tightens the joints. The piping must not be mechanically stressed – if the piping has to be "stretched" to reach the boiler's connection, this is a bad sign and can cause leaks.

Gas connection

The gas connection is made through a gas filter (prevents impurities from entering the boiler's gas valve), a ball valve, and a flexible stainless-steel hose certified for gaseous media. The flexible hose must not pass through walls and must not exceed the recommended maximum length (usually 2 m, depending on regulations). After connection, the seal is tested using a leak detection agent (foam) or a gas meter.

Connecting the flue gas discharge and air supply

This is one of the most important and, at the same time, most frequently underestimated phases. A condensing boiler can operate in two basic configurations:

Closed system (type C) – a coaxial pipe (air/flue gases run in parallel inside one double-walled pipe) passes through the wall or rises into a special chimney. Combustion air is drawn from outside, so the boiler doesn't need air from the room. This is the most common case in typical apartments and houses.
Open system (type B) – the boiler draws air from the room, and flue gases are discharged into a chimney. Less common with condensing boilers, requires special room ventilation.

Coaxial piping has a standardized diameter of 60/100 mm (DN 60/100) or 80/125 mm (DN 80/125) depending on the boiler's output. The maximum length of piping is limited – the manufacturer specifies an equivalent length (e.g., each elbow = 1 m of straight pipe). For a typical apartment, the equivalent length is usually 3–6 m.

Flue gases from a condensing boiler exit the outer end of the pipe at temperatures of only 55–75 °C. For this reason, the outlet of the combustion air must not be near a terrace, parking lot, or staircase, where the steam could bother people nearby, or where the exhaust air could be drawn back into the intake.

Electrical connection

A condensing boiler requires 230 V / 50 Hz power supply, usually with a power consumption of 100–250 W (depending on the model and circulation pump). The boiler must be protected by a circuit breaker (min. 10 A, 16 A recommended), and the outlet must be grounded. When installed in a bathroom, the zone requirements according to STN 33 2000-7-701 must be observed.

In addition to the power supply, the following are connected:

Outdoor temperature sensor (facade-mounted sensor, for weather-compensated control)
Room thermostat or smart thermostat (2-wire or OpenTherm)
Any additional sensors (cylinder thermostat, room temperature sensor)

You can read more about smart control and smart thermostats in the article Smart Control of Condensing Boilers: Regulation and Smart Thermostats. For advanced control of Bosch boilers, the Bosch Easycontrol CT 200 is available – a smart thermostat with WiFi communication that can control not only the boiler but also home zoning.

Phase 3: Commissioning and configuration

After the mechanical installation comes the phase that determines whether the boiler will run efficiently and trouble-free. This isn't just about "turning it on and checking if it's burning."

Filling and bleeding the system

The central heating system is filled with water via the filling valve (usually part of the boiler or its connection), and the cold system pressure is set to 1.0–1.5 bar (cold state). Every radiator, cylinder, and the boiler itself is bled. Modern condensing boilers have a built-in automatic air vent, but the rest of the system must be bled manually via the bleed valves on the radiators.

After bleeding, the system is topped up again to the correct pressure (1.0–1.5 bar cold, 1.5–2.0 bar hot). The expansion vessel must have the correct pre-set nitrogen pressure – usually the same as the operating filling pressure, i.e., 1.0–1.5 bar.

First start-up and gas valve adjustment

The first start-up is performed by the installer. They check:

The gas pressure in front of the boiler (in mbar, according to manufacturer's requirements)
The flame ionization current (safety check for flame detection)
The flue gas temperature and CO₂ or O₂ content in the flue gases (flue gas analyzer)
Combustion efficiency – ideally above 96% for a condensing boiler
Function of safety components (STB – safety temperature limiter, pressure relief valve)

Based on the flue gas analysis, the installer can fine-tune the gas/air ratio in the gas valve assembly. This is a professional operation that directly affects both efficiency and emissions.

Setting up controls and operating parameters

Condensing boilers offer extensive parameter settings. Basic settings that the installer must configure include:

Heating curve (weather-compensated control) – the relationship between the required heating circuit temperature and the outdoor temperature. For underfloor heating, the slope is typically 0.5–0.8, and for radiators, 1.2–1.8.
Maximum flow temperature – for a condensing boiler with underfloor heating, typically 45 °C; for radiators, 70–75 °C (but for efficient condensation, ideally max. 60–65 °C).
Output modulation – condensing boilers usually modulate output in a range of 1:5 to 1:10 (e.g., a boiler with a nominal output of 24 kW can operate down to 2.4 kW). The minimum output must be set so as to avoid unnecessary cycling on/off.
DHW temperature – for combi boilers (with instantaneous heating), the target DHW temperature is set, usually 55–60 °C.
Legionella protection – periodic heating of the cylinder to 70 °C (if a cylinder is present).

For boilers with instantaneous domestic hot water heating – such as the BOSCH Condens GC2300iW 22/25 C Wall-Mounted Condensing Boiler with Instantaneous Hot Water Heating – it's also important to set the DHW priority over heating. During peak demand (e.g., a shower), the boiler increases its output to maximum and temporarily shuts off the heating circuit.

Phase 4: Inspection and documentation

p>After commissioning, documentation must be prepared. This includes:

Installation report – contains equipment identification, installation date, name and registration number of the installer, and measurement results (gas pressure, flue gas temperature, flue gas analysis)
Gas installation inspection report – prepared by an inspection technician with the appropriate license, and is a prerequisite for the first gas filling
Warranty certificate – completed by the manufacturer or installer, registration with the manufacturer (Bosch, Vaillant, and others require online registration to activate the full warranty)
User manual – the installer must familiarize the customer with basic operation, emergency shutdown, and annual servicing

Warning: without the inspection report, the energy company will not connect the new gas appliance, or if you replaced an existing boiler, you may run into problems with an insurance claim if there's no proof of professional installation.

Common installation problems and how to avoid them

From practical experience with dozens of jobs, we know where the most common complications arise:

Undersized expansion vessel

The old boiler had, for example, a 6-liter expansion vessel. The new condensing boiler has an internal 8-liter expansion vessel, but the system is larger (underfloor heating area + hot water cylinder). Result: as pressure rises, the safety valve opens and the system regularly "spits out" water. Solution: calculate the required expansion vessel volume before installation (depends on the system volume and operating temperature) and add an external expansion vessel.

Formula for a rough estimate: V_expansion = V_system × 0.04 × (1.3 + p_max) / (p_max – p_filling). For a system with 200 l of water at a max. pressure of 3 bar and a filling pressure of 1.5 bar, the required volume works out to around 16–20 l.

Neglected system flushing

As mentioned, corrosion in old pipes and sludge (magnetite) from radiators can destroy a boiler within a single season. Solution: chemical flushing (disinfection + pH neutralization), followed by installing a magnetic filter (sludge trap) before the boiler's return, and a corrosion inhibitor added to the system (e.g., Fernox F1, Sentinel X100).

Incorrect length or slope of the condensate pipe

Condensate must drain by gravity; the minimum pipe slope is 2%, i.e., 2 cm of height per 1 m of length. If the installer routes the condensate pipe horizontally or even with a negative slope, the water may stop flowing or freeze in winter when temperatures drop. Result: an error code on the boiler and shutdown of operation.

Incorrect placement of the outdoor temperature sensor

The outdoor sensor for weather-compensated control must be placed on a north or northwest-facing wall (without direct sunlight), at a height of 2–3 m, away from ventilation grilles, exhausts, and other heat sources. If the sensor is in direct sunlight, the boiler "thinks" it's warmer outside than it actually is and underheats the house. This is a very common mistake.

Incorrect coaxial pipe sizing

For outputs above 24 kW, a DN 60/100 coaxial pipe is not sufficient – DN 80/125 is required. If the installer uses a smaller pipe, the boiler experiences increased pressure loss on the flue side, causing alarm states or poor combustion. Always follow the installation manual for the specific boiler.

Combination with underfloor heating and a cylinder

Condensing boilers are an ideal heat source for underfloor heating, since they work efficiently at low temperatures (35–45 °C), where condensation is most intense. If you're combining a condensing boiler with both underfloor heating and radiators, you need a mixing circuit – a three-way valve with an actuator that lowers the water temperature for the underfloor circuit while keeping a higher temperature for the radiators.

A domestic hot water cylinder (50–300 l depending on the number of occupants) is connected via an indirect heat exchanger from the boiler circuit or through a separate boiler outlet. For a typical family home (4 people), a 150–200 l cylinder with a charging output of 20–25 kW is sufficient. An alternative is a combination without a cylinder – a boiler with instantaneous heating, such as the BOSCH Condens GC2300iW 15 P Gas Boiler for smaller homes or apartments with lower hot water consumption.

You can find more about this choice in the article Condensing Boiler with Instantaneous Heating vs. with a Cylinder: Difference and Selection.

What the customer receives at handover: list of documents and information

At handover, the installer must provide the customer with:

User manual in Slovak (or Czech)
Installation report with measured values
Gas equipment inspection report
Warranty certificate (completed and stamped by the installer)
Proof of warranty registration with the manufacturer (if required)
Contact information for the service center

The customer should be familiarized with the following basics: how to check the water pressure in the system and how to top it up, where the emergency gas and electricity shut-off is, what to do in case of a gas leak, when to call for service, and the recommended frequency of annual servicing (once a year, ideally in spring or autumn before the heating season).

You can read more about what and how to check during annual maintenance in the article Servicing and Maintenance of a Condensing Boiler: What and How Often to Check.

Frequently Asked Questions (FAQ)

Can I install a condensing boiler myself?

No. Under Slovak legislation, gas equipment must be installed by a professionally qualified person with the appropriate license for gas installations. Self-installation is not only illegal but also dangerous (risk of gas leaks, carbon monoxide) and automatically voids the manufacturer's warranty. The electrical part can be handled by an electrician, while the gas part must be handled exclusively by a certified gas fitter.

How long does it take to install a condensing boiler?

When replacing an old boiler with a new condensing one (same location, existing piping): 3–5 hours for an experienced installer. For a new installation in a new building or when changing the boiler's location: 6–10 hours, including running new piping. You should also factor in a possible inspection (an additional half day) and preparation of the site before installation (demolition or masonry work, if needed).

Do I need to have the entire gas installation inspected during installation, or just the boiler?

When replacing a boiler without any work on the existing gas pipeline, an inspection of the appliance (the gas boiler) is sufficient. If the installer works on the gas pipeline (extending or relocating the connection), an inspection of the gas pipeline in that section is required. A full inspection of the entire house gas pipeline system is mandatory every 10 years, or after any intervention in the gas pipeline system.

What if the room where I want the boiler doesn't have a sewer connection?

Condensate must drain into the sewer system – this is a legal requirement. Possible solutions: cut a channel in the floor and bring in a sewer branch, or use a condensate pump unit (a mini pump that can pump condensate up to 5 m in height and 20–30 m in distance). These devices are compact and quiet, and are installed directly under the boiler or in the boiler's casing.

What permits do I need to install a condensing boiler?

For replacing an old gas boiler with a new condensing one in a family home, a building permit is typically not required – it's an equipment replacement without any intervention in the building structure. If you're changing the boiler's location, running new gas piping, or changing the flue gas discharge (a new coaxial passage through the wall), it's advisable to consult with the building authority, or inform the gas distributor in advance. In apartment buildings, consent from the building administrator and the homeowners' association is required.

How much does the whole thing cost, i.e., the real installation price?

Prices vary significantly by region, scope of work, and the specific installer. As a rough guide: the installation itself of a wall-mounted condensing boiler (replacing an old one, no demolition work) costs €250–450. An inspection report usually costs €80–150. If demolition work, new piping, or system flushing is needed, the price can rise to €600–1,200 or more. Always request at least two independent quotes and insist on written contractual confirmation of the scope of work.

Conclusion: investing in proper installation pays off many times over

A condensing boiler is a long-term investment – with proper installation and annual servicing, it can operate reliably for 15–20 years. Conversely, cutting corners on installation (e.g., skipping flushing, hiring an unqualified installer, missing documentation) can repeatedly result in breakdowns, a voided warranty, or in the worst case, even a safety incident.

If you're considering which boiler is right for your home, also read the articles How to Choose a Condensing Boiler: What to Focus on Before Buying and What Output Condensing Boiler Do I Need for My Home. And if you're not sure whether to go with a wall-mounted or floor-standing solution, the article Wall-Mounted vs. Floor-Standing Condensing Boiler: Which Is More Suitable will help you decide.

For specific recommendations for your home – what output, what model, and everything the installation will involve – we're here to help. Every installation is a bit different, and the details may vary from the situation we've described here.

Do you have a question on this topic?

Can't decide, or dealing with a specific situation in your household? Write to us - we're happy to help.