Installation of a Condensing Boiler with Instant DHW Heating: Procedure and Requirements

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Installing a Condensing Boiler with Instantaneous DHW Heating: A Comprehensive Expert Guide

The condensing boiler with instantaneous domestic hot water (DHW) heating is today the most widespread type of heat source in family houses and apartments in Slovakia. However, its installation is not just a matter of hanging the boiler on a wall and connecting a few hoses. It is a complex installation process that requires thorough preparation, correct sizing of all pipework, compliance with legal requirements and – crucially – must be carried out by an authorized person with valid certification. In this article we go through the entire procedure step by step, highlight the most common mistakes seen in practice, and provide specific technical values and dimensions that you will encounter in real-world jobs.

If you are still deciding which boiler to choose, we recommend reading the article How to Choose a Condensing Boiler with Instantaneous DHW Heating: What to Focus On and also What Output Condensing Boiler with DHW Do I Need for My House, where you will find guidance on correct output sizing. This article assumes you have already chosen your boiler and want to know what to expect during the actual installation.

Legal Framework and Who Is Allowed to Install a Condensing Boiler

The first thing every customer must understand: installing a gas condensing boiler is not a DIY job. Act No. 251/2012 Coll. on Energy and the related SPP-D decrees clearly state that the installation of gas appliances may only be carried out by a person with professional competence under Decree No. 508/2009 Coll. of the Ministry of Labour, Social Affairs and Family of the Slovak Republic – specifically, a technician with certification for the operation and repair of designated gas technical equipment (Group G). In addition, boiler manufacturers (Protherm, Vaillant and others) make the validity of the warranty conditional on an entry in the service log made by an authorized service technician.

In practice, this means for the customer:

The installation must be carried out by an authorized installer or service technician with the relevant certification.
After installation, the first commissioning ("inbetriebnahme") must take place, during which the technician checks tightness, sets up the boiler and issues a commissioning report.
Installation of the gas connection to the boiler may only be carried out by a holder of authorization for the installation of gas equipment.
All documentation (commissioning report, handover report) must be handed over to the customer and is a mandatory attachment to the warranty conditions.

This does not mean the customer cannot do anything themselves. Preparatory work – removing the old boiler (not disconnecting the gas!), preparing the room, drilling openings – can in many cases be done by a handy homeowner. However, the actual gas and electrical connections must always be carried out by a professional.

Pre-Installation Preparation: What to Find Out and Arrange Before the Technician Arrives

From experience, a large part of the delays in boiler installation arise precisely because of insufficient preparation. The technician arrives, starts measuring, and discovers that the chimney is unsuitable, there is no suitable power outlet, or the gas connection is routed incorrectly. Every extra day also costs the customer money (technician's labor, possibly accommodation in a cold house). Therefore, find out the following in advance:

Installation Location and Load-Bearing Wall

A condensing boiler is heavier than most people think. A typical mid-range boiler weighs between 28 and 42 kg (without water). The wall must be masonry or reinforced concrete with sufficient load-bearing capacity. The boiler cannot be reliably hung on plasterboard without special anchors and a steel bracket – and even then it is problematic. Always consult a technician in advance for installation on plasterboard.

The minimum free space around the boiler is specified by the manufacturer in the installation manual. Typical values for common models are: 100–150 mm on the sides, 300–500 mm in front of the boiler (service space), 50 mm above the boiler. Always check these values in the specific manual for your model, as they vary.

Gas Connection and Gas Pressure

A condensing boiler with instantaneous DHW heating has a higher input than a standard instantaneous water heater. An output of 24–26 kW in DHW heating mode corresponds to a natural gas consumption of approximately 2.8–3.1 m³/h (at a calorific value of 34 MJ/m³). The diameter of the gas connection to the boiler location must match this – typically DN 20 (3/4") for longer runs, DN 15 (1/2") only for short runs up to 3–4 meters. The gas inlet pressure ahead of the boiler should be 18–25 mbar (natural gas G20). The technician will measure this with a manometer during commissioning.

Electrical Connection

Condensing boilers require a separate 230 V / 50 Hz circuit, protected by a 10 A fuse (some models only need 6 A – depending on the power consumption of the electronics and pump). Essential requirement: the boiler must be grounded and the outlet must have a protective contact. Many older apartments have only a two-pin outlet in the boiler room – this is not sufficient and must be replaced before installation. The electrical connection must be within reach of the boiler (max. 1.5 m cable), and an extension cable is not permitted.

Water Connection for DHW

Instantaneous DHW heating in a condensing boiler works by having cold water enter directly through the heat exchanger and flow out heated from the tap. Water hardness is crucial for proper function and long service life of the exchanger. If the water hardness exceeds 15 °dH (German degrees of hardness), a water softener or at least a polyphosphate dosing unit (e.g. BWT Cillit and similar) must be installed ahead of the boiler. Otherwise, the DHW exchanger will scale up within 3–5 years to the point where output drops and an expensive replacement is needed.

The cold water supply to the boiler location should be DN 15 (1/2"), with an operating pressure of 1.5–6 bar. If the pressure in an apartment building is higher (above 6 bar), a pressure reducing valve must be installed ahead of the boiler.

Flue Gas Discharge and Combustion Air Supply: The Most Critical Part of the Installation

Correct installation of the flue gas system is both the most technically demanding and the most safety-critical part of the entire installation. A faulty chimney or incorrectly routed flue system can cause carbon monoxide poisoning – a lethal risk, not just a technical problem.

Closed Combustion System (Turbo) vs. Open

All modern condensing boilers operate as Type C appliances – they draw combustion air from outside and discharge flue gases outside as well, with both ducts sealed off from the boiler room space. Installation in an enclosed space (garage, bathroom, cupboard) is therefore possible, but it must be ensured that the boiler receives sufficient combustion air via external ducting.

Types of Flue Gas Systems and Their Dimensions

The most common type for wall-mounted condensing boilers is a coaxial pipe (air-flue gas in one twin-pipe system), or a parallel twin-pipe arrangement (separate ducts for air and flue gas). Common dimensions:

Coaxial Ø 60/100 mm – most common, max. equivalent length 4–5 m (depending on manufacturer and output). Suitable for a direct passage through the perimeter wall.
Coaxial Ø 80/125 mm – for higher outputs or longer runs, max. equivalent length 8–12 m.
Parallel Ø 80 + Ø 80 mm – for longer runs through the whole house or into an existing chimney, max. equivalent length 20–40 m (depending on manufacturer).
LAS system (Luft-Abgas-System) – for routing through an existing chimney flue, flue gas through a 60 mm pipe inserted into the chimney flue, air drawn from the space around the pipe. Requires professional assessment of the chimney.

Each bend adds to the equivalent length: a 90° bend = approx. 1–1.5 m of equivalent length, a 45° bend = approx. 0.5–0.7 m. When designing the flue pipe route, the technician must calculate the total equivalent length and verify that it does not exceed the maximum permitted value for the given boiler.

Condensate traps must be installed in flue pipes wherever condensate accumulation is likely – i.e. in horizontal sections. The slope of horizontal flue pipe sections must be at least 3° (approx. 5 cm/m) towards the condensate drains.

Installing the Condensing Boiler: Step-by-Step Procedure

The following procedure describes a typical job of replacing an old boiler with a new condensing boiler with instantaneous DHW heating. In most cases, this involves replacing an older boiler with a modern condensing unit – e.g. moving from an old Protherm Medveď to a new Protherm Gepard Condens 18/25 MKV or Vaillant VUW 236/5-3 ecoTEC pro.

Step 1: Shutting Down, Disconnecting and Removing the Old Boiler

Work begins by shutting off the gas (closing the gas valve ahead of the boiler), switching off the electricity (circuit breaker) and shutting off the water (valves on heating and water supply connections). The old boiler is vented and drained. Only a gas fitter may disconnect the gas connection to the boiler – after disconnection, the gas line is capped. The remaining pipework (water, heating) can also be disconnected by a competent homeowner.

If the old boiler was of a different type (e.g. an atmospheric one with a shared chimney), a decision must be made on how flue gas discharge will be handled going forward. The existing chimney flue is either lined with a plastic or stainless steel liner suitable for low-temperature condensate flue gases, or a new coaxial wall penetration is built. Each of these options has different costs and legal requirements.

Step 2: Preparing and Fitting the Mounting Bracket

Modern condensing boilers come with a mounting bracket (template) that precisely determines the position of the anchoring holes and the position of all connections. The bracket is placed against the wall, dimensions are measured and holes marked. The standard mounting height is such that the top edge of the boiler is approximately 1,800 mm from the floor – allowing convenient access to service panels. Anchoring: holes of Ø 10–12 mm are drilled in the masonry, to a depth of 80–100 mm, using chemical or expansion anchors depending on the wall material. Bracket screws are typically M8 or M10.

Step 3: Installing the Flue Gas System

Before hanging the boiler, the flue outlet is usually prepared – a hole is drilled or cut through the wall (Ø 125–150 mm for a 60/100 coaxial pipe), a sleeve is fitted, and sealing is ensured. A flue adapter (included as an accessory) is mounted on the boiler, into which the pipe is inserted. Horizontal sections are laid with a minimum slope of 3° outward (from the boiler towards the outside outlet). The outside outlet must be protected by a rain cover and must not discharge into areas where flue gas could be drawn back in (corners, leeward sides, neighbours' windows within 1 m).

Step 4: Hanging the Boiler and Connecting the Heating Circuits

The boiler is hung on the bracket and secured. The heating pipework – flow and return – is then connected. Here are a few key rules:

The flow pipe (supply) must have a drain valve and a ball valve installed near the boiler.
The return pipe must also have a ball valve and a filling valve (for topping up the system with water).
The expansion vessel must have the correct pre-charge pressure (typically 0.75–1.0 bar for systems up to 7 m in height). Most condensing boilers have a built-in expansion vessel, but its volume (6–8 litres) is only sufficient for smaller systems – for a larger heating system volume (above 100–120 litres) an external expansion vessel must be connected.
The safety valve must be located at the boiler outlet, ahead of any shut-off valves. Typical setting: 3 bar.
System venting: air vents must be installed at the highest points of the heating system.

Step 5: Connecting the Water Supply for Instantaneous DHW Heating

Instantaneous DHW heating in all modern condensing boilers of this type is achieved via a plate heat exchanger (or tubular exchanger), not a storage tank. Cold water enters the exchanger directly and heated water exits to the taps. The cold water connection must be sized for a flow rate of 10–14 l/min (this is typical for 24–26 kW boilers). Before cold water enters the boiler, the following are mandatory:

A shut-off valve (for servicing).
A dirt strainer/filter (mesh size 0.5–1 mm) – protects the DHW exchanger.
A non-return valve (prevents hot water from flowing back into the cold water mains).
A pressure reducing valve, if the mains pressure exceeds 6 bar.
A water softener or polyphosphate dosing unit, where applicable (see above).

Step 6: Gas Connection

The gas connection is carried out exclusively by a certified gas fitter. The boiler's connection to the gas supply is usually G 3/4" or G 1/2" (depending on manufacturer). Between the gas connection and the boiler there must be: a ball valve (gas cock), a gas filter (protecting the boiler's gas valve), and, for some manufacturers' boilers, also a pressure regulator. All joints are tested for tightness after installation – using a manometer (pressure test) or a foaming solution (bubble test). A gas pipeline tightness report must be part of the documentation.

Step 7: Electrical Connection and Thermostat Wiring

The boiler is connected to a grounded 230 V outlet. If the system includes a programmable thermostat or a smart controller (OpenTherm, eBUS), it is connected to the boiler's terminal block according to the wiring diagram in the manual. Most modern boilers (e.g. Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect) support the OpenTherm or eBUS communication protocol, enabling weather-compensated control and significant energy savings. Connecting a wireless thermostat only requires pairing according to the manual – no physical wiring is needed.

If the system includes zone control (multiple circuits with control valves), the control signals are wired to the corresponding boiler terminals. Some boilers require an external module (e.g. Vaillant VR 61, VR 65) for connecting zone control – this needs to be addressed in advance when choosing the boiler.

Step 8: Filling and Venting the System

After complete connection, the system is filled with water. Procedure:

Open the filling valve (on the return), fill slowly with water.
Vent all radiators (from the lowest floor upwards).
Vent the boiler pump (most boilers have a bleed screw on the pump).
Pressurize the system to 1.5 bar (cold system). The boiler usually indicates low pressure on the display.
Check that no joints are leaking.

Step 9: Condensate Discharge

A condensing boiler produces condensate from the condensation of water vapour in the flue gases. The amount of condensate depends on the boiler output and temperature difference, but at full output (e.g. a 24 kW boiler in condensing mode) it can be 1.5–3 litres per hour. Over a full day of winter operation, this amounts to 20–40 litres of liquid. This liquid has a pH of 3.5–5.5 (mildly acidic), which is an issue for drainage. According to STN EN 677 and manufacturers' guidelines:

Condensate may be discharged into the municipal sewer system – this is permitted, as it is sufficiently diluted.
For smaller boilers (up to 25 kW), condensate is usually discharged directly into the drain (sink trap, floor drain).
For higher outputs or specific requirements, a condensate neutralizer is installed (e.g. limestone granules), which raises the pH to a neutral value before discharge into the drain.
The condensate drain must be fitted with a trap (water seal) to prevent odours from the drain being drawn back into the boiler and the room.
Diameter of the condensate drain: min. DN 32 mm, slope of at least 1:50.

First Commissioning: What the Technician Does and Why It Matters

First commissioning ("inbetriebnahme") is a separate, formal step in which an authorized service technician inspects, sets up and hands over the boiler to the customer. Without this procedure, manufacturers' warranty conditions are usually not valid. What takes place during it:

Checking the tightness of the gas pipeline – using a manometer or a foaming solution.
Checking the flue gas system – draft passage, correct slope, joint tightness.
Setting the maximum boiler output – according to the project or the house's calculated heat loss. An oversized boiler cycles unnecessarily (short runs = higher wear).
Setting DHW parameters – hot water temperature (standard 50–55 °C, min. 45 °C due to Legionella risk, max. 60 °C for most models).
Setting pressure parameters – checking expansion vessel pre-charge pressure, system filling pressure.
Combustion analysis – measuring CO, CO₂ and O₂ in the flue gas with an analyzer. Values are recorded in the report. For natural gas G20, the correct CO₂ value in the flue gas is approx. 8.5–10.5% (depending on output and model).
Entry in the service log and issuing of the report.
Customer instruction – boiler operation, thermostat setup, what to do in case of a fault, when to call service.

Special Situations and Common Challenges in Practice

Replacing an Old Protherm Medveď or Gepard Boiler with a Condensing Model

This is probably the most common job in Slovakia. The customer has an old boiler with an open chimney and wants to switch to a condensing model. The main challenge: the new condensing boiler cannot use the existing chimney for open appliances (the so-called draft chimney), because condensing boilers are closed systems (Type C). There are two solutions: (a) lining the chimney – a stainless steel or plastic pipe Ø 80 mm is inserted into the existing masonry flue to carry the flue gases; air is drawn either from the boiler room space (only if it meets the air supply requirements – opening of at least 150 cm²) or through an external pipe; (b) wall penetration – a coaxial or parallel pipe running directly outside. The second option is cheaper and simpler, if the boiler location and the house's architecture allow it.

Boilers such as the Protherm Panther Condens 20/26 KKV or the Protherm Puma Condens 18/24 MKV are designed to be connectable to various types of flue gas systems – however, always check the maximum permitted equivalent length for the given model in advance.

Installing a Condensing Boiler in an Apartment Building

Even stricter rules apply in apartment buildings. Replacing a boiler in an apartment requires the consent of the building manager and, in many cases, also the consent of neighbours (especially if the use of the chimney flue is being changed). The location of the flue gas outlet must meet requirements regarding distances from windows, balconies and ventilation of neighbouring apartments. Typical minimum distances: 600 mm from a window, 1,000 mm from ventilation grilles, 500 mm from the corner of the building. These values vary according to national regulations and local conditions – the technician must assess each case individually.

New Build and First-Time Installation

In a new building, the situation is simpler in terms of flue gas (the chimney is designed from the start), but more complex in terms of coordination. The boiler is usually installed after the shell construction is complete, but before final plastering – the heating and water pipework is prepared by the designer. It is essential here that the designer consult the boiler supplier in advance about the position of the connections – different boiler models have connections in different locations (some at the front, some at the bottom). Changing the position of the pipework after plastering is costly.

Documentation After Installation: What You Must Have

After completing the installation and first commissioning, the customer should have the following documents in hand:

Boiler operating manual (from the manufacturer, in the local language).
First commissioning report (issued by an authorized technician).
Gas pipeline tightness report.
Boiler warranty card, properly filled in and signed.
Service log (containing the entry for first commissioning).
Combustion analysis report (some technicians issue this separately).
Electrical installation inspection report (if the electrical connection was modified).
Chimney or flue gas system inspection report (if carried out by a chimney sweep).

Keep these documents carefully – you will need them when making a warranty claim, when selling the property, and for future servicing. Without the first commissioning report, most manufacturers will not honour warranty repairs.

Maintenance Requirements After Installation and Further Servicing

Installation is not the end of the story. A condensing boiler with instantaneous DHW heating requires regular maintenance to retain its high efficiency and reliability. We cover this in more detail in the article Maintenance and Servicing of a Condensing Boiler with DHW: What and How Often to Check, but in brief:

Annual service inspection – cleaning the burner and heat exchangers, checking tightness, checking pressure, combustion analysis.
Checking and topping up water in the system as needed (when pressure drops below 1 bar).
Checking and flushing filters (cold water strainer, magnetic filter on the heating return).
Checking condensate discharge – the trap must not dry out when the boiler is out of operation for a long time (holiday home).
Inspection of the flue gas system – usually once a year, according to local regulations (chimney sweep inspection).

Regular servicing significantly extends the boiler's service life. A well-maintained condensing boiler will last 15–20 years without major issues. A neglected boiler without servicing may develop serious problems (clogged exchanger, worn pump, burner fault) after just 5–7 years.

Most Common Installation Mistakes with Condensing Boilers – What We See Repeatedly

Over years of field work, we have encountered the same recurring mistakes. Here is a list of the most significant ones:

Incorrect flue pipe slope – horizontal sections without a slope lead to condensate accumulation in the pipe and subsequent damage to the boiler or flue pipe.
Missing cold water filter – impurities from the water mains (sand particles, rust) will damage the DHW plate exchanger within a few months of operation.
Safety valve discharging into an enclosed space – the safety valve must have a discharge pipe visibly routed into a trap or drain, not simply left open into the air behind the boiler.
Expansion vessel with incorrect pre-charge pressure – a vessel designed for a pre-charge of 1.5 bar in a system with a building height of 5 m will fulfil its intended function for only a fraction of its expected lifetime.
Insufficient condensate drain diameter – DN 25 is not enough, DN 32 is the minimum. Clogging of the condensate drain trap then becomes a matter of a few months.
Missing water softening – in areas with water hardness above 15 °dH without a softener, the DHW exchanger will scale up within 2–4 seasons.
Oversized boiler output – a boiler set to full output for a house with low heat loss cycles every 2–3 minutes, which dramatically shortens its service life and increases consumption.
Ungrounded outlet – missing grounding is not just a safety issue, it can also cause interference with the boiler's electronics and false error codes.

Frequently Asked Questions (FAQ)

Do I need a building permit to install a condensing boiler?

When replacing a boiler with the same type (gas for gas, same output), a building permit is in most cases not required – it is considered maintenance work. Exceptions may arise when changing the flue gas system in a way that affects common parts of an apartment building, or if the local building authority states otherwise. For new construction or when changing the type of fuel (switching from solid fuel to gas), a building permit may be required. Always check in advance with the relevant building authority to avoid problems.

How long does it take to install a condensing boiler?

For a standard replacement of an old boiler with a new condensing model in a family house, installation takes 4–8 hours for an experienced pair of technicians. If a new flue gas system is being installed at the same time (chimney lining, a long coaxial run), the work may be spread over 2 days. For new construction, where heating pipework is also being laid, it is a matter of several days of work. First commissioning (after completion of installation and filling) takes a further 1–2 hours.

Can I connect the boiler to an existing chimney?

Yes, this is possible, but only under precise conditions. The existing chimney flue must be of suitable dimensions (at least 100 × 100 mm cross-section), must be in good technical condition, and must be lined with a plastic (PP) or stainless steel pipe Ø 80 mm resistant to condensate. Atmospheric chimneys for open appliances cannot be connected to condensing boilers without lining. Always request a chimney sweep's report on the condition of the chimney before installation.

What DHW temperature is optimal and safe?

The minimum domestic hot water temperature should be 45 °C for hygienic reasons – below

Do you have a question on this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us - we will be happy to help.