Installing a Boiler and Storage Tank Set: Process, Requirements and What to Arrange in Advance

We are still preparing products.

But you can look at other categories.

Installing a Boiler and Storage Tank Set: Complete Procedure, Technical Requirements and What to Prepare in Advance

A set consisting of a condensing boiler and a domestic hot water storage tank is today one of the most common solutions for heating and hot water preparation in family houses. Unlike an instantaneous water heater, a storage tank offers a comfortable and energy-efficient way of heating water, while the condensing boiler ensures maximum combustion efficiency. Installing such a set, however, is not a trivial matter – it requires preparation, expert knowledge, proper project documentation and compliance with several technical standards. This article will guide you through the entire process from the initial site inspection to commissioning the set, drawing on real experience from installation practice.

If you are still considering which set to choose, we recommend first reading the article How to Choose a Condensing Boiler and Hot Water Tank Set, where you will find the selection criteria. For the question of tank capacity, see What Tank Capacity Do I Need: Calculation Based on Number of People and Bathrooms. In this article, we assume you have already chosen your set and will focus on the installation itself.

What Is a Boiler and Tank Set: Basic Principles

Before we get into the installation procedure, let's briefly recap what we are actually installing. The set consists of a condensing boiler (wall-mounted or floor-standing), a domestic hot water storage tank (storage water heater) and the hydraulic connection between them. The boiler heats the tank via a heat exchanger – the tank does not heat the water directly with a flame, but through the boiler's heating circuit. This way, the drinking water is always kept separate from the heating circuit, which is essential from a hygiene standpoint.

In practice, we encounter two main types of connection: direct hydraulic connection (the boiler has a separate outlet for the tank) or connection via a hydraulic connector, also called a Thermolink (typical for the Protherm set). You can read more about how the Thermolink P works and what it offers in the article Thermolink P and Hydraulic Connection of the Boiler and Tank: How It Works. For the purposes of this text, it's important to know that the type of connection directly affects component placement and pipe routing.

What to Arrange and Prepare Before the Installer Arrives

The most common problem we encounter with orders is an unprepared building. The customer orders a set, schedules an installation date, but on the day the installer arrives, it turns out the boiler room isn't plastered, the gas supply is missing, or the electrical wiring isn't in the required configuration. Every such problem extends the installation time and increases the cost of the whole project. So let's go through everything you need to have ready in advance.

Project Documentation and Inspections

If it's a new building or a gas pipeline reconstruction, a gasification project prepared by an authorized person is required. This project must be approved by the gas distributor. Installation may not begin without valid documentation. For condensing boilers, flue gas discharge must also comply with STN EN 483 and the relevant parts of STN EN 15502. If you are replacing a boiler in an existing building where, for example, there was an old atmospheric boiler with a chimney flue, simply connecting to the old chimney is not enough – a chimney inspection is required, and almost always relining or a complete change of the flue gas discharge system (condensate is acidic and would corrode an old brick chimney).

For the electrical installation, the boiler must be connected to a separate protected circuit with a circuit breaker of at least 10 A, with RCD protection (residual current device). Tanks with an electric backup heating element (which some models have) require a separate circuit with higher capacity. Arrange this with an electrician before the boiler installation.

Room and Structural Readiness

The boiler room or the space where the set will be installed must meet several conditions:

Completed plastering work and a dry wall (the boiler is anchored to the wall, and a damp wall will complicate the installation).
Cold water supply with a check valve and a shut-off valve in an accessible position.
Sewage drain for condensate and for the tank's safety valve (the tank's safety valve must have a free drain, not just a closed container).
Sufficient air supply for combustion (for type C – hermetically sealed boilers – air is drawn from outside via a dual-pipe system, so no ventilation is needed, but for older type B boilers – with an open combustion chamber – room ventilation is required).
Room temperature must not drop below 5 °C (frost would damage the equipment).
Minimum ceiling height of 2 m, but for tanks with a larger capacity (over 200 liters) the tank stand can be up to 180 cm high, which must be taken into account.

Gas Supply and Its Parametric Requirements

Condensing boilers with an output of 20–30 kW require a minimum natural gas pressure of 17–20 mbar at rated flow. The gas supply must be sized so that the pressure does not drop below this value even with simultaneous consumption by stoves, appliances, and other devices. The gas valve in front of the boiler must be accessible and functional. A dirt filter is installed on the supply pipe, and for new installations also a gas pressure regulator, unless the distributor already supplies gas at the correct pressure.

Required Materials, Tools and Aids

An experienced installer arrives at the job well equipped, but the customer should know what the installation involves in order to properly understand the price quote and not be caught off guard by "extra work". Here is an overview of what a typical set installation requires:

Materials for the Heating Circuit

Copper or stainless steel pipe connections between the boiler and tank (diameter typically 3/4" or 1", depending on boiler output and tank model).
Ball valves on each connection – both the flow and return of the tank circuit must be shut-off capable so the tank can be disconnected without draining the whole system.
Drain valve at the lowest point of the tank circuit.
Seals, hemp fiber or Teflon tape (depending on joint type).
Expansion vessel for the tank circuit, if the boiler does not have a sufficient built-in expansion vessel (most modern condensing boilers have an expansion vessel for the heating circuit, but the one for DHW must be addressed separately depending on the manufacturer).
Safety valve for the tank circuit (typically set to 6 bar on the DHW side).

Materials for the Gas Section

Flexible gas hose or fixed copper/steel piping from the gas valve to the boiler.
Gas filter (if not already on the supply pipe).
Gas joint sealant (certified for gas, not ordinary hemp thread).

Materials for Flue Gas Discharge

Pipes and fittings for the flue gas and air intake system – diameter 60/100 mm (coaxial) or 80/80 mm (twin pipe) depending on the boiler and project.
Condensate trap or siphon for condensate.
Rain cap for the outlet on the facade or roof.

Tools

An installer's toolkit for such a job typically includes: a hammer drill, wrenches (17–32 mm), pipe cutter, copper bender, soldering iron or press tool, gas detector, pressure gauge, digital thermometer, multimeter, and the essential foam adhesive tape for pipe insulation.

Step-by-Step Installation Procedure

Now let's move on to the procedure itself. We describe a typical installation of a wall-mounted condensing boiler (for example Vaillant VU 246/5-3 ecoTEC pro + VIH R 150 B) with a floor-standing tank in a family house's boiler room.

Step 1: Measuring and Fixing the Boiler Bracket

Every wall-mounted boiler comes with an installation template or bracket. This bracket is placed on the wall at a height that takes into account the space for the tank below or beside it, as well as accessibility for servicing (typically the lower edge of the boiler 1.2–1.5 m above the floor). The wall must be load-bearing – a boiler filled with water weighs 30–50 kg. Anchoring is done with chemical anchors or wall plugs rated for at least twice the weight of the unit. After positioning the bracket, the layout of the connection points (flow, return, gas, condensate, electricity) is usually marked immediately, and wall openings are drilled if the installation goes through the wall.

Step 2: Installing the Tank

The tank is placed in its position – either directly under the boiler (for a set with a small tank, such as Vaillant VU 246/5-3 ecoTEC pro + VIH Q75 B, where the 75-liter tank can hang directly beneath the boiler), or beside the boiler on the floor. Floor-standing tanks (120, 150, 200 liters) are usually placed next to the boiler. The tank is leveled using a spirit level, and floor-standing models may be additionally secured to the wall for stability (especially for tanks over 150 liters filled with water, where the weight exceeds 200 kg). At this step, it is important to consider accessibility to the tank for future magnesium anode replacement – there must be at least 50 cm of free space in front of this opening.

Step 3: Mounting the Boiler and Connecting the Pipework

After the tank is set up, the boiler is mounted. The boiler is carefully placed onto the bracket and secured. Now follows the connection phase, which requires the most expertise and attention. We work in this order:

Connecting the boiler–tank heating circuit: hot water flow from the boiler to the tank's heat exchanger, and return from the tank back to the boiler. These pipes must be thermally insulated (at least 13 mm of insulation). Each side must have shut-off valves.
Connecting the building's heating pipework: flow to the radiators/underfloor heating and return from the radiators. The heating circuit's expansion vessel is also installed here, if not already integrated.
Connecting cold water to the tank: with a check valve and pressure-reducing valve (if the mains pressure exceeds 6 bar). A safety valve (6 bar) with a free drain to the sewer is also installed here.
Connection to the DHW distribution: hot water outlet from the tank to the building's hot water pipework.
Condensate drain: the boiler (and sometimes the tank) produces condensate, which must go to the sewer. Condensate is slightly acidic (pH 3–5), so some municipalities require a neutralization box. The drain must always be trapped to prevent odors from the sewage system.

Step 4: Installing the Gas Supply

The gas connection from the gas valve to the boiler's gas inlet is made with a certified gas hose or fixed piping. A gas hose must not pass through walls or floors – in such cases, fixed piping is required. A gas filter is installed in front of the boiler. Once the gas connection is complete, a pressure test of the gas installation is performed at an overpressure of 150 mbar for at least 10 minutes without any drop.

Step 5: Flue Gas Discharge and Air Supply

Condensing boilers operate exclusively in type C configuration (sealed combustion chamber, air drawn from outside). The flue gas discharge and air intake system is either coaxial (one pipe inside another) or dual-pipe (twin pipe). Configuration C3 (vertical to the roof) or C13 (horizontal to the facade) is the most common. Important parameters:

The maximum length of a 60/100 coaxial run is typically 6–8 m of straight equivalent; each 90° elbow subtracts 1–2 m from this limit.
The flue pipe must have a minimum slope of 3° towards the boiler so that condensate flowing back does not block the flue gas flow.
The outlet on the facade must be at least 0.5 m from any opening into the building (window, door, ventilation) and at least 0.3 m above ground level.
With a twin pipe system (80/80), the runs can be longer – typically up to 40–80 m, which is advantageous for boiler rooms deep inside the building.

Step 6: Electrical Connection and Controls

The boiler is connected to the electrical network via a fixed supply with terminals or via a plug, according to the manufacturer's instructions. Most modern condensing boilers (including Vaillant ecoTEC pro and Protherm Gepard Condens) are equipped with integrated electronics containing the boiler controller. External sensors are connected to it: an outdoor temperature sensor for weather-compensated control, a room thermostat, or smart control units (Vaillant multiMATIC, Vaillant sensoHOME, Protherm Thermolink). Control wiring runs in a conduit along the wall and must be separated from the power wiring (at least 5 cm). A thermostat cable typically has 2–5 wires depending on whether it's a simple thermostat or OpenTherm communication.

Step 7: Filling and Venting the System

Before first startup, the system must be filled with water and vented. The heating circuit is filled with treated water – in hard water areas (over 300 µS/cm), a corrosion inhibitor and softening agent are used to prevent limescale buildup in the boiler's heat exchanger. The water volume in the circuit and the expansion vessel setting must match the project. Cold-state pressure should be 1.5–2 bar. The tank is filled from the cold water side and vented via the DHW outlet valves – open the hot water tap and wait until the water flows smoothly without bubbles.

Step 8: First Startup (Commissioning)

The first startup may only be performed by an authorized service technician of the manufacturer or a certified service partner. This is a contractual condition for warranty validity – manufacturers such as Vaillant and Protherm require commissioning registration in their service system. During the first startup, the following are checked:

Tightness of all gas connections (soap solution test or gas detector)
Correct gas supply setting (pressure, gas volume at rated and partial output)
Ionization current check (flame signal)
Correct function of the flue gas discharge – measurement of draft and flue gas temperature
Boiler parameterization according to the installation (gas type, output, tank heating setting)
Function of the tank circuit – verifying that the boiler actually switches to tank heating mode, the tank heats to the set temperature, and the tank thermostat correctly switches off the boiler
Checking DHW outlet temperature – in case of legionella risk, it is recommended to set the tank temperature to a minimum of 60 °C, but 55 °C is sufficient for normal comfort

Specific Aspects of Installing Vaillant and Protherm Sets

In practice, we most often encounter two brands, so we provide a few practical details for each.

Vaillant ecoTEC pro with VIH Tanks

The set Vaillant VU 246/5-3 ecoTEC pro + VIH R 120 B is a typical example of a compact set for a 3–4 member family. The VU 246/5-3 boiler has an output of 3.2–24.1 kW, and its tank circuit operates with a maximum flow temperature of 80 °C. The VIH R 120 tank has a capacity of 120 liters and is equipped with a stainless steel heat exchanger. An important detail when installing Vaillant sets: the tank pump is integrated into the boiler, so there is no need to install an external tank circuit pump – this significantly simplifies the hydraulics. The tank is connected to the boiler via a direct hydraulic connection using 22 mm copper piping. Vaillant also offers a special configuration for these sets with the multiMATIC 700 controller (see VU 246/5-3 ecoTEC pro + VIH R 150 B with multiMATIC 700), where the controller is included in the package – this is suitable for underfloor heating installations or mixed circuits.

Protherm Gepard Condens with Thermolink P

For sets such as Protherm Gepard Condens 25 MKO + FE 120 BM bottom + Thermolink P, the principle is slightly different. The Thermolink P is a hydraulic connector that also serves as the basic control for the tank circuit. The FE 120 BM tank has a bottom connection (hence the "bottom" suffix) and uses the stratification principle – cold water enters from the bottom, while hot water is maintained at the top. During installation, it's important to maintain the correct tank orientation and ensure the Thermolink P is installed according to the manufacturer's diagram – the direct inlet/outlet ports cannot be swapped, otherwise the tank will not function correctly. The Gepard Condens 25 MKO boiler has a modulating output of 5–25 kW. For smaller houses and apartments, there is also the Protherm Gepard Condens 12 MKO + FE 120 BM bottom + Thermolink P with a maximum output of 12 kW – this solution is suitable, for example, for an apartment or smaller family house with a floor area of up to 120 m².

Pipe Insulation: Why It Matters More Than You Think

Thermal insulation of the pipework between the boiler and the tank is one of the most commonly underestimated aspects of installation. In practice, we see installations where the boiler is separated from the tank by only 1–2 meters of piping, but without insulation, heat losses can reach 5–10% of annual energy consumption for water heating. This can amount to 50–150 € per year in wasted gas. STN EN 12828 recommends insulating heating pipework to a thickness at least equal to the pipe radius – for a DN 22 mm pipe, at least 13 mm of insulation, and for DN 28 mm, at least 20 mm. The material is usually polyethylene foam or rubber insulation (Armaflex). The insulation must be closed with minimal joints and must not be interrupted where it passes through walls.

Commissioning and Handover of Documentation

After a successful first startup, the installer must hand over several important documents and pieces of information to the customer. This part is, unfortunately, often underestimated in practice, but it is key both for warranty and safety reasons.

Documentation You Must Receive

Gas installation inspection report – prepared by an inspection technician authorized under Decree No. 508/2009 Coll. Without this report, the installation is formally illegal.
Commissioning protocol – manufacturers (Vaillant, Protherm) have their own forms, which are filled out by an authorized service technician. This protocol is a warranty condition.
User manual in the local language – the customer must be instructed on operating the boiler, setting temperatures, annual servicing, and what to do in case of a fault.
Warranty card – properly filled out and confirmed.
Wiring/piping diagram – showing how the sources, circuits, and tank are connected, with marked shut-off points and service access points.

Settings That Should Be Explained to the Customer

An experienced installer spends 10–15 minutes explaining basic operation. The customer should know: how to set the heating and tank temperature, how to activate and deactivate holiday mode (frost protection), how to increase system pressure (filling valve), and how to check or replace the boiler fuse. This basic knowledge prevents unnecessary service call-outs when the customer calls the installer just because the boiler "isn't working" – when the actual problem is simply a switched-off thermostat or the tank temperature set to 30 °C.

Hydraulic Diagram of a Typical Installation with One Tank

The following diagram illustrates a typical connection of a condensing boiler with a tank in a family house with two heating circuits (radiators + underfloor heating) and one DHW tank.

Most Common Installation Mistakes and How to Avoid Them

After years in the industry, we know which mistakes recur. We list them here not to demonize installers, but so the customer knows what to look out for when accepting the work.

Tank without pipe insulation – as mentioned, this leads to unnecessary heat losses. Require the installer to insulate all hot water pipework.
Missing tank safety valve or an incorrect drain – the tank safety valve must have a free drain, not just a closed elbow. Otherwise, there is a risk of tank damage at high pressure.
Incorrect flue pipe slope – the flue pipe must slope towards the boiler, not away from it. If the slope is reversed, condensate accumulates in the pipe, causing vibrations, a "gurgling" sound, and gradual boiler damage.
Not using an inhibitor in the heating circuit – in hard water, the boiler's heat exchanger quickly scales up. The inhibitor costs 10–30 €, while replacing the heat exchanger costs 300–600 €.
No commissioning by an authorized technician – the customer starts up the boiler themselves after their neighbor's brother-in-law installed it, not realizing they've voided the warranty. Manufacturers check this when a warranty claim is made.
Undersized expansion vessel – during renovations, when an old boiler is replaced with a condensing one in a larger system, the original expansion vessel is often undersized. A new boiler may have a built-in 10–12 liter vessel, but a system with a large water volume requires an 18–25 liter expansion vessel.

Safety Requirements and Standards the Installation Must Comply With

Installing a condensing boiler and tank set in Slovakia is subject to several regulations. Here are the most important ones:

STN EN 12828 – Heating systems in buildings – Design of water-based heating systems
STN EN 13384 – Flue gas discharge, chimneys
Decree of the Ministry of Labour, Social Affairs and Family of the Slovak Republic No. 508/2009 Coll. – technical equipment, including pressure equipment (a hot water tank is pressure equipment)
STN 38 6405 – Gas equipment – operating principles
Act No. 555/2005 Coll. on the Energy Performance of Buildings – conditions for new installations
Flue system manufacturer standards – every boiler manufacturer specifies maximum lengths and configurations for flue systems; exceeding them is not permitted

The gas inspection technician must hold a valid authorization issued by the Technical Inspectorate (TI SR) under category E1 for gas equipment. Without a valid inspection report from an authorized person, the installation is not legally compliant, and in the event of an insurance claim (e.g., a gas leak or fire), the insurance company could refuse to pay out.

What Affects the Total Installation Cost

Customers often ask us why installation prices vary so much between different companies. Here are the main factors that affect the price:

Do You Have a Question on This Topic?
Can't decide, or dealing with a specific situation in your home? Write to us - we'll be happy to advise.

Full Name Do not fill in this field:

Email

Message