Installation of the ELÍZ electric boiler: procedure and installation requirements

We are still preparing products.

But you can look at other categories.

Installation of the ELÍZ Electric Boiler: Complete Procedure and Everything You Need to Know Before Installation

The ELÍZ electric boiler is a compact, easy-to-install unit – but only if the installation is carried out correctly, in accordance with applicable standards and with sufficient preparation. In practice, we often see customers underestimate either the electrical connection or the hydraulic part, or forget about the expansion vessel and safety valve. The result? The boiler works fine for a few days, then a fault appears, or in a worse case, the insurance company refuses to pay out because the installation does not comply with standards.

This article is intended as a technical guide for experienced installers, but also for technically skilled homeowners who want to know what the installation involves – so they can check the installer's work or prepare themselves. We will go through the whole procedure from A to Z: from receiving the unit through hydraulic connection, electrical connection, pressure testing, start-up, to commissioning.

If you haven't chosen a specific model yet, first read the article How to Choose an ELÍZ Electric Boiler: What to Focus on Before Buying and What Output of the ELÍZ Electric Boiler Do I Need for My House – they will help you choose the right power input before you order the connection from an electrician.

What must be done before the installer arrives

Installation of an electric boiler does not start when the installer arrives, but several weeks or months earlier. You will wait the longest for the electrical connection to be upgraded – and this is exactly where the most common mistake is made when everything is left to the last minute.

Electrical connection and circuit protection

ELÍZ electric boilers operate on a three-phase 3×400 V / 50 Hz power supply. The power input depends on the boiler's output – for example, the ELÍZ EQH 802 has a rated power input of 8 kW. For such a boiler you must count on a current load of:

I = P / (√3 × U × cos φ) = 8,000 / (1.732 × 400 × 1.0) ≈ 11.5 A

We choose a circuit breaker with a margin – typically 16 A (characteristic B or C), cables min. CYKY 4×2.5 mm² or 4×4 mm² for longer runs. More powerful models require a correspondingly stronger circuit breaker and cable. A 30 mA residual current device (RCD) is also always required – this is a Class I appliance (grounding via PE conductor).

Important: the electrical connection must be carried out by a person with electrotechnical qualification according to Decree No. 508/2009 Coll. – at least §21 (a trained person is not sufficient, you need §22 or §23). Once completed, a Report on the Professional Inspection and Test of the Electrical Installation (inspection report) must be issued according to STN 33 1500.

Boiler placement and boiler room

ELÍZ boilers are designed as compact wall-mounted units suitable for a utility room, laundry room, hallway, or cabinet. They produce no combustion gases, so there is no chimney requirement. Nevertheless, several rules apply:

Ambient temperature: 0 °C to +40 °C (the boiler must not be installed in a frost-prone environment without permanent heating)
Relative humidity max. 90% (without condensation)
Access to service panels: min. 30–50 cm of free space (depending on the specific model)
Drip tray or floor drain in the room – recommended, but not always mandatory for apartment buildings
Load-bearing wall: the boiler is relatively light (approx. 15–25 kg depending on the model), most walls can handle the load without problems

Hydraulic preparation of the system

Before installing the boiler, the heating system should be cleaned (flushed), or chemically disinfected if needed. Old sludge, magnetite, and corrosion are the main causes of pump and flow sensor failures. If you are renovating an old system, investing in a magnetic filter and flushing the system will pay off in the boiler's reliability.

Step by step: procedure for installing the ELÍZ electric boiler

Step 1 – Unpacking and checking the delivery

When receiving the unit, check the completeness of the delivery against the delivery note. A standard ELÍZ boiler delivery includes:

The boiler itself with pre-installed hydraulic connections (G ¾" or G 1")
Wall bracket (mounting frame or hooks)
Documentation: installation manual, operating manual, warranty card, EC declaration of conformity
Seals for the hydraulic connections

Note: some ELÍZ models do not include an expansion vessel or a safety valve in the standard delivery – these are separate accessories that you must order and install. These components are, however, a legal requirement according to STN EN 12828. Always read the technical data sheet of the specific model.

If you find mechanical damage when unpacking (cracks in the housing, bent frame, damaged hydraulic connections), do not install the boiler and immediately report the damage to the carrier or seller.

Step 2 – Mounting the boiler on the wall

The boiler is mounted on the wall using the supplied bracket. Procedure:

Determine the mounting height: the supply outlet should be approx. 1.5–1.8 m above the floor (depends on model and connected piping)
Use a spirit level or laser leveling tool – the bracket must be horizontal (max. deviation ±1°)
Drill holes according to the bracket dimensions, use anchors min. ø10 mm (for concrete walls) or chemical anchors (for aerated concrete/ytong)
Screw in the bracket, check the strength by pulling with approx. 20–30 kg on each hook
Hang the boiler on the bracket, secure the locking screw (if included with the bracket)

Practical note: standard anchors are not sufficient in aerated concrete (ytong) walls. For a unit weighing 20–25 kg, we recommend M10 chemical anchors, or mounting through a wooden board screwed through the entire wall thickness (if accessible from both sides).

Step 3 – Hydraulic connection

The hydraulic connections of ELÍZ boilers are mostly located on the bottom of the unit. Typical labels: supply (hot water outlet from the boiler) and return (cooled water inlet back into the boiler). Note – some manuals confuse the terms "supply" and "inlet" from the boiler's perspective, so always follow the technical manual of the specific model rather than a general description.

Hydraulic connection procedure:

Ball valves on both the supply and return – allow the boiler to be disconnected without draining the entire system
Check valve on the supply (recommended) – prevents reverse circulation when the pump is off
Filling and draining valve – at the lowest point of the circuit or directly on the boiler body (if integrated)
Air vent valve – automatic, at the highest point of the circuit (and on the boiler body, if integrated)
Safety valve – set to the maximum boiler pressure (usually 3 bar), directly on the supply or on the boiler body. It must have a free outlet to a collecting vessel or drain!
Expansion vessel – sized according to the volume of the heating system (typically 8–18 l for an average house; calculation: volume of water in the system × 0.08–0.12 when using plain water)
Pressure gauge – recommended (some models have an integrated digital pressure display)

Use PTFE tape or fiber sealant (Loctite 55, hemp + paste) for sealing threaded joints. Never use silicone on threaded joints – it is not designed for this purpose and will crack when tightened. Tighten all joints with a wrench – recommended torque for G ¾" is approx. 25–35 Nm, for G 1" approx. 40–55 Nm (depending on the body material).

Step 4 – Filling and venting the system

After mechanically completing the hydraulic installation, filling follows. The system is filled slowly, via a filling valve connected to the water mains; the maximum pressure during filling should not exceed 2.5 bar (we usually fill to 1.0–1.5 bar cold pressure).

While filling, open the air vent valves on all radiators and on the boiler body. The air is vented once water starts flowing out without bubbles. Then close the air vent valves and check the final pressure. Fill the cold system (at 10–20 °C) to 1.0–1.5 bar – once heated, the pressure will rise to approx. 1.5–2.0 bar (depending on the volume of the expansion vessel and the water content).

After filling, perform a pressure test: maintain a pressure of 1.5 times the maximum operating pressure for at least 30 minutes, and observe whether the pressure drops (leak) or rises (thermal expansion if there is a heat source nearby). Record the pressure test in a report.

Step 5 – Electrical connection

The electrical connection may only be carried out by an electrician with the appropriate qualification. Laypersons must not touch the electrical part of the boiler – this is not just a legal requirement but a matter of safety. An electric boiler with an output of 8–24 kW is comparable, in terms of electrical power, to a small workshop – a fault or electric shock incident can be life-threatening.

Electrical installation procedure in brief:

The boiler is powered from the distribution board via a dedicated circuit (not shared with other appliances)
The circuit is protected by a motor circuit breaker or a breaker with characteristic B/C, sized according to the boiler's power input
A safety disconnector (double-pole/four-pole) with a visible position must be installed upstream of the boiler – in practice this is often the circuit breaker itself in the distribution board or a separate disconnect box near the boiler
The PE, N, L1, L2, L3 conductors are connected to the boiler's terminal block according to the diagram in the manual – pay attention to the phase order
After connection, the insulation resistance (min. 1 MΩ) and the protective circuit (PE) resistance are measured with a testing device
Controller/thermostat: if the boiler does not have an integrated controller, an external room thermostat or control system is connected via the appropriate terminals (LOW VOLTAGE – usually 24 V DC or a voltage-free contact)

Step 6 – First start-up and setup

Before the first start-up, check:

Water pressure in the system: 1.0–1.5 bar (cold system)
All ball valves are open
The system is vented
The electrical connection is correctly wired and checked by an electrician
The safety valve is installed and has a free outlet
The expansion vessel is set to the correct pre-charge pressure before filling (usually 0.5–0.75 bar for most houses)

Turn on the boiler, set the desired heating water temperature (typically 60–75 °C for radiators, 35–45 °C for underfloor heating). Monitor the first 10–15 minutes of operation: rising pressure (acceptable up to 2.0–2.5 bar for a hot system), operation of the circulation pump (audible water circulation), no unusual noises, leaks, or warning signals on the display.

During the first start-up, we recommend venting the radiators once more – even after filling, some air always remains and is released once heated up. Air pockets are indicated by bubbling sounds in the system and uneven heating of the radiators.

For a more detailed procedure on setting up the controller and programming temperatures, read the article Setting Up and Programming the Control System of the ELÍZ Electric Boiler.

Special requirements when renovating: switching from gas to ELÍZ

More and more customers are replacing their gas boiler with an electric ELÍZ boiler – either for economic reasons (combination with photovoltaics) or because they are getting rid of the gas connection. This situation has several specifics:

Hydraulic connections: gas boilers mostly have G ¾" or G 1" connections, same as ELÍZ – there is usually no problem with a direct connection. Check the position of the connections – if they are arranged differently on the boiler, a short section of piping may need to be relocated
Electrical connection: this is usually the biggest problem. A gas boiler had a 230 V / 10 A connection (only for controls and the pump). An electric boiler needs 3×400 V and much higher power – a new cable must be run from the distribution board, and sometimes the main circuit breaker needs to be upgraded or the power input needs to be resolved with the electricity distributor
Chimney flue: after disconnecting the gas boiler, the chimney flue must be sealed – bricked up or fitted with a plastic cap with a seal, to prevent air backflow into the room
Gas connection: disconnection from gas must be carried out by an authorized gas company, and the connection must be capped off

You can find more on this topic in the article Converting from Gas Heating to an ELÍZ Electric Boiler: What You Need to Know.

Combination with photovoltaics and heat storage

More and more ELÍZ boiler installations are being done in combination with photovoltaics. In this case, so-called surplus energy management must also be addressed – i.e., how to make the boiler automatically consume the surplus from the PV panels. There are special controllers for this purpose (e.g., EMMA, Fronius Ohmpilot, and others) that smoothly control the boiler's output according to the available PV power.

ELÍZ electric boilers with continuous power modulation are ideally suited for this combination. Models with power stages (e.g., 2/4/6/8 kW) also work, but the correct stage must be set according to the available PV power. This topic is covered in more detail in the article ELÍZ Electric Boiler and Photovoltaics: Is the Combination Worth It.

Sizing the expansion vessel and safety valve

This is an area where many mistakes are made in practice – either it is underestimated (a small expansion vessel = frequent triggering of the safety valve) or an unnecessarily large vessel is purchased. The principle is simple:

The volume of the expansion vessel (V_exp) depends on:
– the total volume of water in the system (V_sys)
– the temperature range (difference between cold and hot system)
– the pre-charge pressure of the expansion vessel (p0)
– the maximum operating pressure (p_max)

Simplified practical formula for water (without antifreeze):
V_exp ≥ V_sys × 0.04 × [(p_max + 1) / (p_max - p0)]

Example: a system with a water volume of 80 l, p_max = 3 bar (safety valve), p0 = 0.5 bar:
V_exp ≥ 80 × 0.04 × [(3+1)/(3-0.5)] = 80 × 0.04 × 1.6 = 5.12 l
→ choose an 8 l expansion vessel (always choose the next larger size class).

Safety valve: must be certified (CE), set to a pressure not exceeding the boiler's maximum allowed pressure (usually 3 bar). The valve's DN must correspond to the boiler's output – DN 15 (G ½") is sufficient for boilers up to 50 kW. The discharge pipe from the safety valve must not be closed off and must lead to a safe location (collecting tray, drain, outdoors).

Water quality in the heating system

Electric boilers are more sensitive to water quality than gas boilers. The reason is simple: electric heating elements operate at a high surface temperature, and limescale on their surface leads to overheating, shortened service life, and possibly a failure of the safety thermostat.

Water requirements for the heating system with an ELÍZ boiler:

Total hardness: max. 3 °dH (ideally 1–2 °dH) for long-term operation without deposits
pH: 7.5–9.0 (for steel systems), 7.5–8.5 (for aluminum radiators)
Chloride content: max. 50 mg/l
Corrosion inhibitor: recommended, e.g. Fernox F1, Sentinel X100 or equivalent
Magnetic filter on the return before the boiler: highly recommended – captures magnetite (black sludge), the most common problem in old steel radiators

If your water is harder (more than 10 °dH, which is typical for most of Slovakia), we recommend using softened water before filling or adding an inhibitor with a demineralizing component. Some installers use distilled water as a base and add an inhibitor – this is the ideal solution for new systems.

Most common mistakes when installing the ELÍZ electric boiler

Over years of practice, I have seen hundreds of installations. These mistakes recur most often:

Missing safety valve or incorrect connection – the safety valve must be on the supply (boiler outlet), not on the return. If it is located after a shut-off valve, it is practically worthless
Small or incorrectly set expansion vessel – the pre-charge pressure must be set before filling, not after. Release the air from the vessel, set the pre-charge to 0.5–0.75 bar, then fill the system
Wrong cable cross-section or undersized circuit breaker – the electrician forgets that an 8 kW boiler power input = a real current of 11.5 A, and installs a 10 A breaker. The boiler then trips at full output
Absence of ball shut-off valves – during servicing you have to drain the entire system, because it is impossible to disconnect just the boiler
Air in the system after installation – inconsistent venting leads to gurgling radiators and uneven heating. Vent regularly during the first few weeks
Incorrect boiler position – a boiler tilted by more than 2–3 degrees can cause air not to be removed by the automatic air vent (air does not collect at the correct point)
Missing inspection report – without an inspection, the installation is not valid from the insurance company's point of view

Standards and regulations applicable to the installation

The installation of an electric boiler is governed by several standards and regulations. Here is an overview of the most important ones:

STN EN 12828 – heating systems in buildings, design of hot water heating systems (the main standard for hydraulic installation)
STN 33 2000-7-701 and other parts of STN 33 2000 – low-voltage electrical installations
STN 33 1500 – inspection of electrical installations
Decree of the Ministry of Economy of the Slovak Republic No. 508/2009 Coll. – safety of technical equipment, electrotechnical competence
Act No. 321/2014 Coll. on energy efficiency
Government Regulation of the Slovak Republic No. 173/2021 Coll. (implementation of the ErP Directive) – ecodesign requirements for heating appliances
Manufacturer's manual – always takes precedence over general standards where it is more specific

For new installations in family houses, notification to the building authority is usually required (a building permit is not needed if no structural modifications are made), and it may also be necessary to report the change to the electricity distributor if the tariff changes (e.g., switching to dual-tariff metering DD5 or DD6 for low-tariff heating).

Commissioning and documentation

After completing the installation and successful start-up, the installer should hand over complete documentation to the customer:

Pressure test report of the hydraulic system
Electrical installation inspection report (from the electrician)
First start-up and setup report (service report)
Completed boiler warranty card
Operating and installation manual (original)

Without complete documentation, the warranty card is not valid and the insurance company may refuse to cover damage caused by the boiler. Always request these documents from the installer – it is your right and the installer's obligation.

If you are planning to combine the boiler with photovoltaics or want to optimize the settings for energy savings, also check out the article ELÍZ Electric Boiler and Photovoltaics: Is the Combination Worth It and Setting Up and Programming the Control System of the ELÍZ Electric Boiler.

Practical field experience: what practice says

From dozens of jobs where I was either directly present at the installation or dealt with subsequent faults, I can share a few insights that you won't find in the manual:

Underfloor heating and ELÍZ: the combination is trouble-free, but don't forget that underfloor heating has a significantly larger water volume in the system (pipes in the screed) than a radiator system. This affects the sizing of the expansion vessel (larger volume = larger vessel) and also the heating time – the floor doesn't heat up in 20 minutes, but rather over 3–6 hours. Setting up the controller (setback, program) is much more important here than with radiators.

Boiler in a humid room: installation in a laundry room or bathroom is technically possible (IP44 protection is common), but you need to check the specific model's protection rating in the technical data sheet. In very humid spaces (steam sauna, swimming pool hall), an electric boiler is not installed – titanium or plastic boilers designed for specific environments are used for that purpose.

Old cast-iron radiators and ELÍZ: cast-iron radiators have a large water volume and high thermal inertia. This is good for an ELÍZ boiler – the system acts as a storage buffer. The problem may be the poor internal condition of old radiators (corrosion, sludge). Before installing a new boiler, we always recommend chemical flushing and a magnetic filter.

Dual-tariff metering: if you are considering switching to a low-tariff rate (e.g., DD5 – 8 hours low tariff, DD6 – 16 hours), heat storage must be addressed. The ELÍZ boiler itself has no storage tank – you must connect a buffer tank (200–500 l), which is charged during the low tariff period and releases heat during the high tariff period. This significantly changes the hydraulic scheme of the installation and requires a mixing valve and an additional pump.

If you're wondering which specific model to choose for your house, check out the ELÍZ EQH 802 – a popular model with an output of 8 kW, suitable for small to medium-sized houses (up to approx. 100 m² of well-insulated floor area) and compatible with most common control systems. For guidance on choosing the correct output, read the article What Output of the ELÍZ Electric Boiler Do I Need for My House.

Frequently Asked Questions (FAQ)

Can I install the ELÍZ electric boiler myself, without an electrician?

You can carry out the hydraulic part (hanging the boiler, piping, expansion vessel, safety valve) yourself if you have the necessary knowledge and experience with plumbing work. The electrical connection (230 V or 3×400 V, circuit protection, RCD) must be carried out by a person with electrotechnical qualification at least according to §22 of Decree 508/2009 Coll. – and an inspection report must be issued. Without an inspection report, the installation is not legal, and the insurance company may refuse to cover damage. So: hydraulics yes (at your own risk), electrics strictly by a professional.

What cable cross-section do I need for an 8 kW boiler (e.g., ELÍZ EQH 802)?

For the ELÍZ EQH 802 with an 8 kW power input at 3×400 V, the rated current is approx. 11.5 A. The minimum CYKY cable cross-section is 4×2.5 mm² for runs up to 20 m, and 4×4 mm² for longer runs (30–40 m). Choose a 3×16 A circuit breaker with characteristic B or C, preceded by a 25–40 A / 30 mA RCD. Always also follow the specific requirements of the manufacturer in the technical data sheet.

Do I need to connect an expansion vessel and safety valve to the ELÍZ electric boiler if they are already integrated?

It depends on the specific model. Some ELÍZ models have an integrated safety valve and air vent, but not an expansion vessel – this must always be external and sized for the entire volume of the heating system, not just the boiler's volume. An integrated safety valve is sufficient if correctly sized (pressure, DN) – check the values in the technical data sheet. In any case, you must comply with STN EN 12828, which requires external safety devices.

What is the maximum operating pressure of the ELÍZ boiler and what happens if I exceed it?

Most ELÍZ boilers have a maximum operating pressure of 3 bar. If this pressure is exceeded (e.g., due to a too-small expansion vessel or a clogged safety valve), the safety valve opens and releases some water, thereby reducing the pressure. If the safety valve is blocked or malfunctioning, the boiler may be damaged, hoses may burst, or in extreme cases an accident may occur. Therefore, the proper functioning of the safety valve is critically important – test it manually once a year (turn the manual wheel, water must flow out).

Can I connect the ELÍZ boiler to an existing heating circuit with a gas boiler as a backup?

Yes, this is possible – a so-called cascade or backup connection. Both boilers must be connected so that they cannot operate simultaneously (the control system switches between them), or via a hydraulic separator (hydraulic manifold). Such a connection is technically more demanding and requires correct controller setup. If you are working with a gas boiler, both sources must be properly protected and sized from a safety standpoint – consult a specialist.

How long does it take to install the ELÍZ electric boiler when replacing an old boiler?

Under favorable conditions (the electrical connection is already prepared, the existing piping is in good condition), the actual installation of the boiler takes 4–8 hours – 2–3 hours for the hydraulic part, 1–2 hours for electrical work, 1–2 hours for start-up, venting, setup, and documentation. If piping needs to be relocated, a buffer tank installed, or old piping addressed, expect realistically 1–2 days of work. The longest part of the whole project is usually preparing the electrical connection – from ordering to completion, this can take several weeks.

Conclusion: installation is just the beginning

Proper installation of the ELÍZ electric boiler is the foundation – but a foundation that must be solid. Every compromised installation will show sooner or later: pump failure (air in the system), burnt-out heating element (limescale), boiler shutdown (oversized or undersized circuit breaker), or in a worse case, an insurance claim without compensation.

Invest in the right components (expansion vessel, magnetic filter, corrosion inhibitor), require an inspection report from your electrician, and a service report from your installer. The ELÍZ boiler is a reliable unit with a long service life – but only if it is installed and operated correctly.

If you are considering a specific model, check out the ELÍZ EQH 802 or the entire ELÍZ boiler category. For more information on choosing, calculating output, or troubleshooting, read other articles in our Knowledge Center – for example Common Faults of the ELÍZ Electric Boiler and Their Solutions or Maintenance and Service of the ELÍZ Electric Boiler: How to Extend Its Service Life.

Have a question on this topic?

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