Installation of SLOKOV Electric Boiler – Procedure and Requirements

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Installation of the SLOKOV electric boiler – comprehensive guide, requirements and practical tips

The SLOKOV electric boiler is among the popular solutions for central heating in family houses, recreational cottages and apartments. Its installation is simpler compared to gas boilers, but by no means trivial – it requires compliance with a whole range of technical conditions, regulations and manufacturer recommendations. In this article we will go through the entire installation process step by step: from preparing the location through hydraulic and electrical connections to the first start-up and system commissioning. If you have not yet decided which boiler power is suitable for you, I recommend reading the article What power of SLOKOV electric boiler do I need for my house? – the power is the first thing you need to know before you start to deal with the installation.

1. Before installation: what you need to prepare

Preparation is half the success. In practice, we see that most problems during the installation of electric boilers do not arise during the installation itself, but because the customer or installer has not carefully planned the previous steps. Before the arrival of the installer – or before you start the work yourself – go through this list:

Boiler and power selection: Make sure you have chosen the correct model. In the range you will find SLOKOV SL ELEKTRO 10 (10 kW), SLOKOV SL ELEKTRO 15 (15 kW), SLOKOV SL ELEKTRO 18 (18 kW), SLOKOV SL ELEKTRO 23 (23 kW) and SLOKOV SL ELEKTRO 26 (26 kW). Each of these models has different requirements for power supply and protection.
Approval by the electricity distributor: Boilers with a power of 10 kW and more must be reported to the distribution company (ZSD, VSD, SSE). Without the operator's consent, you must not connect the boiler to the electrical grid.
Electrical installation inspection: An old wiring in the house may not meet the requirements for current load. An electrician must assess the condition of the distribution board, cable cross-sections and the grounding system.
Construction work: If you hang the boiler on the wall, the wall must support its weight (including water). The wall should be masonry or concrete; drywall without substructure is not sufficient.
Materials for the hydraulic circuit: Expansion tank, safety valve, thermometer and manometer (or combined thermomanometer), air vent valves, ball valves, dirt filters.

2. Requirements for the electrical installation

The electrical installation is crucial during the installation of the SLOKOV boiler and must be carried out or checked by a person with the appropriate qualification according to Ordinance No. 508/2009 Coll. (electrician with a certificate of at least §21). After completing the work, an inspection report must be prepared.

2.1 Supply voltage and frequency

All models of the SL ELEKTRO range are intended for three-phase supply 3×400 V / 50 Hz. This is important to emphasize, because single-phase connection is not possible – three-phase supply must be ensured even before the boiler installation itself. Check whether your house or apartment has a three-phase supply from the main circuit breaker.

2.2 Dimensioning of the supply cable and circuit breaker

The cross-section of the cable and the value of the circuit breaker depend on the rated power of the boiler. Below are approximate values that are based on common practice (always verify the exact values in the technical documentation of the boiler and in the electrical installation project documentation):

Model	Power	Rated current (A)	Recommended circuit breaker (A)	Minimum cable cross-section
SL ELEKTRO 10	10 kW	~14,4 A	3×16 A	4× 2,5 mm²
SL ELEKTRO 15	15 kW	~21,7 A	3×25 A	4× 4 mm²
SL ELEKTRO 18	18 kW	~26,0 A	3×32 A	4× 6 mm²
SL ELEKTRO 23	23 kW	~33,2 A	3×40 A	4× 10 mm²
SL ELEKTRO 26	26 kW	~37,6 A	3×40 A	4× 10 mm²

The cable must be laid in protected pipes or conduits along its entire route, so that it does not pass through areas with high temperature. In most cases, a cable type CYKY-J 5×(cross-section) is used – i.e. a five-core cable (3× phase + neutral + PE protective conductor). A fixed connection of the cable to the boiler is mandatory – a plug is not acceptable.

2.3 Residual current device (RCD) and overvoltage protection

The electric boiler must be protected by a residual current device with a rated tripping differential current of 30 mA (type A or B). Type B is recommended for devices with frequency converters, but for SLOKOV SL ELEKTRO boilers (resistive coil) type A is sufficient. Overvoltage protection of class T2 is optional, but I strongly recommend its installation in areas with frequent thunderstorms – replacement of heating elements after overvoltage damage is expensive.

3. Requirements for installation location and boiler room space

The electric boiler SLOKOV SL ELEKTRO is designed as a wall-mounted appliance. Wall mounting is standard, but there are several principles that determine the selection of a suitable location:

Protection class IP: The boiler has an IP20 rating – meaning it is not intended for direct use in humid areas (bathrooms, laundries). In the case of installation in a basement or garage, ensure that the wall is dry and the air is not humid.
Ambient temperature: The recommended ambient temperature for operation is +5 °C to +40 °C. In unheated winter spaces, freezing of water in the boiler is a risk if the anti-frost protection is not active.
Installation height: The bottom edge of the boiler should be at least 1.5 m above the floor – for easy access to the expansion tank and safety valves. The top edge should not exceed 2.0 m in height to ensure the control elements are easily accessible.
Distance from flammable materials: There must be at least 200 mm of free space on each side of the boiler. At least 400 mm in front of the boiler for service access to the electrical cabinet and hydraulic valves.
Wall strength: The wall must be able to support the weight of the boiler in its operating condition. An empty boiler weighs 18–28 kg depending on the model, and more when filled with water. On a masonry wall (brick, concrete), the boiler is fixed using the supplied anchors and screws.

4. Hydraulic connection of the boiler to the heating system

The hydraulic side of the installation may be less stressful for a layperson than electricity, but even here, mistakes are hard to correct without draining the entire system. Let's look at the procedure and mandatory components.

4.1 Mandatory safety valves

Neither the manufacturer nor STN EN 12828 accept operation without the following components:

Pressure relief valve: Set to the maximum operating pressure of the system (usually 2.5 bar or 3 bar). It must be placed directly on the boiler circuit, without any shut-off valve between the boiler and the valve. The discharge pipe of the pressure relief valve must be led to a safe area (sewage, catch basin).
Expansion tank: A membrane pressure expansion tank compensates for changes in water volume when heated. The tank volume is dimensioned according to the total water volume in the system and the operating pressure. Roughly: for a system volume of 100 liters and an operating pressure of 1.5 bar, a tank of at least 12–18 liters is required.
Thermometer and manometer: Allow visual monitoring of temperature and pressure in the system without the need to access the boiler's service menu. Many installers today install a combined thermomanometer (so-called thermomanometer) on the boiler's outlet pipe.
Air vent valve: Automatic or manual – at least one piece at the highest point of the circuit. Air in the system causes noise, worsens circulation, and can damage the circulation pump.
Filter/magnetic dirt separator: A slot filter insert protects the circulation pump and at the same time the boiler's heating elements from deposits. I recommend inserting it on the return before the boiler's inlet.

4.2 Boiler connections – inlet and outlet

SLOKOV SL ELEKTRO boilers have hydraulic connections with an external thread of G 1" (inch thread). Pipes in the home distribution are usually 3/4" or 1" – reductions must be used correctly (pay attention to the direction of flow). The standard connection is: boiler outlet (hot water) → heating radiators → return → filter → boiler. The circulation pump is usually an integrated component of the boiler, but in the case of larger buildings or floor heating with higher pressure losses, an additional external pump may be required.

4.3 Filling and air venting of the system

After completing the hydraulic installation and before the first start-up, the system is filled with water. A valve (filling valve) installed in the return line is used for filling, ideally with a pressure gauge. Filling is carried out slowly to allow air to escape through the venting valves. The target filling pressure for a cold system is usually 1.0–1.5 bar. After filling and the first heating, the pressure will increase to the operating range of 1.5–2.0 bar – this is normal.

Important: water for heating systems should not be hard (above 15 °dH). Hard water causes limescale deposits on heating coils, which leads to overheating and reduced lifespan. If hard water is present in your area, use a water softener or desalinator in the filling circuit, or fill the system with deionized water with a corrosion inhibitor.

5. Electrical connection of the boiler – step-by-step procedure

This section is reserved for a qualified electrician with a valid qualification. We describe it for the sake of transparency and customer awareness, not as a guide for independent work without professional supervision.

Disconnect from the power supply: Before any work on an electrical device, switch off the circuit breaker at the distribution board, lock it and label it. Use a voltmeter to confirm that it is indeed de-energized.
Routing the cable: Route the power cable in a conduit (PVC or metal) from the distribution board to the boiler. The cable must not pass through areas with temperatures above 70 °C. At the point of entry into the boiler, the cable must be protected by a cable gland (cable bushing).
Electrical connection in the boiler terminal box: The SLOKOV SL ELEKTRO boiler has an internal terminal box accessible after removing the cover of the electrical part. Phases L1, L2, L3 are connected to the corresponding terminals (marked in the diagram in the manual), the neutral conductor N to terminal N and the protective conductor PE to terminal PE (green-yellow). The sequence of phases is not critical (the boiler has no rotating machines sensitive to phase sequence), but check the assignment of PE!
Connection of the control: The boiler is equipped with an input for an external thermostat or room thermostat (non-powered contact). Terminals T1 and T2 (shorted for direct operation) must be connected through the relay output of the thermostat. A more detailed connection of the thermostat and control is described in the article Connecting the SLOKOV electric boiler to an existing system.
Check before switching on: Check that all terminals are securely tightened, the terminal box cover is in place, the hydraulic part is filled, the system pressure is 1.0–1.5 bar and the safety valve is free (not manually closed).
First switching on: Insert the circuit breaker, test the RCD with the TEST button (it must trip), and then switch the RCD back on. Switch on the boiler with the switch. The display should show the temperature of the circuit and the system pressure.

6. Boiler settings after start-up

After the first start-up, it is necessary to set up the boiler correctly. The basic parameters that the installer usually sets during the first start-up:

Maximum temperature of the supply water: The recommended value for a radiator system is 70–75 °C, for floor heating 35–45 °C (depending on the system design). The SLOKOV SL ELEKTRO boiler allows you to set this limit in the regulator menu.
System pressure: After heating the system to operating temperature, check the pressure – it should be in the range of 1.5–2.0 bar. If it is lower, add water. If it is consistently higher than 2.5 bar, check the expansion tank (the pre-charge pressure must be set correctly).
Power stages: SLOKOV SL ELEKTRO boilers usually have adjustable power stages (e.g. 1/3, 2/3, 3/3 of the nominal power). During the first start-up in a larger family house, set the maximum power and monitor the warm-up time. If the house heats up too quickly and the thermostat turns off the boiler very soon, you can reduce the power – this saves electricity.
Frost protection: Activate the frost protection function, which starts the pump and heating if the water temperature drops below the set value (usually 5–7 °C). This function is essential for boilers in holiday cottages or unheated technical rooms.
Night mode and programming: Details on setting up time programs and using the night rate for DHW heating can be found in the article Electric boiler SLOKOV and night rate – how to set up heating.

7. Typical installation problems from practice

Over the years of experience with customer installations of electric boilers, certain errors repeat themselves, unnecessarily increasing the cost of work or shortening the life of the equipment. Here are a few examples from real cases:

7.1 Inappropriate cable cross-section

A customer in the Banská Bystrica region ordered a SLOKOV SL ELEKTRO 23 and had the boiler installed by an electrician who had previously only done residential installations. A 4×4 mm² cable for a 23 kW boiler? That's not enough. The cable was overheating at full load (33 A), the 3×25 A circuit breaker was tripping, and worse – the cable started to soften in the socket. The correct solution: a 5×10 mm² cable and a 3×40 A circuit breaker. Rewiring the entire cable route (about 12 meters) cost the customer more than the original installation.

7.2 Missing filter on the return

An older house with steel radiators and rusty water – a classic case. The boiler was connected without a filter, and after two years the circulation pump clogged with magnetite (iron oxide) and stopped circulating. The boiler went into a high-temperature fault. Replacing the pump costs hundreds of euros. Installing a filter basket costs 15–30 euros in materials and half an hour of labor. The conclusion is clear.

7.3 Too small expansion tank

A cottage with a system volume of 180 liters and a standard 8-liter expansion tank from "some" warehouse – that's a recipe for regular venting through the safety valve. The pressure during heating jumped to 3.5 bar and the safety valve was leaking water onto the floor. When we dimensioned the correct tank (24 liters), the problem disappeared. Calculating the expansion tank volume is part of the basic hydraulic design – don't underestimate it.

7.4 Boiler mounted on a drywall wall without substructure

No, even if "it looks solid". The weight of the SLOKOV SL ELEKTRO 26 boiler with water easily exceeds 35 kg. A drywall wall without anchoring profiles is not designed for point loading. The boiler must be anchored either into a masonry load-bearing wall or to a steel substructure attached to the building's load-bearing elements.

8. Inspection, documentation and handover of the installation

The installation of the boiler is not complete with the handover of the boiler room keys. Before commissioning for permanent operation, the following documentation must be available:

Electrical installation inspection report: Prepared by an inspection technician. It includes the results of insulation resistance measurements, earth resistance, correct PE connection, RCD testing, and overall inspection results. Without an inspection report, you must not permanently connect the boiler to the grid.
Commissioning protocol for the boiler: Some manufacturers condition the warranty on the existence of this protocol. SLOKOV has warranty conditions tied to professional installation – check the warranty conditions at the time of purchase.
Electricity distributor approval: As we mentioned at the beginning, for boilers with a power of ≥ 10 kW, prior notification/approval from the distributor is mandatory. Keep this document.
User operating instructions: The customer must be familiarized with boiler operation, temperature settings, response to error codes, shutdown procedures (e.g., during long absences in winter), and contact information for the service center.

Further information on recurring faults and service interventions can be found in the article Common faults of SLOKOV electric boilers and their solutions and Maintenance and service of SLOKOV electric boiler.

9. Choosing the right model for your installation

If you are just planning the installation and have not yet selected a model, here is a quick summary of which models are suitable for which range of applications:

SLOKOV SL ELEKTRO 10 – smaller family house (up to 80 m²) or a well-insulated recreational cottage, simpler cable and 3×16 A protection.
SLOKOV SL ELEKTRO 15 – standard family house (80–120 m²) with average insulation or a combination of heating and DHW.
SLOKOV SL ELEKTRO 18 – larger family house (120–160 m²) or older houses with poor insulation up to 120 m² in size.
SLOKOV SL ELEKTRO 23 – large family house or two-generation family villa (160–200 m²), or a combined system with a DHW tank.
SLOKOV SL ELEKTRO 26 – the highest power in the range, suitable for buildings over 200 m² or more energy-intensive systems, requires the most common supply cable 5×10 mm².

A detailed procedure for calculating the required power is the subject of the article What power of SLOKOV electric boiler do I need for my house – including specific formulas and calculation examples for different types of buildings.

10. Installation of SLOKOV SL ELEKTRO vs. other electric boilers – installation specifics

Customers sometimes ask us whether the installation of SLOKOV is more complex or simpler compared to the competition. From practice, we can say that the SLOKOV SL ELEKTRO series is relatively straightforward to install thanks to good access to the terminal block, clear documentation, and unified hydraulic connections G 1". The boiler also has a built-in circulation pump, which eliminates the need for an external pump in single-loop systems. A more detailed comparison with competing brands can be found in the article SLOKOV SL ELEKTRO vs other electric boilers – comparison.

Most frequently asked questions about the installation of SLOKOV electric boiler

Can I install the SLOKOV boiler myself, without an electrician?

The hydraulic part of the installation (mounting the boiler, connecting the pipes, fittings, expansion tank) can be handled by a skilled and technically capable homeowner. However, the electrical connection – supply cable, boiler terminal block, circuit breaker and RCD in the distribution board – must be performed by a person with a valid electrical qualification according to Decree No. 508/2009 Coll., at least §21. After completion, an inspection report must be prepared. Without it, we do not recommend operating the boiler and in most cases it also voids the warranty claim.

How long does the installation of the SLOKOV SL ELEKTRO boiler take?

An experienced two-person team (installer + electrician) can complete the full installation including hydraulics, electrical work, and first start-up in 4–7 hours, depending on the scope of work and the condition of the existing wiring. If significant changes in the distribution board or part of the piping are needed, it may take a full day. When replacing an old electric boiler with a new SLOKOV (same connections), 2–3 hours are realistic.

Do I have to notify the electricity distributor about the boiler? Even 10 kW?

Yes, an electric boiler with a power of 10 kW or more must be notified to the distribution company (ZSD/VSD/SSE) before connection. You follow the procedure of the respective distributor in your region – usually it is enough to fill out an application form for connecting the appliance and attach the technical data sheet of the boiler. The distributor checks whether the existing network can handle the added load and issues approval. Without this approval, you could have problems with insurance claims in case of an issue.

What pressure should I set when filling the system?

When the system is cold (before heating), set the filling pressure to 1.0–1.5 bar. After heating to operating temperature, the pressure will increase, typically to 1.5–2.0 bar. If the pressure exceeds 2.5 bar after heating, check the correct pre-charge pressure in the expansion tank (it should be set to a value corresponding to the boiler's height in the system, typically 0.5–1.0 bar for ground-level installations). The safety valve reacts at 3.0 bar – this is an emergency safety device, not a regulating element.

What to do if the boiler displays an error code after turning it on?

First steps: check the manual – each error code has a specific description. The most common codes during installation of a new boiler are related to low water pressure (add water), temperature sensor failure (check the sensor connection), or a blocked pump (vent it). If the error persists, contact the service center. More about error codes and their solutions can be found in the article Common faults of SLOKOV electric boilers and their solutions.

Is building inspection or notification to the building authority required for the installation of an electric boiler?

For replacing an old boiler with a new electric boiler in an existing boiler room, a building permit is usually not required – it is considered equipment maintenance. However, if you are creating a new boiler room, changing the room layout, or installing a boiler as part of a larger renovation, it is advisable to consult with the local building authority. The electrical part is always subject to inspection regardless of building permits.

Conclusion

The installation of the electric boiler SLOKOV SL ELEKTRO is a manageable task that will provide years of reliable heating, provided that proper preparation and procedures are followed. Let's summarize the key points: correctly dimension the power and electrical installation, do not forget the required hydraulic fittings (safety valve, expansion tank, filter), entrust the electrical part to a qualified person, and ensure regular maintenance. A properly installed boiler will serve you without problems for decades – and if something breaks down later, having an overview of what and how was installed will save service technicians hours of searching.

If you are considering the purchase of a specific model or have questions about selecting the power and suitability for your home, browse our full range of electric boilers SLOKOV or read the article How to choose an electric boiler SLOKOV – what to pay attention to and Common questions about SLOKOV electric boilers in our Knowledge Centre.

Do you have a question about this topic?

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