Frequently Asked Questions about SLOKOV Electric Boilers

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Common questions about SLOKOV electric boilers – a complete overview for buyers and owners

SLOKOV electric boilers are products that customers approach with many questions – and quite rightly so. It is a relatively simple technology, but during the selection, installation and operation, dozens of practical questions arise that are not always clear from the technical specifications or the manual. In this article, I have gathered the questions that most frequently appear in practice – either from customers before purchase, or from installers and homeowners during installation or operation. The answers are intended to be factual and specific, without unnecessary simplification.

What is SLOKOV SL ELEKTRO and how does it work?

SLOKOV SL ELEKTRO is an electric flow-through boiler intended for heating buildings via a classic hot water heating system – that is, radiators, floor heating or a combination of both. It is not a direct heating device (convector, infrared panel), but a full-fledged boiler that heats the heating water and pumps it into the distribution system in the same way as a gas or oil boiler would.

The principle of operation is straightforward: electrical energy passes through resistance (spiral) heating elements immersed in water, the water is heated, and the circulation pump distributes it to the heating system. The boiler contains its own control system – a water temperature regulator at the output, protections (against overheating, low pressure, freezing) and usually also an interface for connecting an external thermostat or busbar control.

It is important to understand that SLOKOV is not a water heater – it does not directly heat hot water for domestic use (HWU), unless it is supplemented by an external storage heater or an indirect water heater. Some installations include this combination, but the boiler itself serves exclusively the heating circuit.

For which houses and buildings is SLOKOV suitable?

This is one of the most common questions and also the question where people make the most mistakes. SLOKOV SL ELEKTRO is suitable mainly where natural gas is not available and where the installation of a condensing gas boiler would be too expensive or complicated – that is, in areas without a gas connection, in mountain chalets, recreational buildings, and also in new buildings where the investor deliberately decided on a gas-free household.

In terms of performance, SLOKOV is available in five power variants:

SLOKOV SL ELEKTRO 10 – 10 kW, suitable for smaller well-insulated houses up to about 80–100 m²
SLOKOV SL ELEKTRO 15 – 15 kW, a typical family house 100–140 m² with average heat loss
SLOKOV SL ELEKTRO 18 – 18 kW, a larger family house or an older building with poor insulation
SLOKOV SL ELEKTRO 23 – 23 kW, a larger object or a combined system with HWU heating
SLOKOV SL ELEKTRO 26 – 26 kW, larger properties, apartment buildings or commercial spaces

The basic rule applies: better insulated house = lower boiler power. For a new build with a heat loss of 50–60 W/m² and an area of 120 m², a boiler of about 8–10 kW is sufficient, while for an older house without insulation of the same area, a power of 15–18 kW may be required. More about calculating the required power can be found in a separate article What power of SLOKOV electric boiler do I need for my house?.

Do I need a special electrical connection for SLOKOV?

Yes – and this is one of the issues that should be addressed before purchasing the boiler. Models with a power of 10 kW can be connected to a single-phase network of 230 V, but in most cases, a three-phase connection of 400 V (3×230 V / TN-S) is recommended and required. The reason is simple: at a power of 15–26 kW, the single-phase load would be unacceptable – the current load on one phase would exceed 100 A, which is beyond the capacity of standard circuit breakers in households.

This practically means:

For 10 kW models: check whether the household has sufficient connection capacity (minimum 16 A circuit breaker, recommended 32 A, single-phase or three-phase)
For 15–26 kW models: mandatory three-phase power supply of 3×16 A to 3×40 A depending on the power
In most cases, it is necessary to request the electricity distributor (VSD, ZSD, SSE) to increase the main circuit breaker or to sign a new contract – this can take 4–12 weeks
For more favorable operating costs, it is advisable to simultaneously request an electric heating tariff (so-called D rate, HDO – night/dual-rate tariff)

The electrical connection of the boiler must be carried out by an electrician with the appropriate qualification according to Decree No. 508/2009 Coll. The boiler requires its own circuit breaker (not shared with other appliances), protection against short circuits and overvoltage, as well as a properly dimensioned cable. Typically, CYKY 5J 4×4 mm² or 5×4 mm² is used for three-phase connections of medium power.

What is the real efficiency of the SLOKOV electric boiler?

Electric resistance boilers have practically 100% efficiency – the entire electrical energy is converted into heat in the water. This is a physical fact, not a marketing claim. No energy escapes through the chimney (there is no chimney at all), no combustion gases are formed, and no condensation causes losses. From the perspective of energy conversion into heat, SLOKOV SL ELEKTRO is essentially perfect.

What complicates things – and what should be considered in comparison with a heat pump or a gas boiler as a whole – is COP (Coefficient of Performance) versus direct energy conversion. A heat pump has a COP of 3–5, which means that from 1 kWh of electricity, it gains 3–5 kWh of heat. An electric boiler has a COP of 1.0 – always. Therefore, SLOKOV is economically advantageous especially in combination with a cheap night tariff (HDO), with photovoltaic panels, or in buildings where the heating season is short and the installation of a heat pump would not be economically justified.

More about the cooperation of the boiler with the night tariff can be found in the article Electric boiler SLOKOV and night tariff – how to set up heating.

Can I connect SLOKOV to floor heating?

Yes, without any problems. SLOKOV SL ELEKTRO works with all types of heating systems – radiators, floor heating, ceiling heating, and fancoils. It is important to set the correct temperature of the output water according to the type of heating system:

Radiators (steel panel or cast iron): output temperature 55–80 °C
Floor heating: output temperature 30–45 °C (maximum 55 °C)
Combined system: hydraulic regulation is necessary, usually a three-way valve and a mixer for the floor heating circuit

The boiler has an adjustable range of output water temperature – usually from 25 to 85 °C. For floor heating, a lower required temperature must be set, and in some cases, a thermostatic valve (three-way or two-way mixing valve) should be installed before the floor heating distributor to prevent the water temperature from exceeding the maximum for the floor. Without this measure, there is a risk of cracking parquet or tiles and damaging PE-RT or PEX pipes.

Do I need an expansion tank and a safety valve for SLOKOV?

Yes, absolutely. This is a technical requirement, not a recommendation. Every closed hot water heating system must have:

Expansion tank – captures the increase in water volume when heated (water expands by approximately 2.5% when heated from 10 °C to 80 °C). Without an expansion tank, the pressure in the system would rise uncontrollably and lead to the opening of the safety valve with every boiler start-up.
Safety valve – protects the system from excessive pressure. It is usually set to 2.5–3 bar. It opens only in an emergency. It must be connected to a safe discharge (not just into the air).
Pressure gauge – allows visual pressure monitoring. The pressure of the cold system should be 1.0–1.5 bar, and when hot, 1.5–2.5 bar.
Automatic air vent – at the highest point of the system or directly on the boiler. Air in the system reduces performance, causes noise, and accelerates corrosion.

SLOKOV SL ELEKTRO has some of these components integrated (circulation pump, pressure gauge, safety valve), but the expansion tank must usually be installed externally – its volume depends on the total water volume in the system and the operating temperatures. Approximately: for a water volume of 50 liters in the system and a temperature of 80 °C, an expansion tank with a volume of about 8–12 liters is required.

How long does it take to heat a house from a cold state?

This is a question I get very often from owners of chalets and recreational properties. The answer depends on several variables, but I'll give a concrete example from practice: a chalet of 90 m², masonry, poor insulation, SLOKOV SL ELEKTRO 15, outside temperature -5 °C, indoor temperature 8 °C, desired 20 °C.

The heat loss of the building is approximately 9–10 kW. The boiler delivers 15 kW. Of that, approximately 10 kW is used to cover heat losses, and the remaining 5 kW is used to heat the building structure and air. Heating the air by 12 °C in 90 m² (volume 225 m³) takes only about 30–45 minutes, but heating the walls, floor, and furniture (the building's thermal capacity) takes 4–10 hours. Result: after 2–3 hours, the air is warm, but the walls are still cold – the building will feel comfortable only after 6–8 hours of continuous heating.

Practical conclusion: if you visit your chalet regularly, it pays off to set up frost protection (e.g., 10 °C) and let the boiler run at a low temperature even during your absence. This is more cost-effective than heating a frozen building to a comfortable temperature every weekend.

What does "cascading" power control mean and does SLOKOV have it?

Cascading or stepped power control means that the boiler does not switch on all heating elements at once, but switches them on gradually – according to the current heat demand. SLOKOV SL ELEKTRO has stepped power control: for example, a model with 15 kW output may have 3 elements of 5 kW each, which are switched on one by one. The result is:

Reduced peak demands on the electrical grid (not 15 kW at once, but 5 → 10 → 15 kW gradually)
Extended lifespan of heating elements (fewer thermal shocks)
Smother regulation of heating water temperature

The number of steps depends on the specific model. The control electronics automatically switch the elements on based on the output water temperature sensor – the user does not need to set anything manually, it is enough to enter the desired output water temperature or connect a room thermostat.

How is SLOKOV controlled – thermostat, room regulator, smart control?

SLOKOV SL ELEKTRO has an input for an external thermostat (typically a two-wire no-voltage contact) or an analog input 0–10 V (some models). This allows connecting practically any room thermostat – mechanical, digital, weekly programmable, or WiFi smart thermostat.

The most practical solutions from practice:

Simple bimetal thermostat – the cheapest, no programming, switches the boiler on/off based on room temperature. Suitable for chalets and simple applications.
Weekly digital thermostat – allows programming different temperatures for individual days and hours. Recommended for regular family homes.
WiFi thermostat (e.g., Honeywell T6R, Salus iT500, Shelly TRV) – control via smartphone from anywhere. Ideal for recreational properties, where you want to turn on heating before arrival.
Bus regulation – OpenTherm or proprietary protocol; allows smooth modulation of boiler power instead of just switching ON/OFF.

The boiler also has its own built-in thermostat for output water temperature – so it maintains the set water temperature even without an external thermostat. However, without a room thermostat, the boiler cannot react to the actual temperature in the rooms – it only maintains water temperature, which is less efficient.

Is the installation of SLOKOV complicated? Can I do it myself?

The hydraulic part can be connected by a skilled DIY enthusiast – it is standard pipe work, threaded connections, sealing. However, the electrical connection must be done by a qualified electrician, otherwise the warranty is void and property insurance may refuse to cover damages in case of fire or electric shock. This is not an administrative formality – at power levels of 15–26 kW, it involves working currents of 20–40 A, where an error can have fatal consequences.

Installer's view: most customers can prepare the location, mount the boiler on the wall, and connect the supply and return. They leave the electrical work to an electrician and welcome a service technician visit at the first start-up – who checks the pressure, purges the system, sets the boiler parameters, and explains the operation to the customer. A more detailed installation procedure is described in a separate article Installation of SLOKOV electric boiler – procedure and requirements.

What is the noise level of the SLOKOV boiler during operation?

Electric boilers are generally quiet. SLOKOV SL ELEKTRO produces only a soft hum from the circulation pump – typically 25–35 dB(A) at nominal speed, which is comparable to the quiet hum of a refrigerator. The heating elements themselves do not produce any noise. Any knocking in the pipes is caused by thermal expansion of the pipes (typical with rapid heating), not the boiler itself.

In practice: a boiler located in a technical room or bathroom is almost inaudible in a residential environment. Noise problems occur only with air bubbles in the system (then the pump makes a characteristic knocking sound) – the solution is to purge the system.

What is the operating cost – real cost example

This is the number one question when deciding to buy. Let's do a simple calculation for a typical family home:

House 120 m², well insulated (new build), heat loss 8 kW
Heating season: 200 days (October–April), average outside temperature during the season -2 °C
Average boiler power used: approximately 40–50 % of rated power = 4–5 kW continuously
Annual electricity consumption: 4.5 kW × 24 hours × 200 days = 21 600 kWh
Electricity price with a two-rate tariff (HDO): night tariff approximately 0.09–0.12 €/kWh (2024, Slovakia)
Annual costs: 21 600 × 0.10 = approximately 2 160 € per year

In older homes without insulation, the costs can be double. For seasonal use (e.g., 60 days per year) the costs will be significantly lower. An important factor is the selection of the right electricity tariff – with electric heating, distributors have special tariffs (D6, D7, D8), where night-time electricity is significantly cheaper. This is discussed in detail in the article Electric boiler SLOKOV and night tariff – how to set up heating.

How long will the SLOKOV boiler last – lifespan and service

Electric boilers have a lifespan of 15–25 years with proper operation and regular service. The main components that wear out:

Heating elements (spiral resistive) – lifespan 8–15 years, depending on water quality (limescale, corrosion). With hard water, regular inspection or softening is advisable.
Circulation pump – lifespan 10–20 years, depending on water quality and frequency of operation. Replacement is relatively simple and inexpensive (50–150 €).
Electronics (regulation module) – usually the most reliable part, but sensitive to overvoltage. I recommend surge protection in the home distribution board.
Expansion tank – the membrane should be checked every 3–5 years, or replaced (30–80 €).

Recommended service frequency: once a year before the heating season – pressure check, bleeding, visual inspection of heating elements and connections. Details in the article Maintenance and service of electric boiler SLOKOV.

Can I connect a hot water storage tank to SLOOV?

Yes – and it is a common configuration in households where there is no gas connection and a separate electric water heater is not desired. The solution is an indirect heating water heater (TÚV storage tank with a heat exchanger) connected as a priority circuit to the boiler. The boiler switches priority to the water heater when heating TÚV is needed – it prioritizes heating water in the tank over space heating.

Practical connection procedure: a branch is led from the boiler output through a circulation pump to the water heater’s heat exchanger, and the return from the water heater back to the boiler. A three-way switching valve (controlled by the water heater thermostat) switches between the TÚV circuit and heating. Tank capacity for a family of four: 150–200 liters.

Important: when dimensioning the boiler for use with TÚV, you must also account for the power needed to heat the tank – typically 1–2 kW more than heating alone.

Most frequently asked questions (FAQ)

Can I use SLOKOV as the main heat source in a year-round occupied home?

Yes, SLOKOV SL ELEKTRO is a full-featured heating boiler suitable for year-round operation. It is not an emergency or supplementary solution – hundreds of installations across Slovakia operate as the primary heat source even in homes permanently occupied throughout the winter. The key to reasonable costs is proper power sizing, good house insulation, and use of a dual-rate electricity tariff (HDO/night tariff).

Do I need a chimney inspection for SLOKOV SL ELEKTRO?

No. The electric boiler does not burn any fuel, no combustion gases are produced, and the boiler does not require any combustion air supply. A chimney is not needed at all – this is one of the main advantages of an electric boiler when renovating old houses or when building, where it is technically impossible to construct a chimney.

What happens if there is a power outage – is there a risk of freezing the system?

During a long power outage, the boiler does not function and the system cools down. For recreational buildings or homes with long-term absence of occupants, it is therefore advisable to use an antifreeze mixture (glycol) in the heating circuit, or to install a backup power source (UPS) for the boiler and pump for short outages. The boiler itself has an anti-freeze function, but it only works when electricity is present – without power, it cannot do anything. In practice, short outages (hours) during mild winter do not pose a freezing risk in a typical occupied home.

What is the difference between SLOKOV SL ELEKTRO and the MINI version?

SLOKOV SL ELEKTRO MINI is a more compact model with lower power, intended for smaller spaces or as a supplementary heat source. It mainly differs in size, power range, and sometimes also in the scope of accessories. A detailed comparison can be found in the article Difference between SLOKOV SL ELEKTRO and SL ELEKTRO MINI, where both models are compared in a table.

Can SLOKOV be programmed to heat only during the night tariff?

Not directly via the built-in controller – the boiler itself does not have a time-based programming function. However, the boiler can be connected to an external programmable thermostat (weekly or WiFi), which will turn the boiler on precisely during the period of the cheap night tariff (typically 22:00–06:00 or according to the HDO signal from the distributor). In combination with thermal storage tanks or just the thermal mass of the house (walls and floors retain heat), this solution works very well and significantly reduces operating costs.

Is it possible to connect multiple SLOKOV boilers to one system?

Technically yes – in larger buildings or when redundancy (backup) is required, it is possible to connect two boilers in parallel with a hydraulic separator (hydraulic arrow or distributor/collector). One boiler covers normal consumption, the second is turned on only during cold spells or when the first is out of operation. Such a solution is common in small apartment buildings or commercial spaces. Details on such a connection can be found in the article Connecting an electric boiler SLOKOV to an existing system.

Conclusion – what to take away

SLOKOV SL ELEKTRO is a proven, technically reliable and relatively easy to install solution for heating family homes, chalets and small commercial buildings. Its strength is absolute simplicity – no chimney, no fuel, no emissions, minimal maintenance. A weakness is higher operating costs compared to a gas condensing boiler or heat pump at high electricity prices – but with the right tariff and a well-insulated house, the differences are manageable.

If you are in the decision-making process, I recommend starting with the article How to choose an electric boiler SLOKOV – what to pay attention to, where the key decision-making criteria are summarized. And if you know the area of your house and have at least a rough idea of its thermal performance, the article What power of electric boiler SLOKOV do I need for my house will guide you through the calculation quite reliably.

To directly compare the individual power variants, take a look at the specific models: SLOKOV SL ELEKTRO 10, SLOKOV SL ELEKTRO 15, SLOKOV SL ELEKTRO 18, SLOKOV SL ELEKTRO 23 and SLOKOV SL ELEKTRO 26. Each model lists technical specifications, dimensional drawings and installation documentation in detail.

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