How to choose an electric boiler SLOKOV – what to pay attention to

We are still preparing products.

But you can look at other categories.

How to choose an electric boiler SLOKOV – what to pay attention to

Electric boilers SLOKOV belong to proven solutions for heating of family houses, chalets, commercial premises and apartments without access to gas or where natural gas is too expensive to install. Over the years of practice I repeatedly encounter the situation where customers come with a simple question – "which SLOKOV should I buy?" – and upon closer inspection it turns out that behind this question there is a whole range of decisions that have a direct impact on the comfort, efficiency and reliability of the entire system. This article will help you go through these decisions step by step, with specific numbers, examples from practice and explanations of what individual parameters mean in real operation.

What is SLOKOV SL ELEKTRO and why people choose it

SLOKOV SL ELEKTRO is a series of electric flow/accumulation boilers for heating, which operate on the principle of resistance heating of water in a closed circuit. The boiler heats the water, which circulates through radiators or floor heating and transfers heat to the rooms. Unlike heat pumps or solid fuel boilers, electric boilers are structurally simple – they contain a heating element (cartridge), a pump, an expansion module, a temperature sensor and control electronics. Thanks to this, they are compact, do not require a chimney, combustion chambers or fuel tanks.

Why do customers choose SLOKOV specifically? From practice I can say that the most common reasons are these:

A chalet or recreational building without gas, where it would not be worth building a boiler room
A temporary solution during reconstruction, until the new system is completed
Supplementary heating for a heat pump (bivalent system)
A small apartment or studio, where another boiler would be oversized
A building with access to only electrical energy (mountain areas, new construction)
House reconstruction, where the existing piping remains, but the old boiler has served its purpose

SLOKOV SL ELEKTRO is produced in several power variants – SLOKOV SL ELEKTRO 10, SLOKOV SL ELEKTRO 15, SLOKOV SL ELEKTRO 18, SLOKOV SL ELEKTRO 23 and SLOKOV SL ELEKTRO 26 – where the number after the name indicates the nominal power in kilowatts. This is the first and most important parameter to think about.

Boiler power: the most important parameter and how to estimate it

Choosing the power is the place where mistakes are most often made – in both directions. Sometimes a customer wants a 26 kW boiler for a three-room apartment in a panel building (unnecessarily oversized), sometimes someone buys a 10 kW boiler for a 200 m² unheated chalet (tragically undersized). Both scenarios have real negative consequences.

The basic rule says that the required boiler power depends on the building's heat losses. Heat losses are influenced by these factors:

Enclosed volume and building envelope area
Thickness and type of thermal insulation of the outer walls, roof and floor
Type of windows and doors (single, double, triple glazing)
Climatic area (design outdoor temperature)
Required indoor temperature
Ventilation and air tightness of the envelope

For an approximate calculation without an energy audit, a rather rough estimate is used in practice according to the specific heat loss of the building in [W/m²]:

Old, non-insulated house (before 1980, without insulation): 80–120 W/m²
Partially insulated house: 50–80 W/m²
New construction after 2000, standard norms: 35–55 W/m²
Low-energy building: 20–35 W/m²
Passive house: under 15 W/m²

Practical example: A house with a floor area of 120 m², built of brick masonry, partially insulated facade (80 mm EPS), PVC windows with double glazing, located in an area with a winter design temperature of –12 °C. Estimated specific heat loss of about 60 W/m². Calculation: 120 m² × 60 W/m² = 7 200 W = 7.2 kW. With a reserve of about 20 % for adverse conditions and hot water heating: required power ≈ 8.5–9 kW. In this case, SLOKOV SL ELEKTRO 10 is the right choice – it is not oversized, but has sufficient reserve.

Another example: A chalet of 75 m², wooden frame construction with 120 mm mineral wool insulation, triple-glazed windows, located in a mountain area with a design temperature of –18 °C, but the chalet is used only on weekends and the customer needs to heat it quickly from 5 °C to 20 °C. In this case, it is not enough to calculate only heat losses, but also the power needed for rapid heating. The result can be 10–12 kW, so SLOKOV SL ELEKTRO 15 for faster heating.

A more detailed calculation procedure can be found in the article What power of SLOKOV electric boiler do I need for my house? in this Knowledge Center.

Electrical connection and protection – critical installation condition

This is the point where customers most often get unpleasant surprises after purchase. The electric boiler SLOKOV with a power of 15 kW, 18 kW, 23 kW or 26 kW is not a standard appliance that you plug into an outlet. It requires a three-phase supply of 3× 400 V and adequate protection.

Specific current values for individual models (at 400 V three-phase supply):

Model	Power (kW)	Current (A, 3×400V)	Min. circuit breaker (A)	Cable cross-section (min.)
SL ELEKTRO 10	10	14,4	16 A (3-phase)	2,5 mm²
SL ELEKTRO 15	15	21,7	25 A (3-phase)	4 mm²
SL ELEKTRO 18	18	26,0	32 A (3-phase)	6 mm²
SL ELEKTRO 23	23	33,2	40 A (3-phase)	10 mm²
SL ELEKTRO 26	26	37,6	50 A (3-phase)	10 mm²

Consequences in practice: If you have an old family house with a main circuit breaker of 3×25 A and you want to install SLOKOV SL ELEKTRO 23, you must address the replacement of the main circuit breaker and possibly also the incoming cable from the distribution substation before installing the boiler. This is a matter for the distribution company (ZSD, SSE, VSE), which can take months and cost several hundred euros extra. Therefore, always check your connection capacity before ordering the boiler.

A customer from the Banská Bystrica region ordered a 26 kW boiler to heat a larger building. He came to us because the electrician told him that the connection was insufficient. After a joint reassessment, we found out that the house was better insulated than estimated, and the actual need was around 18 kW – which the connection could handle without modifications. Lesson learned: First check the status of the connection, then choose the boiler power.

Type of heating system and heating water temperature

The electric boiler SLOKOV SL ELEKTRO is a full-featured boiler – it heats water in a closed circuit to the set temperature and distributes it through a pump to the system. Here we encounter an important difference between types of heating systems:

High-temperature radiator systems – old steel or cast iron radiators that were designed for 75/65 °C (supply/return temperature). SLOKOV SL ELEKTRO has no problem here, as it can heat water to the required temperature.
Low-temperature floor heating – operates at 35–45 °C. SLOKOV is also compatible here, but the regulation must be set correctly and a mixing group must be connected if you have a combination of floor and radiators.
Combined systems – floor heating + radiators. They require hydraulic balancing and proper zoning. For these cases, I recommend also reading the article Connecting the SLOKOV electric boiler to an existing system.

Important notice: If you are renovating an older house and plan to add underfloor heating, but keep the old radiators, the entire system must be designed to be hydraulically balanced. A simple connection without a mixing valve will end with either the radiators not heating sufficiently or the floor overheating and the tiles cracking.

Integrated control, thermostat and programming

SLOKOV SL ELEKTRO is equipped with built-in control that allows you to set the heating water temperature and basic operating parameters directly on the boiler. However, for comfortable and energy-efficient heating, it is almost always suitable to connect a room thermostat, or a programmable thermostat or smart thermostat (e.g. via OpenTherm or dry contact, depending on the model).

From practice: A customer who does not heat continuously (e.g. a cottage, a house where no one is at home during the day) can save 15–25 % on electricity just by setting a night or setback mode via a programmable thermostat. The boiler, for example, maintains only 16 °C instead of 21 °C from 22:00 to 6:00 – this is a huge difference in electricity consumption over the season. Read more in the article Electric boiler SLOKOV and night rate – how to set heating.

When choosing a thermostat, make sure it is compatible with your boiler. Cheap analog thermostats with dry contact (ON/OFF) work with SLOKOV without problems, but OpenTherm communication (if supported by the boiler) allows much finer control – the boiler does not run at full power/off, but modulates the output, which saves energy and prolongs its life.

Night rate and dual tariff electricity meter

An electric boiler is an energy-intensive appliance, so it is almost essential to equip yourself with a dual tariff electricity meter with a night rate (NT) from your electricity supplier. During night hours (usually 8 hours a day, the exact schedule is determined by the distributor), the price of electricity is lower – depending on the contract, the difference can be 30–50 % compared to the day rate. A boiler with an accumulator tank (boiler or buffer tank) can draw most of the energy during these cheaper hours.

SLOKOV SL ELEKTRO boilers can be connected to HDO (remote mass control), which controls tariff switching. The relay controlling the boiler reacts to the HDO signal and turns the heating on/off according to the price window. This requires proper wiring, more about it you can read in the article Electric boiler SLOKOV and night rate – how to set heating.

Integrated storage tank TÚV or combination with a boiler

Basic models of SLOKOV SL ELEKTRO are heating boilers – they do not directly heat hot water (TÚV) in a tank. If you want to supply hot water for the bathroom and kitchen with the boiler, you have two options:

External storage water heater TÚV (indirect preparation): The boiler heats the TÚV tank via a heat exchanger (coiled pipe). A 120–200 liter tank is sufficient for most family homes. The advantage is that the tank can be heated at night at a lower rate.
Electric flow water heater TÚV (direct preparation): A cheaper solution in purchase price, but more expensive in operation, since an electric flow heater does not store heat and draws full power whenever water is drawn.

I recommend a storage solution with a volume of at least 150 liters, ideally 200 liters, connected via a heat exchanger to the boiler for a family home with 3–4 people. Morning peak consumption of TÚV (0.5 hours) is covered by the stored supply, the boiler will add heat during calmer times.

Hydraulics, pump and expansion tank

SLOKOV SL ELEKTRO boilers are delivered with a circulation pump – this is a big advantage compared to boilers where the pump has to be purchased separately. The pump is sized for standard residential systems. If you have a larger system (large house, long pipes, multiple circuits), additional secondary pumps for individual circuits may be needed.

The expansion tank is either built-in (compact models) or external. The correct size of the expansion tank depends on the volume of water in the system – for a standard family house (radiators + pipes ≈ 50–80 liters of water), an 8–12 liter tank is sufficient. If the system is larger, this must be considered during planning. An incorrectly sized expansion tank will be indicated by the safety valve discharging water – this is the first sign of a problem that customers ignore, and later they ask for service. More about diagnostics in the article Common faults of electric boilers SLOKOV and their solutions.

Installation location and physical dimensions

SLOKOV SL ELEKTRO is a compact wall-mounted boiler, installed on the wall similarly to a gas boiler. The weight ranges from approximately 12 to 18 kg depending on the model. It does not require a chimney, special ventilation or any safety cutout – the installation can be carried out in a technical room, at the entrance, in the bathroom (with IP protection) or in the kitchen.

What to pay attention to when choosing a location:

Closeness to the distribution board – the high-current cable must not be too long (losses and cable costs increase with length)
Service access – the boiler must be accessible from the front with at least 60 cm of free space
Frost protection – if the boiler is in an unheated space, freezing is a risk in cold weather (boilers have anti-freeze protection, but only if connected to the electrical grid)
Sound insulation – the circulation pump emits a slight noise during operation; this should be considered for installation in a bedroom or children's room

Overview of the model range and when to choose which model

To give you a clear idea, here is a practical summary of which model is suitable for whom:

Model	Typical use
SL ELEKTRO 10	Apartment up to 70 m² (new build), small cabin up to 60 m², garden house, well-insulated recreational building
SL ELEKTRO 15	Family house 80–120 m² (new build, low-energy), apartment 80–100 m² (insulated), cabin 70–90 m²
SL ELEKTRO 18	House 120–150 m² (standard thermal performance), house 100–120 m² (older building), combined system with DHW
SL ELEKTRO 23	Larger family house 160–200 m², older building without insulation up to 140 m², building with lower thermal load
SL ELEKTRO 26	Large house over 200 m², building with high thermal losses, commercial building, bivalent source for HP

If you are on the border between two models, I usually recommend choosing the higher power. The reason is not unnecessary spending – an electric boiler simply runs shorter cycles at lower loads, which does not mean higher consumption (unlike, for example, a pre-installed gas boiler). You will appreciate the extra power reserve during prolonged cold weather, when the system needs to quickly make up for the thermal deficit.

What to do before purchase – checklist

Based on hundreds of resolved customer cases, I recommend going through this checklist before ordering the boiler:

✔ I have determined the area and thermal standard of the building and made an approximate power calculation
✔ I have checked the capacity of the electrical supply (main circuit breaker, phases)
✔ I know whether I have or want a two-rate electricity meter with HDO
✔ I have decided how I will solve DHW heating
✔ I know where the boiler will be physically installed and the distance from the distribution board
✔ I know what type of heating system I have (radiators, floor, combined)
✔ I know whether I need a thermostat and which type
✔ I plan to have the installation done by an electrician and a heating technician who are familiar with such boilers

If you have answered yes to all points, you are ready to make a purchase. If any point is unclear, do not rush – a wrongly chosen boiler can be a problem for years.

Installation, commissioning and warranty conditions

SLOKOV SL ELEKTRO must be installed and commissioned by a qualified electrician (for high-current connection) and a heating technician or plumber (for hydraulic connection). Self-installation without professional supervision may affect warranty conditions and could be problematic in the case of an insurance claim. A detailed installation procedure is described in the article Installation of SLOKOV electric boiler – procedure and requirements.

During commissioning, the following must be ensured:

Depressurizing the entire system (air in radiators hinders circulation)
Checking the water pressure in the system (typically 1.0–1.5 bar in a cold system)
Setting the temperature of the heating water according to the type of system
First start-up and verification of the pump, thermostat and safety valve function
Recording in the warranty certificate and installation registration

Electric boiler vs. other alternatives – when SLOKOV is yes and when not

An electric boiler is not always the best choice. Here is a brief look at scenarios when SLOKOV makes sense and when it does not:

SLOKOV makes sense when:

The building has no access to natural gas and the cost of a gas connection is high
Investment costs must be low (electric boiler is cheap to purchase)
The building is used seasonally or on weekends – does not require constant supervision
It is a supplementary (bivalent) source for a heat pump
You have a favorable electricity contract (e.g., renewable sources, night tariff)

SLOKOV may not be the best choice when:

Electricity is expensive and the house has high heat losses – operating costs can be high
You have access to cheap natural gas or biogas
You are planning to install a heat pump – then it is better to wait and invest directly in a heat pump
The house is not insulated – invest in thermal insulation before the boiler

More about the comparison with other electric boilers can be found in the article SLOKOV SL ELEKTRO vs other electric boilers – comparison.

Most common mistakes when choosing an electric boiler SLOKOV

To conclude this section, I have summarized the mistakes I see repeatedly:

Selecting power "by eye" without calculation – the customer buys a 26 kW boiler "just in case" for a house where 12 kW is sufficient. The boiler runs in short cycles, unnecessarily stressing the control electronics.
Not checking the capacity of the electrical connection – an ordered 23 kW boiler cannot be connected because the connection is only 25 A.
Ignoring DHW (Domestic Hot Water) – the customer buys a heating boiler and forgets that hot water must be handled separately.
Installation without a qualified electrician – warranties and safety are compromised.
No regulation – a boiler without a thermostat runs at full power until someone manually turns it off.
Not using the night rate – operating costs are unnecessarily high.

Frequently asked questions (FAQ)

Can the SLOKOV SL ELEKTRO boiler be connected to a single-phase network 230 V?

Models with a power of 10 kW and higher require three-phase power supply 3×400 V. Single-phase supply (230 V) is not standard or recommended for these models. If you have only a single-phase supply available, you must either expand the connection (depending on the distributor) or choose a different type of heating. If you are interested in a smaller variant, read the article Difference between SLOKOV SL ELEKTRO and SL ELEKTRO MINI, where more compact power variants are also described.

How long does it take to heat up the house from zero after turning on the boiler?

It depends on the boiler power, the heat loss of the house, and the initial temperature. Roughly: for a house with a heat loss of 10 kW (at -12 °C outside), starting from 10 °C to a target of 20 °C, you need a boiler with at least 15–18 kW power and the heating time will be 3–6 hours. Therefore, for weekend cottages, I recommend preheating via a thermostat with a program – set it to start heating several hours before arrival.

What happens if the SLOKOV boiler freezes during a longer absence?

SLOKOV SL ELEKTRO boilers have an anti-freeze protection that automatically starts the pump and short-term heating when the water temperature drops below about 5 °C. This protection only works if the boiler is connected to the electrical network and not manually turned off. During a longer absence in freezing weather, never unplug the boiler – keep it connected at least in this anti-freeze mode.

Can I connect a DHW tank to one SLOKOV boiler?

Yes, a DHW tank with indirect heating (with a coil/heat exchanger) is a common way to prepare hot water with an electric boiler. The tank is connected to the boiler circuit as an additional heat consumer. It must be correctly connected via a three-way valve or a priority circuit so that the boiler first heats the DHW tank and then switches to heating. The tank size depends on the number of people – for 3–4 people, I recommend 150–200 liters.

What is the lifespan of the heating element (heating rod) in a SLOKOV boiler?

With proper operation and good water quality (hardness below 20 °fH, ideally softened water in a closed circuit), the heating element can last 10–15 years. With hard water, limescale deposits form, which reduce efficiency and shorten the lifespan. Therefore, use treated water when filling the system for the first time and regularly check the condition. More about maintenance can be found in the article Maintenance and service of the SLOKOV electric boiler.

Is an annual service inspection necessary?

An electric boiler does not have a combustion process, so an annual inspection is not mandatory as with a gas boiler. I still recommend a basic inspection every 2–3 years: visual inspection, pressure check, condition of the expansion tank, function of the safety valve, resistance of the heating element. These inspections significantly extend the boiler's lifespan and prevent unexpected failures in the middle of winter.

Conclusion: what to take away from this article

Do you have a question about this topic?

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