How to choose a Vaillant eloBLOCK electric boiler – what to watch out for

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Why choose an electric boiler and why specifically the Vaillant eloBLOCK

Electric boilers have made a significant comeback in recent years. While ten years ago they were mostly seen as an emergency solution when gas wasn't available, today more and more households choose them deliberately – as part of an energy-independent home with photovoltaics, as the main heat source in low-energy buildings, or simply as a replacement for an old gas boiler in an apartment building, where replacement is neither technically nor economically straightforward. The Vaillant eloBLOCK has long been one of the best-selling model ranges in this segment on the Slovak market, and not by accident.

Although the eloBLOCK looks simple at first glance – after all, it's "just" an electric boiler – a closer look reveals that choosing the right model and sizing it correctly requires fairly careful consideration. Output, power supply, controls, compatibility with underfloor heating systems, the possibility of combining it with photovoltaics, fuse requirements – all these factors determine whether the boiler will operate reliably, efficiently and economically.

This article will guide you through the entire selection process step by step, with concrete numbers, practical examples and warnings about the mistakes people make most often.

What is the Vaillant eloBLOCK and how does it work

The Vaillant eloBLOCK is an electric flow boiler that heats water in a closed heating system. Unlike storage water heaters or direct electric heating convectors, the eloBLOCK operates in a classic central heating system – it has a circulation pump, an expansion vessel, a safety valve, a manometer and all the components needed for the boiler to operate in a closed circuit. It doesn't need a chimney, a gas connection or a heat pump – just an electrical connection of the correct capacity.

Inside the boiler are electric heating elements (resistance coils made of stainless steel) that directly heat the water passing through the heat exchanger. Efficiency is practically 100% – all the electrical energy is converted into heat. This is both an advantage and a disadvantage: an advantage, because nothing is lost through a chimney or flue gas leakage, and a disadvantage, because the cost of heat depends solely on the price of electricity.

It's important to know that the eloBLOCK does not heat domestic hot water (DHW). It is exclusively a heating boiler. If you need DHW heating, it must be solved separately – either with an electric water heater or a solar system. Some customers are surprised by this, so we highlight it right at the beginning.

The eloBLOCK model range – overview of available outputs and variants

The Vaillant eloBLOCK is available in several output variants, each designed for a different type of building and heating system. The following models are currently available on the Slovak market:

Vaillant eloBLOCK VE 6/14 EU III electric boiler 6 kW – the smallest variant, suitable for low-energy houses, apartments or as a supplementary source
Vaillant eloBLOCK VE 9/14 EU – medium output, very popular for apartments and smaller well-insulated houses
Vaillant eloBLOCK VE 12/14 EU – suitable for larger apartments and smaller family houses
Vaillant eloBLOCK VE 14/14 EU III electric boiler 14 kW – for family houses with standard thermal insulation
Vaillant eloBLOCK VE 18/14 EU III electric boiler 18 kW – for larger or older houses with higher heat loss

The "14" designation in the name (e.g. VE 9/14) does not indicate the maximum output, but the maximum heating water temperature, i.e. 85 °C – which is standard for most heating systems. This temperature is important: the eloBLOCK can heat water up to 85 °C, which means it can also be used in older systems with a hot-water circuit designed for higher temperatures (e.g. 75/60 or 80/60 °C). For underfloor heating, on the other hand, temperatures of 35–45 °C are usually used.

Generation III models (marked "III") are the current series with modernized electronics and an updated display design. Functionally they are very similar to previous series, but have better compatibility with external controllers and the eBUS bus system.

How to determine the correct output – this is the most important thing when choosing

Choosing the wrong output is by far the most common mistake I see in the design of electric heating. The boiler's output must match the building's heat loss – not its area, not the number of rooms, not the owner's subjective feeling. Heat loss depends on how well the house is insulated, what windows it has, where it's located, and what the winter design temperature is for that particular location.

Simplified, a rough rule applies: an older uninsulated family house consumes 80–120 W/m², an insulated house 40–60 W/m², a low-energy building 15–30 W/m², a passive house under 15 W/m². So for a family house with a living area of 150 m², the following applies:

Old, uninsulated house: 150 × 100 W = 15,000 W = 15 kW → eloBLOCK VE 18
Standard insulated house (after renovation): 150 × 50 W = 7,500 W = 7.5 kW → eloBLOCK VE 9
Low-energy house: 150 × 25 W = 3,750 W = 3.75 kW → eloBLOCK VE 6

For an accurate heat loss calculation, we also recommend our other article in the Knowledge Center titled What eloBLOCK output do I need for my house – calculating kW according to area, which provides a much more detailed calculation procedure, including correction coefficients for construction type, building orientation and Slovakia's climate zone.

In practice, it's important to oversize by at least 10–15%, because the boiler shouldn't operate permanently at 100% output – heat loss figures are calculated for extreme winter conditions, not for a typical winter day. At the same time, the eloBLOCK allows output regulation – it can also work at partial output, so oversizing by one level usually isn't a problem. Interestingly, the eloBLOCK regulates output in stages: the VE 18 model operates at either 6, 12 or 18 kW, not continuously. This should be kept in mind when choosing a controller.

Single-phase vs. three-phase power supply – when it concerns you

This is a technical matter that surprises many customers. eloBLOCK models with an output of up to 3.5 kW can run on single-phase 230 V power, but the VE 6 and higher require a three-phase 400 V supply (3 × 400 V, 50 Hz). This means that if you only have a single-phase connection available in your house or apartment, you simply won't be able to connect an eloBLOCK VE 6 or higher without modifying the electrical connection.

In older apartment buildings, it sometimes happens that a flat has only a single-phase 230 V connection with a fuse rating of 25 A – that's a maximum of 5.75 kW. This wouldn't be sufficient, either in terms of output or technically, for installing an eloBLOCK VE 6. In this case, you either need to change the connection (which requires the grid operator's consent and rewiring in the distribution board), or choose a different heating solution.

In family houses, a three-phase connection is almost standard today, but the main fuse rating must still be checked. For example, the VE 18 at full 18 kW output draws a current of about 3 × 26 A on a three-phase connection, which requires a fuse rating of at least 3 × 32 A. If you only have 3 × 25 A in your house, you'll need to arrange this with the electricity distributor. You can find more on this topic in the article Converting to three-phase power supply for the eloBLOCK – when it's necessary and how to do it in the Knowledge Center.

Control and regulation – a key factor for comfort and savings

The Vaillant eloBLOCK has its own display and internal thermostat, which allows you to set the desired heating water temperature. However, this basic control is insufficient for most households – an external controller with a room thermostat or an intelligent wireless controller is far more comfortable and energy-efficient.

Vaillant offers its own VRC series controllers for the eloBLOCK – for example the VRC 700, VRC 720 or the ambiSENSE system. These controllers communicate with the boiler via the eBUS bus (a data cable with two wires), enabling full communication – the boiler "knows" what the controller requires and can react more flexibly. The advantage is weather-compensated control (the heating water temperature adapts to the outdoor temperature), time programs, and, in the case of modern models, WiFi control via the myVAILLANT app.

If you plan to use the eloBLOCK with underfloor heating, we recommend investing in a quality controller with weather compensation, because underfloor heating is very sensitive to fluctuations in heating water temperature. You can find more on this combination in the article Connecting the Vaillant eloBLOCK to an underfloor heating system – what you need to know.

A separate topic is compatibility with third-party thermostats. In addition to eBUS, the Vaillant eloBLOCK also has a classic input for a dry contact (ON/OFF), which means it can be controlled by practically any room thermostat – including cheap wireless models. This contact simply switches the boiler on or off, without more sophisticated communication. This is sufficient for standard radiator heating, while an eBUS controller is better for underfloor heating.

What to watch out for during installation – technical requirements

Installing the eloBLOCK is not a DIY job – the law requires that the electrical part of the installation be carried out by an authorized electrician and the hydraulic part by a heating engineer. Warranty repairability depends on whether the installation was carried out professionally. Vaillant places emphasis on having the boiler commissioned by an authorized service technician, or at least in accordance with the manual by a professional – otherwise the extended warranty may not apply.

Technical requirements for installation include:

Electrical connection: three-phase 400 V (for models of 6 kW and higher), fuse rating according to the boiler's output (for the VE 6: 3×10 A, for the VE 9: 3×16 A, for the VE 12: 3×20 A, for the VE 14: 3×25 A, for the VE 18: 3×32 A), cable with at least 5 conductors (3× phase, neutral, earth)
Hydraulic connection: supply and return 3/4" or 1" (depending on the model), minimum water flow through the boiler
Expansion vessel: the eloBLOCK has an internal expansion vessel (usually 8 liters), but for larger systems an external one must be added
Filling valve and air vent: the system must have the ability to be filled and vented
Operating pressure: the boiler operates at a pressure of 1–3 bar; before first start-up, the system must be filled to about 1.5 bar
Mounting position: exclusively on a wall, in a vertical position, with sufficient service access (min. 50 cm in front)

A much more detailed procedure and electrical diagrams can be found in the article Installing the Vaillant eloBLOCK – procedure, electrical wiring requirements and fuse rating.

Operating economics – what you'll actually pay for electricity

Electric heating is more expensive than gas heating – that's a fact that needs to be stated openly. But it depends significantly on the electricity price you pay and whether you can take advantage of a cheaper tariff (night rate, multi-band tariff). In Slovakia, distribution tariff DD7 (or DD6) is available for customers with electric heating, allowing a lower electricity price in blocks (usually 8–22 hours a day), but it requires heat accumulation – either in an accumulation tank or in massive underfloor heating.

A practical example: A family house with a heat loss of 8 kW, insulated, 130 m². eloBLOCK VE 9 boiler. The average annual heat consumption in a design region with 2,800 degree-days is about 22,400 kWh. At an electricity price of €0.22/kWh (including distribution and taxes, DD7 tariff), the annual heating cost is about €4,900. Comparison: gas heating at a price of €0.07/kWh (thermal content of natural gas, 90% efficiency) would cost about €1,740. The difference is real and needs to be taken into account.

The situation changes fundamentally if you have your own photovoltaics. With a 10 kWp PV system, you can cover household consumption in summer and accumulate surpluses; in the transitional periods (spring/autumn) you can run the boiler for free on sunny days. In winter, when sunshine hours are shorter, self-sufficiency is lower, but even then you can save 30–50% of heating costs. More on this combination in the article Vaillant eloBLOCK and photovoltaics – is combining it with your own electricity worth it.

Physical dimensions and installation conditions – what you need to know before the boiler arrives

The Vaillant eloBLOCK is a compact wall-mounted boiler. All models have the same external dimensions: height 720 mm, width 440 mm, depth 160 mm. Weight is 22–24 kg depending on the model. The boiler is mounted exclusively on the wall – it's a wall-hung boiler with a typical Vaillant hanging system that allows easy removal and re-hanging during servicing.

The water connections are at the bottom: the hot water outlet (supply to the heating circuit) and the return inlet are next to each other, with a distance of 50 mm between the pipe centers. The electrical connection is brought in from below via a terminal box. When planning the installation, you need to think in advance about where the pipes and electrical cable will come from – the boiler cannot simply be moved sideways once it's installed.

Minimum space required for service access: 500 mm in front of the boiler (for removing the casing and accessing the heating elements), 200 mm above the boiler (for access to the hydraulic valves). The boiler must not be installed in a dusty environment or in areas where an explosive atmosphere may occur. The ambient humidity must be within the range of 0–95% relative humidity (non-condensing). The boiler is suitable for indoor use, not for external installation or an unheated garage.

Warranty, service and reliability of the eloBLOCK in practice

The standard warranty period for the Vaillant eloBLOCK is 2 years from the date of sale. Vaillant offers the option of extending the warranty to 5 years, provided the product is registered in the Vaillant system and regular annual servicing is carried out by an authorized service technician. This warranty extension is beneficial and we recommend taking advantage of it.

In terms of reliability, the eloBLOCK is an above-average quality boiler in its class. The weak point that technicians occasionally complain about is the heating elements – with hard water, they can become scaled prematurely, leading to overheating and failure. The solution is regular monitoring of water hardness and possibly the use of a softening filter or corrosion inhibitor in the circuit. Another common source of faults is the pump – after years of operation, it can become clogged with sludge, which shows up as an error code and stops circulation. More on error codes and their solutions can be found in the article Common Vaillant eloBLOCK faults and their solutions – error codes and failures.

Annual servicing is not a special procedure – the technician checks the system pressure, the condition of the expansion vessel, the function of the safety valve, the condition of the heating elements, the cleanliness of the filter and the fuse setting. More details on servicing can be found in the article Maintenance and servicing of the Vaillant eloBLOCK – what to check and how often.

eloBLOCK vs. the competition – where it excels and where it has limits

On the Slovak market, the eloBLOCK's main competitors are models from Protherm (Protherm Ray), Bosch (Tronic Heat 3500), Dimplex and several less well-known brands. The Vaillant eloBLOCK is generally in the mid to upper price category – you don't buy it because it's the cheapest, but because it offers a good combination of build quality, service availability, control compatibility and warranty policy.

Advantages of the eloBLOCK in practice: a dense Vaillant service network in Slovakia (a technician usually arrives within 2–5 business days), availability of spare parts even for older models, a quality internal pump (Grundfos or Wilo), eBUS communication for advanced control, compact dimensions, a clear display and error code diagnostics.

Limits: a higher initial price compared to "no-name" competitors, the need for a three-phase power supply already from 6 kW (some competing models offer single-phase variants up to 4 kW), no DHW heating (must be solved externally). A detailed comparison with the competition can be found in the article Vaillant eloBLOCK vs. other electric boilers – comparison of parameters and price.

Step by step: how to proceed when choosing the right model

To conclude this section, here is a summary of the recommended selection procedure we use for every order:

Determine the building's heat loss – either by calculation (based on area, construction, insulation), or even better, through an energy audit.
Check the electrical connection – whether you have a three-phase 400 V supply, what the main fuse rating is, and what the diameter of the cable from the distribution board to the boiler installation site is.
Decide on the control system – a basic ON/OFF thermostat, or an eBUS controller with weather compensation and WiFi? Underfloor heating almost always requires eBUS.
Assess the hydraulic system – type of radiators or underfloor heating, simple direct connection or mixing circuits, need for an accumulation tank.
Consider combining with PV – if you're planning photovoltaics, take this into account when choosing the output and control system.
Choose the specific model – with an output about 10–15% above the calculated heat loss.
Arrange professional installation and first commissioning – by an authorized technician, to preserve the warranty and ensure safe operation.

Frequently Asked Questions (FAQ)

Is the eloBLOCK suitable as a replacement for an old gas boiler in an apartment building?

This depends on the flat's electrical connection. If the flat has a three-phase 400 V connection with sufficient capacity (at least 3 × 16 A for the 9 kW model), the eloBLOCK is an excellent replacement – it connects hydraulically to the same piping as the original gas boiler, and no chimney is needed. The higher price of electricity compared to gas can be a problem, which needs to be weighed economically. In many flats, however, this replacement is very simple – reasonable boiler dimensions, no flue gas paths, no gas inspection.

Can I connect the eloBLOCK to solar panels or a heat pump?

The eloBLOCK is a purely electric boiler – it has no input for a solar circuit or hydraulic connection with a heat pump as a bivalent source. If you want to combine it with photovoltaic panels (not solar collectors), the eloBLOCK is an ideal partner – the photovoltaics generate electricity that the boiler uses to heat the water. Combining it with a heat pump (air-to-water) as a bivalent connection is technically possible via separate hydraulic circuits, but requires detailed design work.

What happens if the power goes out? Does the boiler restart automatically?

Yes, the eloBLOCK has an automatic restart function after a power outage. When power is restored, the boiler continues operating according to the last setting without any need for manual intervention. During a longer power outage in winter, there is a risk of the system freezing – the eloBLOCK has a built-in frost protection program that activates if the water temperature drops below a set value (typically 5 °C), but this program naturally only works if electrical power is available.

What is the minimum heating water temperature for the eloBLOCK?

The Vaillant eloBLOCK allows the heating water temperature to be set within a range of 20–85 °C. For underfloor heating, an operating temperature of 30–45 °C is recommended, while for radiator heating it's typically 55–70 °C (depending on radiator sizing). A lower operating temperature of the heating water with underfloor heating is more energy-efficient, because the heating elements work more effectively at a lower temperature difference.

Is it possible to get a subsidy or grant for the eloBLOCK in Slovakia?

Currently, there are no subsidy programs available in Slovakia for electric boilers comparable to those that exist for heat pumps (the Green Households program). Electric boilers generally don't fall into the category of supported devices, since they don't bring about a reduction in primary energy consumption within the meaning of European directives. An exception may be combining the eloBLOCK with photovoltaics – the PV system itself may be supported, but not the boiler. More on subsidies and warranty service in the article Frequently asked questions about the Vaillant eloBLOCK – warranty service, spare parts, subsidies.

How much does annual servicing of the eloBLOCK cost, and is it really necessary?

Annual servicing of the eloBLOCK by an authorized technician typically costs €60–120, depending on location and scope of inspection. It's not mandatory in a legal sense (unlike gas appliances), but it is a condition for maintaining the extended Vaillant warranty. Beyond the warranty, servicing has real value – it detects problems before they cause a failure at the most inconvenient time (in the middle of winter), and checks the pressure, condition of the expansion vessel and filters. In the long run, it pays off.

Conclusion – what really matters when choosing

The Vaillant eloBLOCK is a quality, reliable electric boiler suitable for a whole range of buildings – from flats to family houses, from new low-energy buildings to renovations of older houses. The key to satisfaction with this boiler is not just the product choice itself, but above all correct output sizing, a suitable electrical connection and a well-chosen control system.

Investing in the eloBLOCK makes sense especially when a gas connection isn't available or isn't worthwhile, when you have or are planning photovoltaics, or when you're looking for simple, low-maintenance, chimney-free heating. If you're facing this decision, start with your house's heat loss and your electrical installation – these two figures will tell you which eloBLOCK model is right for you.

Do you have a question about this topic?

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