Frequently Asked Questions about Thermona THERM EL Boilers – Selection, Installation, Operation and Faults

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Introduction: Why Thermona THERM EL boilers, and the questions customers ask most often

Thermona THERM EL electric boilers are among the proven and popular solutions on the Slovak market for apartment buildings, family houses, cottages and commercial premises. Even though they are relatively straightforward devices compared to a gas boiler or a heat pump, customers still run into a whole range of questions before purchase, during installation, and during operation. This comes from everyday practice – once you have been working in this field for a while and have handled dozens of jobs, you see that the same doubts and problems keep repeating.

This article was created precisely to answer, in one place, the most frequent questions related to choosing the right model, correct installation, everyday operation, and troubleshooting common faults. It is not a marketing text but a practical manual based on hands-on experience. If you are looking for more detailed information on a particular sub-topic, we also recommend other articles in our Knowledge Center – for example Installing the Thermona THERM EL Electric Boiler – Procedure, Electrical Wiring Requirements and Inspection or Setting Up and Commissioning the Thermona THERM EL Boiler – First Start-Up Step by Step.

Choosing the right model – what output do I need?

The first and most important question a customer faces is the boiler's output. The THERM EL range covers outputs from 8 kW to 38 kW, with the individual models being:

Thermona THERM EL 8 – maximum output 8 kW, suitable for smaller apartments, cottages or supplementary heating
Thermona THERM EL 15 – output 15 kW, the most popular model for family houses with an area of 100–150 m²
Thermona THERM EL 23 – output 23 kW, suitable for larger family houses or smaller apartment buildings
Thermona THERM EL 30 – output 30 kW, a solution for larger buildings with higher heat demand
Thermona THERM EL 38 – maximum output 38 kW, intended for more demanding applications, larger buildings or industrial premises

Choosing the right output is not rocket science, but it does require knowing a few basic parameters of the building. A rough calculation suggests 50–70 W per cubic metre of heated space with standard insulation. So for a family house with a usable floor area of 120 m² and an average ceiling height of 2.6 m, you get a building volume of roughly 312 m³. The required output would be about 312 × 60 W = 18,720 W, i.e. around 19 kW. In practice, in this case you would reach for the THERM EL 23 model, because it is sensible to have some output reserve.

An important factor is also whether the boiler will serve only for heating, or also for heating domestic hot water (DHW) via an external tank. In the latter case, the required output naturally increases, because on top of the building envelope's heat loss you add the load of heating water – especially during morning peaks, when households are waking up.

You can find a more detailed calculation procedure and all related criteria in the articles How to Choose a Thermona THERM EL Electric Boiler – Output, Room Size and Other Criteria and What Output of Thermona THERM EL Electric Boiler Do I Need for My House?

You can find a detailed calculation procedure and all related criteria in the articles How to Choose a Thermona THERM EL Electric Boiler – Output, Room Size and Other Criteria and What Output of Thermona THERM EL Electric Boiler Do I Need for My House?

Electrical connection and protection – what must be in place before installation?

This is an area where customers most often underestimate the preparation, and the installation then runs into a problem right on site. THERM EL electric boilers are three-phase devices (400 V / 50 Hz). This means you cannot connect the boiler to a single-phase socket – this is a fundamental point that must be verified right at the start.

Each model has its own requirements for protection and cable cross-section:

THERM EL 8: input approx. 8 kW, current ~11.5 A / phase → circuit breaker min. 16 A / type B or C, cable CYKY 4×2.5 mm²
THERM EL 15: input approx. 15 kW, current ~21.7 A / phase → circuit breaker min. 25 A / type C, cable CYKY 4×6 mm²
THERM EL 23: input approx. 23 kW, current ~33.3 A / phase → circuit breaker min. 40 A / type C, cable CYKY 4×10 mm²
THERM EL 30: input approx. 30 kW, current ~43.5 A / phase → circuit breaker min. 50 A / type C, cable CYKY 4×16 mm²
THERM EL 38: input approx. 38 kW, current ~55 A / phase → circuit breaker min. 63 A / type C, cable CYKY 4×25 mm²

These values are indicative – you should always follow the manufacturer's actual technical data sheets and the electrical installation project documentation. In practice we come across customers who have an older home distribution board with a total capacity of only 3×25 A and want to install a THERM EL 23. In such a case it is necessary to replace the main circuit breaker and reinforce the connection from the distributor – and that is not a matter of days, but of weeks, sometimes even months.

Besides correct sizing, it is a legal requirement to produce an electrical installation inspection report before the boiler is commissioned. Without it, the appliance is not legally in operation, and a warranty repair can be complicated. You can find more on this topic in the article Installing the Thermona THERM EL Electric Boiler – Procedure, Electrical Wiring Requirements and Inspection.

Hydraulic connection – what must be in the circuit and why?

An electric boiler is not an isolated appliance – it is part of the heating hydraulic circuit. Correct hydraulic connection is just as important as correct electrical wiring, and this is exactly where many installers make unnecessary mistakes that later show up as faults or reduced output.

The following must be present in the THERM EL boiler's circuit:

Expansion tank: electric boilers do not have a built-in expansion tank (unlike many gas boilers). The expansion tank must be sized correctly according to the system's water volume – roughly 10% of the total water volume in the system. For a typical family house, this is usually an 18–35 litre tank.
Circulation pump: the circulation pump is also not included with the boiler. It must be designed according to the hydraulic resistance of the network. In practice, for domestic heating a class A pump (energy-efficient) is usually sufficient.
Safety valve: set to 3 bar (for most household systems), installed on the boiler's supply line.
Air vent: an automatic air vent prevents the formation of air pockets, which reduce flow and can cause overheating of the heating elements.
Dirt filter (magnetic filter): a strongly recommended component, especially during renovations of old systems. A magnetic filter captures corrosive products from old radiators and pipes, which would otherwise damage the boiler's heating elements.
Thermometer and pressure gauge: for monitoring operation – although the boiler has an integrated temperature display, a physical pressure gauge in a visible location is a practical aid during service checks.

The system's working pressure should be 1.5–2.5 bar in cold water; the maximum permitted operating pressure for the THERM EL boiler is 3 bar. If your system includes underfloor heating, a mixing unit is required, since underfloor heating operates at temperatures of 35–45 °C, while the boiler can reach up to 85 °C.

Detailed wiring diagrams for various configurations (radiators only, radiators + underfloor heating, combined with a DHW tank) can be found in the article Connecting the Thermona THERM EL to the Heating System – Wiring Diagrams and Hydraulic Requirements.

Stepwise output control – how does it work and why does it matter?

One of the characteristic features of THERM EL boilers is stepwise output control. This is not a shortcoming – on the contrary, when properly understood, it is an efficient and reliable solution. The electric heating elements (cartridge-type resistive heating elements made of durable material) in THERM EL boilers switch on in groups, achieving several output stages.

For example, the THERM EL 15 kW model typically operates in stages of 5 – 10 – 15 kW. The THERM EL 23 kW model may have stages of 7.67 – 15.33 – 23 kW. The exact number and arrangement of stages depends on the specific model and how its heating elements are wired. This is why the boiler sometimes "jumps" while maintaining temperature – this is normal behaviour.

Advantages of stepwise control:

Simpler and more reliable design than continuous control (modulation)
Longer lifespan of the heating elements
Lower price of the appliance
Fewer components prone to failure

Disadvantages (good to be prepared for):

Higher short-term power spikes when a heating stage switches on
Not ideal for customers extremely sensitive to fluctuations in indoor temperature (a weather-compensating controller would help here)

You can find more about the principle of stepwise control in the article Stepwise Output Control in Thermona THERM EL Boilers – How It Works and Why It Matters.

First start-up and setup – how to do it?

Commissioning the boiler is a critical phase, which, if performed incorrectly, can lead to a fault or void the warranty. The entire procedure should always be carried out by an authorized technician or at least an experienced professional, but customers often come to us asking exactly what happens during the first start-up.

The basic sequence of steps is as follows:

Checking the hydraulics before filling: all joints, shut-off valves, filters, safety valve – visual inspection of sealing and assembly.
Filling the system: gradual filling with water, venting the radiators and the expansion tank. Pressure must be 1.5–2 bar in cold water.
Checking the electrical connection: measuring voltage at the terminals (must be 400 V ± 10%), checking phase sequence, checking PE grounding.
Switching on the boiler: after the first switch-on, the boiler initializes. Basic parameters are set – the desired water temperature (usually 70–75 °C for a radiator system or 50–55 °C for underfloor heating), hysteresis, and possibly weather-compensation control parameters.
Checking operation: monitoring whether the boiler switches correctly between stages, whether the circulation pump is working, and whether there is any water leak or pressure drop.
Recording in the service log: the technician records the initial setting parameters and signs the warranty certificate.

You can find a detailed procedure in the article Setting Up and Commissioning the Thermona THERM EL Boiler – First Start-Up Step by Step.

Common faults and error codes – what appears on the display?

THERM EL boilers are equipped with a digital display, which shows operating parameters as well as error codes in the event of a fault. From experience, we know which faults recur most often.

Error code E1 – flow temperature sensor fault

Most common cause: a broken or short-circuited sensor cable, or a faulty sensor itself. The boiler shuts down for safety reasons. Solution: measuring the sensor's resistance (an NTC sensor at 25 °C has a resistance of approx. 10 kΩ), and replacing the sensor if there is a deviation. It is not an expensive repair, but it requires a professional.

Error code E2 – safety thermostat (STB) fault

The STB (safety temperature limiter) is triggered by overheating – typically caused by poor water flow, a low water level in the system, or a clogged filter. The boiler locks out. After eliminating the cause of overheating and manually resetting the STB (reset button on the boiler), the boiler restarts. If the fault recurs, the flow, pump function and filter condition must be checked.

Error code E3 – insufficient water pressure

The boiler reports low pressure when the pressure drops below a minimum value (typically below 0.8 bar). Cause: a water leak in the system, an incorrectly set expansion tank, or a worn membrane. Solution: topping up water, checking for leaks, and possibly replacing the expansion tank.

Error code E4 – phase or power supply fault

The boiler reports a problem with the phase power supply. This can be caused by a loss of one phase in the mains, a faulty circuit breaker, or a fault in the boiler's internal power module. Measuring the voltage at the boiler's terminals with an external voltmeter immediately reveals whether it is a mains issue or an internal problem.

Fault: the boiler is running, but the space is not heating up

A relatively common call. The boiler is working, the outlet temperature is 70 °C, but the rooms are cold. Causes seen in practice: a non-working or poorly set circulation pump, thermostatic heads closed on all radiators simultaneously (hydraulic short-circuit), air in the system blocking flow, or an incorrect setting of the three-way valve when combined with underfloor heating.

Fault: the boiler keeps switching on and off (short cycling)

When the hysteresis is set too narrow, or the water volume in the system is too small (low thermal capacity), the boiler falls into short cycling – even several times a minute. Solution: widening the hysteresis in the boiler settings (e.g. from 2 °C to 5 °C), or installing a buffer tank in the system.

You can find a detailed overview of error codes and their solutions in the article Common Faults in Thermona THERM EL Boilers – Causes, Error Codes and Their Solutions.

Regular maintenance – what, when, and why?

Electric boilers are generally far less demanding in terms of maintenance than gas boilers, since they have no combustion process, requiring no yearly burner cleaning or electrode replacement. Nevertheless, regular maintenance extends the boiler's lifespan and prevents most faults.

Recommended service interval: 1–2 years.

What is checked during a service inspection:

Heating elements: checking for limescale deposits. In areas with hard water (hardness above 2 mmol/l), scale can build up quickly, reducing thermal conductivity and, in extreme cases, causing overheating and cracking of the heating elements. The recommended maximum calcium and magnesium content in heating water is below 1 mmol/l (i.e. approx. 5.6 dH).
Dirt filter (magnetic filter): cleaning at least once a year; during the first year of operation of a new system, even 2–3 times, since the system releases the most impurities during the run-in period.
Expansion tank: checking the gas pressure in the expansion tank (with the system drained, it should be 0.5–0.8 bar, depending on the height of the building). The membrane degrades and the pressure must be topped up.
Electrical connections: checking terminal tightness – vibrations and thermal expansion gradually loosen terminals, leading to contact resistance and overheating.
Safety thermostat (STB): functional test – simulating overheating to verify that the STB actually shuts down the boiler at the correct time.
Pump: checking noise level and speed. A worn pump usually has reduced output and runs noisier – signs of an approaching end of service life.

Regarding the quality of the heating water – if your system combines different metals (e.g. steel radiators + aluminium manifolds + copper pipes), a corrosion inhibitor must be added and the water's pH kept between 7.5–9.5. If this measure is not followed, electrolytic corrosion can damage even the boiler's heating elements over the years.

Further details on servicing procedures can be found in the article Maintenance and Servicing of the Thermona THERM EL Electric Boiler – What and How Often to Check.

Operating costs – how much electricity do THERM EL boilers consume?

This is a question with no single clear answer, but customers ask it regularly. Electric heating is generally more expensive than gas heating if calculated purely per kWh of energy. However, electricity has one big advantage – you can buy it at night or via a D6 rate (dual-tariff rate), when the price is significantly lower.

A practical example for a family house with an area of 120 m² in central Slovakia:

Average annual heat consumption: approx. 15,000–20,000 kWh (depending on insulation)
At an average rate of €0.18/kWh (daytime tariff): annual costs €2,700–3,600
Combining with the D6 night tariff (€0.09/kWh at night, €0.18 during the day, with a suitable tank): annual costs can be reduced to €1,800–2,500

To optimize costs, it is very important to combine the THERM EL boiler with a properly sized buffer tank and an intelligent controller that maximizes heating during the night tariff and stores heat for daytime use. This combination can bring savings of 30–40% compared to purely daytime operation of the boiler.

The Thermona THERM EL 15 model is economically the most advantageous for most well-insulated family houses with an area of 100–150 m² – sufficient output without unnecessary oversizing.

Comparison with other electric boilers – how is THERM EL different?

Customers often ask us why Thermona THERM EL and not some cheaper alternative from Asia, or conversely, why not a premium European brand. From experience, we know that Thermona is a Czech brand with more than 30 years of history on the Central European market, which is reflected above all in the availability of spare parts and the service network.

Specific advantages over cheaper alternatives:

Availability of spare parts even 10 years after production
A service network covering the whole of Slovakia and the Czech Republic
Technical documentation in Slovak/Czech
Proven lifespan of heating elements (15–20 years with correct handling)

Compared to premium European brands, THERM EL is significantly more affordable, while the quality of components and reliability are comparable. You can find a more detailed comparison in the article Thermona THERM EL vs. Other Electric Boilers – What Makes It Different and Who Is It Suitable For?

Installation during renovations of old houses – what needs attention?

A special situation that comes up very often in practice: a customer is replacing an old gas boiler with an electric one, or renovating the entire heating system in an old house from the 1980s. This is a typical scenario where several things need to be addressed at once.

Common complications during renovations:

Old electrical connection: houses from the 1980s–1990s often have only a single-phase 230 V connection or a three-phase 3×25 A connection, which is not sufficient for larger models. Consultation with the distributor is necessary.
Contaminated hydraulic circuit: after years of operation of the old system, the pipes and radiators are contaminated with sludge, rust, and manganese. Before installing a new boiler, it is always recommended to flush the system (chemically or mechanically), because a new boiler can "release" these impurities and clog its own heating elements.
Differences in connection dimensions: the old boiler may have had different connection sizes, requiring adapters and fittings.
Incorrectly sized expansion tank: during renovation, the old expansion tank is often left in place, which may be oversized, undersized, or have a worn membrane.

In such cases we recommend choosing a model with an output reserve, e.g. Thermona THERM EL 23 instead of the EL 15, because old systems have greater hydraulic losses and worse pipe insulation than new ones.

Frequently asked questions (FAQ)

Can I install the Thermona THERM EL boiler myself, without a professional?

The electrical part of the installation (connecting to the 400 V power supply) must be carried out exclusively by a person with electrical qualification in accordance with the applicable regulations (a qualified electrician). The hydraulic connection can be carried out by an experienced heating technician. Upon completion of the installation, it is a legal obligation to produce an electrical installation inspection report. Without it, the boiler is not legally in operation and the warranty terms will not apply. In practice, the answer is: hydraulics yes (if you have experience), electrics – no, not without the appropriate qualification.

What is the minimum system pressure and what to do if it drops?

The minimum operating pressure in the system for THERM EL boilers is usually 0.8–1.0 bar. If the boiler reports low pressure (error code E3 or similar), check the pressure gauge. If the pressure has dropped, top up water in the system via the filling valve until you reach 1.5 bar in cold water. Recurring pressure drops indicate a leak somewhere in the system – an inspection and repair are necessary. Never run the boiler for a long time at low pressure, as this risks a fault from overheating.

How long does it take to heat up the house after a cold start of the boiler?

This depends on several factors – the size of the house, outdoor temperature, and type of heating system. As a rough guide: for a family house with an area of 120 m² at an outdoor temperature of 0 °C, with a THERM EL 15 boiler, heating the interior from 10 °C to 20 °C takes roughly 3–6 hours with radiator heating. Underfloor heating has a significantly longer response time (6–12 hours) due to its thermal inertia, but it keeps the heat longer after the boiler is switched off. That is why a good thermostat with a weekly program is very important for electric heating – do not let the house cool down to 10 °C and then heat it up again, as this is extremely energy-inefficient.

Can I connect a domestic hot water tank to the THERM EL?

Yes, THERM EL boilers are also designed for heating domestic hot water (DHW) via an external tank with a heat exchanger. In this case, the boiler must be sized so that it can cover the heating output for both space heating and heating the tank at the same time. As a rough guide: a standard 200-litre DHW tank requires an additional output of approx. 5–8 kW. This is one of the reasons why customers with family houses that need DHW frequently choose the EL 23 instead of the EL 15. The tank is connected via a three-way valve or a separate circuit with DHW priority.

What to do if the boiler makes unusual noises (crackling, popping)?

Crackling or popping from the boiler during heating is usually caused by limescale deposits on the heating elements. Limescale has lower thermal conductivity than metal, so the heating element overheats and thermal stress occurs. If the noises are regular, it is time for chemical descaling of the system (circulating an acidic solution, e.g. citric acid). In more serious cases, replacing the heating element is necessary. Crackling from the pipe network (not from the boiler itself) is usually normal thermal expansion of the pipes when heating up – the solution is to allow the pipes to move freely where they pass through walls or floors.

Is it possible to operate the THERM EL boiler together with a photovoltaic system?

Yes, combining an electric boiler with a photovoltaic system is very popular and economically attractive today. The electric boiler acts as a "consumer of surplus energy" – when the photovoltaic system produces more than the household directly consumes, the boiler uses the surplus to heat water or charge a buffer tank. For this purpose, an intelligent energy manager is needed (e.g. Fronius Smart Meter, Victron, or other solutions), which switches the boiler's stages according to the available PV output. The Thermona THERM EL 38 model, thanks to its stepwise control, is particularly suitable for larger PV systems (10+ kWp), where each of the boiler's output stages can be matched perfectly to the available PV output.

Conclusion – key takeaways from this article

Thermona THERM EL boilers are technically proven, reliable, and relatively simple devices, but their correct operation depends on choosing the right output, professional installation of both the electrical and hydraulic systems, and regular maintenance. Most of the problems customers encounter stem from one of these three areas – and all of them can be prevented with proper preparation.

Once you have chosen the output, check the availability of a three-phase connection and the capacity of the home distribution board. If you are planning a renovation, do not neglect flushing the old system. And remember that an annual or biennial service inspection is not a wasted expense – it is an investment in the lifespan of the appliance, which, with correct handling, should last 15–20 years without major problems.

For specific technical details on individual topics, continue reading the other articles in our Knowledge Center – each of them covers one area in depth, which this summary FAQ article could not provide due to space constraints.

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