Thermona THERM EL vs. other electric boilers – what's the difference and who is it suitable for?
Thermona THERM EL vs. other electric boilers – what makes it different and who is it suitable for?
Electric boilers have made a significant comeback in recent years. Rising gas prices, increasingly strict emission requirements, and interest in maintenance-free heating without a chimney have led many homeowners and apartment owners to reconsider their heating systems. On the Slovak market, we encounter several dozen models of electric boilers from various manufacturers – from Chinese no-name brands through German and Austrian premium brands to proven Central European manufacturers. One of the most frequently mentioned names is the Czech company Thermona with its THERM EL range. In this article, we will look in detail at how this series differs from the competition, where its strengths lie, where it reaches its limits, and who we recommend it to – and who we don't.
Brief overview of the Thermona THERM EL range
The THERM EL range is essentially a line of flow-through electric boilers for central heating. The boiler does not heat water in a tank, but on a flow-through basis – water passes through electric resistance coils (heating elements) that are submerged directly in the boiler body. Thermona offers five power levels: THERM EL 8 (8 kW), THERM EL 15 (15 kW), THERM EL 23 (23 kW), THERM EL 30 (30 kW), and THERM EL 38 (38 kW). The boilers are designed for three-phase 400 V power supply (models from 8 kW and up), and it is recommended that installation be carried out only by a qualified electrician with the appropriate inspection report.
Production takes place in Kuřim in the Czech Republic, where Thermona has its headquarters and main production facilities. This has a practical impact: spare parts are quickly available, the service network in Slovakia and the Czech Republic is dense, and communication with the manufacturer takes place in the native language.
What sets THERM EL apart from cheap electric boilers on the market
The first and most obvious difference becomes apparent when you open the documentation. Cheaper electric boilers – and you can find them from various manufacturers from Asia and Europe – often have minimalist documentation, machine-translated into Slovak with obvious errors. Thermona provides complete technical documentation, wiring diagrams, service manuals, and datasheets directly in Slovak or Czech. For the electrician installing the boiler and the inspector approving it, this is a huge convenience.
The second difference lies in the construction. THERM EL uses stainless steel heating elements and a stainless steel heat exchanger body. Some cheap boilers have heating elements made of copper alloy, which are more sensitive to water quality and react more aggressively to limescale. With hard water (a typical value in most of Slovakia is 15–25 °dH), this difference is crucial – a stainless steel boiler body with regular maintenance will last 15 or more years without problems, while service issues appear sooner with cheaper alternatives.
The third difference is in the stepped power regulation. For example, THERM EL 23 does not only work at full power of 23 kW – it can work at lower power levels (for example 7, 15, or 23 kW) depending on the current heating needs. This is not a given. Many cheap boilers simply switch on fully or are off. You can find more on this topic in a separate article: Stepped power regulation in Thermona THERM EL boilers – how it works and why it matters.
Thermona THERM EL vs. premium European brands (Stiebel Eltron, Kospel, Protherm)
If we compare THERM EL with premium European competitors such as Stiebel Eltron, Kospel, or Protherm Ray, we enter interesting territory. These brands are undoubtedly high quality – they have a long history, an extensive service network, and their products are technically advanced. So where does the difference lie?
Price and value for money: THERM EL is typically more affordable than Stiebel Eltron or Protherm Ray at comparable power. The difference can be 15 to 35%, depending on the model and current prices. For a customer looking for a reliable boiler with proven components who doesn't want to pay for marketing premium, THERM EL is a rational choice.
Smart function integration: Stiebel Eltron and Protherm Ray have more sophisticated native smart integration in some models – Wi-Fi modules, app control, integration with heat pumps. THERM EL is more conservative in this respect: control via an external thermostat (including OpenTherm controllers) is fully functional, but a native app in the sense of modern smart home products is missing. For most ordinary households, this is not a problem, but anyone who wants detailed consumption graphs directly on their phone without an external meter must reach for a different solution or connect an external smart controller.
Service availability: This is where Thermona scores. The service network in Slovakia is relatively dense, and spare parts – heating elements, pumps, expansion vessels, sensors – are available in stock from several distributors. With some smaller European brands, waiting for a spare part can extend to several weeks, which is not pleasant in winter.
Kospel (Polish manufacturer): Kospel is a direct competitor in the mid-price segment. Their EKCO.L2 and EKCO.T2 boilers are popular and comparably priced to THERM EL. Kospel has a slightly higher switching frequency of power levels (some models offer more switching stages), but in terms of structural reliability and service availability, it's an even match. It depends on the regional distributor and their service readiness.
Thermona THERM EL vs. heat pump – asking the right question
This comparison comes up increasingly often. The customer faces a decision: electric boiler or heat pump? It's a legitimate question, but it needs to be framed correctly.
An air-to-water heat pump today has a coefficient of performance (COP) of 3.0 to 4.5 under optimal conditions (outdoor temperature around 7 °C) – meaning it produces 3 to 4.5 kWh of heat from one kilowatt-hour of electrical energy. An electric boiler has an efficiency close to 100% – one kWh of electrical energy becomes almost one kWh of heat. At first glance, the heat pump is the undisputed winner.
However, reality is more complex. A heat pump is an investment of 6,000 to 15,000 euros (installation + equipment), requires space for an outdoor unit, generates noise of around 45–55 dB(A), and its output drops significantly at temperatures below -10 °C – exactly when you need it most. The THERM EL 15 electric boiler costs a few hundred euros, doesn't take up outdoor space, doesn't require special hydraulic system modifications, and has no moving parts that would wear out.
Conclusion from practice: if you have a well-insulated house with low annual heat consumption (for example a class B or better single-family house, annual consumption below 10,000 kWh), low storage electricity tariffs, and you're considering a long-term investment, a heat pump makes sense. If you're looking for a fast, cheap, reliable solution for backup, a transitional period, heating a smaller space, or you want to eliminate dependence on the gas grid without a large investment, THERM EL is a rational alternative.
Construction details of THERM EL – what's inside
Understanding the boiler's construction is important not only for technicians but also for the customer who wants to know what they're paying for. Boilers in the THERM EL range are built with the following key elements:
- Stainless steel heat exchanger: The boiler body, in which the heating elements are located, is made of stainless steel. It eliminates corrosion and is less susceptible to limescale than copper. The exchanger is compact and easily accessible when replacing heating elements.
- Resistance heating elements: These form the heart of the boiler. They are submerged directly in the system water (not through the exchanger wall), ensuring excellent heat transfer. Power is regulated by their gradual switching on.
- Circulation pump: The boiler is equipped with a built-in circulation pump that ensures water circulation in the system. The customer does not need to deal with an external pump – it's a complete, compact unit.
- Expansion vessel: The integrated expansion vessel (standard 6–8 litres depending on the model) compensates for volume changes in water during heating. For larger systems, an additional external expansion vessel may be needed – this is important to assess when designing the system.
- Safety valve: Standard equipment, set to 3 bar. Protects the system against overpressure.
- Electronic control unit: Controls the switching of the heating elements, communicates with an external thermostat or weather-compensation controller, and displays diagnostic information. In the event of a fault, it displays error codes that facilitate diagnostics.
- Water temperature sensor: An NTC sensor measures the outlet water temperature and prevents overheating. The adjustable range is typically 30–85 °C.
The boiler's physical dimensions are compact – for example, the THERM EL 15 measures approximately 450 × 300 × 200 mm (height × width × depth, exact values are in the datasheet). The boiler is designed for wall mounting. Weight depends on power output, ranging from approximately 15 kg (8 kW model) to approximately 25–30 kg (38 kW model). For larger models, it is a good idea to plan for an assistant during installation.
Who is Thermona THERM EL really suitable for?
From practical experience, we can quite accurately define the customer for whom THERM EL fits their needs. It's not for everyone – and that's fine. Every boiler has its niche.
Ideal customers for THERM EL
Owners of well-insulated smaller houses or apartments: If you have a house up to 100 m² with insulation and triple-glazed windows, the calculated heat loss in a mild climate zone is typically 4–8 kW. The THERM EL 8 or THERM EL 15 model will suffice with a margin. Operation is quiet, maintenance-free, without the need for a chimney. Ideal for apartments, smaller flats, or holiday cottages that are heated only during the season.
Customers without access to gas: There is still a large number of properties in Slovakia (mainly rural areas) where the gas network is not available or its installation would be disproportionately expensive. An electric boiler is a natural choice here. If you have access to a storage tariff (low tariff at night or during the day for customers with electric heating), operating costs are significantly reduced.
Customers looking for a backup to another heating source: A very common solution – primary heating is provided, for example, by a pellet boiler, fireplace insert, or heat pump, but for the transitional period (spring, autumn) or as a backup in case of primary source failure, THERM EL serves the purpose. Installation is simple, the price is low, and the system is always ready.
Renovations and quick solutions: A heating system renovation must take place over the summer, and the new boiler must be functional by September. An electric boiler can be installed in one day, does not require a chimney inspection, and does not require project documentation to the extent required for a gas boiler. This is a huge advantage for renovations under time pressure.
Owners of properties with photovoltaic panels: If you have a PV system with surplus power, an electric boiler is an elegant way to convert this surplus into heat. Some customers connect THERM EL via a smart relay or surplus power controller (such as WATT Router or SolarReg systems), which automatically switch on the boiler when the PV panels produce more electricity than the household consumes. Result: cheap heat from your own panels during the day, regular tariff only at night.
Who THERM EL is not suitable for
Large uninsulated houses with high heat loss: If you have an older 250 m² house with a heat loss of 20–25 kW, an electric boiler without significant insulation upgrades will be very energy-intensive. Here, a more suitable answer is either insulation + heat pump, or a condensing gas boiler (if gas is available). The THERM EL 38 model technically covers the power requirement, but the monthly electricity bill will be painful during all-day operation.
Customers looking for a primary heater for large volumes and low operating costs: If the primary criterion is the lowest possible annual operating cost regardless of investment cost, a heat pump wins in the long run. The payback period for the higher investment in a heat pump for a typical Slovak house with an annual heat consumption of 15,000 kWh is roughly 8–12 years.
Customers without a sufficient electrical connection: Larger THERM EL models (23, 30, 38 kW) require fusing and cable cross-section corresponding to their power input. If your connection is not designed for such a load, an increase in fusing needs to be arranged with the distributor – which can take time and cost money. Before purchasing, therefore, check the state of your electrical connection. You can find more about these requirements in the article Installation of a Thermona THERM EL electric boiler – procedure, electrical installation requirements, and inspection.
Control and integration with smart home systems
THERM EL supports connecting an external thermostat via a standard voltage-free contact (ON/OFF control) or via an OpenTherm interface. OpenTherm is a modulating communication protocol that allows the controller (thermostat) to dynamically communicate with the boiler – setting the desired water temperature, reading the current boiler status, and modulating power. This is significantly more efficient than a simple ON/OFF thermostat, as it prevents cyclic switching (so-called short cycling), which shortens the boiler's lifespan and increases consumption.
For weather-compensated control – that is, regulating the boiler water temperature according to outdoor temperature – a weather-compensation controller with an outdoor temperature sensor needs to be connected. The principle is simple: the colder it is outside, the higher the boiler water temperature must be so that the radiators can release enough heat into the room. This feature significantly improves operating efficiency and is recommended for all installations with underfloor heating or larger radiator panels, where water temperature is a critical parameter.
For those interested in integration into smart home systems (KNX, Loxone, Home Assistant): THERM EL does not have a native smart home interface, but via an external controller (e.g. Honeywell, Siemens, Bosch) with Wi-Fi or Ethernet output and a relay for a voltage-free contact, full integration can be achieved. Some technicians use smart plugs or Shelly relays combined with simple automation – although this solution is less elegant, it works reliably for less demanding customers.
Real-world scenarios – how THERM EL works in the field
Scenario 1 – Garden cottage, 55 m²: The owner of a cottage in Turiec purchased a THERM EL 8 as the main heat source for the cottage, which he uses from March to November. The cottage is 55 m², the windows have been replaced, and the roof is insulated. A qualified electrician installed the boiler in one day, connected to a simple digital thermostat. Electricity consumption over a March weekend (Friday–Sunday) at an outdoor temperature of -2 °C and a desired indoor temperature of 21 °C was around 45 kWh. At a rate of €0.20/kWh, this represents €9 per weekend – the customer is satisfied and is also considering connecting solar hot water heating for the summer.
Scenario 2 – Family house 140 m², backup to pellets: A house near Nitra has primary heating provided by a pellet boiler. The owner had a THERM EL 23 installed as a backup and supplementary source. During the transitional period (September, October, April, May), when firing up the pellet boiler would be uneconomical, THERM EL runs at low power levels. The pellet boiler is only started up from December to February. The customer saved on pellets since the season is shortened, and has the assurance that in the event of a pellet boiler failure, the house will be without heating for a maximum of a few hours – until a service technician is called and arrives.
Scenario 3 – Apartment in a residential building, disconnection from central heating: The owner of a 3-room, 78 m² apartment in Bratislava decided to disconnect from the district heating supply (CZT). Reasons: high fixed charges, inability to individually regulate temperature, a new building management company that increased payments. After consulting a designer and structural engineer (load-bearing walls, penetrations), he installed a THERM EL 15 with a weather-compensation controller and outdoor sensor. Monthly heating payments in winter range from €80–110 (at a low electricity tariff of €0.15/kWh), which represents a saving of around 30–40% compared to the original district heating payments.
Safety, standards, and certifications
Electric boilers are subject to strict safety standards. THERM EL is certified according to European directives – the CE marking guarantees compliance with the Low Voltage Directive (LVD), the Electromagnetic Compatibility Directive (EMC), and the ErP Regulation (ecodesign). For the customer, this practically means that the boiler has passed certification tests and its placing on the market is legal throughout the EU.
The following requirements apply to installation in Slovakia: the electrical part must be carried out by an electrician with the appropriate professional qualification, and after installation, a Report on Professional Inspection and Professional Test (inspection report) must be prepared in accordance with STN 33 1500. Without this inspection, the insurance company may not accept insurance claims related to the electrical installation. Details on the inspection can be found in the article Installation of a Thermona THERM EL electric boiler – procedure, electrical installation requirements, and inspection.
The boiler has multi-stage thermal protection: a temperature limiter (STB) with automatic or manual reset in case of overheating, a safety valve, and electronic protection against dry running. These safety features are standard for boilers of this category, but their proper functioning depends on correct filling and venting of the system during installation.
Frequently Asked Questions (FAQ)
Is Thermona THERM EL suitable for underfloor heating?
Yes, provided the boiler water temperature is set correctly. Underfloor heating typically operates with a water temperature of 30–45 °C, while radiator systems run at 55–75 °C. THERM EL allows the maximum outlet temperature to be set, so with the correct configuration (weather-compensation controller or thermostat with the correct characteristic), it is fully compatible with underfloor heating. For a mixed system (underfloor + radiators), hydraulic separation with a mixing valve is required.
How much does it cost to run an electric boiler per month?
It depends on the size and insulation standard of the house, the electricity tariff, and the outdoor temperature. Illustrative example: a 120 m² house with average insulation, annual heat consumption of 14,000 kWh. At an average price of €0.18/kWh (low overnight tariff), the annual heating cost is around €2,520, which is a monthly average of about €210. At a standard rate of €0.23/kWh, this rises to an average of about €270 per month (zero in summer, up to €450–500 in January). You can find a more precise calculation for your specific case in the article What power output of the Thermona THERM EL electric boiler do I need for my house?
Do I need a chimney to install THERM EL?
No. The electric boiler does not burn any fuel, so no flue gases or combustion products are produced. A chimney is not needed. This is one of the main advantages over gas or solid fuel boilers. The installation room does not need to meet requirements for combustion air supply. A regular boiler room or utility room with access to the electrical connection and heating system is sufficient.
How long will a THERM EL boiler last without repair?
With correct installation, regular maintenance (annual inspection of heating elements, venting, pressure check), and good water quality in the system, a lifespan of 15–20 years is realistic. The most common fault encountered in practice is limescale buildup on the heating elements in hard water areas – this problem manifests as increased consumption and louder operation. The solution is regular descaling or water treatment in the system (softening). More information can be found in the article Maintenance and servicing of the Thermona THERM EL electric boiler – what and how often to check.
Can I connect THERM EL to a domestic hot water (DHW) tank?
Yes. THERM EL can control the charging of a DHW tank via an additional output (depends on the model and control version). The tank is connected to the system with DHW priority – during hot water heating, space heating is temporarily suspended, the boiler preferentially heats the tank, and then returns to heating. This is a common solution in new buildings where a gas boiler with integrated DHW preparation is not available.
What's the difference between THERM EL 30 and THERM EL 38 – is it worth paying extra for a higher output?
The THERM EL 30 and THERM EL 38 models are also covered in other articles. Short answer: 30 kW output covers most family houses up to 200 m² with a standard insulation level. The 38 kW model is intended for larger houses, multi-unit dwellings, or systems with greater heat loss. Overpaying for higher output that you will never actually use in practice is not efficient – the boiler will only work at a low power level and its efficiency will not be optimal. Correct sizing is key, and this is covered in detail in the article How to choose a Thermona THERM EL electric boiler – power, room size, and other criteria.
Conclusion – where THERM EL excels and where it has limits
Thermona THERM EL is a well-constructed, practically proven mid-range electric boiler that offers a good balance between price, build quality, service availability, and ease of installation. It's not the cheapest boiler on the market – and that's a good thing, because the lowest price usually hides something. At the same time, it's not a premium technological flagship with full smart home integration – and that's fine, because most customers don't need that functionality and don't want to pay for something they won't use.
If you're looking for reliable, maintenance-free heating without a chimney, with the option of stepped power regulation, excellent availability of spare parts and service, and your heating needs fall within the range of 8 to 38 kW, the Thermona THERM EL range is definitely worth your attention. You can find more details on choosing the right model, installation, and initial commissioning in the articles Setting up and commissioning a Thermona THERM EL boiler – first start-up step by step and Frequently asked questions about Thermona THERM EL boilers – selection, installation, operation, and faults.
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