How to Choose a Thermona THERM EL Electric Boiler – Output, Room Size and Other Criteria
How to choose an electric boiler Thermona THERM EL – power, room size and other criteria
Selecting the right electric boiler is not something you can just tick off with a few clicks based on price. A too weak boiler will not be able to heat up the client from Liptov, who bought an 8-kilowatt unit for a 180 m² older house and then wondered why the boiler was running at full power non-stop and still couldn't manage. On the other hand, a boiler that is too powerful unnecessarily loads the electrical installation, is more expensive to buy and wears out faster due to shorter cycling. In this guide, we will go through all the criteria that determine the correct choice of boiler from the Thermona THERM EL series, and we will go through the entire decision-making process step by step – from the heat loss of the building to the types of heating systems and regulation options.
What is Thermona THERM EL and how it differs from other electric boilers
Thermona THERM EL is a series of wall-mounted electric boilers from the Czech manufacturer Thermona, intended for heating of family houses, apartments, chalets and smaller commercial spaces via a hot water heating system. Unlike direct heating electric convector heaters or storage stoves, THERM EL works on the same principle as a classic gas boiler – it heats water that circulates in the radiators or floor heating circuits.
The THERM EL series is produced in power variants THERM EL 8, THERM EL 15, THERM EL 23, THERM EL 30 and THERM EL 38, where the number in the name corresponds to the maximum electrical power in kilowatts. A key feature is the stepped switching of power – the boiler does not behave as "either everything or nothing", but activates only as many heating elements as it needs at the moment. How exactly this stepwise regulation works and why it is so important in terms of operation, you will find in detail in the article Stepwise power regulation in Thermona THERM EL boilers – how it works and why it is important.
Who is this series suitable for and where there are better alternatives, you will find out in the article Thermona THERM EL vs. other electric boilers – what is the difference and for whom is it suitable? Here we focus exclusively on how to choose the right model within this series.
The first and most important step: calculating the heat loss of the building
The entire boiler selection stands and falls with one number – the heat loss of your building expressed in kilowatts. It is the amount of heat that the house loses in the coldest weather (so-called design outdoor temperature) through the surrounding structures, windows, roof and ventilation air. The boiler must be able to replace this heat – neither more nor less.
Heat loss is influenced by a number of factors at once:
- Area and shape of the building envelope – the larger the surface and the smaller the volume, the higher the losses
- Thickness and type of insulation – an uninsulated masonry house can have a loss up to 5 times higher than a new construction in a passive standard of the same area
- Type of windows and doors – old double-glazed windows have a U-value of around 2.8 W/m²K, modern triple-glazed windows drop below 0.7 W/m²K
- Climatic zone – the design outdoor temperature varies in Slovakia from −12 °C in warmer areas to −18 °C and less in higher altitudes
- Type and condition of the roof – an unheated attic air layer vs. an inhabited attic with an insulated roof
- Method of ventilation – natural ventilation vs. recuperation with an efficiency of 75–90 %
An accurate calculation according to the standard STN EN 12831 is done by a designer or energy auditor. If your house's energy certificate does not exist and you do not have the design documentation, you can use approximate specific values of heat loss:
| Type of building and condition | Specific heat loss (W/m²) | Example: 120 m² |
|---|---|---|
| Passive house, new construction after 2020 | 15–25 | 1.8–3.0 kW |
| New construction with ETICS, year 2010–2020 | 30–50 | 3.6–6.0 kW |
| House renovated after 2000 | 50–70 | 6.0–8.4 kW |
| Older masonry house, partial insulation | 70–100 | 8.4–12 kW |
| Older house, no insulation, leaky windows | 100–150 | 12–18 kW |
Practical example: A customer from Banska Bystrica has a brick family house built in 1978, with an area of 148 m². In 2012, the façade was insulated with 8 cm of polystyrene and the windows were replaced with plastic double glazing. The house has a loss somewhere between 70–90 W/m². At 80 W/m² × 148 m² = 11,840 W ≈ 12 kW. For such a house, the correct choice is THERM EL 15 – it covers the heat loss with a small reserve and the remaining kilowatts will be appreciated in the case of hot water preparation or in extremely cold weather.
A deeper look at the calculation methodology, formulas and tables for different climatic zones of Slovakia can be found in the follow-up article What power of the electric boiler Thermona THERM EL do I need for my house?
Approximate rules power → area (and where they fail)
On the internet, there are many "guaranteed" rules such as "1 kW per 10 m²" or "1 kW per 8 m²". These numbers are at best approximate and in practice very unreliable, as they ignore all the factors mentioned above. Despite this, we mention them here as a quick first filter, after which a more detailed estimate must follow:
Note: These ranges apply to a house with standard insulation (approx. 70–80 W/m²). A new building with a passive standard can operate in the same area range with one step lower power. Conversely, an older house without insulation may require a boiler that is one to two steps stronger.
Type of heating system and heating medium temperature
The choice of boiler power is also linked to the type of heating system you have or plan to install. The differences are significant:
Classic radiators (high-temperature system)
An older panel radiator designed for 70/55 °C (supply/return) works properly at this temperature. If you operate it at a lower temperature, it must be either larger or you will feel cold. The electric boiler THERM EL can supply a supply temperature up to 85 °C, so it has no problem with old radiators. Consequence for selection: here you need exactly as many kW as corresponds to the heat loss – without reduction.
New low-temperature radiators and panel convectors
Modern steel radiators designed for 55/45 °C or even 45/35 °C have a significantly larger surface area. They are more efficient and suitable for an electric boiler, because the lower medium temperature means lower pipe losses. In combination with a modern new build, you can consider a lower boiler power.
Floor heating
Floor heating works with supply temperatures of 30–45 °C, which is advantageous for an electric boiler – lower losses, longer warm-up times, greater thermal inertia of the system. The disadvantage is that the floor slowly reacts to changes in outdoor temperature, so the regulation must be set with a lead time. This does not, however, mean a change in the required boiler power for the electric boiler – it still corresponds to the heat loss of the house.
Combined systems (radiators + floor heating)
A common case in practice – the ground floor has floor heating (temperature 35–40 °C), the upper floor has radiators (temperature 55–65 °C). In such a case, a hydraulic manual or motorized reduction is installed, the boiler heats to a higher temperature and a mixing set is installed before the floor heating circuit. The boiler power must cover the total sum of both circuits.
Preparation of hot utility water – boiler or tank?
Thermona THERM EL is primarily a heating boiler – it does not prepare hot utility water (HUC) by itself. There are two common solutions for the preparation of HUC:
- Electric storage water heater – a separate device that is independent of the boiler. The tank (80–150 l for a family of 3–4 people) has its own electrical connection. Advantage: the boiler does not need a higher power due to HUC, the tank is charged during the night in a lower tariff.
- Tank connected to the heating circuit – indirect heating of the tank via a heat exchanger. The boiler charges the tank with priority over heating. In this case, you must add the power required to heat the water to the boiler power, or choose a boiler with sufficient power for both tasks. A 150 l tank from 10 °C to 55 °C requires approximately 6.5 kW additionally during charging within 1 hour.
Practical example: A customer from Trenčín had a house with a heat loss of 9 kW and was considering THERM EL 15 with a connection to a 120-liter tank. The correct solution – 15 kW comfortably covers 9 kW of heating + 6 kW of tank charging with a slight reserve. If he had chosen THERM EL 8, the tank would have charged very slowly and in the coldest weather it would not have been able to heat simultaneously.
Electrical connection and tariffs – limiting factors
An electric boiler is not only about choosing a model – your electrical connection must be able to handle it. Here are the hard technical limitations that you need to consider even before choosing the power:
Important: All models of the THERM EL series are three-phase devices. If you have only a single-phase connection (230 V) in your apartment or cottage, these boilers are not suitable for you – look for an alternative solution. Most single-family homes have a three-phase connection, but its capacity (main circuit breaker limit and reserved capacity with the distributor) may limit the choice of power. A typical main circuit breaker in a single-family home in Slovakia is 3×25 A or 3×32 A. This is sufficient for the THERM EL 15 without any problems. With the THERM EL 23, you will reach the limit of 3×25 A, and for higher capacities, you will need to request an increase in reserved capacity from the distributor (VSD, ZSD, SSE). This is an administrative and financial process that can take several months – it should be factored into your planning. More detailed requirements for the electrical installation can be found in the article Installation of the Thermona THERM EL electric boiler – procedure, requirements for electrical installation and inspection.
Cost-effective night tariff (HDO)
For an electric boiler, the use of the night (low) tariff is very important, within the so-called HDO (remote load management). The distributor offers a significantly lower electricity price during the night tariff than the daytime rate. The THERM EL boiler can be set up to mainly charge the TÚV tank or increase the temperature in the system during the night (when the low tariff is active), and only maintain it during the day – thus reducing operating costs. Details on the setup can be found in the article Setting up and commissioning the Thermona THERM EL boiler – first start-up step by step.
Installation and space requirements
Thermona THERM EL is a compact wall-mounted boiler – all models in the series have similar cabinet depth and height, differing only in weight and the number of heating elements inside. The boiler can be mounted on a standard wall mounting strip. A minimum service clearance in front of the boiler (for opening the cover and possible service) is 60 cm. At least 20–30 cm of free space should be left above and below the boiler for the electronics and access to the hydraulic connections.
Unlike a gas boiler, the THERM EL does not require any chimney opening, exhaust flue, or air supply hose. This significantly simplifies the choice of installation location – the boiler can be placed in the kitchen, hallway, utility room, or even in a larger built-in cabinet, provided that service access is ensured. No building permit is required, which would be necessary in some cases for passing through fire-rated partitions.
Control and intelligent management – what the THERM EL series offers
The boiler's power is just one side of the coin. Equally important is how the boiler manages its work. Thermona THERM EL is equipped with an integrated control system with the option to connect an eCOthermic regulation – the boiler measures the outside temperature via a sensor and automatically adjusts the heating medium temperature according to how cold it is outside. Result: the system cycles less, heating is smoother, and consumption is lower.
Other regulation options include:
- Room thermostat (ON/OFF or modulating) – the basic and most widespread form of regulation
- Weekly programmer – different temperature programs for weekdays and weekends
- OpenTherm communication – modern thermostats (e.g. Honeywell Evohome, Heatmiser, Nest) communicate with the boiler digitally and allow more precise regulation
- External HDO input terminals – the boiler can respond to the distributor's signal and limit consumption during peak times or, conversely, charge the tank at night
Decision-making process step by step
Concrete practical scenarios – which model for which building
Scenario 1: Small apartment or cottage up to 60 m²
A cottage in the Nízkе Tatry, area 55 m², wood-concrete construction, 12 cm insulated façade, triple glazing. Heat loss is approximately 50–60 W/m², i.e. 3.0–3.3 kW. TÚV is handled by a separate 80-liter electric water heater. The right choice: THERM EL 8 – it will cover the heat loss with a large reserve and will operate at only 40–50% capacity, which is energy-efficient. A 3×16 A connection is sufficient.
Scenario 2: Renovated single-family home, 130 m²
Brick from 1985, 10 cm ETICS, plastic windows with double glazing, attic insulated with 20 cm mineral wool. Heat loss is approximately 65 W/m² × 130 m² = 8.5 kW. A 150-liter TÚV tank connected to the boiler. Required power for heating the tank in 1 hour: 7 kW. Total during simultaneous operation: 8.5 + 7 = 15.5 kW. Conclusion: THERM EL 15 with priority for the tank – during tank charging, heating is temporarily limited, which is acceptable, as the house has sufficient thermal inertia. Alternative: THERM EL 23, if you want more comfort and simultaneous operation without limitations.
Scenario 3: New build, 200 m², floor heating
Brick house, built in 2019, 16 cm ETICS, triple glazing, heat recovery ventilation with 80% efficiency. Heat loss is approximately 25 W/m² × 200 m² = 5 kW. TÚV is separate (heat pump). Conclusion: THERM EL 8 will easily cover even the coldest winter and will still have a reserve. Some customers in this case consider a heat pump as the primary source with an electric boiler as a backup – this is a technically elegant solution, but a different topic.
Scenario 4: Older house without insulation, 180 m², replacing a gas boiler
Brick from 1970, no insulation, older wooden-aluminum double glazing, roof without insulation. Estimated heat loss is 120–140 W/m². At 130 W/m² × 180 m² = 23.4 kW. TÚV is separate for now. Conclusion: THERM EL 23 at the limit of capacity, ideally THERM EL 30 with a reserve. Note: the connection must be verified, an increase in reserved capacity will likely be required. At the same time, I recommend considering insulating the house in such a case – the investment in insulation will pay off faster than the long-term electricity overpayment due to such heat loss.
Scenario 5: Larger commercial space or office, 350 m²
Office space in an older building, ceiling height 3.0 m, partial insulation. Estimated heat loss is 85 W/m² × 350 m² = 29.75 kW. Conclusion: THERM EL 30 or THERM EL 38 according to the precise calculation. At such power levels, it is essential to have design documentation, a statement from the distributor, and an inspection of the electrical installation.
What to avoid when choosing – most common mistakes
- Selecting based on area without considering insulation – the most common mistake. A house of 120 m² may require 5 kW or 18 kW depending on the condition of the building envelope.
- Ignoring the electrical connection – the customer orders THERM EL 30, the electrician arrives and finds out that the main circuit breaker is 3×20 A. The boiler has to be returned.
- Forgetting about DHW (Domestic Hot Water) – the customer calculates a loss of 8 kW, orders THERM EL 8, and then wants to connect a storage tank. The boiler is not sufficient.
- Overdimensioning "just to be safe" by two power levels – a boiler with too high a power output cycles, meaning it turns on and off unnecessarily in short cycles. This accelerates the wear of the heating elements and electronics. A correct reserve is one power level higher – no more.
- Not securing the right tariff for the electric boiler – without a suitable tariff for heating (e.g., DD5 or DD6 in Slovakia), you pay significantly more. It is necessary to agree on the correct tariff with the electricity supplier before installation.
Model comparison – summary overview
| Model | Max. output | Recommended area* | Circuit breaker | Typical use |
|---|---|---|---|---|
| THERM EL 8 | 8 kW | up to 80–100 m² | 3×16 A | Apartment, cottage, new build up to 150 m² |
| THERM EL 15 | 15 kW | 80–150 m² | 3×25 A | Medium single-family house, renovation with a storage tank |
| THERM EL 23 | 23 kW | 150–230 m² | 3×40 A | Larger single-family house, combination of heating + DHW |
| THERM EL 30 | 30 kW | 200–300 m² | 3×50 A | Large single-family house, older house without insulation, commercial use |
| THERM EL 38 | 38 kW | 300–400 m² | 3×63 A | Large building, commercial spaces, backup |
* Recommended area applies to buildings with average insulation (70–80 W/m²). For poorly insulated buildings, use a model one step more powerful, for passive houses one step less powerful.
Most frequently asked questions (FAQ)
Can I use THERM EL 8 as a backup for a heat pump?
Yes, using an electric boiler as a backup for a heat pump is a common solution. The heat pump covers most of the season, and the electric boiler kicks in during the cold when the COP of the pump drops. In this case, you choose the boiler power according to the heat loss of the house in extreme cold (design temperature), because it must function on its own at that time. The hydraulic scheme must be designed so that the boiler and pump do not operate simultaneously in the same circuit without proper control – more in the article Connecting Thermona THERM EL to the heating system – connection diagrams and hydraulic requirements.
How much electricity does THERM EL 15 consume per month?
It depends on the heat loss of the house, outside temperature, and the type of regulation. A rough estimate: with an average heat consumption of 8,000–12,000 kWh per heating season (typical renovated house of 120–150 m²) and an average electricity price of 0.18–0.22 €/kWh, the annual heating costs amount to 1,400–2,600 €. Exact figures depend on local conditions, the set temperature, and the use of night tariffs.
Is a building permit required for the installation of an electric boiler?
Usually not for the boiler itself – within an apartment building or a single-family house, it is considered a replacement of the heating equipment, which does not require a building permit. However, professional electrical installation by a person with a valid electrical qualification (min. § 21 of Government Regulation 508/2009 Coll.) and a subsequent electrical inspection are required. If there is a significant increase in power (e.g., from 15 to 38 kW), a request to the electricity network distributor for a change in reserved capacity is necessary.
Can I install the boiler myself?
The hydraulic part (pipes, radiators, expansion tank) can be done by a skilled owner with some experience, although we recommend a professional. The electrical part – connecting the boiler to the distribution board, dimensioning the cabling, and installing circuit breakers – must be done by a qualified electrician, otherwise an electrical inspection cannot be performed and the boiler may not be under warranty for repairs by the manufacturer.
What is the lifespan of the electric heating elements in the THERM EL boiler?
With proper water quality (pH 7–9, hardness up to 2.0 mmol/l) and regular maintenance, the heating elements can last 10–15 years. Aggressive water with high hardness (calcium) or low pH causes rapid limescale buildup and corrosion, which shortens the lifespan. Therefore, it is important to fill the system with treated water and add a corrosion inhibitor. More on maintenance can be found in the article Maintenance and service of the Thermona THERM EL electric boiler – what and how often to check.
What if I find out that I chose a weak boiler – can it be replaced?
Yes, the mounting is standard and the THERM EL models have the same hydraulic connections, so replacing it with a more powerful model is relatively simple from a mechanical point of view. The problem arises with the electrical installation – a more powerful boiler requires a thicker cable and a larger circuit breaker. Therefore, it is better to do the calculation properly from the start and not to skimp on the one-step reserve.
Conclusion
Choosing the right model from the Thermona THERM EL range is not a mystery, but a systematic process. Start with the heat loss of the building – either get an energy audit or use approximate values according to the type and condition of the house. Add DHW preparation if you want to handle it via the boiler. Check the capacity of the electrical connection and fusing. Only then choose a specific model – and choose with one power reserve upwards, not more. The THERM EL series is designed to cover almost all common applications from a small apartment (THERM EL 8) through a typical single-family house (THERM EL 15 or THERM EL 23) to larger buildings (THERM EL 30, THERM EL 38). If you are unsure, also check other articles in this Knowledge Center – for example, Common questions about Thermona THERM EL boilers – selection, installation, operation, and faults – where you will find answers to many other specific situations.
Do you have a question on this topic?
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