How to Choose a Thermona THERM EL Electric Boiler – Output, Room Size and Other Criteria
How to choose a Thermona THERM EL electric boiler – output, room size and other criteria
Choosing the right electric boiler isn't something you can settle with a few clicks based on price alone. A boiler that's too weak won't fully heat your home in January – as one customer from Liptov found out after buying an 8-kilowatt unit for an older 180 m² house, only to wonder why the boiler was running at full power around the clock and still couldn't keep up. A boiler that's too powerful unnecessarily strains the electrical wiring, costs more to buy, and wears out faster due to shorter cycling. In this guide we'll go through all the criteria that determine the correct choice of boiler from the Thermona THERM EL series, and walk through the entire decision-making process step by step – from the building's heat loss to the types of heating systems and control options.
What is Thermona THERM EL and how does it differ from other electric boilers
Thermona THERM EL is a series of wall-mounted electric boilers made by the Czech manufacturer Thermona, designed for heating family houses, apartments, cottages and smaller commercial spaces via a hot-water heating system. Unlike direct-heating electric convectors or storage heaters, THERM EL works on the same principle as a classic gas boiler – it heats water that circulates through radiator circuits or underfloor heating.
The THERM EL series is manufactured in the following 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 input in kilowatts. A key feature is stepwise output switching – the boiler doesn't behave in an "all or nothing" way, but instead activates only as many heating elements as it currently needs. You can find out exactly how this staging works and why it's so important for operation in the article Stepwise output control in Thermona THERM EL boilers – how it works and why it matters.
Who this series is suitable for, and where better alternatives exist, is covered in the article Thermona THERM EL vs. other electric boilers – what's different and who is it suitable for? Here we'll focus solely on how to choose the right model within this series.
The first and most important step: calculating the building's heat loss
The entire boiler selection stands or falls on a single number – your building's heat loss, expressed in kilowatts. This is the amount of heat the house loses through its outer walls, windows, roof and ventilation air during the coldest weather (the so-called design outdoor temperature). The boiler must be able to replace this heat – no more, no less.
Heat loss is affected by many 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 brick house can have up to 5× higher heat loss than a new passive-standard building of the same floor area
- Type of windows and doors – old double-glazed windows have a U-value of around 2.8 W/m²K, while modern triple glazing drops below 0.7 W/m²K
- Climate zone – the design outdoor temperature in Slovakia ranges from −12 °C in warmer regions to −18 °C or lower at higher altitudes
- Roof type and condition – unheated attic air layer vs. a habitable loft with an insulated roof
- Ventilation method – natural ventilation vs. heat recovery with 75–90% efficiency
An accurate calculation according to the STN EN 12831 standard is performed by a designer or energy auditor. If your house doesn't have an energy certificate and you don't have project documentation, you can use the following approximate specific heat loss values:
| Building type and condition | Specific heat loss (W/m²) | Example: 120 m² |
|---|---|---|
| Passive house, new build after 2020 | 15–25 | 1.8–3.0 kW |
| New build with ETICS, 2010–2020 | 30–50 | 3.6–6.0 kW |
| House renovated after 2000 | 50–70 | 6.0–8.4 kW |
| Older brick house, partially insulated | 70–100 | 8.4–12 kW |
| Older house, uninsulated, leaky windows | 100–150 | 12–18 kW |
A practical example: A customer from Banská Bystrica has a brick family house built in 1978, with a floor 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 heat loss of somewhere between 70–90 W/m². At 80 W/m² × 148 m² = 11,840 W ≈ 12 kW. For a house like this, the correct choice is the THERM EL 15 – it will cover the heat loss with a small reserve, and you'll appreciate the extra kilowatts for domestic hot water preparation or during extremely cold weather.
A deeper look at the calculation methodology, formulas and tables for various climate zones in Slovakia can be found in the follow-up article What output of a Thermona THERM EL electric boiler do I need for my house?
Rule-of-thumb ratios of output to area (and where they fail)
There are many "guaranteed" rules circulating online, such as "1 kW per 10 m²" or "1 kW per 8 m²". These figures are at best a rough guide and are very unreliable in practice, because they ignore all the factors mentioned above. Nevertheless, we present them here as a quick first filter, which must be followed by a more detailed estimate:
Note: These ranges apply to houses with standard insulation (approx. 70–80 W/m²). A new building with a passive standard can operate with one level lower output for the same floor area. Conversely, an older uninsulated house may need a boiler one or two levels more powerful.
Type of heating system and heating medium temperature
The choice of boiler output is also linked to the type of heating system you have or are planning to install. The differences are significant:
Classic radiators (high-temperature system)
An older panel-house radiator designed for 70/55 °C (flow/return) works correctly at this temperature. If you operate it at a lower temperature, it either has to be larger, or you'll be cold. The Thermona THERM EL electric boiler can supply a flow temperature of up to 85 °C, so it has no problem with old radiators. Consequence for the selection: here you need exactly as many kW as correspond to the heat loss – with no 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 economical and better suited for an electric boiler, because a lower medium temperature means lower heat loss in the piping. Combined with a modern new building, you can consider a boiler with lower input power.
Underfloor heating
Underfloor heating works with flow temperatures of 30–45 °C, which is advantageous for an electric boiler – smaller losses, longer start-up times, greater thermal inertia of the system. The disadvantage is that the floor reacts slowly to outdoor temperature changes, so the controls must be set with a lead time. For an electric boiler, however, this doesn't change the required output – it still corresponds to the building's heat loss.
Combined systems (radiators + underfloor heating)
A common case in practice – the ground floor has underfloor heating (temperature 35–40 °C), the upper floor has radiators (temperature 55–65 °C). In this case a manual or motorized hydraulic reduction is installed, the boiler heats to the higher temperature, and a mixing unit is placed ahead of the underfloor circuit. The boiler's output must cover the total sum of both circuits.
Domestic hot water preparation – boiler or storage tank?
Thermona THERM EL is primarily a heating boiler – it does not prepare domestic hot water (DHW) on its own. There are two common solutions for DHW preparation:
- Electric storage water heater – a standalone appliance independent of the boiler. The tank (80–150 l for a family of 3–4) has its own electrical connection. Advantage: the boiler doesn't need extra output for DHW, and the tank is charged overnight at a lower tariff.
- Tank connected to the heating circuit – indirect tank heating via a heat exchanger. The boiler charges the tank with priority over heating. In this case you must add the output required for water heating to the boiler's output, or choose a boiler with sufficient output for both tasks. A 150 l tank heated from 10 °C to 55 °C requires approx. 6.5 kW extra when charged over 1 hour.
A practical example: A customer from Trenčín had a house with a heat loss of 9 kW and was considering a THERM EL 15 connected to a 120-litre tank. Correct solution – 15 kW comfortably covers 9 kW of heating + 6 kW of tank charging with a slight reserve. Had he chosen a THERM EL 8, the tank would have charged very slowly, and in the coldest weather the heating would have fallen behind at the same time.
Electrical connection and tariffs – limiting factors
An electric boiler isn't just about choosing a model – your electrical connection must be able to handle it. Here are the hard technical limits to consider before selecting the output:
Important: All models in the THERM EL series are three-phase appliances. If you only have a single-phase connection (230 V) in your apartment or cottage, these boilers are not suitable for you – look for a different solution. Most family houses do have a three-phase connection, but its capacity (main breaker limit and contracted reserved capacity with the distributor) may limit the choice of output.
A typical main breaker for a family house in Slovakia is 3×25 A or 3×32 A. This is sufficient for the THERM EL 15 without any problem; with the THERM EL 23 you reach the limit of 3×25 A, and for higher outputs you will need to apply for an increase in the 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 Installing a Thermona THERM EL electric boiler – procedure, electrical installation requirements and inspection.
Off-peak tariff (HDO)
For an electric boiler, it's very important to make use of the night (low) tariff, within the so-called HDO (remote-controlled load switching) system. During the low tariff period, the distributor offers an electricity price significantly lower than the daytime rate. The THERM EL boiler can be set so that it mainly charges the DHW tank or raises the system temperature during the night (when the tariff is low), and only maintains it during the day – which can reduce operating costs. Details on 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 a similar casing depth and height, differing only in weight and the number of internal heating elements. The boiler can be hung on a standard wall mounting bracket. The minimum service clearance in front of the boiler (for opening the cover and any servicing) is 60 cm. At least 20–30 cm of free space should be left above and below the boiler for cooling the electronics and access to the hydraulic connections.
Unlike a gas boiler, the THERM EL requires no chimney flue, no flue gas discharge, and no 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 servicing access is ensured. No building permit is required, which in some cases would be needed for passage through fire-separated areas.
Control and smart management – what the THERM EL series offers
The boiler's output alone is only one side of the coin. Equally important is how the boiler manages its work. Thermona THERM EL features a built-in control system with the option to connect weather-compensated (equithermal) control – the boiler measures the outdoor 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 control options include:
- Room thermostat (ON/OFF or modulated) – the basic and most widespread form of control
- Weekly programmer – different temperature programs for weekdays and weekends
- OpenTherm communication – modern thermostats (e.g. Honeywell Evohome, Heatmiser, Nest) communicate with the boiler digitally, allowing more precise control
- External HDO input terminals – the boiler can respond to a signal from the distributor and reduce consumption during peak periods, or charge the tank at night
Step-by-step decision-making process
Real-life scenarios – which model for which building
Scenario 1: Small apartment or cottage up to 60 m²
A cottage in the Low Tatras, floor area 55 m², timber-concrete construction, 12 cm insulated façade, triple glazing. Heat loss approx. 50–60 W/m², i.e. 3.0–3.3 kW. DHW is handled separately by a small 80-litre electric boiler. Correct choice: THERM EL 8 – it will cover the heat loss with a large reserve, running at only 40–50% output, which is economical. A 3×16 A connection is sufficient.
Scenario 2: Renovated family house, 130 m²
Brick, built in 1985, 10 cm ETICS, plastic double-glazed windows, loft insulated with 20 cm mineral wool. Heat loss approx. 65 W/m² × 130 m² = 8.5 kW. A 150 l DHW tank connected to the boiler. Required output to heat the tank in 1 hour: 7 kW. Sum for simultaneous operation: 8.5 + 7 = 15.5 kW. Conclusion: THERM EL 15 with tank priority – during tank charging the heating is temporarily reduced, which is acceptable, as the house has sufficient thermal inertia. Alternative: THERM EL 23, if you want greater comfort and simultaneous operation without limitation.
Scenario 3: New building, 200 m², underfloor heating
Masonry house, built in 2019, 16 cm ETICS, triple glazing, heat recovery ventilation with 80% efficiency. Heat loss approx. 25 W/m² × 200 m² = 5 kW. DHW handled separately (heat pump). Conclusion: THERM EL 8 comfortably covers even the coldest winter and still has a reserve. Some customers in this case consider a heat pump as the primary source with an electric boiler as backup – this is a technically elegant solution, but a different topic.
Scenario 4: Older uninsulated house, 180 m², replacing a gas boiler
Brick, built 1970, uninsulated, older wood-aluminium double glazing, uninsulated roof. Estimated heat loss 120–140 W/m². At 130 W/m² × 180 m² = 23.4 kW. DHW handled separately for now. Conclusion: THERM EL 23 at the limit of capacity, ideally THERM EL 30 with a reserve. Note: the connection must be verified, and an application to increase reserved capacity will likely be needed. I would also recommend considering insulating the house in this case – the investment in insulation will pay off faster than years of overpaying for electricity with such heat loss.
Scenario 5: Larger commercial space or offices, 350 m²
Office space in an older building, ceiling height 3.0 m, partially insulated. Estimated heat loss 85 W/m² × 350 m² = 29.75 kW. Conclusion: THERM EL 30 or THERM EL 38, depending on the precise calculation. At such outputs, project documentation, a statement from the distributor and an electrical installation inspection are essential.
What to avoid when choosing – the most common mistakes
- Choosing by floor area without considering insulation – the most common mistake. A 120 m² house may need anywhere from 5 kW to 18 kW depending on the condition of the envelope.
- Ignoring the electrical connection – a customer orders a THERM EL 30, the electrician arrives and finds the main breaker is 3×20 A. The boiler has to be returned.
- Forgetting about DHW – a customer calculates a loss of 8 kW, orders a THERM EL 8, and then wants to connect a tank. The boiler isn't enough.
- Oversizing "just to be safe" by two output levels – a boiler with too much output cycles, i.e. switches on and off unnecessarily in short cycles. The heating elements and electronics wear out faster as a result. The correct reserve is one extra level – not more.
- Not securing a suitable tariff for an electric boiler – without a rate suited to heating (e.g. DD5 or DD6 in Slovakia), you pay considerably more. Make sure to arrange the correct rate with your electricity supplier before installation.
Model comparison – summary overview
| Model | Max. output | Recommended area* | Breaker | Typical use |
|---|---|---|---|---|
| THERM EL 8 | 8 kW | up to 80–100 m² | 3×16 A | Apartment, cottage, new building up to 150 m² |
| THERM EL 15 | 15 kW | 80–150 m² | 3×25 A | Medium-sized house, renovation with a tank |
| THERM EL 23 | 23 kW | 150–230 m² | 3×40 A | Larger house, combined heating + DHW |
| THERM EL 30 | 30 kW | 200–300 m² | 3×50 A | Large house, older uninsulated house, commercial use |
| THERM EL 38 | 38 kW | 300–400 m² | 3×63 A | Large building, commercial spaces, backup |
* The recommended area applies to buildings with average insulation (70–80 W/m²). For poorly insulated buildings, use a model one level more powerful; for passive houses, one level less powerful.
Frequently Asked Questions (FAQ)
Can I use a THERM EL 8 as a backup to a heat pump?
Yes, using an electric boiler as a backup source to a heat pump is a common solution. The heat pump covers most of the season, while the electric boiler kicks in during frosts, when the pump's COP drops. In this case you choose the electric boiler's output based on the house's heat loss during extreme frosts (design temperature), because that's exactly when it must operate on its own. The hydraulic scheme must be designed so that the boiler and pump don't work simultaneously into the same circuit without proper control – more in the article Connecting Thermona THERM EL to the heating system – wiring diagrams and hydraulic requirements.
How much electricity does a THERM EL 15 consume per month?
This depends on the house's heat loss, outdoor temperature and control method. Rough estimate: with 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 cost comes to €1,400–2,600. Exact figures depend on local conditions, the set temperature and use of the off-peak tariff.
Is a building permit required to install an electric boiler?
Generally not for the boiler itself – within an apartment building or family house, it's a replacement of a heating appliance, which doesn't require a building permit. However, professional installation of the electrical part by a person with valid electrotechnical qualification (min. § 21 of Decree 508/2009 Z.z.) is required, along with a subsequent electrical inspection. If there's a significant increase in input power (e.g. from 15 to 38 kW), an application to the electricity distributor for a change in reserved capacity is needed.
Can I install the boiler myself?
The hydraulic part (piping, radiators, expansion tank) can be done by a handy owner with experience, although we recommend a professional. The electrical part – connecting the boiler to the switchboard, sizing the wiring, installing protective devices – must be carried out by an electrician with the appropriate professional qualification; otherwise the electrical inspection cannot be performed and the boiler may not be covered by the manufacturer's warranty repairs.
What is the lifespan of the electric heating elements in a 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 faster limescale buildup and corrosion processes that shorten the lifespan. It is therefore important to fill the system with treated water and add a corrosion inhibitor. More on maintenance can be found in the article Maintenance and servicing of the Thermona THERM EL electric boiler – what to check and how often.
What if it turns out I chose a boiler that's too weak – can it be replaced?
Yes, the wall bracket is standard and models in the THERM EL series share the same hydraulic connections, so replacing it with a more powerful model is relatively straightforward mechanically. The problem arises with the electrical installation – a more powerful boiler requires a thicker cable and a larger breaker. That's why it's better to do the calculation properly right from the start and not skimp on the one-level reserve.
Conclusion
Choosing the right model from the Thermona THERM EL range isn't a mystery, but a systematic process. Start with the building's heat loss – either have an energy audit calculated, or use approximate values based on the type and condition of the house. Add domestic hot water preparation if you want to handle it via the boiler. Check the capacity of the electrical connection and its protection. Only then choose a specific model – and go with one output level of reserve upward, not more. The THERM EL series is designed to cover the vast majority of common applications, from a small apartment (THERM EL 8) through a typical family house (THERM EL 15 or THERM EL 23) to larger buildings (THERM EL 30, THERM EL 38). If you're not sure, check out other articles in this Knowledge Center – for example Frequently asked questions about Thermona THERM EL boilers – selection, installation, operation and faults – where you'll find answers to many other specific situations.
Do you have a question on this topic?
Can't decide, or dealing with a specific situation in your household? Write to us – we'll be happy to help.