Installation of the Thermona THERM EL electric boiler – procedure, electrical installation requirements and inspection

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Installing the Thermona THERM EL Electric Boiler – a Complete Technical Guide

The Thermona THERM EL electric boiler is a technically undemanding device when it comes to installing the heating circuit itself – but the electrical part of the installation is exactly where the most common mistakes occur, leading to malfunctions, breaker trips, or even rejection by the inspecting electrician. This article is intended for installation technicians, electricians, and skilled DIY enthusiasts who want to understand what happens behind the distribution board and in the heating circuit. We will cover both mechanical and electrical installation, cable sizing requirements, overcurrent protection, grounding, protection against dangerous contact, and the procedure for a professional inspection of the installation.

If you're still deciding which boiler model to choose for your home, first read our articles How to Choose a Thermona THERM EL Electric Boiler – Output, Space Size and Other Criteria and What Output of the Thermona THERM EL Electric Boiler Do I Need for My Home? Installation only makes sense once you have correctly sized the power input.

Overview of Models and Their Electrical Parameters

The THERM EL series includes five power levels. Each model has slightly different electrical wiring requirements, so these values need to be clearly understood before purchasing materials.

Model	Rated output (kW)	Power supply	Rated current (A)	Recommended circuit breaker (A)	Min. Cu cross-section (mm²)
THERM EL 8	8	3×230 V / 400 V	11.5	3×16 A	2.5
THERM EL 15	15	3×230 V / 400 V	21.7	3×25 A	4
THERM EL 23	23	3×230 V / 400 V	33.3	3×40 A	6
THERM EL 30	30	3×230 V / 400 V	43.5	3×50 A	10
THERM EL 38	38	3×230 V / 400 V	55.1	3×63 A	16

Note on current values: The values are calculated for cos φ = 1 (resistive load, which applies to an electric boiler), with a symmetrical three-phase load and a voltage of 400 V phase-to-phase (230 V phase-to-neutral). The actual rated current stated in Thermona's technical documentation may differ slightly from the theoretical calculation – always follow the value shown on the rating plate of the specific boiler.

Installation Location – What Must Be Fulfilled Before Bringing the Boiler to the Site

Before physically mounting the boiler, several structural and spatial conditions must be checked. In practice, at least a third of problems arise precisely because this is overlooked, and the distribution board later has to be rebuilt, cables extended, or the unit relocated.

Wall surface: The boiler is hung on a solid masonry, concrete, or otherwise load-bearing wall. Plasterboard without sufficient anchoring is not enough – a boiler filled with system water can weigh 25–35 kg, and even more for larger models once the system is filled.
Distance from flammable materials: The minimum distance from flammable structures is 200 mm on the sides and 500 mm above the boiler. The boiler must not be installed in closed cabinets without adequate ventilation.
Access to service panels: At least 600 mm of free space must be left in front of the boiler for servicing, replacing heating elements, and inspecting the electrical connection.
Ambient temperature: The minimum room temperature during operation is +5 °C, the maximum is +40 °C. The boiler is not suitable for unheated utility rooms where the temperature drops below zero in winter.
Humidity: Relative humidity must not permanently exceed 80%. The boiler is not suitable for rooms with condensing moisture (bathroom, unventilated laundry room).
Aggressive environment: The space must not contain chlorine, sulfur, or other aggressive vapours – these would cause corrosion on the electronics and contacts.

Mechanical Installation – Step-by-Step Procedure

Step 1: Unpacking and Inspection

After unpacking, check that neither the packaging nor the boiler itself is damaged. Check the contents of the package – boiler, mounting template, operating manual, and any accessories (the expansion tank, safety valve, and air vent are in most cases supplied separately and need to be purchased additionally). Check the rating plate and make sure the output, voltage, and frequency match your conditions (230/400 V, 50 Hz).

Step 2: Marking the Position and Anchoring

Use the supplied mounting template or hold the boiler against the wall and mark the holes for the mounting hooks. The boiler must be vertical – use a spirit level. Drill the holes according to the wall material (concrete: diameter 10 mm, length 80 mm, S10 or SK10 Fischer anchors). Make sure the hooks are evenly loaded – incorrect anchoring leads to mechanical stress on the connection ports of the hydraulic circuit.

Step 3: Hydraulic Connection

The boiler has the heating circuit inlet and outlet marked on the lower part of the body. Before connecting, install service ball valves (shut-off fittings) on both sides – this will greatly simplify replacing a heating element without draining the entire system. It is also advisable to install a Y-strainer (filter) on the return line, right before the boiler inlet, to prevent sediment from the system clogging the heating elements and circulation pump.

The pipe connections are usually threaded G ¾" or G 1" (depending on the model). Use hemp sealant with paste or Teflon tape – not rubber seals in direct contact with brass without a suitable surface. Watch the tightening torque: do not exceed 25–30 Nm for plastic couplings, and max. 50 Nm for brass ones.

The system pressure in the cold state must be 1.0–1.5 bar. You must connect the safety valve (usually 3 bar) and the expansion tank externally to the system – the boiler does not have them integrated (unlike some combined units). More detailed hydraulic diagrams can be found in the article Connecting the Thermona THERM EL to the Heating System – Wiring Diagrams and Hydraulic Requirements.

Electrical Installation – Requirements, Regulations, and Procedure

This is the core of the whole article. The electrical installation of an electric boiler is not ordinary socket-outlet wiring – it is a permanently connected Class I load, supplied from a three-phase network, with a rated input of up to 38 kW. Mistakes here have a direct impact on the safety of people and property.

Legal Framework

In Slovakia, the installation and inspection of electrical equipment for permanently connected loads is governed primarily by:

STN 33 2000-7-701 to 7-705 – special locations (bathrooms, outdoor areas)
STN 33 2000-4-41 – protection against electric shock
STN 33 2000-5-52 – selection and installation of electrical wiring systems
STN 33 1500 – inspection of electrical equipment
Decree of the Ministry of Labour, Social Affairs and Family of the Slovak Republic No. 508/2009 Coll. – occupational health and safety when working with electrical equipment

The electric boiler is a designated technical device (VTZ) of group E – its installation and commissioning may only be carried out by a person with the relevant professional qualification under §22 (electrician) or §23 (independent electrician) of the aforementioned decree. The inspection must then be carried out by an inspection technician holding a certificate under §24.

Sizing the Supply Cable

The supply cable must be sized for continuous current with a margin, taking into account:

Cable type: For fixed installation in a wall or conduit, use CYKY-J (copper, with a protective conductor). For underground installation, use CYKY-O or armoured cables. Aluminium cables are technically permissible for inputs above 15 kW, but for reasons of contact reliability and mechanical strength, they are not recommended for family homes.
Number of conductors: A 5-core cable – three phases (L1, L2, L3), a neutral conductor (N), and a protective conductor (PE). TN-C-S or TN-S system – the protective and neutral conductors must always be separate from the distribution board to the boiler.
Installation method: A cable installed in a wall (covered by plaster) can carry more current than a cable in a bundle or a cable duct with other cables – always apply the correction factors according to STN 33 2000-5-52.
Cable length: With a long supply line (over 20–25 m), an excessive voltage drop can occur – the maximum permitted voltage drop for the load is 4% from the connection point to the load. For a 30 kW boiler at a distance of 30 m, a cross-section of 16 mm² will be needed instead of the theoretical 10 mm².

Overcurrent Protection and Protective Devices

Each boiler must have its own dedicated outgoing circuit in the house distribution board, fitted with:

A three-pole circuit breaker: Characteristic B for common resistive loads (the Thermona THERM EL boiler has no motor starting currents, so characteristic C is not necessary, though it doesn't hurt). The rated current of the breaker is selected according to the table above.
A residual current device (RCD): Sensitivity 30 mA, type A (minimum) or type B (for devices with frequency converters – for THERM EL boilers without a VFD, type A is sufficient). The RCD must be four-pole (3P+N). Note: some older types of RCDs react to the switching pulses of the boiler's electronics – in practice, RCDs with a delay (super-immune types) or with impedance blocking, such as the SI type from Hager, ABB, or Eaton, have proven effective.
A main disconnector (service switch): A multi-pole disconnector must be installed near the boiler (no further than 3 m away) that safely disconnects all poles from the mains during servicing. This can be a combined breaker-disconnector in the distribution board, but only if the board is within reach of the operator from the boiler location (in most family homes this is not the case). The solution is a service disconnector right at the boiler – for example a Moeller FAZ or similar 4-pole 63 A unit.

Grounding and Equipotential Bonding

The PE protective conductor must be safely routed to the boiler's PE terminal block, and from there the boiler must be bonded to the entire equipotential bonding network (heating circuit piping, metal structures). The cross-section of the protective conductor must be at least equal to the cross-section of the phase conductor up to 16 mm², and at least half for larger cross-sections. In practice, a green-yellow PE conductor of the same cross-section as the phases is used for THERM EL boilers.

Surge Protection

The electronics of modern boilers are sensitive to surge pulses (lightning strikes, switching surges in the network). We recommend installing Class D surge protection (SPD Type 3) directly in the distribution board on the boiler's outgoing circuit. THERM EL boilers have their own electronics and controller – damage caused by a surge is not covered under warranty if surge protection is not installed.

Wiring the Boiler Terminal Block

After opening the boiler's terminal block cover, you will find the terminal block for the power supply and, in most cases, terminals for an external thermostat (a two-wire low-voltage circuit). The power supply is connected according to the diagram in the manual: L1, L2, L3, N, PE. The phase sequence is not decisive for the boiler's function with a resistive load, but for the sake of order in the distribution board and any future inspection, it is good practice to follow the standard colour coding: L1 – brown, L2 – black, L3 – grey, N – blue, PE – green-yellow.

Tighten the terminals to the torque recommended by the terminal block manufacturer (usually 2.5–4 Nm for terminals at 10–16 mm²). An under-tightened connection causes contact heating and gradual oxidation – for high-output boilers, this is a real fire risk factor.

An external room thermostat or programmer is connected to the low-voltage terminals marked TA (ambient thermostat) – this is a voltage-free contact (230 V must not be connected to these terminals!). Mistakenly connecting mains voltage to the thermostat terminals will destroy the boiler's controller.

Filling and Venting the System Before First Start-up

Before switching on the boiler for the first time, the heating system must be properly filled, vented, and pressurised. Procedure:

Close all air vents on the radiators and the heating system.
Open the filling valve and slowly fill the system with water, watching the pressure gauge – the target cold pressure is 1.0–1.5 bar.
Gradually open the air vents on the radiators – start from the highest points in the system and continue downwards. Close the valves once water starts flowing out without air.
Check the filling pressure on the gauge – after venting it usually drops slightly, so top up the water.
Visually check all connections, boiler connections, and fittings for leaks.
Switch on the circulation pump (if external) and let it run for 5–10 minutes – this will release any remaining air from the boiler's heating elements.

Water quality for the heating system: water hardness should not exceed 2.5 mmol/l (14 °dH). For harder water, treatment is recommended (adding a corrosion inhibitor and carbonates, or installing a softening filter). Untreated hard water causes limescale on the heating elements – this reduces their output and shortens their service life. Detailed instructions for the first start-up and controller setup can be found in the article Setting Up and Commissioning the Thermona THERM EL Boiler – First Start-up Step by Step.

Professional Inspection of the Electrical Installation

After completing the installation, a professional inspection of the electrical part is required, carried out by an inspection technician certified for group E5 (electrical installations and equipment). The inspection is legally required for every new installation of a permanently connected device over 3.5 kW. Without a valid inspection report, the insurance company may not cover the damage in the event of an insured event (fire, short circuit).

What the Inspection Includes

Visual inspection: Correctness of wiring, adherence to colour coding, mechanical protection of cables, correct sizing of wiring and protective devices, presence of the protective conductor.
Insulation resistance measurement: Between phase conductors and between phases and PE – minimum value 1 MΩ at a 500 V DC test voltage (according to STN 33 1500).
Protective conductor resistance measurement: From the boiler to the main earthing terminal – max. 1 Ω for standard residential installations.
Fault loop impedance measurement (Zs): Verifies that in the event of a fault (phase short-circuit to PE), the fault loop current actually trips the breaker within the required time (< 0.4 s for socket-outlet circuits, < 5 s for fixed installations – though with a sensitive 30 mA RCD, protection is faster).
Verification of RCD function: Measurement of tripping current and time – a 30 mA RCD must trip at 30 mA within < 300 ms, and at 150 mA within < 40 ms.
Functional test: Verification that the boiler starts up, the controller works, the circulation pump is controlled correctly, and the thermostat shuts down the boiler.

Periodicity of Repeat Inspections

For electrical installations in family homes (areas without increased risk), the periodicity of repeat inspections is 5 years. If the boiler is installed in a higher-risk environment (damp, dust, aggressive substances), the periodicity is shortened to 3 years. The inspection of the boiler itself as an appliance (VTZ type) is carried out at the same time or may be part of a service inspection – see the article Maintenance and Servicing of the Thermona THERM EL Electric Boiler – What and How Often to Check.

Common Mistakes in Practice – What Installers Do Wrong

Over the years of dealing with electric boiler installations, the same mistakes keep repeating. Don't repeat them:

Undersized cable over a long run: A customer buys a THERM EL 23, and the installer runs a 6 mm² cable over a length of 35 m – the voltage drop is 6.8%, which exceeds the limit. The boiler shows "outages" and doesn't heat efficiently. The correct choice is 10 mm².
Missing or incorrectly sized RCD: A single-pole or two-pole RCD is installed instead of a four-pole one – the system is not properly protected.
Service switch far from the boiler: The electrician puts everything in the distribution board in the garage, 15 metres from the boiler room. During servicing, the technician has nowhere nearby to make an emergency disconnection.
Unconnected PE conductor: Even though the cable has a PE core, the installer doesn't connect it (not enough time, didn't seem important). The boiler works, but in case of insulation failure of the heating element, a life-threatening voltage appears on the boiler body.
Overly rigid piping at the hydraulic connection: Without flexible compensators or slightly bent piping, thermal expansion creates stress on the boiler's threaded connections – water gradually leaks out.
Connecting the thermostat to power terminals: Confusing the TA terminals (24 V logic) with the mains supply destroys the control module – repair costs 30–60% of the boiler's price.
Not performing a pressure test before start-up: The system is filled, the boiler is switched on, heating begins – and a warm joint with a leak starts dripping significantly. Water and nearby electrical components are not a good combination.

Coordination for Connecting a Higher Power Input – Západoslovenská energetika, MH Teplárenský holding, and Other Distributors

Installing a boiler with an input above 3.68 kW (which applies to every model in the THERM EL series) requires the consent of the electricity distributor. In practice this means:

For inputs up to approx. 13.8 kW (three-phase 3×20 A): Most consumers have a sufficient reserved capacity in their contract – a notification of the change in consumption and the new equipment installation to the distributor is enough.
For inputs above 13.8 kW: An application for consent to increase the power input must be submitted (change of main breaker, change of connection agreement). The process takes 30–90 days and should be arranged in advance, not after purchasing the boiler.
The THERM EL 23, 30, and 38 boilers almost always require an increase of the main breaker – for smaller family homes the standard main breaker is 25 A or 32 A three-phase, which is not sufficient for 23 kW.

This administrative requirement is one of the most commonly underestimated aspects when choosing the boiler's output. This topic is covered in more detail in the article What Output of the Thermona THERM EL Electric Boiler Do I Need for My Home?

Special Situations: Renovation vs. New Construction

During renovations (replacing an old boiler with an electric one), it is necessary to check not only the capacity of the distribution board but also the condition of the existing installation – old aluminium wiring from the 1980s is not suitable for such high-power loads and must be replaced. It is advisable to flush the existing heating system piping (radiators, pipes) before connecting the new boiler – sediment and corrosion from the old system can clog the heating element in the very first season.

In new construction, the situation is more comfortable – the installation is planned from scratch, so you can correctly size the wiring, the location of the distribution board, and the board itself. In new buildings, a Home Energy Management System is increasingly being installed, which can regulate the boiler's power input depending on the household's total consumption and photovoltaic production. THERM EL has an output for an external thermostat that can be connected to such control systems.

Frequently Asked Questions (FAQ)

Can I install the Thermona THERM EL electric boiler myself, without an electrician?

The hydraulic part of the installation (piping, fittings, filling) can, in most cases, be carried out by a skilled DIY enthusiast or a heating installer. The electrical part – the supply, overcurrent protection, and terminal block wiring – may only be carried out by a person with the relevant professional electrical qualification (§22 or §23 of Decree 508/2009 Coll.). Without a valid inspection report, the installation is not legal, and the insurance company may refuse to pay out in the event of a claim.

How long does it take to get an increased power input approved by the electricity distributor?

Usually 30–60 days, and in more complicated cases (new connection, necessary reconstruction of the distribution line) even 90–120 days. Submit the application before purchasing the boiler. Larger models in the series – for example the THERM EL 30 or THERM EL 38 – almost always require an increase in power input.

Is a pressure test of the heating system required before starting up the boiler?

Yes, even when renovating an existing system. The test pressure for a new system is 1.5 times the maximum operating pressure, but at least 0.6 MPa (6 bar), and the system must hold the pressure for at least 60 minutes without a drop. For smaller residential installations, this is sometimes skipped, but for higher-output boilers, the test is clearly recommended.

Can the THERM EL boiler be powered from solar photovoltaics via a hybrid inverter?

Yes, the THERM EL is a purely resistive load (cos φ = 1), which is ideal for hybrid inverters and off-grid inverters. However, you need to verify that the inverter's rated output power covers the boiler's power input – for the THERM EL 15, you need an inverter capable of continuously supplying 15 kW. Another advantage is the stepped power regulation of the THERM EL, thanks to which the boiler can be directly limited to lower stages according to the inverter's available output. More on this topic can be found in the article Stepped Power Regulation in Thermona THERM EL Boilers – How It Works and Why It Matters.

How long does the installation of the THERM EL electric boiler take?

With a prepared installation (finished wiring, distribution board, piping), the mechanical installation and connection of the boiler takes 2–4 hours. The hydraulic connection, including filling and venting, takes another 1–3 hours. The inspection, if the inspection technician comes to the site, takes another 1–2 hours. Overall, realistically plan for one working day for a complete installation from scratch.

What should I prepare before the installer arrives?

Finished wiring from the distribution board to the boiler installation location, finished piping with service valves and an air vent, an installed expansion tank and safety valve, and access to a water tap for filling the system and to the electrical distribution board. The better everything is prepared, the shorter and cheaper the installation itself will be.

Conclusion

Installing the Thermona THERM EL boiler is a relatively straightforward matter if the correct procedures are followed. The key is correctly sizing the electrical installation – underestimating conductor cross-sections, missing residual current protection, or the absence of an inspection are mistakes that are only worth making if you want trouble with your insurance company or a safety incident. Invest time in a proper design before ordering the boiler and in choosing an experienced electrician and installer.

The Thermona THERM EL series covers outputs from 8 to 38 kW – from a small apartment with the THERM EL 8 to a larger family home with the THERM EL 38. Every model deserves a correctly sized and safely carried out installation, because only then will you fully utilise the potential of the device while complying with applicable legislation and insurance conditions.

Do you have a question on this topic?

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