Common Faults of Thermona THERM EL Boilers – Causes, Error Codes and Solutions

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Common Faults of Thermona THERM EL Boilers – Causes, Error Codes and Solutions

Thermona THERM EL electric boilers are among the reliable and proven heating appliances that, with correct installation and regular maintenance, serve for many years without major issues. Nevertheless, faults do occasionally occur – some are a direct result of incorrect installation, others arise from neglected maintenance, and others are simply a manifestation of natural wear of components after years of operation. In this guide, we will look at the most common faults, explain their causes, and show what the user can do themselves and when it is necessary to call a professional service.

The article is written to be useful both for technicians working with the boiler for the first time and for homeowners who want to understand what is happening with their heating system. If you are more interested in the preventive side of things, we also recommend our article Maintenance and Service of the Thermona THERM EL Electric Boiler – What and How Often to Check.

How the Thermona THERM EL Boiler Is Designed – Basic Technical Context

Before we move on to specific faults, it is good to understand how the boiler basically works. Thermona THERM EL is an electric flow-through boiler (in some configurations a storage type) with resistance heating elements – so-called coils or rod elements. Output is regulated in stages by switching individual elements on via contactors. The boiler contains control electronics, temperature sensors, a safety thermostat (limiter), a circulation pump, and an expansion vessel.

The THERM EL series is available in several power variants – from the compact Thermona THERM EL 8 with an output of 8 kW, through the mid-range THERM EL 15 and THERM EL 23, up to the more powerful THERM EL 30 and THERM EL 38 models. The power rating also directly affects the type and number of components that can be a source of a fault – higher output means more contactors, more heating elements, and more demanding requirements for the electrical installation.

Each of these components can be a source of a fault. Knowing their function helps to determine faster where the problem occurred.

Error Codes of Thermona THERM EL Boilers – What They Mean

Thermona THERM EL boilers display fault states either by means of a flashing indicator light or – depending on the version and accessories – via the regulator's display. The specific signalling method depends on the type of control panel or external regulator connected to the boiler. In the basic version, THERM EL boilers use simple signalling, while when connected to advanced regulators (e.g. Thermoreg or weather-compensation regulators), diagnostics are more detailed.

Below is an overview of typical fault states and their codes as they appear in the technical documentation and service manuals for the THERM EL series:

Code / Signal	Fault Description	Typical Cause	User-Resolvable
E1 / red indicator light	Activation of the safety thermostat (limiter)	Boiler overheating – insufficient water circulation	Partially – reset after cooling, check the pump
E2	Interrupted temperature sensor (NTC probe)	Faulty sensor or broken wire	No – service required
E3	Temperature sensor short circuit	Damaged NTC sensor or moisture at the connector	No – service required
Flashing indicator light	Pump fault (no-flow condition)	Stuck pump, closed valve, air in the system	Partially – check valves, bleed air
No display, boiler not working	Power outage / circuit breaker tripped	Electrical installation fault or breaker in the panel	Checking breakers – yes; otherwise service
Contactor fault	Boiler switches on but does not heat to full output	Worn or burnt-out contactor of a power stage	No – service required

Important: In case of any doubt regarding an error code, always consult the operating manual of the specific model. Boiler versions may differ, and an older THERM EL 8 model has slightly different signalling than, for example, the THERM EL 38 with the current firmware version.

Fault No. 1 – Activation of the Safety Thermostat (Limiter)

This is by far the most common fault encountered by service technicians with Thermona THERM EL boilers. The safety thermostat – professionally referred to as the "limiter" – is a safety component that switches off the boiler the moment the water temperature in the boiler reaches a critical value, usually around 95–100 °C. Its purpose is to prevent damage to the heating elements or an explosion due to overheating.

Why does this happen?

The limiter activates whenever the heat cannot be dissipated by the water quickly enough. The most common causes are:

Stuck circulation pump – the pump does not move, the water is not flowing but static. This typically happens after a longer boiler shutdown, for example after summer. The pump rotor gets stuck due to limescale or corrosion. Solution: unscrew the pump's service plug and manually rotate the rotor.
Closed shut-off valve – a simple mistake that nevertheless occurs surprisingly often at first start-up after a winter shutdown or after a renovation. Just one forgotten closed ball valve on the circuit is enough for the boiler to overheat.
Air in the system – an air pocket prevents water circulation. The boiler heats, but the heat has nowhere to go. The solution is to bleed the radiators and the air vent on the boiler itself.
Scaled heating elements – after years of operation, limescale builds up on the surface of the coils. A scale layer only 1–2 mm thick can worsen heat transfer to such an extent that the boiler starts to overheat. This is typical for long-neglected hard water.
Incorrectly set thermostat – if the desired temperature is set too high and the control does not work correctly, the boiler may overshoot beyond the maximum operating temperature.

What to do after the limiter activates?

After the safety thermostat activates, the boiler remains in a blocked state. Before resetting, it is necessary to:

Let the boiler cool down for at least 20–30 minutes.
Check that the shut-off valve is open and that the circulation pump is running (listen for the characteristic quiet hum or buzz).
Bleed the system if necessary.
Only then press the limiter's reset button (usually located behind the boiler's cover panel, marked "RESET" or in red).
Start the boiler and observe whether the fault recurs.

Warning: If the limiter activates repeatedly, never repeat the reset several times in a row without determining the cause. Repeated resets without correcting the cause lead to permanent damage to the heating elements.

Fault No. 2 – Boiler Does Not Switch On, No Power, No Signalling

If the boiler does not respond to any control elements and there is no activity on the display or indicator light, the cause is almost always on the power supply side. Electric THERM EL boilers are three-phase appliances (an exception may be only the THERM EL 8 model with single-phase connection), so a power supply fault can take several forms:

Tripped circuit breaker in the electrical distribution board – the most common and simplest cause. A residual current device or circuit breaker tripped due to a transient short circuit or overvoltage. Check the distribution board and try switching the breaker back on. If it trips immediately again, do not touch the boiler – an electrician is needed.
Fault of the residual current device (RCD) – if the RCD trips repeatedly, this usually signals a leakage to earth, which may indicate damaged insulation of a heating element. This is a serious condition and the boiler must not be operated until repaired.
Fault of the boiler's main circuit breaker – the boiler's own breaker itself may be faulty and stop switching, while visually appearing switched on.
Interrupted phase – in three-phase boilers, one phase may be interrupted (e.g. due to a fault in the grid or at the terminal block). The boiler may not work at all or may work only at partial output.

Practical example: In one case, the owner reported that a THERM EL 23 boiler stopped working overnight, although it had worked normally the day before. Upon inspection we found that the breaker in the distribution board was indeed switched on, but measurement showed that one phase was down – the fault was in the street cabinet, not in the boiler. After the electricity distributor made the repair, the boiler started immediately.

Fault No. 3 – Boiler Works, but Does Not Heat Sufficiently

This fault is tricky because the boiler outwardly "works" – it lights up, hums, heats the water partially – but the output is significantly lower than the set output would suggest. The causes can be:

Faulty or worn contactor

A contactor is an electromechanical switch that turns individual power stages (heating elements) on and off. Each power stage has its own contactor. If one contactor stops switching, the corresponding heating element remains permanently off. In the THERM EL 30 model with three power stages, this may mean the boiler works, for example, at only 2/3 of output – heating the water fully on mild days, but unable to keep up during frost.

Symptoms of a faulty contactor:

The boiler does not heat the water to the set temperature even at maximum setting
Increased electricity consumption relative to output (some contactors remain permanently closed – the boiler heats non-stop)
A faintly audible click when stages switch – one stage stays silent

Damaged heating element (coil)

Heating elements are tubular resistance coils. They can fail in two ways: either the resistance wire breaks (interrupted circuit – the given stage does not heat at all), or there is a leakage to the casing (RCD trips). Both conditions require replacement of the element, which is a service job requiring at least partial draining of the boiler.

Limescale on the heat exchangers

In areas with hard water (hardness above 2.5 mmol/l, which is common for example around Záhorie, near Nitra, or in the Spiš region), limescale builds up quickly. A layer of scale acts as a thermal insulator – it reduces heat transfer, and the boiler has to run longer for the same effect. Besides reduced output, there is a risk of overheating the elements and their premature failure.

The chart shows that already at a limescale thickness of 2 mm, heat transfer drops to less than 60% of the original value. This means the boiler has to run considerably longer, increasing electricity consumption and component wear.

Fault No. 4 – Noisy Operation, Banging, Rattling

Noises from the boiler during operation are unpleasant but important diagnostic indicators. Different types of noise signal different problems:

Regular banging or bubbling – a typical manifestation of air pockets in the system. Air gets trapped in radiators, pipes, or the boiler itself. Solution: systematic bleeding of the whole system, starting from the boiler through the nearest radiators.
Loud boiling sound with pressure pulsation – this sound (similar to water boiling in a kettle) indicates local overheating of a heating element. Causes: limescale, air in the boiler, too low pressure in the system. This is a serious signal requiring the boiler to be stopped and a service technician called.
Mechanical rattling when switching – contactors make a quiet click during normal operation. Loud banging when engaging can indicate a worn or oxidised contactor contact.
Continuously running pump with cavitation noise – a characteristic hissing sound occurring at low pressure. Check the pressure in the system (pressure gauge on the boiler) – when heated, it should be 1.5–2.0 bar. If the pressure keeps dropping, the system is likely losing water.

Fault No. 5 – Pressure Drop, Insufficiently Filled System, Water Leak

Electric boilers are closed systems with an expansion vessel. The pressure in a cold system should be 1.0–1.2 bar, and in a heated operating state 1.5–2.0 bar. If the pressure is dropping, the causes may be:

Burst or expired expansion vessel – the expansion vessel has a rubber membrane. After years of operation, the membrane hardens or bursts. As a result, the vessel stops fulfilling its function and the system pressure fluctuates or drops. The vessel's pre-charge pressure should be around 0.8–1.0 bar (check via the valve on the vessel). If water leaks from the valve, the membrane is punctured and the vessel must be replaced.
Leakage from fittings or joints – seeping spots on pipes, seals on filters, and others. Regularly visually check the accessible parts of the installation.
Leakage through the safety valve – if the safety (pressure relief) valve keeps dripping, it signals either that the maximum pressure has been exceeded (incorrect refilling or a faulty expansion vessel) or that the safety valve itself is faulty. Never block the safety valve – it is a safety component!

Fault No. 6 – Fault of the Regulator or Temperature Sensor

Modern THERM EL boilers can be equipped with various types of regulators – from a simple bimetallic thermostat to programmable weekly regulators or weather-compensation regulators with an outdoor sensor. Any link in this chain can fail.

Faulty NTC temperature sensor

The NTC sensor measures the water temperature in the boiler and sends the information to the control electronics. If the sensor fails (open circuit or short circuit), the boiler displays a fault code (E2 or E3, depending on the firmware version). The sensor is usually replaced as a whole – a simple, yet still professional service job.

Fault of the external room thermostat

If an external room thermostat is connected to the boiler, a fault may also occur here. Symptoms: the boiler does not heat at all even though it is switched on, or conversely, it heats constantly. Test: bridge the external thermostat terminals on the boiler's control board – if the boiler starts, the thermostat is faulty, not the boiler.

Weather-compensation regulator and outdoor sensor

With a boiler using weather-compensation control (for example, the THERM EL series combined with a Thermoreg regulator or similar), failure of the outdoor temperature sensor can cause the boiler to control the temperature incorrectly – for example, heating at full output even when it is 10 °C outside. Outdoor NTC sensors are sensitive to mechanical damage (frost, UV radiation) and often need to be replaced after several years.

Fault No. 7 – Boiler Heats, but Temperature Fluctuates or the System Oscillates

Fluctuating temperature of the heating medium is typical of incorrectly set control parameters or a hydraulically unbalanced system. In THERM EL boilers, this manifests as the boiler alternately switching on and off, the room temperature fluctuating, and comfort decreasing.

The causes can be:

Too large a power jump between stages – in the THERM EL 38 model with three power stages (e.g. 12/25/38 kW), the transition between two adjacent stages can be steep, causing unwanted oscillations. This is not a boiler fault but a matter of correct sizing – discussed in more detail in the article Stepped Output Control in Thermona THERM EL Boilers – How It Works and Why It Matters.
Too small water volume (low thermal mass) – if the system has a small water volume and the boiler has relatively high output, the system reacts quickly and the control becomes "jittery".
Incorrectly set thermostat hysteresis – many room thermostats allow you to set the hysteresis (the difference between switching on and off). If the hysteresis is too small (e.g. 0.5 °C), the boiler switches on and off too often.

Fault No. 8 – Water Leaking from the Safety Valve

The boiler's safety valve is set to 3 bar (standard). If water drips or flows from it during normal operation, this signals one of the following conditions:

The system pressure is too high – check the pressure gauge and, if necessary, release the excess pressure via the fill valve
The expansion vessel is faulty or too small for the volume of the heating system
The safety valve itself is worn – the seal in the valve seat has been damaged and the valve leaks even at normal pressure

Important notice: Never secure the safety valve mechanically (for example by wiring it shut or screwing it closed). It is a safety component protected by law, and tampering with it is a serious violation of regulations with potentially tragic consequences. Replace a faulty valve with the same type having the same pressure rating.

Preventive Measures – How to Avoid Faults

Customer service experience shows that most faults in THERM EL boilers could be prevented by regular and simple maintenance. Here are the key points:

Once a year – check the system pressure, bleed the system, visually inspect visible pipework for leaks, check the operation of the circulation pump
Every 2–3 years – have the expansion vessel pre-charge checked, check the condition of the safety valve, have the water hardness measured, and if necessary have the boiler chemically descaled
Every 4–5 years – we recommend having the condition of the contactors checked, testing the insulation resistance of the heating elements, and carrying out an overall service inspection
Before each heating season – start the boiler early (September), check all control elements, set the regulator, check the shut-off valves

More on this topic can be found in a separate article Maintenance and Service of the Thermona THERM EL Electric Boiler – What and How Often to Check.

When Professional Service Cannot Be Avoided

Some faults can be resolved by a handy homeowner themselves (bleeding air, resetting the limiter, checking the breaker). However, most repairs require professional qualification, because THERM EL boilers work with three-phase 400 V power, which is deadly dangerous for a layperson. Professional service is mandatory for:

Replacing a heating element
Replacing a contactor
Replacing the control board or NTC sensor
Replacing the expansion vessel or safety valve
Any intervention into the electrical part of the boiler
Repeated RCD tripping

Remember that unauthorised intervention in the boiler may result in the loss of warranty service, and in the event of an insurance claim (for example, damage caused by a fire), the insurance company may refuse to pay out.

Frequently Asked Questions (FAQ)

Why does my THERM EL boiler repeatedly "reset" itself and switch off again after a short period of operation?

If the limiter activates repeatedly shortly after a restart, the cause is most likely insufficient water circulation – a stuck pump, a closed valve, or heavy limescale. Never let the boiler run in this state, and repeated resetting without finding the cause is dangerous. Call a service technician.

My THERM EL boiler is working normally, but one radiator remains cold. Is this a boiler fault?

Most likely not. A cold radiator with a functioning boiler and circulation pump indicates a local problem – air in the radiator (bleed it), a closed thermostatic valve (check the setting), or hydraulic imbalance in the system. A boiler fault would affect the whole system, not just one radiator.

The RCD in the distribution board trips repeatedly right after switching on the boiler. What could this be?

This is a serious condition signalling a leakage to earth – most often damaged insulation of a heating element (coil) due to corrosion, overheating, or prolonged moisture. The boiler must not be operated until the fault is fixed. Call an electrician or an authorised Thermona service, as working on a three-phase circuit is legally reserved for professionally qualified persons.

Can I refill water into the THERM EL boiler myself if the pressure has dropped?

Yes, a one-off refill of water is a task the owner can perform themselves – most THERM EL boilers have a fill valve with a pressure gauge. Fill slowly, watch the gauge, and stop at 1.0–1.2 bar (cold system). However, if it is necessary to refill repeatedly (several times per season), this is a sign of a leak or a faulty expansion vessel, requiring professional diagnostics. Constant refilling with hard water also accelerates limescale build-up.

My THERM EL 15 boiler makes regular bubbling sounds. Is this dangerous?

Mild bubbling sounds when the boiler starts up are normal and related to the thermal movement of water. However, loud, persistent bubbling or boiling-like sounds (similar to intense boiling) are a warning sign of local overheating – usually due to limescale or insufficient flow. In such a case, switch off the boiler and call a service technician.

After a renovation, I left my THERM EL 30 boiler switched off for three months. Now it won't start. What should I check first?

First step: check the breakers in the electrical distribution board. Second step: check that all shut-off valves are open (they may have been closed during the renovation and forgotten to be reopened). Third step: try manually unblocking the circulation pump rotor (unscrew the pump's service plug and turn the rotor with a screwdriver – after a long shutdown, the rotor may have stuck). If the boiler still won't start, call a service technician.

Conclusion – Diagnostics Requires a Systematic Approach

Thermona THERM EL boilers are structurally simple appliances with no flue gases, no combustion process, and no need for a chimney – this gives them a significant reliability advantage compared to gas boilers. Most of the faults encountered by service technicians in practice are not caused by the boiler itself, but by an external cause: poor installation, neglected maintenance, unbalanced hydraulics, or inadequate electrical installation.

The key to long and trouble-free operation is a combination of correct installation (details in the article Installation of the Thermona THERM EL Electric Boiler – Procedure, Electrical Installation Requirements, and Inspection), regular annual checks, and a quick response to the first warning signs. Whether you choose the compact THERM EL 8 for a smaller family house or the powerful THERM EL 38 for a larger building, the principles of proper boiler care are the same – and investing in prevention is always cheaper than an unplanned service call in the middle of the January frost.

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 will be happy to advise.