Common faults of SLOKOV electric boilers and their solutions

We are still preparing products.

But you can look at other categories.

Common faults of electric boilers SLOKOV and their solutions

Electric boilers SLOKOV series SL ELEKTRO are reliable devices that operate for years without major problems when installed correctly and maintained regularly. Despite this – and this applies to any electrical device working with water, pressure and high temperatures – sooner or later situations arise when the boiler stops working as expected. From the experience of dozens of customer cases, I know that most faults are repeated and have quite clear causes. This article will help you recognize the fault, understand why it occurred, and in many cases eliminate it yourself – or at least know when to call a technician.

Before we get into specific faults, one basic warning: an electric boiler is a device under voltage of 400 V (three-phase connection) and simultaneously under hydraulic pressure. Never interfere with electrical parts without switching off the circuit breaker, and never open the hydraulic circuit without first releasing the pressure. Safety first.

How are SLOKOV SL ELEKTRO boilers built – the basis for understanding faults

In order to understand why a fault occurs, you need to know what the boiler is made of. Boilers of the SLOKOV SL ELEKTRO 10 to SLOKOV SL ELEKTRO 26 series are built on the same concept – they differ only in power and the number/power of heating elements.

The basic functional units of each SL ELEKTRO boiler:

Heating tank (boiler) – a steel tank in which water is heated by electric heating elements (cartridges). In more powerful models, such as SLOKOV SL ELEKTRO 23 or SLOKOV SL ELEKTRO 26, there are more heating elements.
Circulation pump – ensures circulation of water in the heating circuit.
Expansion tank – compensates for thermal expansion of water.
Pressure relief valve – protects the system from excessive pressure (standardly set to 3 bar).
Control electronics and thermostat – regulate the power, protect the boiler from overheating.
Manometer – displays the current pressure in the system.
Safety thermostat (STB) – a non-return emergency thermostat that shuts down the boiler at dangerous temperatures.

This knowledge of the construction is key: when the boiler is not working, the problem is always in one of these units. Diagnosis is not a guess, it is a systematic process of going through the options from the simplest to the most complex.

Fault No. 1: The boiler does not start, no indications, darkness on the display

This is the most common type of report: "the boiler is not working, no indicator lights are on". From experience, I know that in the vast majority of cases it is a problem on the power supply side, not a fault of the boiler itself.

Causes and solution steps

Blown fuse or tripped circuit breaker in the distribution board – Check the home electrical distribution board. An electric boiler (especially more powerful models such as SLOKOV SL ELEKTRO 18 and higher) is powered from a separate three-phase circuit protected by a B or C type circuit breaker with the appropriate ampere rating. If the circuit breaker is off or the fuse is blown, the boiler does not work. Solution: reset the circuit breaker (if it trips again immediately, there is a deeper problem – call an electrician).
Incorrect wiring or fault in the supply cable – Less common in new installations, but after long-term operation, a terminal in the boiler cabinet may become loose. This is resolved exclusively by an electrician.
Control electronics failure – If the power supply is in order and the control board is unresponsive, it may be a fault in the electronics. This is resolved by a service technician.
Frozen protection activated – Some models have a control lock function if the protection is activated. Check the user manual and possibly perform a reset according to the manufacturer's instructions.

Practical scenario: A customer called saying that the boiler was not working after a vacation. No indicator light was on. Cause: the circuit breaker in the distribution board had tripped – likely the main circuit breaker was turned off before the vacation and when turned on again, a short current surge tripped the circuit breaker. Resetting the circuit breaker, the boiler started normally.

Fault No. 2: The boiler is running, but the heating system does not reach the desired temperature

The boiler is on, the pump is running, but it is cold in the house and the water temperature in the system does not reach the set value. There can be several causes, and their seriousness varies.

Low power or heating element (cartridge) failure

Heating elements (cartridges) are consumable materials. After years of operation, they may fail – either the resistance wire breaks (the element stops heating completely) or a partial short occurs (the element heats less efficiently). SL ELEKTRO boilers have a higher number of heating elements and a stepped power output – if one element fails, the boiler still heats, but at a lower power. This is often identified by customers as "the boiler is not working properly".

Diagnosis: A professional technician will measure the resistance of each heating element with a multimeter. The measured value must correspond to the calculation from the rated power and supply voltage (R = U²/P). For example, a 2 kW/230 V element has a resistance of approximately 26.5 Ω. A significant deviation or infinite resistance (break) indicates a faulty element.

Improperly set thermostat or regulator

Common cause – the customer set the thermostat to 60 °C, but the equithermal regulation (if used) automatically lowers the temperature according to the outside temperature. In the transitional period, the regulator may set a lower temperature, which the customer perceives as a fault. Solution: check the regulator settings and understand how equithermal regulation works.

Undersized boiler power compared to the actual heat loss of the building

This is not a boiler fault, but a selection error. If the SLOKOV SL ELEKTRO 10 was installed in a house with a real heat loss of 12–14 kW, the boiler will never be sufficient during a severe winter. More about choosing the correct power can be found in the article "What power of SLOKOV electric boiler do I need for my house?"

Failure No. 3: Boiler repeatedly shuts off – safety thermostat (STB)

One of the most common faults I encounter in the field is the activation of the safety thermostat (STB – Sicherheits-Temperaturbegrenzer). This component is designed to permanently shut down the boiler when a dangerous temperature is reached (typically 95–100 °C). Unlike a regular thermostat, it does not reset automatically – it requires a manual reset, which is usually accessible only to a technician (behind a cover panel).

Why does the STB activate?

Insufficient water flow through the boiler – This is by far the most common cause. If the circulation pump is not working (fault, jammed, air in the pump), the water in the boiler overheats. Result: STB is triggered. Solution: check the pump (see below).
Closed outlet valve on the boiler – A novice or someone after service forgot to open the shut-off valve. The water in the boiler heats up, the flow is zero, and the STB activates.
Clogged or blocked filter (strainer) – If there is a strainer before the pump or at the boiler inlet, and it is clogged with dirt, the flow decreases. Cleaning the strainer is a simple task that even a layperson can do.
Air in the system – Air bubbles in the boiler or pipes cause the pump to "pump air" and not pressurize the water sufficiently. The result is the same – localized overheating.
STB itself is faulty – Less common, but the STB can fail on its own (especially after years of operation). A technician will test it and replace it if necessary.

Practical scenario: At a customer with SLOKOV SL ELEKTRO 15, the STB repeatedly activated – always in the morning after the first start-up. Cause: the pump had jammed during the summer break (the pump shaft corroded while idle). Solution: the pump was unjammed via a service screw and corrosion inhibitor was added to the system. Since then, the customer manually turns the pump every summer or sets a summer program to ensure the pump runs at least once a week.

Failure No. 4: Low or zero pressure in the system, manometer shows less than 1 bar

The correct operating pressure in a closed heating system is 1.0 to 1.5 bar when the water is cold. When the water is hot (during operation), the pressure rises to 1.5 to 2.5 bar, depending on the temperature. If the manometer shows less than 0.5 bar or zero, the boiler has low pressure and most modern control systems will shut it down (to prevent dry running).

Causes of pressure drop

Leak in the heating system – Check all connections, radiators, and seals. A small leak may not be immediately visible (it may evaporate). Systematically inspect the entire circuit.
Defective or worn expansion tank – The expansion tank has an internal membrane that separates air from water. If the membrane bursts, the tank no longer performs its function. Symptoms: pressure rapidly increases during heating and rapidly drops during cooling, the safety valve opens with every heating cycle. Solution: replace the expansion tank or inflate nitrogen/air into the pre-chamber (this is done by a technician).
Open or faulty safety valve – The safety valve (3 bar) should not open during normal operation. If it opens, it is either faulty (does not open at the correct pressure – it is more sensitive) or the pressure in the system is actually exceeding 3 bar. A leaking safety valve must be replaced.
Forgotten or omitted system refilling – At the beginning of each heating season, the system usually requires water to be added (small leaks through seals and evaporation are normal). Refilling is done via a filling valve (usually located under the boiler).

Failure No. 5: Noisy circulation pump or the pump does not run

The circulation pump ensures water circulation. Its failure can manifest in various ways: noise, vibrations, or, on the contrary, silence – when it does not run at all.

The pump makes noise (grinding, squeaking, knocking)

Air in the pump – Air bubbles passing through the pump cause characteristic gurgling or grinding. Solution: bleed the system (radiators, the highest point of the system, bleed valve on the pump).
The pump is operating at too high a speed – If the speed is set unnecessarily high, the noise increases. Most pumps have 3 speed levels. Check the setting and try a lower level.
Worn bearings – After years of operation, the pump bearings wear out. The result is grinding and eventually seizing. Solution: replace the pump.
Impurities (sludge, rust) in the bypass circuit – Corrosion residues and sludge circulate through the system and damage the pump. Therefore, it is important to use a magnetic or sediment filter and regularly clean the system (more in the article "Maintenance and service of the electric boiler SLOKOV").

The pump does not run at all

Stuck rotor (shaft) – A classic issue after the summer shutdown. The pump rotor corrodes while stationary. Solution: most pumps have a service screw on the side (white or red), after unscrewing it with an adjusting screwdriver, manually turn the shaft to free the jam.
Motor failure of the pump – The pump motor is either burned out (burned windings) or there is a fault on the control board (the pump does not receive a start signal). Both situations require a technician.
Interrupted pump power supply – Check whether the control unit is in heating mode (not summer mode with the pump turned off). Some controllers turn off the pump at low temperatures as an economy function.

Failure No. 6: Boiler leaks or drips water

A water leak from the boiler or from the surrounding connections is a problem that must be addressed immediately – further operation with a water leak can damage the electronics and cause a short circuit.

Where to look for the leak

Seals of hydraulic connections – Boiler inlet and outlet connections, filling cock, safety valve. Hemp seals or silicone can dry out and crack after years. Solution: drain the system and replace the seals.
Heating element (patron) – Heating elements are screwed into the heating tank. The element seals (brass or plastic gasket) can wear out. Signs include white deposits (limescale) around the element screw heads. Solution: replace the seal or the entire heating element (technician).
Corrosion of the heating tank – If the system was not protected with a corrosion inhibitor and the water pH was out of range, corrosion can cause the tank to puncture. This is a serious fault requiring tank or entire boiler replacement.
Dripping safety valve – If the valve drips at normal pressure, it is faulty. If it drips at pressure above 3 bar, it is a sign of a faulty expansion tank. Solution: replace the valve, or the expansion tank.

Failure No. 7: Error codes on the SLOKOV boiler display

Newer control units of SLOKOV boilers display error codes that significantly simplify diagnostics. Specific codes and their meanings can be found in the user manual provided with the boiler. Here we list the most common categories:

Temperature sensor (NTC) error – The control board reports that the sensor reading is outside the real range (sensor is broken or shorted). NTC sensors are cheap and easily replaceable.
Flow / circulation error – If a flow sensor is installed, it signals zero flow despite the pump running. It may be a jammed pump, closed valve, or faulty sensor.
Communication error with the controller – If the boiler is connected to an external controller (OpenTherm, 0–10 V, thermostat), a communication break will be displayed as an error. Check the controller wiring.
Overheating (HIGH TEMP) – The control board records an excessively high output temperature. The cause is the same as when STB is activated – insufficient flow.

Important: Never ignore error codes by simply disconnecting and reconnecting the boiler (so-called hard reset) without finding the cause. The error will return and repeated resets can cause further damage.

Failure No. 8: Air in the system – gurgling, cold upper parts of radiators

Air in the heating system is a chronic issue that is not a boiler fault in itself, but the boiler senses its presence – it increases the risk of overheating, reduces efficiency, and burdens the pump.

Signs of air pockets:

The radiator is cold on top, warm at the bottom – air rises and stays in the upper part.
Audible gurgling or grumbling in the pipes.
The circulation pump makes noise, although it is quieter when the boiler is turned on.
The system pressure fluctuates more than usual.

The solution is bleeding: each radiator has a bleed valve (a small screw on the top side of the radiator). Open it with a small key or screwdriver until water starts to flow (not air) – then close it. Start with the radiators located highest. After bleeding, check the pressure and, if necessary, add water to the system.

Failure No. 9: Boiler does not reach the set DHW temperature (domestic hot water)

Some SLOKOV boiler installations are combined with DHW tanks (hot water heaters). If the hot water does not meet the requirements, the problem may be:

DHW priority is turned off or incorrectly set – The control unit must have DHW heating priority set before heating. Check the settings.
Faulty DHW tank sensor – If the tank sensor reports an incorrect temperature, the boiler does not heat or heats unnecessarily.
Limescale in the tank or in the heating coil – In hard water, limescale forms, which significantly reduces heat transfer. The tank must be regularly descaled (professional service).
Faulty mixing valve – If there is a mixing valve (thermostatic) behind the tank, its failure causes the water to be too cold or dangerously hot.

Prevention of failures: What to do to make the boiler last for years

The cheapest repair is the one that does not happen. From practice, I know that the majority of failures could be prevented by simple regular maintenance. More specific procedures can be found in the article "Maintenance and service of the electric boiler SLOKOV," here is a brief overview:

Every year before the season: Check the system pressure. Manually turn the pump (if it was in summer shutdown). Bleed the radiators. Visually check all connections for tightness.
Every 2–3 years: Chemical system service – add a corrosion inhibitor (e.g., Fernox, SENTINEL). Check the pressure in the expansion tank. Clean the filter/sieve.
Every 4–5 years: Professional inspection of heating elements (by measuring resistance). Check STB. Replace seals on hydraulic connections.
Water in the system: Use softened water (less than 5 °dH hardness) with pH 7–8. Never add large amounts of untreated tap water – this accelerates corrosion and limescale formation.

When to Call a Technician and When You Can Do It Yourself

Distinguishing between tasks a layperson can handle and those that require a technician is practically important. Incorrect intervention can worsen the problem or create a dangerous situation.

What You Can Do Yourself (Without Professional Qualification):

Check and reset the circuit breaker in the distribution board
Defrost radiators
Refill water into the system via the filling valve (to the correct pressure)
Clean the strainer/filter (on a closed and drained circuit)
Check the setting of the thermostat and regulator
Manually release a jammed pump via the service screw

Requires a Professional Technician (Electrician or Heating Engineer):

Reset the safety thermostat STB (located behind the boiler cover panel)
Replace the heating element (cartridge)
Replace or repair the pump
Replace the expansion tank
Repair or modify the electrical wiring and control electronics
Any intervention in a closed hydraulic system under pressure
Repair or replacement of the safety valve

If you have any doubts, always choose a technician. An electric boiler operating at 400 V is not a device where improvisation is advisable.

Most Frequently Asked Questions (FAQ)

The SLOKOV boiler does not start in the morning, but after a while it works normally – what could be the issue?

This is a typical sign of a faulty temperature sensor NTC or a cold joint on the terminals of the control board. At low ambient temperatures, the material resistance changes enough for the control board to report an error, but after warming up, the situation normalizes. It can also be a consequence of condensation of moisture on the electronics in a boiler room without sufficient ventilation. We recommend a professional inspection.

The safety valve regularly releases water – is that normal?

No, it is not normal. The safety valve should open only exceptionally in case of a real dangerous overpressure. Regular release indicates either a faulty (cracked membrane) expansion tank or too low nitrogen pressure in the expansion tank. The result is that the system is constantly overpressurized during heating. You need to check and possibly replace the expansion tank. Unqualified intervention is not sufficient – call a technician.

Can I replace the heating element (cartridge) in the SLOKOV boiler myself?

Technically yes, if you have experience with hydraulics – the heating element is screwed into the tank and can be replaced after draining the system. However, the electrical connection of the new element is the responsibility of an electrician. You also need to know what type of cartridges the boiler uses (power, voltage, thread, dimensions) and order the correct one. We do not recommend replacing it without experience – there is a risk of leakage or incorrect electrical connection.

The boiler is running, but the radiators are cold – what to do first?

Check the following in order: 1) Is the pump running and rotating? (Place your hand on the boiler – you can feel vibrations if the pump is running.) 2) Is the system pressure at least 1 bar? 3) Are the radiators and boiler circuit valves open? 4) Do you have air in the radiators? Bleed them. If the radiators remain cold after these steps, call a technician.

The SLOKOV boiler makes strange noises – what kind of noise indicates what?

Gurgling/whirring: air in the system or pump. High-frequency whistling: mechanical wear of the pump bearings. Muffled booming: pump running at too high a speed or narrowing in the circuit (filter, valve). Cracking sounds from the boiler tank: limescale deposits on the heating elements (scale cracks during temperature changes) – the boiler needs descaling.

What is the lifespan of heating elements (cartridges) in SLOKOV boilers?

With proper water quality (hardness under 5 °dH, pH 7–8, corrosion inhibitor), heating elements last 8–15 years. In hard water without treatment, they may fail after 3–5 years – limescale forms an insulating layer, the heating element overheats, and the resistance wire cracks. This is the most common reason for premature replacement of cartridges. Therefore, water treatment is really important, not just recommended.

Conclusion: Most Failures Can Be Predicted

Electric SLOKOV boilers are structurally relatively simple devices without gas valves, flame igniters, or flue gas paths – thus, they have fewer potential sources of failure than gas or pellet boilers. Nevertheless, no device lasts forever without maintenance. From practice: 80% of the failures I encounter with SL ELEKTRO boilers are caused by neglected maintenance or poor water quality in the system.

If you are considering purchasing a new boiler, take a look at the full range in the SLOKOV category on atria.sk – from the compact SL ELEKTRO 10 for smaller homes to the powerful SL ELEKTRO 26 for larger or poorly thermally insulated buildings. You can find more practical information in related articles from our Knowledge Center – for example, "Installation of the SLOKOV Electric Boiler – Procedure and Requirements", "Connecting the SLOKOV Electric Boiler to an Existing System", or "SLOKOV Electric Boiler and Night Rate – How to Set Up Heating".

Do You Have a Question on This Topic?

Not sure or dealing with a specific situation in your household? Write to us – we are happy to help.