Maintenance and Servicing of the Thermona THERM EL Electric Boiler – What to Check and How Often

We are still preparing products.

But you can look at other categories.

Why is regular maintenance of an electric boiler important at all?

At first glance, an electric boiler seems like a completely maintenance-free appliance – no combustion, no chimney, no exhaust gases. Many owners therefore assume that it's enough to connect the boiler, switch it on, and forget about it. This idea is partly true (compared to gas or solid-fuel boilers, maintenance really is significantly simpler), but not entirely correct. Even an electric boiler needs regular attention – and neglecting its upkeep can result in increased energy consumption, premature wear of the heating elements, pump failure, or in extreme cases, flooding of the boiler room.

Thermona THERM EL belongs to the category of industrially robust, reliable appliances. The series includes models from the Thermona THERM EL 8 (8 kW, suitable for smaller apartments and family houses) through the Thermona THERM EL 15 and Thermona THERM EL 23 up to the more powerful Thermona THERM EL 30 and Thermona THERM EL 38. Despite the high-quality construction, the lifespan of the boiler still depends on how you treat it. From practical experience, I know that a boiler which receives regular attention at least once a year can easily last 15–20 years without major intervention. A boiler that no one has checked for 7 years will typically fail at the first severe frost with a cracked pump or burnt-out heating elements.

In this article, we'll go through in detail what specifically needs to be checked on Thermona THERM EL boilers, how often, what you can handle yourself, and what belongs in the hands of a professional service technician. If you're interested in how to properly set up the boiler after installation, also check out the article Setting up and commissioning the Thermona THERM EL boiler – first start-up step by step. For solving specific faults, we recommend the article Common faults of Thermona THERM EL boilers – causes, error codes and their solutions.

Overview of components affected by maintenance

Before we move on to the schedule and procedures, let's briefly outline what the Thermona THERM EL boiler consists of and which parts deserve attention. Understanding the basic structure of the boiler will help you better understand why each task is important.

Basic components we focus on during maintenance:

Heating elements (resistance coils / rod heating elements) – the primary heat source, sensitive to limescale deposits and dry running
Circulation pump – ensures water circulation in the system; in many failures, the pump is the first victim
Expansion tank – compensates for temperature-related changes in water volume; its malfunction stresses the entire system
Safety valve – a safety element that requires regular inspection and occasional manual activation
Electronic control board and thermostats – the brain of the boiler, sensitive to moisture and voltage surges
Electrical installation and connectors – terminal blocks, circuit breakers, connections; neglect leads to failures or safety risks
Temperature and pressure sensors – NTC sensors measure temperature, the pressure sensor monitors system pressure
Filter/magnetic dirt separator – if installed at the boiler inlet, needs to be cleaned

Maintenance schedule – what to do and when

The most common mistake owners make is not setting any regular cycle. As a result, they only get to the boiler once something stops working. So make yourself a simple plan – ideally write it down in the boiler's service log or in a calendar.

Every month – quick visual check (5 minutes)

Every owner can handle this without any technical knowledge. Just look at the boiler once a month and note a few basic parameters:

System pressure: Check the boiler's pressure gauge to see whether the pressure is within the set range. For most closed systems with radiators, the correct pressure when cold is 1.0–1.5 bar, and 1.5–2.2 bar during operation. If the pressure has dropped below 0.8 bar, the system needs to be topped up. If it keeps dropping regularly, there is a leak somewhere.
Visual leak check: Inspect the joints, fittings, and the area around the boiler. Any water stains, wet spots, or limescale deposits on connections are a sign of a problem.
Checking error messages on the display: If the boiler shows a fault code, note it down immediately and look up its meaning (details can be found in the article Common faults of Thermona THERM EL boilers – causes, error codes and their solutions).
Listening to the boiler: Unusual sounds – gurgling, bubbling, loud pump rattling – are warning signs. Under normal conditions, the boiler is almost silent.

Before the heating season – complete seasonal check

The ideal time is September or early October, i.e. before you actually need the boiler. A basic seasonal check includes all the monthly tasks plus:

Bleeding the radiators and the boiler: Air in the system is a common issue after the summer break. Bleed each radiator (start with the furthest one and work toward the closer ones), then check the automatic air vent on the boiler.
Testing the circulation pump: Turn on the boiler and listen to how the pump starts up. After the summer break, the pump may be stuck (this is a common issue, see the solution in the next section).
Checking thermostats and sensors: Set a higher temperature and check whether the boiler responds. If the outdoor thermostat or controller doesn't respond, check the batteries and connection.
Checking control settings: Verify that the programs, temperature curves, and schedules are set according to your current needs. Parameters may reset after a power outage.
Visual inspection of the electrical installation: Check the supply cables and terminal blocks. If you notice charring, surface discoloration of the insulation, or mechanical damage, call an electrician – do not intervene in the boiler yourself.

Once a year – professional service inspection

Once a year (or according to the manufacturer's instructions), the boiler should be visited by an authorized service technician. This task is not always legally mandatory (unlike with gas boilers), but it significantly extends the appliance's lifespan and maintains the warranty. The technician performs:

Checking and measuring electrical parameters (insulation resistance of heating elements, power input)
Cleaning limescale from the heating elements (if necessary)
Checking and possibly replacing seals and the expansion tank membrane
Inspecting the safety valve
Updating the control board firmware (if available)
Making an entry in the service log

Every 2–3 years – deeper maintenance

Water quality analysis: Have the heating water analyzed (pH, hardness, chloride content). The ideal pH is 7.0–8.5, and hardness should not exceed 2–3 °dH (soft water). If the water is hard, consider installing a softener or treating it with additives.
Checking the expansion tank pressure: The pre-charge pressure in the expansion tank should be equal to, or 0.2 bar higher than, the set cold system pressure. It is checked at the tank's air valve using a pressure gauge (like on a tire).
Chemical system cleaning: If the system is old or has circulation problems, consider flushing and applying inhibitors to the heating system.

Heating elements – how they work, why they fail, and how to prevent it

The heating elements (resistance rod elements) are the heart of the boiler. Their reliability is most affected by the quality of the heating water. If the water is hard (containing calcium and magnesium), a layer of limescale gradually forms on the surface of the heating elements. This layer acts as thermal insulation – the heating element must heat up to a higher temperature to transfer the same amount of heat to the water. The result? Increased energy consumption and premature burnout of the element.

From practical experience, I know that in areas with truly hard water (for example southern Slovakia, some areas of the Danube Lowland, where hardness exceeds 20–25 °dH), heating elements may show visible deposits after just 2–3 years of operation. In the soft water of mountain regions, this is practically a non-issue for the entire lifespan of the boiler.

So how do you prevent deposits?

Treat the water before the first filling of the system: If you know you have hard water, ask your installer to fill the system with treated (demineralized or softened) water containing a corrosion inhibitor.
Don't refill the system unnecessarily: Every new filling of the system brings fresh, untreated hard water. The less often you change the water, the fewer new deposits form.
Monitor the outlet temperature: The higher the temperature of the heating element itself (not the water, but the surface), the faster limescale forms. If possible, limit the maximum boiler temperature to 70–75 °C instead of 85–90 °C.
Chemical treatment: Inhibitors and additives for heating systems are available on the market (e.g. Fernox, Sentinel products) that slow down the formation of deposits and protect metal parts from corrosion.

Circulation pump – the most common problem after summer shutdown

The pump is a mechanical component that, unlike purely electronic components, requires real attention. The most common problem I encounter in practice is a stuck pump after a longer summer break. The pump has been motionless all summer, the rotor has slightly stuck to the shaft (oxidation, deposits), and when first started up in autumn, it fails to spin. The boiler reports a pump fault or simply doesn't heat up, even though the heating element is working.

The solution is simple if you know about it in advance: on the pump housing (usually on the side or front) there is a screw (nut) with a slot for a flat screwdriver. After unscrewing it (careful, a bit of water may leak out – have a cloth ready), you can turn the rotor directly with a flat screwdriver. Do this before the first seasonal start-up if the boiler has been shut down for longer than 3 months.

Safety valve – a safety element that must not be ignored

The safety valve is a simple but critically important device. It opens when the set pressure is exceeded (usually 3 bar for most home systems) and releases water to prevent an accident. The problem is that a safety valve that has never been used can seize up over time. The valve seat may become clogged with limescale or corrosion, so that at truly high pressure the valve fails to open – and that is a dangerous situation.

That's why the manufacturer recommends manually activating the valve once a year: turn the lever (present on most safety valves) and let a bit of water flow out. This verifies that the valve is functional and removes any deposits from the seat. At the same time, check the discharge pipe (siphon) – it must be unobstructed and must lead to the sewer or a collection container.

Warning: If the safety valve regularly (e.g. every day or every week) releases water on its own, this is not normal. Possible causes include: too high operating pressure, a malfunctioning expansion tank, or a damaged (not airtight) valve. In such a case, call a technician.

Expansion tank – the silent safeguard of the whole system

The expansion tank is a hermetically sealed container divided by a rubber membrane into two parts: an air part (with nitrogen or air pre-charge) and a water part (connected to the system). When water expands as it heats up, it pushes into the water part of the tank and compresses the membrane into the air part. This absorbs the pressure increase and protects the system from excessive pressure.

If the membrane bursts, the air and water parts mix. The tank loses its cushioning function – the pressure in the system rises sharply during heating, the safety valve opens and releases water. The result: the system is repeatedly topped up with fresh hard water, which accelerates the formation of deposits and corrosion.

Check the condition of the expansion tank every 2–3 years. Procedure:

Shut down the boiler and let the system cool down.
Close the ball valve between the tank and the system (if installed).
Check the pressure at the tank's air valve (as with a tire). It should match the set cold system pressure (e.g. 1.0 bar) plus 0.0–0.2 bar extra.
If water (not air) comes out of the valve, the membrane is damaged and the tank must be replaced.
If the pressure is low, add air using a standard tire pump.

Electrical installation and safety aspects

The Thermona THERM EL electric boiler operates at 400 V (three-phase connection), which puts it among appliances where any intervention in the electrical part is reserved exclusively for a qualified electrician. This is not marketing or unnecessary caution – three-phase voltage is lethal on contact, without exception.

As for the electrical part, a layperson can only monitor the following external symptoms and immediately call an electrician or service technician:

Smell of burnt plastic material or "burnt wires"
Charring, darkening, or melting of insulation near the boiler
The boiler's circuit breaker tripping every time you try to turn it on
Sparking or shocks when touching the appliance (dangerous – immediately switch off the main circuit breaker)
Increased electricity consumption without explanation (e.g. 15–20% higher than the previous season with the same usage)

A professional service technician (an electrician with the necessary qualification) should check the terminal block connections, the insulation resistance of the heating elements with a megohmmeter, and the condition of the circuit breakers and residual current device once a year. You can find more about electrical installation requirements in the article Installation of the Thermona THERM EL electric boiler – procedure, electrical installation requirements and inspection.

Heating water quality – the most important factor in boiler longevity

We can't stress this enough: the water in the system is for an electric boiler what motor oil is for a combustion engine. Bad water can destroy a boiler in 3–5 years; good water can keep it running for 15–20 years.

In practice, I often encounter situations where a customer has topped up the system from the tap without any water treatment, repeatedly, every time the pressure dropped. The result after 5 years: a thick layer of limescale on the heating elements, muddy sediment at the bottom of the boiler, a blocked pump, and a final service bill 4 times higher than regular maintenance would have cost. Don't underestimate this aspect.

Cleaning the filter and magnetic dirt separator

If your system has a mechanical filter (mesh filter or Y-strainer) or a magnetic dirt separator installed (e.g. Fernox TF1, Spirovent), they need to be cleaned regularly. The frequency depends on the condition of the system – in the first year of operation of a new system, we recommend cleaning every 3 months, later once a year.

Procedure for cleaning a mechanical filter:

Close the valves before and after the filter.
Unscrew the collection basket (watch out for leaking water).
Clean the mesh under running water or with a soft brush.
Reassemble and reopen the valves. Check for leaks.

The magnetic separator is cleaned in a similar way – by pulling the magnet out of the housing, cleaning off the collected sludge, and reassembling it. Sludge and magnetic particles are a normal occurrence, especially in old cast-iron systems. Capturing them before they reach the boiler significantly extends the lifespan of the pump and heating elements.

What to do when shutting down the boiler for a longer period (holiday, empty house)

If you shut down the boiler for a longer period (more than 2–3 weeks), consider the following:

Don't turn off the frost protection with the main switch: The Thermona THERM EL has a built-in frost protection program that automatically activates the pump and minimal heating when the temperature drops low enough. Leave the boiler on in this mode; don't rely on a switched-off boiler in an unheated house.
Check the pressure before leaving: Fill the system to the correct pressure (1.2–1.5 bar) and check for any leaks.
If the house will be empty for a long time and heating isn't worthwhile: Consider draining the system, but this is always a last resort – refilling brings in fresh hard water and air.

Service log – your most important document

Every THERM EL boiler comes with an installation and warranty certificate. Every service intervention should be recorded in the service log (part of the boiler's technical documentation): date, task performed, technician's name, and their stamp/signature. This record is important for several reasons:

Preserving the manufacturer's warranty (Thermona conditions the warranty on installation and servicing by an authorized person)
Proving proper care in the event of an insurance claim (insurance events due to appliance failure)
A history of faults helps a new technician quickly identify chronic problems
When selling a property, it increases the value of the installation, as the buyer sees that the boiler has been professionally maintained

When to call a service technician – the line between DIY and professional

Below is an overview of what an informed owner can handle themselves and what belongs exclusively in the hands of a professional.

Task	Owner	Technician
Checking pressure, topping up water	✓
Bleeding radiators	✓
Manual activation of the safety valve	✓ (carefully)
Cleaning the filter	✓
Unscrewing/freeing up the pump	✓ (if you know how)
Checking and topping up expansion tank pressure	✓ (carefully)	✓
Replacing the expansion tank		✓
Replacing the pump		✓
Replacing the heating element		✓
Intervention in the electrical part (terminal blocks, board)		✓
Measuring insulation resistance of heating elements		✓
Replacing the safety valve		✓

Maintenance costs and their real-world evaluation

A regular annual service inspection of an electric boiler is significantly cheaper than for a gas boiler – typically in the range of €60–120 including the call-out, depending on distance and scope of the inspection. Replacing a pump on a THERM EL usually costs €150–280 for labor and materials. Replacing a heating element costs roughly €80–200 per piece, and with larger models (THERM EL 30 or Thermona THERM EL 38) there may be several elements, each counted separately.

For comparison: if you neglect water quality and deposits cause the heating elements to burn out 5 years earlier than they should, and this also damages the pump and control board, the total repair costs can easily exceed €600–1,000. And that's not even mentioning the interruption of heating in winter and all the difficulties that come with it.

Investing in preventive maintenance clearly pays off – not only financially, but also in terms of comfort and heating reliability. If you have a smaller model like the Thermona THERM EL 8 or the mid-range Thermona THERM EL 15, service costs are even lower, since the appliance is more compact and takes the technician less time to check.

Frequently Asked Questions (FAQ)

How often should the THERM EL electric boiler be serviced by a professional?

The manufacturer recommends an annual service inspection by an authorized technician. Unlike gas boilers, this is not legally mandatory, but it is recommended to maintain the warranty and extend the appliance's lifespan. In practice, many owners only call a service technician when there's a fault, which is understandable but economically disadvantageous. The ideal rhythm is once a year before the heating season, i.e. in September or October.

The system pressure keeps dropping regularly. Do I need to have this fixed, or is it normal?

Slight pressure drops over a season (0.2–0.3 bar) can be caused by bleeding of the system and are to some extent normal. However, if the pressure drops faster (for example 0.3 bar per month or more than 0.5 bar per season), there is a leak somewhere. Visually check all joints, fittings, radiators, and the boiler itself. If you can't find an obvious leak, call an installer to run pressure tests – the leak may be behind plaster or in underfloor heating.

Can I top up a closed heating system with regular tap water?

Yes, in an emergency this is an acceptable solution for a one-time top-up. The problem arises if you do this repeatedly or in large quantities. Regular tap water is hard in many parts of Slovakia and contains dissolved oxygen, which upon heating causes corrosion and limescale formation. If you need to top up the system more than once a season, look for the cause of the pressure drop instead. Ideally, top up with demineralized water containing a corrosion inhibitor.

The boiler reports a pump fault right after switching on in autumn. What should I do?

This is a typical case of a pump stuck after the summer shutdown. Before calling a service technician, try the following: turn off the boiler, find the service screw (nut) on the pump housing with a slot for a flat screwdriver, unscrew it (watch for leaking water), turn the rotor with a flat screwdriver, and screw it back in. Turn the boiler back on. If the pump starts up, the problem is solved. If not, or if you're not comfortable doing this yourself, call for service.

Do I need to completely switch off the THERM EL boiler during summer?

It's not necessary and in many cases not even advisable. If the boiler remains in standby mode, the control electronics stay active, the frost protection remains functional, and the pump will periodically perform a short self-test cycle (depending on the model), preventing it from getting stuck. If you switch off the boiler completely using the main switch, the frost protection will not work. We recommend leaving the boiler in standby mode throughout the year.

Does regular maintenance reduce electricity consumption?

Yes, and quite noticeably. A limescale layer just 1 mm thick increases the heating element's consumption by 7–10%. A 3 mm layer can mean up to a 20–25% increase in consumption. Clean heating elements, properly set controls, and a well-bled system can reduce annual heating costs by 10–20% compared to a neglected system. For powerful models like the Thermona THERM EL 23, this can represent several hundred euros per season.

Conclusion – maintenance as an investment, not a burden

The Thermona THERM EL electric boiler is a robust and reliable appliance. If you treat it responsibly – regularly checking the pressure, having an annual service inspection done, and monitoring the water quality in the system – it will serve you for 15–20 years without major problems. The monthly visual check takes 5 minutes. The annual service inspection costs significantly less than a premature replacement of a heating element or pump would.

Key takeaways: monitor the pressure, protect the system from hard water, don't leave the pump stuck after the summer break, activate the safety valve at least once a year, keep a service log, and never intervene in the electrical part yourself. These simple rules will save you worries, money, and a winter without heating.

If you're interested in how to properly connect the boiler to the heating system or what the hydraulic requirements are, we recommend the article Connecting the Thermona THERM EL to the heating system – wiring diagrams and hydraulic requirements. For further questions about choosing the right model, there's also the article How to choose the Thermona THERM EL electric boiler – output, space size and other criteria.

Do you have a question about this topic?

Can't decide or dealing with a specific situation in your household? Write to us - we're happy to help.