Most Common Faults in Electric Boilers and How to Fix Them
Most Common Faults in Electric Boilers and How to Fix Them
An electric boiler is one of the most reliable and structurally simplest heat sources you can install in a household – it has no burner, no flue gases, and no need for a chimney, so it also has fewer parts that can break. Even so, it's still dedicated, high-power electrical equipment with its own typical, well-documented faults. Some of them the owner can fix in a few minutes; others require an electrician or a qualified service technician. In this article we go through six of the most common situations we come across with electric boilers – from a complete failure to start, through weak output, frequent shutdowns, and dropping pressure, to problems with the HDO tariff and a noisy circulation pump. For each fault we explain what's actually happening inside the boiler, what you can check yourself, and when it's time to call a professional. The goal is for you to be able to recognise the fault, describe it correctly to a service technician over the phone, and above all avoid interventions that can be dangerous on an electrical device.
Overview of the faults covered in this article.
1. The boiler won't start at all
The most common, and also the easiest to solve. Electric boilers have a considerably higher power draw than typical household appliances – standard single-phase models range roughly from 6 to 12 kW, and three-phase models for larger houses commonly from 15 to 24 kW or more. With a draw like that, it's entirely normal for the breaker to trip on the first start, after a longer shutdown, or after adding another appliance to the same circuit. If the wiring or the breaker isn't correctly sized for the boiler's output, trips will happen repeatedly, sometimes even several times a day. The first thing worth checking is therefore the distribution board – look at whether the relevant breaker is in the "off" position or in the middle, tripped position. If so, try switching it back on once.
The second common cause of a complete failure to start is a fault in the contactor or the control relay that switches the individual power stages inside the boiler. A contactor is essentially an electrically operated switch through which the large current needed to feed the heating elements flows. After years of switching, its contacts wear out – they can fuse together (the boiler then can't be switched off at all) or, conversely, lose contact, so the boiler doesn't start at all, or only partly. If the breaker trips again immediately after being switched back on, or trips repeatedly with no obvious reason, it's no longer ordinary overload but very likely a short circuit or fault directly in the boiler's electrical section. In that case, don't force the breaker back on repeatedly – a breaker tripping repeatedly is its protective function signalling a real fault, not a whim. Bypassing this protection in an emergency is a direct fire risk.
What you can handle yourself: checking the breaker's position on the distribution board and trying to switch it back on once. What's not for a layperson: opening the boiler's casing, checking or replacing contactors, taking measurements at the terminal block. This is always a job for an electrician or a service technician with the relevant qualification.
2. The boiler doesn't reach the set temperature or heats weakly
An electric boiler usually doesn't have one large heating element, but several smaller power stages (for example three stages of 2 kW each for a total output of 6 kW), which switch in one after another as needed – first one, and if more heat is needed the control system adds a second and third. This principle has the advantage of smoother output control, but also a downside: if one of the power stages fails (a burnt-out heating rod/coil for that stage, or a fault in the contactor that switches it), the boiler still starts and runs, just with lower total output. In practice this looks like the boiler working seemingly normally, but in colder weather the house can't be heated up to the set temperature, or it takes noticeably longer than usual. This is one of the trickiest faults precisely because it doesn't show up as a complete shutdown – the boiler "runs", just with part of its real output, and many owners assume for a long time that it's just colder weather.
A second possible cause of insufficient or incorrect heating is a fault in the temperature sensor. This sensor continuously measures the water temperature in the boiler and sends the reading to the control electronics, which decide how many power stages to switch on based on it. If the sensor is damaged, poorly seated in its pocket, or its wire is damaged, the control system receives distorted or no data – the boiler might switch off too early (thinking the temperature has already been reached), or conversely fail to reach the actual set value.
As a model example of a multi-stage electric boiler, we can mention the Protherm Ray 14KE – a boiler with a 14 kW output split across several power stages that switch in one after another exactly according to the principle described above.
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Protherm Ray 14KE – an electric boiler with a 14 kW output and gradual switching of several power stages, a typical example of the design discussed above – if one stage fails, the total output noticeably drops, but the boiler keeps running. Price from €961.00. |
What you can handle yourself: noticing that the heating is weaker than usual for the same setting, and remembering/writing down this information for the service technician (since when it's been happening, at what outdoor temperature). Diagnosing a specific power stage or sensor is already a job for a service technician – it requires measuring current and voltage directly at the boiler's terminals.
3. The boiler shuts off or restarts frequently
This fault needs to be distinguished from the first one – here the boiler starts and runs normally for a while, then shuts itself off or restarts, often repeatedly at regular intervals. One of the most common causes is thermal overload of the breaker. If the wiring or cable cross-section is undersized for the boiler's power draw, the breaker thermally trips itself after a certain time of continuous operation (typically tens of minutes, as long as it takes the conductors to heat up) – this isn't an instant short circuit, but gradual overheating that protects against fire. It shows up as the boiler running, switching itself off after a while, being switchable back on once the breaker has cooled down, and the whole cycle repeating. This is a clear signal that either the wiring doesn't match the boiler's output or the breaker isn't correctly sized (more on this in our separate article on what breaker and wiring an electric boiler needs).
The second common cause is the activation of a safety cut-out against overheating (often called an STB – safety temperature limiter). This is an independent safety component that hard-cuts the power supply to the heating elements if the water temperature in the boiler exceeds a safe limit, regardless of what the normal control system says. This cut-out is usually activated when the water in the system, for some reason, isn't drawing heat away quickly enough – most commonly due to insufficient circulation (a closed or partly closed valve on some circuit, air in the system blocking flow, a non-functioning circulation pump), or a fault in the control system itself. In that case the boiler shuts down to protect against overheating the water and damaging the heating elements.
What you can handle yourself: noticing the shutdown pattern (for example always after the same running time, always during harder frost), checking whether any radiator valves are fully closed, and listening for air bubbling in the system. What's not for a layperson: repeatedly forcing the breaker or boiler to restart without finding the cause – if thermal overload or activation of the safety cut-out is suspected, call an electrician, since these are protective mechanisms against a more serious fault or fire.
4. Low system pressure or air in the system
This fault works on exactly the same principle as with gas boilers, because it has nothing to do with how the water is heated, but with the hydraulics of the whole closed heating circuit. The heating system is filled with water under slight overpressure (commonly 1 to 2 bar when cold – check the exact value in your specific boiler's manual). A gradual, slow drop in pressure over the months is to some extent normal – a small amount of air and water is naturally lost over time through vents and joints. It shows up as the pressure gauge reading dropping below the recommended value, or as bubbling or knocking in the pipes, and radiators that are cold at the top despite being warm at the bottom – a typical sign of air trapped in the upper part of the radiator.
You can fix both of these situations yourself: topping up water through the boiler's fill valve according to the manufacturer's instructions until the pressure gauge shows the recommended value, and bleeding radiators with a bleed key – just turn it until air starts escaping and then a continuous stream of water flows, then tighten the little valve back up. This is normal, safe maintenance that practically any owner can manage.
A problem arises when you have to top up water repeatedly – for example more than once a month, or several times over a heating season. At that point it's no longer a normal small leak, but a signal of either a hidden leak somewhere in the system (a joint, a valve, the pump housing), or a worn-out expansion tank. The expansion tank is a pressure vessel with a rubber diaphragm that accommodates the water's volume changes as it heats and cools. If the diaphragm loses its elasticity over time, or the nitrogen cushion inside it is lost, the tank stops damping the water's thermal expansion, pressure in the system then fluctuates more and often escapes through the safety valve, which outwardly shows up exactly as a repeated pressure drop that you have to top up. In that case it's worth considering replacing the expansion tank with a new one.
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IVAR.ER 12 pressure expansion tank (12 l) – a common replacement for a worn-out diaphragm expansion tank in small to medium heating systems with an electric boiler. If you keep having to top up water and haven't found a visible leak, this tank is the most common hidden cause. Price from €47.73. |
What you can handle yourself: topping up water, bleeding radiators, a visual check of visible joints and valves for leaks. What's not for a layperson: replacing the expansion tank (requires draining part of the system and working with a pressure vessel) and any intervention inside the boiler itself – leave those to a heating technician.
5. HDO/contactor not working correctly (low or high tariff)
HDO (ripple control) is a signal that the electricity distributor sends to households, used to switch between the low (cheaper) and high (more expensive) tariff, or in some cases to directly block selected appliances during the high tariff. For an electric boiler, the HDO signal is typically fed to a dedicated HDO receiver, which then, via a contactor, allows or blocks power to the boiler according to the current tariff. If the boiler is set to heat only during the low tariff, and yet it also runs during the high (more expensive) tariff, or conversely doesn't start at all even during the cheap window, the problem isn't in the boiler itself – its electronics and power stages work fine, they just aren't receiving, or are misinterpreting, the permission from the HDO circuit.
The cause is usually either an incorrectly set HDO time schedule (at the distributor or directly in the receiver), a fault in the HDO receiver itself, or a wiring fault in the contactor between the HDO circuit and the boiler's power supply. This is always a job for an electrician – it requires measuring the signal, checking the wiring in the distribution board, and possibly communicating with the electricity distributor about the tariff time schedule. The boiler owner can't fix this themselves and shouldn't try to, since it involves working directly on the mains wiring.
What you can handle yourself: noticing and writing down the exact times when the boiler runs and doesn't run, and comparing them with the low-tariff time window shown on your electricity bill – this information will significantly help the electrician. Leave the actual repair, adjustment, or replacement of the HDO receiver or contactor to a professional. You can read more about how HDO works with an electric boiler in a separate article.
6. Circulation pump is noisy or not working
The circulation pump has the same job in an electric boiler as in any other type of heat source – it forces the heated water to circulate from the boiler to the radiators or underfloor heating and back. Without a working pump, heat from the boiler doesn't get where it's needed, even if the power stages themselves are working perfectly. Typical signs of a pump fault are audible noise (buzzing, rattling, knocking), radiators or underfloor circuits that stay cold or lukewarm despite the boiler's display showing it's actively heating, or complete silence instead of the usual quiet hum of the pump running.
The most common cause of noise is air trapped directly inside the pump housing – solved by bleeding the pump via the bleed screw on its front, which in many cases is a task a handier owner can manage by following the manufacturer's instructions. A second, somewhat more serious cause is mechanical dirt and deposits in the circulating water – rust, limescale, or residue left from installation or ageing pipework. Over time these particles settle in the pump's rotor, grind the bearings, and can even completely jam the rotor, especially after a longer summer shutdown when the pump hasn't run for a while. The same deposits also gradually reduce the efficiency of the boiler's heating elements themselves and shorten their lifespan.
The most effective prevention for this type of fault is a mechanical filter at the water inlet to the system, which catches exactly these impurities before they reach the pump or the boiler's heating elements.
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NANO HOT 3/4" - 1" 500 mcr water filter – a brass mechanical filter that catches deposits and impurities in the circulating water before they damage the circulation pump or the boiler's power stages. A sensible precaution, especially for older pipework. Price from €65.50. |
What you can handle yourself: bleeding the pump according to the instructions (if it has an accessible bleed screw), listening for whether the pump makes unusual noise while running. What's not for a layperson: taking the pump apart, replacing the rotor or bearings, wiring in a replacement pump – that's a job for a service technician.
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Thermona THERM EL 8 – a model with built-in contactors for safe, smooth switching of the individual phases without inrush current spikes, directly addressing fault type 1 described above (a worn-out contactor causing the breaker to trip repeatedly). Price from €709.53. |
The order of steps to take before calling a service technician.
When to call an electrician or a service technician
One simple rule always applies to electric boilers, and it's worth keeping in mind: an electric boiler is dedicated, high-power electrical equipment, so any intervention inside it – power stages, contactors, control electronics, temperature sensors, HDO wiring – belongs exclusively to a person with professional electrotechnical qualification. This isn't just a formality; an incorrect intervention in such equipment can cause an electric shock or a fire, and can also void the boiler's warranty.
Conversely, routine hydraulic maintenance of the heating system is entirely safe, and it's fine for the owner to handle it themselves: topping up water to the correct pressure, bleeding radiators, checking and switching a tripped breaker back on once, and observing and writing down the symptoms of a fault (when they appear, at what temperature, how long the boiler had been running before). This last activity in particular – careful observation and recording of symptoms – is what helps a service technician the most and shortens both the repair time and its cost.
As an illustration, let's look at a model situation that service technicians come across fairly often with electric boilers. A family house has an electric boiler installed with three power stages of 3 kW each, so a total output of 9 kW, connected to a single-phase breaker. The owner notices that on freezing days, for several weeks now, the house hasn't been reaching the set room temperature, even though the boiler's display shows it's "heating", and there was no problem in summer or autumn. The first step they can handle themselves is checking the system pressure (it's fine) and bleeding the radiators (no effect on the main problem). So they write down that the problem only appears during harder frost, which suggests the boiler is missing part of its output exactly when it's needed most – a typical sign of a failed power stage. On the service technician's visit, this suspicion is confirmed by measuring the current on the boiler's individual phases: two stages draw current normally, the third doesn't at all – the cause is a worn-out contactor on the third stage, which the technician replaces. After the replacement, the boiler again reaches its full declared output of 9 kW even in freezing weather. Without a record of the exact circumstances (when exactly the problem occurs), the diagnosis itself would have been somewhat more time-consuming – which is why carefully observing symptoms is always worth doing.
Telling routine maintenance apart from work requiring professional qualification.
Simplified wiring diagram of an electric boiler
Below is a simplified diagram showing the path of electric current from the house's distribution board all the way to the boiler's individual power stages – exactly the elements (breaker, contactor, power stages) discussed in most of the faults above.
If the breaker trips, current doesn't reach the contactor or the power stages at all – the boiler doesn't start at all (fault 1). If only the contactor or the heating element of one of the stages fails, the other stages keep working, just with lower total output (fault 2). If the problem is in the HDO relay, the power stages themselves may be fine, but they don't get permission to run at the right time (fault 5).
Frequently Asked Questions
Why does my breaker trip on my electric boiler?
Most often because the wiring or the breaker itself isn't sized for the boiler's actual power draw – electric boilers have a high draw (commonly 6–24 kW), and if the breaker was chosen without regard to this value, it will trip on every start-up or whenever other large appliances switch on at the same time. If the breaker keeps tripping, don't force it back on repeatedly – contact an electrician.
Can I repair my electric boiler myself?
Routine hydraulic maintenance, yes – topping up water, bleeding radiators, checking and switching the breaker back on once. Anything inside the boiler (power stages, contactors, sensors, HDO wiring) is dedicated electrical equipment, and only a person with electrotechnical qualification may work on it. A DIY intervention is a safety risk and usually also grounds for voiding the warranty.
How often do I need to top up water in the heating system?
A slight, occasional pressure drop over the months is normal. But if you have to top up water repeatedly, for example several times a season, it's a signal of either a leak in the system or a worn-out expansion tank whose diaphragm has lost its elasticity – in that case it's worth replacing the tank.
What does it mean if the boiler also runs during the more expensive (high) tariff?
It means a problem in the HDO circuit – either in the time schedule setting, a fault in the HDO receiver, or in the wiring of the contactor that's supposed to block the boiler during the high tariff. The boiler itself can be completely fine. This is an electrician's matter, not a boiler matter.
Why doesn't the boiler reach the set temperature, even though it's clearly running?
Most often because one of the boiler's several power stages has failed (a fault in the contactor or the heating element of that stage), so the boiler runs, but with lower total output than what's set. This shows up mainly during harder frost, when the house needs full output.
Is it normal for the circulation pump to occasionally make noise?
Brief noise after bleeding or after a longer shutdown (for example after summer) isn't unusual and can often be fixed by bleeding the pump itself. Persistent or growing noise, or cold radiators while the boiler is running, already signal wear or deposits in the pump and should be checked.
How long does an electric boiler last without a major fault?
Thanks to their simple construction (no burner or flue path), electric boilers generally have a long service life, but the individual power stages, contactors, and the pump are parts subject to ordinary wear from switching and operation. Regular pressure checks, bleeding, and water filtration significantly extend the interval between major service interventions.
Related topics
- What circuit breaker and wiring does an electric boiler need
- How HDO (ripple control) works with an electric boiler
- Comparison of electric boiler brands
- Frequently asked questions about electric boilers and convectors
Have a question about choosing a boiler?
Can't decide, or dealing with a specific situation in your household? Write to us – we're happy to help.