Common Faults of Attack Electric Boilers and How to Fix Them

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Common problems with Attack Electric electric boilers and how to solve them

Attack Electric electric boilers are among the reliable and technically well-developed devices – but like any heating equipment, they can encounter problems during operation. From practice, I know that most of the problems customers call or write about are not caused by a product defect, but by incorrect settings, neglected maintenance, or problems with the electrical installation or heating system. In this article, I systematically cover the most common faults, their symptoms, causes and – most importantly – specific steps on how to diagnose and eliminate them.

This article is primarily intended for boiler owners who want to understand their device and be able to assess what they can do themselves and when to call a service technician. If you are still planning the installation, also see the topics Installation of Attack Electric electric boiler: procedure, requirements and connection and Control and setting of Attack Electric electric boiler: how to save on electricity?, which are part of this Knowledge Center.

Overview of faults by symptoms and severity

Before we get into the details, it is useful to have an overall overview. Electric boiler faults can be divided into several main categories according to what the owner sees or feels at first glance:

The boiler does not start at all – no reaction, the display does not light up or it does, but the boiler does not heat
The boiler starts, but shuts off shortly – safety fuse, overheating, insufficient flow
Incorrect temperature in the heating circuit – the boiler heats too little or, on the contrary, too much
Noise and vibrations – hissing, knocking, bubbling, noise from the circulation pump
Error codes on the display – the boiler itself signals a specific problem
Problems with the preparation of hot water (HW) – if the HW module set (HW preparation in an external tank) is installed
Water leakage or pressure problems – pressure drop in the system, moisture around the boiler

Boiler does not start – diagnostics step by step

This is the most common reason for panic, but at the same time, most cases have a trivial solution. Before you call a service, go through the following procedure – in practice, most of these cases can be resolved by yourself in 10–20 minutes.

1. Check the power supply and circuit breaker

The Attack Electric electric boiler operates on three-phase power supply (400 V / 3+N+PE) and requires a separate circuit breaker dimensioned according to its power. For example, Attack Electric Excellent 15 kW requires a circuit breaker of at least 25 A (B-characteristic), while Attack Electric Excellent 24/22 kW requires a 40 A circuit breaker. The first step is to go to the distribution board and check whether the circuit breaker is in the "on" position and whether someone has not turned off the boiler via the main switch.

If the circuit breaker is tripped (in the middle or in the off position), do not force it to turn on immediately. First, find out the cause. The circuit breaker can trip due to a short circuit, overvoltage, or if it is too weakly dimensioned. You can tell this by the fact that it trips again immediately after turning on – in that case, the problem is either inside the boiler or on the supply cable, and the solution belongs to the hands of an electrician.

2. Check the fuse inside the boiler

Attack Electric boilers have internal fuses and thermal fuses (thermal fuses) on the heating elements (electric heating elements). The thermal fuse is a one-time safety element that irreversibly "burns out" in the event of boiler overheating – most often due to insufficient water flow through the boiler. After the thermal fuse is tripped, the boiler does not heat and appears from the outside as if it had no power supply, although the circuit breaker in the distribution board is in order. Replacing the thermal fuse is a relatively inexpensive service matter, but it is always necessary to find out and eliminate the cause of the overheating – otherwise the situation will repeat itself.

3. Thermostat and regulator setting

This sounds trivial, but in practice, it is one of the most common "problems": the boiler works, but it is set to too low a desired temperature. If the water temperature in the system is higher than the set desired temperature, the boiler will not start – this is a normal state, not a fault. Check the regulator setting and make sure that the desired temperature of the heating circuit is actually higher than the current water temperature in the boiler. More about the correct setting of the regulation can be found in the topic Control and setting of Attack Electric electric boiler: how to save on electricity?

The boiler starts but shuts down shortly after starting – safety lock

This problem is a bit more tricky because the boiler seems to be working, but in reality it cycles between starting and shutting down. Usually, four causes are behind this behavior:

Insufficient water flow through the boiler

Attack Electric boilers have a built-in flow or outlet temperature sensor that stops the heating elements in case of insufficient flow – to prevent overheating of the water in the boiler and damage. If the circulation pump is jammed, the filter is clogged, or the shut-off valves are not properly open, the boiler will heat up, the safety thermostat will shut off the heating elements, and it will wait until the temperature drops – then the cycle starts again.

Solution: Check whether all shut-off valves are fully open. Manually check the rotation of the circulation pump (usually has a bleed screw – turning it on and off will start the pump if it is jammed after a long period of inactivity). Check and flush the magnetic filter on the boiler return line.

Air in the system

Air in the circuit causes the pump to be unable to maintain sufficient flow and the boiler behaves similarly to when the pump is jammed. Air also causes characteristic gurgling and knocking in the pipes. Bleed the radiators one by one, starting from the furthest to the nearest to the boiler, then top up the system pressure to 1.2–1.5 bar (when the system is cold).

Overheating – safety thermostat setting

Attack Electric boilers (including the model Attack Electric Easy 8) have an adjustable or fixed safety thermostat on the outlet pipe. If the maximum required temperature is set too high (over 80 °C in older, less insulated systems), the boiler may reach a safety limit. Check whether the outlet temperature setting is within a reasonable range – 35–45 °C for floor heating, 55–70 °C for radiators.

Defective temperature sensor

If the outlet temperature sensor shows incorrect values (usually either very low or very high), the boiler may think it is overheating, even though it is not actually overheating. This is indicated by an error code on the display – check what the code means (see below).

Error codes on the display – what they mean and what to do

Attack Electric Excellent series boilers have a digital display that shows error codes in case of a fault. The Easy series have a simpler indication (LED indicators). Here are the most common codes and their explanations:

Code / Signal	Likely cause	Solution
E1 / Err1	Fault in the outlet temperature sensor (NTC sensor)	Check the sensor connection, resistance at 25 °C is approx. 10 kΩ; if deviating – replace the sensor
E2 / Err2	Overheating – safety thermostat tripped	Let the boiler cool down, check the flow and pump, reset manually using the reset button
E3 / Err3	Circulation pump fault or no flow	Check the pump, make sure it is not jammed and is properly powered (230 V)
E4 / Err4	Fault in the DHW tank temperature sensor	Applies when the DHW module is installed; check the tank sensor and its wiring
LED blinking / no code	Thermal fuse tripped (one-time)	Replace the thermal fuse by a qualified technician; eliminate the cause of overheating before replacement
Fluctuating values / unstable display	Phase drop in power supply	Check the three-phase power supply, all three phases must be present; problem for an electrician

Note: Exact codes may slightly differ depending on the specific version of the control electronics. Always compare with the user manual for your boiler.

Noise and vibrations – causes and solutions

Properly installed and bled Attack Electric electric boiler is practically silent. If any sound appears, it is almost always a signal of a specific problem.

Gurgling and knocking in the pipes

The most common source of noise. Cause: air in the system. Resolved by bleeding the radiators and possibly installing an automatic air vent at the highest point of the system. Do not forget to top up the pressure via the filling valve after bleeding.

Roaring or buzzing of the pump

If the circulation pump is jammed or worn, it produces a characteristic roaring sound. Sometimes the pump can be "freed" by manually turning the bleed screw, but if the sound persists, the pump is a candidate for replacement. Pumps are a part that is replaced first in boilers with a longer lifespan – the typical lifespan is 8–12 years under normal operation.

Knocking of heating elements (electric heating elements)

During rapid temperature rise, the heating elements may produce knocking sounds – this is thermal expansion. It is usually harmless, but if the sound is pronounced and regular, it may indicate limescale deposits on the heating elements, which reduce their efficiency. Heating elements with a thick layer of limescale heat up much faster, which shortens their lifespan.

Hissing – pump cavitation

Hissing is a typical sign of cavitation – the pump is sucking in air bubbles. The cause may be low pressure in the system (under 0.8 bar), a clogged filter before the pump, or too fast flow through a narrowed section. Check the pressure and filter.

Pressure drop in the system and water leaks

An electric boiler is not often the direct source of water leaks, as many might think – most leaks occur at pipe connections, valves, or the expansion tank. However, the boiler can be indirectly involved in the issue.

Expansion tank – a hidden problem

The expansion tank (pressure type) is equipped with a membrane that compensates for water volume when heated. If the membrane is perforated or the tank is overfilled, the cold system pressure may be normal, but it will exceed 3 bar when heated, causing the safety valve to discharge water – this is why you may see moisture or dripping under the boiler's safety valve. Check the pre-charge pressure of the expansion tank (air chamber on the opposite side of the membrane) – it should be 0.8–1.0 bar in a cold system.

Safety valve – it is not a boiler fault

A safety valve set to 3 bar will open if the system pressure exceeds this value when heated. If you see the safety valve dripping, it is not a valve fault – it is a signal that the system has excessive pressure. The cause may be an overfilled system (too high cold pressure – above 1.5–1.8 bar) or a faulty expansion tank.

Pressure drop without visible leak

If the system pressure regularly drops and no visible leak is found, look for a leak in hidden piping, in floor heating (a cracked pipe), or in an automatic air vent that may be leaking water. A pressure drop of 0.1–0.2 bar per month is normal when topping up; if it drops faster, there is a leak somewhere.

Problems with hot water preparation (DHW)

If you have a DHW module set (DHW preparation in an external tank) connected to your electric boiler, another category of potential problems may arise. This topic is discussed in detail in the article DHW module for an electric boiler: how does hot water preparation work in an external tank?. Here are the most common faults:

DHW does not heat up to the desired temperature

There are several possible causes: the boiler may be set so that DHW preparation has low priority and switches back to heating before the tank reaches the desired temperature. Check the DHW priority setting in the regulator menu. Another cause may be a clogged heat exchanger in the tank (limescale), which slows down heating – in areas with hard water, we recommend cleaning or descaling the tank every 2–3 years or replacing the sacrificial anode.

Temperature fluctuations in hot water during use

If hot water alternates between hot and cold when the tap is opened, the cause may be a low tank capacity relative to consumption, or a faulty tank temperature sensor that incorrectly informs the boiler of the achieved temperature.

The tank is heating, but DHW temperature is low

Here, the problem may be in the mixing: a thermostatic mixing valve at the tank outlet (a mandatory safety component, set to a maximum of 55–60 °C for legionella prevention) may be set too low or may be damaged and not mixing cold and hot water accurately.

Limescale on electric heating elements – a silent enemy

In areas with hard water (hardness above 15–20 °dH), limescale deposits on electric heating elements. A layer as thin as 1 mm increases electricity consumption by about 10 %, a 3 mm layer means a performance loss of up to 20–25 % and significantly shortens the lifespan of the element due to localized overheating under the limescale layer.

Signs of clogged elements:

The boiler takes longer to reach the desired temperature than at the beginning of operation
Increased electricity consumption without changes in external conditions
Increased boiler noise (knocking) during heating
Repeated tripping of the thermal fuse

Solution: Professional degreasing and descaling of the elements or their replacement. Preventively, we recommend using a magnetic water softener or a corrosion and limescale inhibitor in the system in areas with hard water. Installing a water softener is cost-effective in the long run – it extends the lifespan of the elements and the entire system. More on preventive maintenance in the article Maintenance and service of the Attack Electric boiler: what and when to check?

Electrical faults – problems with the control electronics

The control electronics of Attack Electric boilers are robust and designed for long life, but they are not indestructible. The most common electrical faults:

Failure of one phase in three-phase power supply

Boilers such as Attack Electric Excellent 8 / 7.5 kW have heating elements evenly distributed across three phases. If one phase fails (e.g., a circuit breaker blows or there is a fault in the distribution board), the boiler will only operate at two-thirds of its capacity. This is noticeable when the boiler cannot maintain temperature during cold weather, even though it functions well in milder conditions. Diagnosis: measure the voltage on all three phases (phase-neutral: 230 V on each phase). This issue belongs to an electrician.

Regulator failure – freezing or incorrect values

A digital regulator can occasionally freeze during power outages, overvoltage, or electrostatic faults. The first thing to try is to reset the regulator – usually, turning the boiler off for 30 seconds and turning it back on (via the circuit breaker or main switch) is sufficient. If the problem persists, the control board may need to be replaced.

Back surge from the power grid

During thunderstorms or when large loads are switched on nearby (machining tools, compressors, welders), a surge impulse can enter the grid and damage the sensitive electronics of the regulator. Protective measure: installation of surge protection (SPD – Surge Protection Device) in the distribution board, ideally class T1+T2 for single-family homes.

Uneven heating of rooms – not always a boiler problem

Customers often attribute a problem to the boiler, while the cause lies elsewhere. If some radiators are heating poorly, the reasons may be:

Insufficient hydraulic balancing of the system: The nearest radiators receive too much flow, while the distant ones get too little. The solution is hydraulic balancing – setting the preset on thermostatic valves or installing balancing valves.
Air in the radiator: The radiator is cold only in the upper part – a classic sign of air. Bleed it using a bleeding key.
Clogged thermostatic valve: The thermostatic head or valve may be stuck in the closed position, especially after summer, when the boiler was not in use. Try temporarily removing the thermostatic head and manually opening the valve stem.
Undersized boiler: If the boiler is too small for the heated area, it will never reach sufficient temperature during frost. Read the article What power of electric boiler do I need for my house? and Attack Electric Excellent vs. Easy: in what they differ and which one to choose?

When to call a service technician and when you can handle it yourself

This is a practical question every reasonable boiler owner asks. Summary:

Problem	Can you handle it?	Note
Bleeding radiators	✅ Yes	Routine maintenance, no professional qualification required
Pressure topping up (filling valve)	✅ Yes	Top up to 1.2–1.5 bar, monitor the pressure gauge
Boiler reset after a fault	✅ Yes	After identifying and eliminating the cause
Manual pump release	✅ Yes	Loosen the pump locknut, manually rotate
Thermal fuse replacement	⚠️ Professional	Requires disassembly, professional testing
Electric element replacement	❌ Service	Professional work, draining, sealing, electrical work
Circulation pump replacement	❌ Service	Electrical + hydraulic work, warranty conditions
Control board failure	❌ Service	Diagnosis + replacement, warranty conditions
Electrical – fuses, power supply	❌ Electrician	Legal obligation – only a qualified person

Preventive protection – preventing failures

From practice, I can say that boilers whose owners perform regular annual checks run without major problems for 15 or more years. On the contrary, neglected boilers without any maintenance often experience their first major failure (element or pump replacement) already after 5–7 years. Preventive steps:

Once a year (ideally at the end of summer, before the heating season): system bleeding, pressure check, visual inspection of seals and connections
Once every 2–3 years: check the expansion tank pre-charge pressure, check the TÚV tank anode, check the safety valve setting
Once every 3–5 years: professional inspection of electrical system, condition of elements, pump and sensors; filter cleaning
Ongoing: monitor system pressure (pressure gauge), monitor electricity consumption (significant increase = clogged elements or another problem)

More details about planned maintenance can be found in the article Maintenance and service of the Attack Electric boiler: what and when to check?

Most frequently asked questions (FAQ)

Attack Electric boiler reports an error and does not heat – should I call a service immediately?

Not necessarily. Most error states are caused by removable issues: air in the system, low pressure, jammed pump or incorrect thermostat setting. Go through the control steps described above step by step. If the problem persists or recurs after resetting the boiler and checking basic parameters (pressure, bleeding, settings), it is time to call a service technician.

My Attack Electric boiler consumes more electricity than last year – what is happening?

Increased consumption without changes in the heated area or outdoor temperature most often indicates clogged electric elements due to limescale (in hard water areas), or heat loss due to building envelope issues (insulation failure, leaky windows). Another possible cause is suboptimal regulation settings – for example, an excessively high heating setpoint. Compare consumption with the same month of the previous year under similar temperatures.

Thermal fuse on the Attack Electric boiler has blown – can I replace it myself?

Technically, thermal fuse replacement is relatively simple, but we recommend entrusting it to a professional for two reasons: first, working inside the boiler involves live 400 V conductors – this requires electrical qualifications; second, before replacement, it is essential to eliminate the cause of overheating, otherwise the new thermal fuse will blow again. A service technician will also check whether the elements themselves were damaged during the overheating.

Why is air still remaining in my radiators, even though I bleed them regularly?

If air keeps returning to the system despite bleeding, there are several possible causes: the system has a non-functional automatic air vent that is letting air in instead of out; the system has corrosion (rusty water) generating gases; or there is a problem with the expansion tank, which is not allowing proper compensation and water pressure is drawing air in through seals. Check the color of the water when bleeding – rusty or dark water indicates corrosion and requires a corrosion inhibitor.

The boiler is heating, but the floor heating remains cold – what is the problem?

With floor heating, the most common cause is a jammed distributor (valves on the floor heating distributor) or a thermoelectric actuator on the circuit that is not opening (faulty actuator, broken wiring, problem with the room thermostat of the respective circuit). Check whether the actuators on the distributor are in the correct position and whether 230 V is supplied to the actuator during an active heating request.

Is it normal for the Attack Electric boiler to make a soft clicking sound when turning on and off?

Yes, a soft clicking sound when switching on and off of elements (relays or contactors) is completely normal and is not a sign of a fault. Similarly, slight vibrations in the pipes when the temperature changes are caused by thermal expansion and are normal, provided the pipe runs are properly supported with expansion compensators. Problems arise if the clicking is loud, metallic, and repeats even without a temperature change – this may indicate loose connections or a faulty contactor.

Conclusion

Attack Electric boilers are relatively simple devices in terms of construction – electric heating elements, a circulation pump, temperature sensors, control electronics, and hydraulic components. This simplicity means that most faults can be diagnosed systematically without special tools. The key is knowing what a sound, error code, or boiler behavior means – and this is exactly what this article is for.

If you are deciding whether to repair an older boiler or invest in a new one, consider the repair costs in relation to the device's lifespan. A boiler older than 12–15 years with recurring element or pump faults is better replaced – modern models such as Attack Electric Excellent 24/22 kW or Attack Electric Excellent 24/22 kW offer improved efficiency and reliability.