Common faults of electric water heaters and how to fix them

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Common faults of electric water heaters and how to fix them

An electric water heater is one of those appliances we quickly get used to – until it stops working. A cold shower in the morning before work is an unpleasant experience that motivates you to solve the problem immediately. The good news is that most faults of electric water heaters – whether tank-type or tankless – are quite typical, can be diagnosed on your own, and many of them can be fixed by a more skilled layperson. Some, however, require the intervention of a professional, and it is important to know when to call a service and not expose yourself to unnecessary risk.

In this article, we will systematically go through the most common faults of both types of heaters, explain what causes them, how to recognize them, and – where safety allows – how to eliminate them step by step. If you are interested in which type of heater is more suitable for you, also see our article How to choose an electric water heater: tank-type vs. tankless.

Fundamental differences between tank-type and tankless water heaters in terms of faults

Before we get into specific faults, it is important to understand why tank-type and tankless water heaters break down differently. They have different constructions, different operating conditions, and therefore different weak points.

Tank-type water heater has a tank inside (5 to 200 liters), in which water is heated using an immersed electric element – most commonly a coiled tubular heating element. The water is heated gradually, kept at the set temperature, and drawn as needed. Since the water is constantly present in the tank, the heater is prone to limescale deposits, corrosion, and anode rod fatigue. The thermostat controls the heating, so its failure has a direct impact on the water temperature or the safety of the appliance.

Tankless water heater heats water on the fly – water flows around the heating element and is immediately hot at the outlet. The heating element is more powerful (3.5 to 12 kW) and is only loaded when water is flowing. Weak points include the switching mechanism (hydraulic or electronic), flow sensor, and the heating element itself, which is subjected to much higher thermal loads than in a tank.

Faults of tank-type water heaters

1. The heater does not heat the water at all – the water remains cold

This is the most common reason why customers call for service. There can be several causes, and the diagnosis should be done systematically, starting with the simplest ones.

Cause A: Tripped circuit breaker or RCD. Check the distribution board. A tank-type water heater is most often protected by a separate circuit. If the circuit breaker is off or the RCD has tripped, try to turn it on. If it trips again immediately, do not turn it on repeatedly – this indicates either a fault in the appliance or a problem with the installation. Call an electrician.

Cause B: Burned-out heating element. The heating element is a consumable part with a limited lifespan – typically 5 to 10 years, and even less in hard water with calcium content above 300 mg/l. A damaged element may have a broken resistance wire, or it may be so corroded that the electrical circuit is interrupted. Replacing the element is a task that can be done by an electrician or a more skilled owner: drain the water, disconnect the electricity, remove the old element, apply Teflon tape to the thread, and install the new one. The type of element depends on the heater's power – for tank-type heater BD 2.0 kW / 30 l, it is an element with 2000 W power at 230 V voltage.

Cause C: Faulty thermostat. The thermostat controls the switching on and off of the element. If the thermostat is stuck in the "off" position (so-called open-circuit fault), the heater will never turn on. The thermostat can be tested with a multimeter when the device is disconnected – in the cold position, it should measure zero resistance (closed contact), and open when the temperature exceeds the set value. Replacing the thermostat is a cheap and readily available service task.

Cause D: Tripped safety fuse (temperature limiting thermostat – TOT). Tank-type heaters have, in addition to the operating thermostat, a safety temperature limiter that usually trips at overheating above 85–95 °C. After the heater has cooled down, it can be manually reset by pressing the button through a small hole in the device body. If the TOT keeps tripping, the cause (faulty control thermostat) must be addressed, not just reset.

2. The water is lukewarm, not hot enough

If the heater produces lukewarm water instead of hot water, the causes are as follows:

Thermostat set too low – check the set temperature. The recommended operating temperature is 55–60 °C to prevent the growth of legionella. Many users lower the temperature to 40–45 °C in an attempt to save electricity, which is hygienically risky.
Thick layer of limescale on the element – limescale is an excellent thermal insulator. A layer as thin as 3 mm can extend the heating time by 15–25 % and reduce the final water temperature. The element has to work harder, which accelerates its degradation. Solution: descaling (see our article Maintenance and descaling of electric water heaters: how to extend their lifespan).
Excessive consumption compared to the tank capacity – a 30-liter tank is not sufficient for a household with multiple people if they shower one after another without breaks. This is not a fault, but a sizing issue – the tank simply cannot heat the next batch of cold water quickly enough.
Cold water bypassing the cylindrical element – if the element's seal leaks, cold water "short-circuits" around the heating and mixes with the hot water before the outlet.

3. Water is leaking from the safety valve (pressure relief valve)

The safety valve is a safety component on the cold water inlet pipe. Its function is to discharge excess pressure caused by water expansion during heating. A normal drip of 1–2 dl per day during heating is normal – this is thermal expansion. However, if the valve is leaking continuously or a large amount of water is escaping, the causes may be:

Excessive pressure in the network – water supply pressure above 4 bar without a pressure-reducing valve. Solution: install a pressure-reducing valve on the inlet.
Defective (faulty) safety valve – after years of operation, the valve may wear out and leak. Replacing the valve is a simple task.
Missing expansion tank – in pressure installations without an expansion tank, all expansion is controlled by the safety valve, which wears out faster. We recommend installing an expansion tank of 2–8 liters depending on the tank volume.

4. Water smells like sulfur (rotten egg smell)

This problem occurs exclusively in storage water heaters with a magnesium (magnesium) anode. The anode is a magnesium rod built into the tank, whose purpose is to protect the steel tank from corrosion using an electrochemical principle – Mg is sacrificed instead of steel. In some types of water (soft water, water with a higher content of sulfates), a reaction occurs in which sulfate-reducing bacteria convert sulfates into hydrogen sulfide. Result: the water smells like spoiled eggs.

Solution: replace the magnesium anode with a titanium anode with powered current protection (active cathodic protection) or with an aluminum anode. A titanium active anode does not cause this phenomenon. We also recommend flushing and disinfecting the device once a year.

5. Rusty water from the heater

Rusty water indicates corrosion inside the tank. This means that the magnesium anode is depleted and has stopped protecting the tank – the steel begins to corrode. Alternatively, the surface glaze layer inside the tank is cracked. Unfortunately, the solution to cracked glaze is the end of the water heater – repair is not cost-effective. Preventively, the anode should be replaced every 2–4 years (depending on water hardness) and its condition should be periodically checked.

Water flow heater faults

6. The flow heater does not turn on – water is cold

A flow heater is activated by water flow. If water is flowing, but the heater does nothing, check the following:

A: Insufficient water flow. Every flow heater has a minimum flow required to activate the switching mechanism – typically 1.5 to 3.5 l/min. If the water pressure is low or the inlet mesh (filter) is clogged, the heater will not receive a signal to start. Clean the inlet filter, check the water pressure (must be at least 0.1–0.3 MPa).

B: Faulty hydraulic switch (in hydraulically switched models). Flow heater PM 3.5 kW with hydraulic switching uses a membrane that is compressed by the water flow, closing the electrical circuit. The membrane may be cracked, clogged, or deformed. If I measure 230 V at the switching block with a multimeter, but the switch does not make contact, it is a fault in the switching block – it needs to be replaced.

C: Faulty electronic flow sensor (in electronically switched models). The heater MK1 3.5 kW and MK1 4.5 kW use an electronic flow sensor – a Hall sensor or thermal flow sensor. If the sensor fails, the control electronics will not receive a signal and the heater will not turn on even at full flow. Diagnosis and replacement of the sensor is performed by a service technician.

D: Circuit breaker or RCD tripped. Same as with a storage heater – check the distribution board.

E: Problem with the electrical wiring. In new installations, it may happen that the heater is not properly connected to a three-phase power supply, or it is missing a neutral conductor. We recommend reading the article Installation of an electric flow water heater: procedure, connection and electrical installation requirements.

7. The flow heater turns on, but the water is not sufficiently hot

This is a very common complaint, and in most cases it is a mistake in choosing the power, not a fault. A flow heater can heat water by a certain temperature difference (delta T) at a given flow rate. If you want 40 °C at the outlet and the cold water is 12 °C, you need a heating of 28 °C. At a flow rate of 4 l/min, this requires a power of 7.8 kW – which can only be achieved by a more powerful three-phase model, not a 3.5 kW heater.

If the power is correctly chosen and the temperature is still insufficient, check:

Thermostat setting (if the heater has a thermostat).
Scale buildup in the flow chamber – reduces heat transfer.
Low pressure/flow – paradoxically, at low temperature, there may also be too high a flow, water flows faster than the heater can heat it.

More on this topic can be found in the article What power flow water heater do I need: 3.5 kW, 4.5 kW or 5.5 kW?

8. The flow heater turns on and off in short intervals (so-called "temperature flickering")

This is an unpleasant phenomenon in which the water is sometimes warm, sometimes cold, in a cyclic manner. Causes:

Fluctuation of water pressure in the network – if the pressure oscillates around the minimum activation threshold, the heater turns on and off alternately. Solution: pressure-reducing valve with damping function.
Worn membrane of the hydraulic switch – the membrane loses elasticity and no longer reliably maintains contact. Replace the switching block.
Clogged inlet filter – a partially clogged filter causes a pressure drop exactly at the moment when someone opens another tap in the house. Clean the filter.

9. Water is leaking from the flow heater (leak)

Water leakage from the heater is always a serious situation – water near electrical installation is dangerous. Immediately turn off the water supply and disconnect the heater from the power supply.

Most common leak locations:

Seals on inlet/outlet connections – common wear of O-rings or sealing surfaces. Replace the seal after disconnecting the device.
Cracked heating element – when water freezes in the chamber, a cracked element can cause a leak directly into the electrical part. This is a failure – the heater must be disconnected and replaced.
Crack in the flow block – less common, but possible under excessive pressure (over 6 bar). For pressure models MK1 4,5 kW, the maximum operating pressure is 6 bar – if the pressure in the network is higher, a pressure-reducing valve must be installed.

Common faults for both types – electrical and safety

10. FI switch (residual current device) keeps tripping

If the FI switch (residual current device, usually 30 mA) keeps tripping when the heater is turned on, it indicates a current leakage to the device housing or ground. Causes:

Moisture inside the heater (e.g., after installation without drying).
Broken insulation of the heating element – the element has burned through the insulating ceramic and has contact with water or the housing.
Damaged power cable.
Problem with the grounding of the installation (faulty grounding of the neutral conductor).

This is a situation where you must not continue using the device under any circumstances. Leave the entire diagnosis and repair to an electrician.

11. The heater is too noisy

Humming, clicking or noise during the heating cycle:

Cracking and clicking (storage heater) – limescale is falling off the element or the walls of the tank during thermal expansion. Normal with heavy scaling, solution is descaling.
Humming (storage heater) – vibration of the element caused by a loose mounting nut or water flow resonance. Tighten the element nut after draining the tank.
Noise (flow heater) – turbulent water flow in narrowed channels, especially with limescale deposits. Descale the flow chamber with a 5–10% citric acid solution.

12. Increased electricity consumption

If you notice that your monthly electricity consumption has increased without any obvious change in household behavior, the heater may be the culprit:

A thick layer of limescale on the element reduces heating efficiency – the heater runs longer to achieve the same temperature.
A faulty thermostat inaccurately regulates and keeps the element on longer than necessary.
Poor thermal insulation of the storage tank (damaged foam insulation) – the tank loses heat faster and the element turns on more frequently.
Leak of hot water through a dripping tap – it may seem trivial, but a dripping tap with a rate of 1 drop/second releases about 30 liters of water per day, which the tank constantly refills with cold water.

How to Prevent Failures – Preventive Maintenance

Most of the faults described in this article are not sudden breakdowns, but the result of neglected maintenance. An electric water heater is a device that operates 365 days a year and is often forgotten in most households right after installation. Regular preventive care will cost you much less than an unplanned replacement of the appliance.

We recommend the following for tank water heaters:

Every 12 months, visually inspect the safety valve and the condition of the anode.
Every 2 years, descale the tank and the heating element (using citric acid or special descaling agents for water heaters).
Every 2–4 years, replace the magnesium anode (depending on water hardness – annually if hardness exceeds 400 mg/l).
Check seals and connections during each planned shutdown.

For tankless water heaters:

Every 12 months, clean the inlet filter/screen.
Every 2 years, descale the flow chamber (using citric acid via circulation method or by disassembling the body).
Check the condition of the power cable and electrical connections.
For non-pressurized models (PM 4,5 kW), check the condition of the shower head (pearlizer, hose) – sediment increases flow resistance and reduces comfort.

A detailed descaling procedure and the entire routine maintenance are described in the article Maintenance and Descaling of Electric Water Heaters: How to Extend Their Lifespan.

When to Repair and When to Replace the Water Heater

This question always arises when the fault is more serious. A general rule says: if the repair costs exceed 40–50% of the value of a new device of the same category, the repair is not worth it. This is especially true when the water heater has been in operation for 10 or more years and when multiple faults occur simultaneously.

A tank water heater with a rusted tank should be replaced – repairing the tank is not feasible. A tankless water heater with a cracked flow chamber or a burned control board (in electronic models) is also a borderline case where the cost of parts and labor may exceed the price of a new appliance.

On the other hand, replacing the heating element, thermostat, anode, safety valve, or switching block are common service tasks that can be carried out at a fraction of the cost of a new device, and the water heater will serve you for many more years.

Questions and Answers (FAQ)

Why is my tank water heater losing hot water pressure after some time of heating?

This is a typical phenomenon when the shut-off valve is partially open or when the outlet pipe is partially clogged with limescale. Check whether the outlet ball valve is fully open. If the problem persists, inspect the entire outlet pipe for deposits – if there is a flexible connection (flexible hose) between the tank and the plumbing, the inside surface may be narrowed due to corrosion. It is also possible that the anode rod was replaced with the wrong size and partially blocks the tank's outlet.

My tankless water heater heats water to only 30 °C even at maximum setting – what is wrong?

The most common cause is a mismatch between the heater's power, water flow, and the temperature of the cold water at the inlet. If the cold water is 8 °C in winter and you want 38 °C at the outlet, with a flow rate of 4 l/min, you need at least 8.4 kW. A 3.5 kW heater physically cannot do that – reduce the flow (close the valve to 40–50%) or invest in a more powerful model. A more detailed calculation can be found in the article What Power of a Tankless Water Heater Do I Need for My Apartment?.

Water from my tank smells like sulfur – is it dangerous to health?

Hydrogen sulfide (H₂S) is a toxic gas, but the concentrations formed in a home water heater are much lower than harmful levels – the problem is primarily aesthetic and hygienic. Nevertheless, we recommend disinfecting the device (heat to 70 °C for 30 minutes), replacing the magnesium anode with a titanium one, and rinsing the device. Repeated bacterial issues require regular disinfection cycles.

Can I repair a tank water heater myself, or do I need an electrician?

Certain tasks – such as replacing the heating element, thermostat, or anode – are technically accessible to a skilled layperson as long as safety rules are followed (disconnect from the power supply, drain the water, and work only on a cooled device). Other tasks – diagnosing the electrical installation, replacing a residual current device (RCD), or any work when there is a suspicion of a current leak – must be performed exclusively by a person with electrical qualifications (at least §21 of Government Regulation No. 508/2009 Coll.).

My tankless water heater only turns on occasionally after years of operation – could it be a worn-out switch?

Yes, exactly. Hydraulic switching membranes have a lifespan of 8–15 years under normal operation, and less under hard water or pressure fluctuations. The membrane loses elasticity, and the switch contacts become worn. The entire switching block is sold as a replacement part and the replacement is not complicated – refer to your model's service documentation or contact us for help identifying the correct replacement part.

How long can a well-maintained electric water heater last?

With regular maintenance (descaling, anode replacement, seal checks), a tank water heater can last 12–18 years, and a tankless one 10–15 years. Without any maintenance in hard water conditions (over 300 mg/l CaCO₃), a tank water heater may fail before 5 years – more likely due to an exhausted anode and tank corrosion than an electrical fault. A tankless water heater without descaling will become clogged with limescale and the heating element may crack due to further freezing or thermal shock.

Conclusion: Systematic Diagnostics Saves Time and Money

Electric water heaters are reliable devices that usually give early warning of faults – through reduced performance, increased consumption, irregular operation, or unusual behavior of the safety valve. The key to a long lifespan is regular preventive checks, descaling, and timely replacement of consumables such as the anode, seals, and, if necessary, the heating element.

If you are considering a new water heater and are interested in the difference between pressurized and non-pressurized operation, we recommend our article Pressurized vs. Non-Pressurized Water Heater: What Is the Difference and What to Choose?. If you are thinking about installing a tankless water heater and want to correctly size the electrical connection, see Installation of an Electric Tankless Water Heater: Procedure, Connection, and Requirements for Electrical Installation.

The full range of tank and tankless water heaters can be found in the category electric water heaters, where models for various household needs are included – from compact non-pressurized tankless heaters through pressurized electronically controlled models to tank water heaters with various tank sizes.

Do you have a question about this topic?

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