Common faults of boiler and storage tank sets and how to recognise them

We are still preparing products.

But you can look at other categories.

Common faults in boiler and storage tank sets and how to recognize them

A boiler and hot water storage tank set is one of the most reliable solutions for preparing hot water in family houses. Nevertheless, in practice we encounter a whole range of faults that can compromise household comfort, increase gas consumption, or in extreme cases cause a safety incident. Most faults do not appear suddenly – they have warning signs and early symptoms that an observant owner can catch before the situation worsens. In this article we go systematically through the most common problems, their causes, symptoms, and diagnostic procedures. If you're looking for information on preventive maintenance, we also recommend the article Service and Maintenance of a Boiler and Storage Tank Set – What and How Often in our Knowledge Center.

Why boiler + storage tank sets fail – basic context

Before we move on to specific faults, it's good to understand how the whole set works. The boiler – whether atmospheric or with forced flue gas discharge – heats water in the primary circuit. The storage tank is a separate reservoir in which a supply of domestic hot water (DHW) is stored. Water from the primary circuit circulates between the boiler and the storage tank through a heat exchanger in the tank, where it transfers heat. This seemingly simple system has several critical points: hydraulic connections, control valves, pumps, expansion vessels, temperature sensors, and the heat exchanger surfaces themselves.

When one of these elements fails, it shows up either in comfort (cold water, insufficient DHW), in economy (higher gas consumption), or in safety (increased pressure, water leakage). Long-term experience shows that more than 60% of service interventions on boiler + storage tank sets are related to limescale, corrosion, or neglected maintenance. The rest consists of electrical and electronic faults, installation errors, and mechanical wear.

Insufficient or interrupted hot water supply

This is by far the most common problem customers call the service for. It manifests as follows: when you open the tap, hot water flows at first, then cools down, then warms up again – or the water is never hot enough. There can be several causes and they need to be distinguished:

Undersized storage tank or boiler

If the storage tank was chosen too small relative to the household's actual consumption, it simply runs out before it can reheat. A typical example: a family of 4 with a morning peak draw (2 showers in quick succession, dishwashing) with an 80-liter tank. The tank empties, and the boiler has to heat the entire volume from cold – which takes considerably longer than most people expect. We cover the topic of storage tank sizing in detail in the article What Storage Tank Volume Do I Need for My Household.

Storage tank heat exchanger clogged with limescale

Limescale (calcium and magnesium carbonate) settles preferentially on the hottest surfaces – i.e., on the storage tank's heat exchanger coils. A layer of scale only 2–3 mm thick can reduce heat transfer by 20–30%. The tank then heats up considerably longer, even when the boiler is running at full capacity. In areas with water hardness above 20°dH (German hardness degrees) – such as many areas of the Danubian Lowland – the heat exchanger can become critically fouled after just 3–4 years without maintenance.

Symptoms of a fouled heat exchanger:

The heating time of the tank has increased (e.g., from 45 minutes to 80–90 minutes)
The boiler runs longer, gas consumption has risen
The tank doesn't reach the set temperature even with long boiler operation
Crackling or bubbling sounds are heard in the tank during heating

Non-functional storage tank charging pump

Most sets use a separate charging pump or diverter valve to transfer heat from the boiler to the storage tank (in some sets, tank charging is integrated directly into the boiler). If the pump stops working – whether due to seizing (sediment, corrosion), a burnt-out motor, or a faulty capacitor – the water in the tank simply doesn't heat up, even though the boiler is firing.

Diagnosis: Check whether the pump emits a quiet vibrating hum during tank heating. If it's silent and the tank is cold, the pump is probably not working. Note – some pumps may be mechanically stuck but electrically fine (manually release it using the screw on the pump's front).

Pressure fluctuations in the system – too low or too high pressure

The pressure gauge on the boiler is the first indicator of the entire system's health. Normal operating pressure in a closed heating circuit is usually 1.2–1.8 bar when cold and 1.8–2.2 bar at operating temperature. Deviations are always a signal of a problem.

Too low pressure (below 0.8 bar)

Causes: water leakage in the system (at the boiler, storage tank, fittings, radiators), non-functional expansion vessel, damaged expansion vessel membrane. In practice, we most often encounter a dripping joint on the storage tank (screw connection, safety valve, drain cock) or overflow through the DHW tank's safety valve during overheating.

Consequences: when pressure drops below approx. 0.5 bar, the boiler automatically stops and displays an error code (e.g., F.22 for Vaillant, or a similar code for Protherm). The system cannot be started until pressure is topped up. If you need to top up water more than twice a year, it indicates a genuine leak – it needs to be found and fixed, not just topped up.

Too high pressure (above 2.5–3 bar)

Causes: non-functional expansion vessel (lost its charge, ruptured membrane), closed expansion valve, or a fault in the pressure reducing valve during filling. High pressure opens the safety valve – 3 bar is the standard opening pressure. Water then drips or flows from the safety valve, which owners usually discover as a wet floor or dripping at the safety valve's discharge outlet.

Practical warning: Sometimes customers plug or "stop" a dripping safety valve because they don't know what's dripping from it. This is dangerous – the safety valve is the last line of protection against excessive pressure, and disabling it can lead to the storage tank or boiler bursting.

DHW storage tank faults – corrosion, anode protection, microbiological risks

Corrosion inside the storage tank

DHW storage tanks are enamelled on the inside or lined with a special coating (e.g., duplex enamel, titanium enamel). This coating protects the steel shell from corrosion. If the enamel is damaged (e.g., by thermal shock when first filling a hot tank with cold water, or mechanically during installation), corrosion spreads quickly. The result is brown/rusty water from the hot water tap.

The line of defense is the magnesium anode. This sacrificial electrode (a rod made of magnesium alloy) slowly dissolves instead of the steel shell. Its inspection and replacement are a key servicing task – for most tanks every 2–3 years, in soft water even annually. A completely consumed anode = an unprotected tank = corrosion begins within months.

Practically verified symptoms of storage tank corrosion:

Brown, rusty water from the hot tap (not the cold one)
Foul smell from the hot tap water
Visible corrosion around the tank's service opening
Significant reduction of anode material during a service inspection (less than 1/3 of the original thickness remaining)

Legionella risk at low storage tank temperatures

Legionella pneumophila bacteria multiply in hot water at temperatures of 25–45°C. A storage tank operated below 50–55°C poses a potential risk, especially if the water stands for a longer period (holidays, an empty house). Modern boilers and controllers therefore include a thermal disinfection function – every few days, the tank automatically heats up to 70°C and holds that temperature for 30–60 minutes. If this function is not active or the boiler is set too low, the risk exists.

Verification: Check the storage tank temperature setting on the controller – it should be at least 55–60°C during normal operation. The thermal disinfection function should be turned on, especially in systems where the tank remains unused for long periods.

Electrical and control faults

Faulty temperature sensors (NTC sensors)

The DHW storage tank has its own temperature sensor – an NTC resistance sensor – which informs the boiler controller of the current water temperature in the tank. If the sensor fails (short circuit, open circuit, corroded contacts), the controller either does not start heating the tank at all, or heats it uncontrollably (without knowing when to stop).

A typical symptom of a faulty tank sensor: the tank is warm, but the boiler keeps heating it (continuous operation), or conversely the tank is cold even though the boiler is fine and does fire up at least for heating. An error code on the boiler display (e.g., F36, F75 for Protherm, F.75, d.90 and similar for Vaillant) usually refers directly to the storage tank sensor.

Fault in the three-way valve or diverter unit

In sets where the boiler alternates between heating and DHW preparation, switching is provided by a three-way valve (or a diverter hydraulic unit). If the valve gets stuck in the "heating" position, the storage tank doesn't heat up at all – and vice versa. Mechanical seizing of the valve is typically caused by sediment or corrosion. Electrical failure (burnt coil, broken cable) is another common cause.

How to recognize it: The boiler is running (the burner fires), but the storage tank stays cold – even though the water temperature in the boiler is fine. Or the opposite: the tank heats up, but the radiators stay cold. You can physically check whether the three-way valve is warm on both outlets or only on one.

Noise symptoms – what sounds mean and what they signal

Noises from the heating system are a diagnostic tool that experienced technicians use right at the first visit. Each type of sound tells you something different:

Crackling and bubbling in the storage tank during heating: A typical sign of limescale on the heat exchanger. The scale contains air pockets that crack during thermal expansion. When you hear crackling, the heat exchanger is probably fouled enough that we recommend chemical descaling.
Squealing or high-pitched sound from the pump: The pump bearings are worn, or the pump is running dry (air in the circuit). The system needs bleeding and the pump needs to be checked.
Knocking in the pipes when opening the DHW tap: Water hammer – too rapid closing of the lever mixer tap, or high pressure in the cold water supply (above 4 bar). Solution: a pressure reducing valve at the cold water inlet.
Burner cycling (short, frequent ignition and extinguishing – "short cycling"): Too small a hydraulic volume in the system, faulty control, or a clogged filter. The burner doesn't like having to constantly restart – it shortens the life of the ignition electrode and valve.
Loud rumbling or bang when the burner ignites: Contaminated ignition electrode, debris in the gas pipe, or delayed gas supply. Requires immediate service intervention.

Faults specific to atmospheric boilers in the set

Atmospheric boilers (e.g., Vaillant atmoTEC plus VU CZ/SK 200/3-5 + VIH CK 70) have different weak points than turbo boilers. Flue gases exit through natural draft via the chimney, and combustion air is drawn in from the installation space.

Typical faults of atmospheric boilers:

Contaminated burner (strips): Cobwebs, dust, and combustion residues settle on the burner strips. The result is a yellow or orange flame (instead of blue), soot formation, and CO fumes.
Insufficient chimney draft: The boiler won't ignite, the flame goes out, the flue gas discharge safety device (thermal fuse FDD) trips a fault. Causes: blocked chimney, bird's nest, condensate in the chimney throat in winter.
Chimney backdraft: Flue gases return into the room – an extremely dangerous condition. You can recognize it by the smell of combustion fumes and repeated tripping of the thermal safety device. Never operate a boiler with backdraft.

Turbo boiler faults in the set – specific to sealed combustion

Turbo boilers (e.g., Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH CK 70 or Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH R 120) have a sealed combustion chamber and a concentric flue (air/flue gas). Their faults are different:

Condensate in the coaxial flue pipe: In cold weather, condensate may form in the horizontal section of the flue. Proper slope (min. 3° towards the outside) is key. If the slope is incorrect, condensate flows back into the boiler and damages the fan and heat exchanger.
Debris on the fan: The fan draws air from outside and can become clogged with dust, insects, or leaves. Reduced fan efficiency shows up as fluctuating output and combustion-related error codes.
Clogged secondary heat exchanger (condensing boilers): In condensing turbo boilers, condensate from the aluminum heat exchanger is drained via a siphon. If the siphon is clogged, condensate accumulates, the boiler stops, and displays a fault.

Faults in Protherm Panther + FE storage tank sets

The sets Protherm Aqua Complet Panther 25KOO + FE 120 BM and Protherm Aqua Complet Panther 25KTO + FE 120 BM are among the best-selling combinations on the Slovak market. In practice, we record several recurring faults specific to this type of set:

Storage tank charging pump fault: The Panther uses an internal pump and three-way valve to charge the storage tank. If the valve is stuck or the pump is seized, the FE tank doesn't heat up. Error code E26 or E28 on Protherm Panther boilers specifically signals problems in the DHW circuit.
Fouling of the FE tank's temperature sensor: FE series tanks use an immersion NTC sensor. In older installations in hard water, the sensor can become fouled with limescale, which changes its resistance and thus reports an incorrect temperature to the boiler.
Corrosion of the magnesium anode in the FE tank: FE BM (without magnesium) tanks are equipped with active cathodic protection (electric current) instead of a passive magnesium anode. If this protection is deactivated or the transformer fails, corrosion sets in quickly. Regularly check whether the active anode indicator light is on.

When to call service and when you can handle it yourself

This is one of the most practical questions for every set owner. The basic rule is: everything related to gas, pressure vessels, safety fuses, and electrical wiring of the boiler belongs exclusively in the hands of an authorized service technician. A handy owner can manage the following tasks themselves:

Topping up water pressure via the fill valve (after confirming that pressure has dropped and there is no active leak)
Bleeding radiators and manifolds (with a bleed key)
Manually releasing a stuck pump (by turning the screw on the pump's front)
Checking and cleaning the Y-filter before the boiler (after closing the shut-off valves)
Resetting the boiler after an error code (once – if the code repeats, call the service)
Checking the DHW tank's safety valve – by manually turning it once a year

Everything else – anode replacement, chemical descaling, pump replacement, controller reconfiguration and boiler adjustment, sensor replacement, repair of leaks in the pressure circuit – requires a professional company. See also our article Service and Maintenance of a Boiler and Storage Tank Set – What and How Often, where you'll find a complete service calendar.

Error codes – how to read them and what they signal

Modern boilers display error codes on an LCD display. Each manufacturer has its own nomenclature, but patterns exist. A few examples from practice:

Code (Vaillant)	Code (Protherm)	Problem description	Typical cause
F.22	E05	Low water pressure	Leak, damaged expansion vessel
F.75	E26	Non-functional pump / pressure sensor	Stuck pump, air in the system
d.90	E28	Storage tank sensor fault	Faulty NTC sensor, broken cable
F.28 / F.29	E08	Ignition / ionization fault	Contaminated electrode, weak gas pressure
F.36	E09	Heat transfer fault	Limescale on heat exchanger, low flow rate

Important: If the code appears once and the boiler works normally after a reset, it may be a one-off outage (e.g., a brief power failure, an air bubble). If the code repeats or the boiler immediately stops again after a reset, it's always a reason for a service intervention.

Preventive diagnostics – what to check once a year yourself

Experienced homeowners have gotten into the habit of a simple annual inspection before the heating season (September–October), which helps detect minor problems before they grow:

System water pressure: Check the pressure gauge on the boiler when cold (in the morning before starting up). It should be 1.2–1.5 bar. A drop of more than 0.3 bar per year without topping up = a hidden leak.
Visual inspection of the storage tank: Examine the tank from the outside – look for discoloration, damp spots around fittings, drips at the safety valve.
Storage tank heating time: Let the tank cool down (e.g., after a vacation) and measure the time from cold to reaching 55°C. A longer time than in the previous season indicates a fouled heat exchanger.
Qualitative check of hot water: Let hot water flow through a white glass – look for discoloration, cloudiness, or sediment.
Storage tank safety valve: Open it manually once a year – the valve must be able to open and close without leaking after closing. If it drips after closing, the valve is damaged and needs replacing.
Filters and strainers: Check the Y-filter before the boiler and the strainers on the taps (fouling indicates corrosion in the system).

The most common installation errors and their delayed consequences

A large part of the faults we encounter in 2–5 year old sets have their roots in incorrect installation. The most common problems:

Incorrect connection of the storage tank return: If the tank is connected so that the return goes back into the primary circuit without proper hydraulic separation, hydraulic short circuits occur. The tank doesn't heat up or heats only partially.
Missing filter before the cold water inlet to the storage tank: Without filtration, impurities from the water supply enter the tank and settle on the heat exchanger and sensor.
Storage tank filled "dry" or with thermal shock: If you fill a hot tank with cold water (e.g., during initial commissioning without pre-cooling), the enamel may crack. Correct procedure: always fill the tank slowly, with cold water, until water flows out of the bleed valve.
Missing expansion vessel on the DHW circuit: Water expands when heated. If there is no expansion vessel on the DHW side, repeated pressure spikes damage the fittings and the tank.
Incorrect controller settings: Too low a storage tank temperature (for "savings") increases the risk of Legionella and shortens anode life.

We also cover the correct installation procedure in the article How to Connect a Boiler to a Storage Tank – Installation Procedure.

Service life of sets and when to consider replacement

A quality DHW storage tank will last 15–20 years with proper maintenance. A boiler has a service life of 15–25 years depending on the brand, intensity of use, and quality of servicing. In practice, however, we find that after 12–15 years, repair and service costs begin to exceed reasonable limits. Several signals indicate it makes sense to consider replacing the whole set:

Repeated faults within a short time (more than 2 service visits per year)
Rusty water despite anode replacement – corrosion of the tank shell is irreversible
Repair costs exceed 40–50% of the price of a new unit
Significant increase in gas consumption (by more than 15–20% without changes in habits)
Unavailability of spare parts (typically for units older than 15 years)

In this case, the article How to Choose a Boiler and Storage Tank Set – What to Pay Attention To can help you, where we discuss the criteria for choosing a new set.

Frequently Asked Questions (FAQ)

Why does cold water flow from the hot water tap first, then hot, then cold again?

This phenomenon is called the "cold water sandwich." It's characteristic of storage tanks with a shorter heat exchanger or incorrectly set tank priority. The tank is filled with hot water, but then the boiler briefly heats freshly introduced cold water from the inlet. The solution may be better insulation of DHW piping, correct storage tank temperature setting, or a DHW circulation pipe. If this phenomenon occurs significantly and repeatedly, it may indicate an undersized storage tank.

My storage tank cools down overnight even though the boiler works normally – what's the problem?

If a well-insulated tank loses more than 5–8°C overnight, check: 1) the tank insulation – visible damage, condensation on the shell, 2) the check valve at the tank inlet – if it's missing or not working, water from the hot tank freely flows back into the cold pipe, 3) the DHW circulation pump – if it's running continuously (without a timer), it carries heat back into the piping. In practice, the most common cause is precisely a missing or non-functional check valve.

The storage tank's safety valve drips during every heating cycle – is that normal?

Not entirely. When water heats up, pressure genuinely rises and the safety valve may release a few drops – that's normal function. But if it drips significantly (tens of milliliters per cycle) or continuously, it means either too high a pressure at the cold water inlet (solution: pressure reducer), a missing expansion vessel on the DHW circuit, or a worn safety valve that no longer seals tightly. It's recommended to replace the safety valve preventively every 5–7 years.

The boiler shows an error right after charging the storage tank – where should I look for the cause?

It depends on the error code. If it's a pressure-related code (e.g., F.75 for Vaillant), the pump probably couldn't fill the tank without air bubbles, and the pressure sensor detected the drop. Try bleeding the system. If it's a storage tank sensor code, check the cable and the cleanliness of the sensor. If the code is related to combustion or overheating, it may be a fouled storage tank heat exchanger – the boiler works at full power for too long without sufficient heat transfer and overheats the boiler's heat exchanger.

The storage tank emits a strange smell with hot water – is it safe?

It depends on the type of smell. A sulfur or "burnt rubber" smell may signal a chemical reaction of the magnesium anode with the water (typical with higher sulfate content in the water) – the solution is to replace the anode with a special type with reduced reactivity. A stagnant water or earthy smell may indicate biological growth (including Legionella) – the tank needs to be thermally disinfected immediately (heated to 70°C for at least 30 minutes) and the minimum temperature setting should be checked. A chemical or plastic smell in a new tank usually disappears after a few days of operation.

How long will a DHW storage tank last if I respond correctly to faults?

With regular anode replacement (every 2–3 years), an annual service inspection, and descaling every 5–7 years, a quality DHW storage tank (e.g., Vaillant VIH R or VIH CK) can function for 18–22 years. Tanks without active anti-corrosion protection or in areas with exceptionally hard water have a shortened service life of 10–15 years. The key factor is precisely the anode – neglecting it can shorten the tank's service life to 5–8 years.

Conclusion – a fault as information, not a catastrophe

Every fault in a boiler and storage tank set is, first and foremost, information about what isn't working or has been neglected in the system. Most faults that service technicians encounter can be caught at an early stage through routine monitoring – pressure on the gauge, water temperature, heating time, visual inspection. The sooner you catch the fault, the cheaper and simpler the repair.

If you're considering a new set or want to understand the differences between individual models, check out our other articles in the Knowledge Center: Protherm Panther vs. Vaillant turboTEC – Comparison of Sets, Atmospheric vs. Turbo Boiler in a Set – What's the Difference, or Correct Sizing of Boiler Output to Storage Tank Volume. Good information is the best prevention against faults.

Do you have a question on this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us - we'll be happy to advise.