Common Faults in Low-Temperature Boilers with Storage Tanks and How to Recognize Them

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Common faults of low-temperature boilers with a storage tank and how to recognize them

Low-temperature boilers with a built-in domestic hot water storage tank are popular in Slovak households for their compactness and convenient operation – heating and hot water preparation in one unit, without external tanks, without complicated hydraulic schemes. However, this "dual role" brings specific fault conditions that differ from ordinary flow-through boilers or condensing units without a tank. Over years of practice with equipment handovers, service visits, and warranty claims, it turns out that most faults are either predictable or announce themselves well in advance through minor symptoms – and that's the key: knowing how to read the signals before something more serious and expensive happens.

In this article, we will systematically look at all the main areas of faults – from the storage tank itself, through the heating section, control system, to safety components. For each fault, we explain the symptoms, causes, and a rough diagnostic procedure. If you're more interested in the preventive side, we also recommend the article Maintenance and service of a low-temperature boiler with a storage tank: what and how often to check and Rust, limescale, and the tank anode – how to extend the boiler's lifespan.

Domestic hot water tank faults

1. Insufficient volume or temperature of hot water

This is by far the most common reason customers call for service – or why they think the boiler is broken, when in fact it's an incorrect setting or a worn component. The symptoms are clear: you have hot water in the morning, but after five minutes in the shower it starts getting cold, or the hot water runs out sooner than it used to.

Most common causes:

Limescale build-up on the tank's heat exchanger – in areas with hard water (hardness above 2.5 mmol/l, which covers most of central and eastern Slovakia), limestone deposits build up on the coil heat exchanger. A layer just 3 mm thick increases the exchanger's thermal resistance by 20–30%, meaning slower heating and a lower final temperature. The tank will eventually heat the water to the required 60°C, but it takes considerably longer – and if consumption exceeds the heating rate, the water "runs out."
Low tank temperature setting – modern boilers such as the Thermona THERM PRO 14 KX allow the target tank temperature to be set, usually within the range of 45–75°C. When the set temperature is too low (for example, 45°C for the sake of saving energy), the actual comfortable outlet temperature after mixing with cold water in the pipes may be only 35–38°C – which many people consider "cold" water.
Faulty tank temperature sensor (NTC probe) – if the sensor reads an incorrect value, the boiler's controller thinks the tank is heated, even though its actual temperature is lower. The boiler ends the tank heating cycle prematurely. This fault is tricky because it doesn't announce itself with an error code – the tank simply doesn't heat up to the required temperature.
Faulty three-way valve – the valve that switches the medium between tank heating and space heating gets "stuck" in the heating position. The tank doesn't heat up at all. This condition most often shows up in winter, when the boiler is heating intensively and the tank doesn't get enough heating capacity.

2. Discharge from the tank's safety valve

A small amount of water dripping from the tank's safety valve (usually set to 6 bar) is normal during water heating – the water expands and excess pressure is released. The problem arises when the valve leaks continuously or discharges a larger amount of water.

Symptoms and causes:

Missing or damaged DHW expansion vessel – this is the most common cause of continuous leakage. Every storage tank must have its own expansion vessel for the DHW circuit (not to be confused with the heating circuit's expansion vessel!). When this vessel is damaged, doesn't have the correct pre-charge pressure, or its membrane has ruptured, the safety valve takes over all the water's expansion.
Too high pressure in the cold water supply – in Slovakia, mains water pressure can reach 5–6 bar in some locations, which is close to the safety valve's opening pressure. With every heating cycle, the pressure rises further and the valve opens. The solution is to install a pressure reducing valve on the cold water inlet, set to 3–3.5 bar.
Worn safety valve – after years of operation, the valve's membrane or seat loses tightness and the valve starts dripping even under normal pressure conditions. The safety valve is a cheap component that should be preventively replaced every 5–7 years.

3. Cloudy, smelly, or brown-colored hot water

This is one of the symptoms that genuinely worries boiler owners – and rightfully so. If the shower produces water with a brownish tint, a metallic taste, or an unpleasant sulfurous smell, it's always an issue that requires immediate attention.

Depleted magnesium anode – the anode is a sacrificial protection against corrosion for the tank. A standard magnesium anode in an 80–150 l tank lasts 4–6 years with soft water, but only 2–3 years with hard water. When the anode is completely dissolved, the tank starts rusting from the inside. Result: brown water with a metallic smell. If the anode was replaced in time, the problem never reaches this stage.
Sulfurous smell – at a low tank temperature (below 55°C) or during prolonged water stagnation (for example, during a holiday), sulfate-reducing bacteria can multiply in the tank. They produce hydrogen sulfide, characterized by a "rotten egg" smell. Solution: heat the tank to 70°C for 1 hour (a thermal disinfection cycle – a legally required procedure for eliminating legionella).
Corrosion of the tank's inner surface – if the tank doesn't have an enameled or stainless steel inner surface, or if the enamel has cracked due to thermal shocks, corrosion progresses quickly. A customer with a slight smell and mild cloudiness will often be satisfied with an anode replacement – but when a cross-section of the tank is visibly rusted, the situation requires replacing the entire tank or boiler.

Heating section faults

4. Boiler doesn't ignite or shuts down shortly after ignition

Ignition faults are among those most owners can't explain – the boiler displays an error code, but the reason isn't obvious at first glance. The symptoms are either that the boiler doesn't release gas at all and doesn't attempt ignition, or it tries to ignite but the flame goes out within 3–5 seconds and the boiler locks out.

Faulty ionization electrode – the ionization electrode is a cheap but critical component. It measures the electric current passing through the flame and confirms that the burner is actually burning. If the electrode is fouled (black carbon coating), the porcelain body is cracked, or the gap is incorrect, the signal is weak and the control board locks out the boiler. Diagnosis: measure the ionization current (normal value: 4–8 µA, fault below 1 µA).
Gas valve problem – a clogged gas valve or incorrectly set supply gas pressure. In Slovakia, the natural gas distribution pressure is usually 18–25 mbar before the boiler. If the pressure is lower (for example, at the edges of the network in winter during high demand), the boiler may have ignition problems.
Fan / exhaust system fault – modern low-temperature boilers are equipped with a fan, and the control board checks whether the fan has reached the required speed before ignition. If the fan is fouled, its bearing is worn, or the exhaust route is partially blocked by a bird's nest (a common problem in spring), the boiler refuses to ignite and reports a flue gas discharge fault.
Lockout due to insufficient chimney draught – low-temperature boilers with a shared combustion air/flue gas duct (type C) are less sensitive to chimney draught than old boilers with natural draught. However, in more complex installations (long horizontal ducting, unsuitable terminal), backflow of flue gases can occur and the boiler locks out via the safety thermostat.

5. Boiler knocks, buzzes, or makes unusual noises

Acoustic symptoms are underestimated – as long as the boiler "knocks" for a while, most households ignore it. Experience shows that noises are a reliable warning of an approaching more serious fault.

Boiling in the tank or heat exchanger (knocking) – a typical sound with very heavy limescale build-up. The deposits insulate the metal from the water, causing local overheating and the formation of steam bubbles. This sound is the same as a pot of water just before boiling. A tank with heavy knocking needs chemical cleaning or heat exchanger replacement.
Buzzing circulation pump – a circulation pump normally runs quietly. Buzzing or vibration indicates either air in the system (the pump is pumping air instead of water), a worn pump bearing, or too high a differential pressure (closed balancing valves, clogged filter).
Clicking three-way valve – a quiet click is normal when switching between tank heating and space heating. Repeated clicking every few minutes or irregular clicking means the valve can't engage properly – typically due to a worn motor or a valve body mechanically blocked by limescale.

6. Boiler heats unevenly or shuts off too frequently

The so-called "short cycling" is a state where the boiler ignites and shuts off at short intervals – for example, every 2–3 minutes. Each such cycle includes ignition, brief heating, and shutdown. Result: increased consumption, wear of ignition components, and uneven heating.

Too small or depleted heating circuit expansion vessel – when pressure in the system rises quickly during heating, the boiler's safety thermostat or overpressure protection shuts it off. After cooling, the pressure drops and the boiler ignites again. The cycle repeats. Common cause: the expansion vessel's membrane has ruptured, or the vessel is too small for the given system.
Incorrectly set thermostat hysteresis – if the room thermostat is set with too small a hysteresis (for example, 0.1°C), the boiler switches on and off with even minor room temperature fluctuations. The recommended hysteresis for low-temperature boilers is 0.5–1°C.
Air in the system – air in the heating system causes some radiators not to heat up, the circulation pump to draw air, and pressure to fluctuate. Simple solution: bleed all radiators and top up the pressure (correct operating pressure for low-temperature boilers: 1.5–2.5 bar when cold).

Control system and electronics faults

7. Error codes on the display – what they mean

Modern boilers such as the Thermona THERM 28 LXZ.A 5 or Thermona THERM PRO 14 TKX display error codes that precisely identify the type of fault. Here is an overview of the most common codes in Thermona boilers (codes may differ by model – always compare with the service manual):

Error code	Fault description	Typical cause	Reset / procedure
E01	Ignition fault	Ionization electrode, gas	Reset button, service
E02	Fan fault	Fan, speed sensor	Service technician
E05	Boiler overheating	Pump, flow, limescale	Cool down, check the pump
E07	DHW NTC sensor fault	Break, short circuit in NTC probe	Sensor replacement
E10	Low system pressure	Leak, system not topped up	Top up water, find the leak
E14	Three-way valve fault	Valve motor, position	Service technician

Important note: some boilers reset themselves after displaying an error code (automatic reset), while others require a manual reset via a button. If the same error code appears repeatedly – for example, several times a day – resetting alone is never enough. The cause persists and must be resolved.

8. Boiler is losing pressure – repeated water top-ups needed

If the heating circuit needs to be topped up with water more than once a year, a leak exists. A small, slow leak can go undetected for a long time – the water evaporates or drips into an inaccessible spot. With this type of fault, it's crucial not to postpone action: corrosion around the leak spreads quickly, and water near electrical components is dangerous.

Loose joints and fittings – in boilers where installation was carried out incorrectly (gasket not replaced, threaded connections without PTFE tape or hemp sealant), minor leaks may appear after the first heating season. Visually: water stains or a white limescale coating around the joint.
Cracked heat exchanger – in older boilers or boilers exposed to freezing (water froze inside the boiler during a power outage in an unheated building), a cast-iron or steel heat exchanger can crack. Symptoms: leak inside the boiler, increased water consumption, pressure drop.
Worn heating circuit safety valve – similar to the tank, a worn safety valve (usually 3 bar) drips even at normal pressure. System pressure drops because water leaks out through the valve.

Mechanical and hydraulic faults

9. Circulation pump failure

The circulation pump is the "heart" of the heating system. When it fails, heating stops working even though the boiler ignites and burns. Symptoms: radiators are cold or only slightly warm at the top of the distribution, the boiler overheats quickly and locks out (because the heat isn't being carried away into the system).

Common pump faults:

Blocked rotor – on first startup after a summer pause, the pump rotor may become blocked by limescale deposits. Many pumps have a service screw on the front cover that allows the rotor to be manually released. Power must be disconnected before opening it.
Worn bearing – bearing wear is common in boilers older than 10–12 years. Symptoms: noisy operation, vibrations, eventually complete stoppage.
Incorrect pump output – during renovations where new radiators were added or the system was expanded, the original pump may become insufficient. Symptom: distant radiators are noticeably cooler than nearby ones.

10. Pressure and expansion system problems

The expansion vessel in a low-temperature boiler with a storage tank dampens pressure changes caused by temperature fluctuations of the liquid in the system. The typical air pre-charge pressure in the expansion vessel is 0.5–0.75 bar (depending on the height of the system). If the pre-charge pressure is zero (ruptured membrane), the vessel doesn't perform its function – pressure in the system rises dramatically during heating and drops during cooling. Result: the safety valve opens regularly and discharges water, pressure drops, and the boiler reports a low-pressure fault.

Measuring the expansion vessel's pre-charge pressure is a simple service task – an ordinary tire pressure gauge is enough. If the pre-charge pressure is zero and water (not air) comes out of the valve, the membrane has ruptured and the vessel must be replaced. This is typically one of the first checks performed at every service inspection.

Prevention and proper diagnostics – practical tips from experience

Over years of working with customers dealing with issues in boilers with storage tanks, a few rules consistently and dramatically reduce the number of service interventions:

Check the boiler's pressure once a year – the pressure gauge is right on the boiler. Correct pressure when cold: 1.5–2 bar. Below 1 bar – top up; above 3 bar – find the cause.
Listen to the boiler – a change in the character of the sound (new knocking, buzzing, whistling) is almost always an advance warning sign. Problems addressed in time are cheaper.
Check the anode every 2–3 years – with hard water, even every 2 years. Replacing the anode costs a fraction of the price of a new tank.
Bleed the radiators every autumn – before the heating season, not during it. After bleeding, top up the pressure.
Don't ignore yellow water – yellow or brown hot water is never "normal." It always indicates corrosion processes that require immediate action.
Regular service every 2 years – a legal requirement for gas appliances, but also a purely practical step. A service technician's inspection catches 80% of problems before they turn into a breakdown.

If you're interested in the extent of maintenance you should perform yourself versus leaving it to a service technician, we cover this in detail in the article Maintenance and service of a low-temperature boiler with a storage tank: what and how often to check. For those considering a new boiler and who don't want to complicate matters with recurring service issues, we also recommend the comparison Condensing vs. low-temperature boiler with a storage tank: which is more worthwhile.

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

There are tasks that a layperson can safely perform themselves – and there are interventions where an unqualified attempt can worsen the situation or lead to a dangerous condition. The law is clear on this: any intervention on gas components may only be carried out by a person with valid authorization for gas appliance installation.

You can do it yourself:

Check and top up the water pressure in the heating circuit
Bleed the radiators
Reset the boiler after an error code (once, not repeatedly)
Check whether the gas valve before the boiler is open
Check the outdoor flue gas terminal (presence of a bird's nest, etc.)
Check the anode visually after unscrewing it (without further disassembling the boiler)

Call a service technician if:

There is any smell of gas near the boiler
The same error code appears repeatedly after a reset
There is a water leak inside the boiler
Any gas component needs replacing (valve, burner, ionization electrode)
The electronic control board is faulty
The safety valve keeps opening repeatedly (more than once a month)
The hot water is yellow or brown

Frequently Asked Questions (FAQ)

The boiler reports error E10 – low pressure. How long can I wait before calling a service technician?

If the pressure dropped suddenly (for example, from 2 bar to 0.5 bar overnight), there is an active leak, and you shouldn't wait more than one business day to call a technician. If the pressure is dropping slowly – for example, by 0.2–0.3 bar per month – it may be a minor leak or a worn expansion vessel. Even in this case, the service visit shouldn't be delayed more than 2–4 weeks, because repeatedly topping up with fresh water accelerates fouling and corrosion of the heating system.

Water drips from the tank's safety valve every day. Is this normal?

Dripping with each heating cycle (a few deciliters) is normal and related to the thermal expansion of water. However, if the safety valve leaks continuously or discharges more than 1–2 liters daily, the problem is either a missing or damaged DHW expansion vessel, or too high cold water pressure. Both are easily fixed – but it needs to be addressed, because repeated opening of the safety valve gradually wears it out until it leaks even at rest.

Hot water from the shower only lasts 5 minutes and then turns cold. The tank is 100 liters – is this normal?

A 100-liter tank, when functioning correctly, is usually enough for 15–20 minutes of comfortable showering (40–42°C). If your hot water runs out after 5 minutes, there could be several causes: the tank's heat exchanger is fouled with limescale and heating takes too long, the tank's temperature sensor is reading an incorrect value, or the tank is set to too low a temperature. With boilers such as the Thermona THERM PRO 14 TKX, we recommend checking the set tank temperature in the boiler menu – it should be at least 55–60°C for comfortable consumption.

The boiler is making loud knocking noises. It was quiet a year ago. What happened?

Knocking in the tank or heat exchanger that has developed gradually almost always signals growing limescale deposits. A 3–5 mm layer of scale can change the sound from barely noticeable to quite disruptive. First, have a technician check the tank and consider chemical cleaning. If the knocking comes from the heating heat exchanger, it may also signal air in the system – try bleeding the radiators.

The hot water smells like "rotten eggs." The boiler is 3 years old. What's causing this?

The sulfurous smell (hydrogen sulfide) is produced by bacteria that multiply in the tank at low water temperatures or during prolonged stagnation. Solution: increase the set tank temperature to 65–70°C and let it run this way for 1–2 hours. If the tank has an antibacterial heating function (thermal disinfection), activate it manually. For a recurring problem, check the anode – a worn or incorrectly chosen anode can also contribute to hydrogen sulfide formation through a reaction with magnesium.

The boiler switches on every 3 minutes and burns briefly. Gas consumption has increased. What should I do?

This is a classic case of "short cycling." The most common cause is a damaged membrane in the heating circuit's expansion vessel. It's easy to verify: turn off the boiler, let the system cool down, disconnect the expansion vessel, and measure the pre-charge pressure with a pressure gauge. If it's zero or water comes out of the valve, the membrane has ruptured. The second possibility: too small a hysteresis on the room thermostat – check the thermostat setting and increase the hysteresis to 0.5–1°C. You can find more detailed technical installation requirements in the article Low-temperature boiler with a storage tank – requirements for the boiler room, dimensions, and space.

Conclusion

Low-temperature boilers with a built-in storage tank are reliable units with a long service life – easily 15–20 years with proper care. Most of the faults we encounter in practice are neither mysterious nor unpredictable. Knocking, pressure drops, brown water, short cycling – all of these are symptoms with a clear cause and a clear solution. The key is not to avoid regular maintenance and not to view these units as "something that either works or doesn't." The boiler will tell you about every problem in advance – you just need to know how to listen.

If you're looking for a quality low-temperature boiler with a built-in storage tank, take a look at our range in the boilers with a built-in storage tank category – you'll find proven Thermona models with a long-established service network in Slovakia and readily available spare parts.

Do you have a question on this topic?

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