Common faults of low-temperature boilers with connected storage tanks and their solutions
Common Faults in Low-Temperature Boilers with Connected Storage Tanks and Their Solutions
The combination of a low-temperature boiler with a hot water storage tank is very common in Slovak households – and that is precisely why it is associated with a whole range of specific faults that simply do not occur in boilers without a storage tank. After years of working in the field, I know that most service call-outs for this configuration have a common denominator: either incorrect settings, or neglected maintenance. Less often, it's a genuine hardware fault. This article will guide you through all the essential faults, their causes and – most importantly – their solutions. Where a user intervention is sufficient, I'll say so plainly. Where a technician is needed, I'll warn you against improvising.
If you're more concerned with choosing the right boiler and storage tank, check out our other topics in the Knowledge Center: How to choose a low-temperature boiler with the option of connecting a storage tank or What storage tank volume do I need for a low-temperature boiler. Here we'll focus exclusively on faults and their solutions.
Why the boiler + storage tank combination is more prone to faults than the boiler alone
A low-temperature boiler working only for heating has a relatively simple hydraulic circuit. As soon as you connect a storage tank to it, you add another variable load to the system, a switching valve, a storage tank thermostat, a circulation pump for the storage tank circuit (or a three-way valve), and an electrical connection to the control unit. Any of these components can fail or be set incorrectly. Moreover, low-temperature boilers – unlike condensing boilers – operate with higher heating water temperatures, which creates specific conditions when heating the storage tank: faster buildup of limescale deposits, greater thermal expansion of pipes, and higher demands on regulation.
Fault #1 – The storage tank does not heat up, or only heats up partially
This is by far the most common complaint. Cold water comes from the tap even though the boiler is running. There can be several causes, and they need to be checked step by step.
Cause A: Faulty three-way switching valve
Most low-temperature boilers with a connected storage tank use a three-way motorized valve to switch the boiler's output between the heating circuit and the storage tank circuit. When the valve stops switching correctly into the "storage tank" position, heating simply does not occur. In practice, this manifests as the boiler firing and the pump running, but the storage tank remaining cold while the radiators are hot. The typical lifespan of a motorized valve is 8 to 15 years, depending on water quality and the number of switching cycles.
Solution: Verifying the correct function of the valve requires measuring the supply voltage (usually 230 V AC or 24 V DC depending on the manufacturer) and checking the mechanical movement of the spindle. For Vaillant boilers (including models atmoTEC plus VU 200/3-5 and turboTEC plus VU 202/3-5), the three-way valve is supplied as a separate part, and if the motor fails, it's enough to replace just the valve actuator, not the whole valve – which saves money. Always drain the storage tank and close the relevant ball valves before disassembly.
Cause B: Faulty storage tank thermostat or temperature sensor
If the storage tank thermostat shows an incorrect temperature (e.g. it measures 45 °C, even though the actual water temperature in the tank is only 25 °C), the boiler thinks the storage tank is heated and does not switch its output to it. This fault is tricky – the boiler appears to function normally, the storage tank is cold, and there is no error message.
Solution: Measure the storage tank thermostat or NTC temperature sensor with a multimeter – the resistance of the NTC sensor at 25 °C is usually around 10 kΩ, dropping to about 2.5 kΩ at 60 °C (values vary by manufacturer, always check the technical data sheet). If the values are out of range, replace the sensor. Sensor prices are low – usually 10 to 30 €.
Cause C: Incorrectly set storage tank heating priority
The boiler's control electronics allow you to set whether the storage tank has "priority" – i.e. whether the boiler interrupts heating when there is a demand to heat the storage tank and focuses its output on the storage tank. If the storage tank priority is switched off or set to parallel operation, in cold weather with high heating demand the storage tank may not heat up sufficiently. We covered this topic in detail in the article Setting the storage tank heating priority on a low-temperature boiler.
Solution: Check the priority setting in the control unit menu. For Vaillant atmoTEC and turboTEC boilers, this function is set in the service menu under parameter d.17 or similar (the exact designation depends on the firmware version). We recommend setting the storage tank priority to "absolute priority" (1), with the heating interruption limit in winter mode set to 30 to 60 minutes.
Fault #2 – The storage tank cools down too quickly
The storage tank was full of hot water in the morning, but by the afternoon it's cold – even though nobody took a long shower. This fault has two main causes: heat losses from the storage tank, or continuous draw via return circulation.
Cause A: Faulty check valve on the circulation pipe
If a hot water circulation pipe is connected to the storage tank (for the comfort of instant hot water at more distant taps), a check valve must be fitted on it. When this valve stops sealing properly, a thermosyphon effect occurs – hot water moves through the pipe even without the pump running, cools down in the pipe, and returns to the storage tank as cold water. The storage tank can thus lose its thermal energy within a few hours, even without a single draw-off point being used.
Solution: Turn off the circulation pump, place your hand on the circulation pipe and check whether it stays warm (which should not normally happen when the pump is not running). Alternatively, you can briefly close the ball valve on the circulation circuit and observe whether the storage tank holds its temperature better. Replace the check valve – it's a cheap part, usually costing 5 to 15 €.
Cause B: Insufficient thermal insulation of the storage tank
Older storage tanks with damaged or thin insulation lose heat much faster than they should. The standard heat loss of a modern 150-liter storage tank should not exceed 1.5 to 2 kWh per 24 hours (at a water temperature of 65 °C and an ambient temperature of 20 °C). A storage tank without insulation or with damaged insulation can lose 5 to 8 kWh per day – which is more than the heating itself.
Solution: Inspect the storage tank thoroughly. The surface should feel relatively cool (lukewarm) to the touch. If the storage tank is hot all over its casing, the insulation has failed. The solution is either new insulation (a jacket for the storage tank) or replacing the tank with a modern one. When replacing the storage tank, read the article What storage tank volume do I need for a low-temperature boiler.
Fault #3 – The boiler overheats or displays an error code while heating the storage tank
During storage tank heating, the boiler shuts off on the safety thermostat, displays an error code (e.g. F.22, F.28, F.75 on Vaillant units) or behaves unstably. This is a more serious fault that requires immediate diagnosis.
Cause A: Storage tank heat exchanger clogged with limescale
Low-temperature boilers operate with an outlet water temperature of 60 to 80 °C. With hard water (over 15 °dH), intensive precipitation of CaCO₃ occurs on the walls of the storage tank's heat exchange coil. A clogged heat exchanger increases the hydraulic resistance of the circuit, reduces flow, and the boiler overheats during power transfer. In practice, I see storage tanks with a completely calcified coil after just 5 years with hard water and no regular maintenance.
Solution: The storage tank needs to be descaled – by a chemical flush with citric acid (10% solution) or a special descaling agent. Procedure: close the valves, drain the storage tank, connect a pump unit with the solution, flush for 2 to 4 hours, rinse with clean water, and put back into operation. We describe this procedure in detail in the article Maintenance and servicing of a low-temperature boiler with a storage tank – what and when to check. If the boiler's own heat exchanger is also clogged, it needs to be removed and cleaned separately.
Cause B: Incorrect circulation – air in the storage tank circuit
Air trapped in the storage tank circuit disrupts circulation, and the boiler starts to overheat and reacts with a safety shutdown. Typical symptoms include gurgling and bubbling sounds in the pipes during storage tank heating, or loud knocking.
Solution: Venting the storage tank circuit – there should be an automatic air vent at the highest point of the circuit. Perform manual venting by opening the bleed valve (usually with a screwdriver). Repeat the process several times until the water coming out contains no bubbles. Then check the system pressure – it should be 1.2 to 1.5 bar when the boiler is cold.
Fault #4 – The storage tank is too noisy – bubbling, knocking, banging
Noise from the storage tank is one of those faults that people initially ignore – and then have to deal with in an acute state. Knocking or banging during heating (so-called "kettle noises") are almost always caused by limescale on the heating coil or an electric heating element (if the storage tank is also equipped with electric backup heating).
Noise levels increase with temperature – the higher the heating temperature, the more intense the bubbling. The problem is solved by regular descaling (once every 2 years with hard water, once every 4 years with soft water) and by installing a water softener or a polyphosphate dosing device on the cold water inlet to the storage tank. Particularly loud banging (the "hammer effect" – water hammer) can also be caused by incorrectly set water pressure in the storage tank circuit or a faulty expansion vessel.
Fault #5 – Fluctuating hot water temperature during draw-off
The storage tank heated the water to 55 °C, but the water coming from the tap is sometimes hot, sometimes lukewarm, sometimes cold – without any change in the position of the thermostatic mixer lever. This is a typical stratification and hydraulic imbalance problem.
Cause A: Incorrect connection of the storage tank fittings
The storage tank connections must be wired correctly: cold water enters from the bottom, hot water exits from the top. The storage tank's heating coil has its own specific inlet and outlet ports – boiler water must enter from the top and exit from the bottom (counter-flow exchanger). If any of these connections are reversed, the heat exchanger's efficiency drops by 30 to 50%, and the hot water temperature becomes unstable. For Vaillant boilers, this wiring is referenced in the installation manual – see also our article Connecting a Vaillant atmoTEC and turboTEC boiler with a hot water storage tank.
Solution: Check the wiring diagram according to the installation manual of the storage tank and boiler. If the connection is incorrect, the piping needs to be reworked.
Cause B: Undersized storage tank or boiler
If the storage tank has a volume of 80 liters, but a family of 4 people plans to shower one after another, the hot water will simply run out – and temperature fluctuation occurs when switching from heated storage tank water to cold makeup water. This is not a technical fault, but a sizing error. General rule: 40 to 60 liters of storage tank per person for a low-temperature boiler.
Fault #6 – The boiler heats, but the storage tank never heats up, either at night or during the day
This fault differs from Fault #1 in that it is permanent – the storage tank never heats up. The most common cause is an error in the electrical wiring or a fault in the boiler's control board.
Cause A: Faulty electrical connection between the boiler and the storage tank
The boiler communicates with the storage tank thermostat via a control signal – either a voltage-free contact or a 24 V control circuit. If this cable is swapped, interrupted, or connected to the wrong terminals, the boiler will never receive an instruction to heat the storage tank. I've even seen cases where an electrician doing a home renovation cut the storage tank cable and forgot to reconnect it.
Solution: Check the wiring diagram in the boiler's manual (for example, with the Vaillant turboTEC plus VU 122/3-5, these are the eBUS terminals or terminals 3/4/5 for the storage tank, depending on the version). A break in the circuit is verified with a multimeter on the boiler's terminal block.
Cause B: Control board fault
The boiler's control board may have a faulty output for the storage tank circuit. This is the most expensive fault – replacing the board costs 200 to 600 € plus labor. Before diagnosing a board fault, rule out all previous causes. Diagnosing the control board requires a service technician with appropriate diagnostic equipment.
Fault #7 – System pressure drops – water leak from the expansion vessel or storage tank
Low-temperature boilers with a connected storage tank have a larger volume of water in the system – which places higher demands on the expansion vessel. If the expansion vessel (membrane type) loses pre-charge pressure or has a damaged membrane, the system pressure will fluctuate and the safety valve may start venting water.
Symptoms: The boiler's pressure gauge shows pressure below 0.8 bar after cooling down; the safety valve is dripping; the boiler regularly requires refilling with water (you top up the pressure more than twice a year).
Solution: Measure the pre-charge pressure in the expansion vessel – it should be 0.5 to 0.8 bar with a cold system (for a boiler with a circuit height of up to 5 m). Measure using a pressure gauge on the vessel's air valve (like checking a tire). If the pressure is zero or water is leaking from the valve (damaged membrane), the expansion vessel needs to be replaced. For a larger volume of water in the system (boiler + storage tank), it's advisable to install an additional expansion vessel.
Fault #8 – Bad smell from the hot water or discolored water from the storage tank
A hydrogen sulfide (rotten egg) smell, cloudy or discolored water are symptoms of biological or corrosive activity in the storage tank. This is not a fault of the boiler, but a fault of the storage tank itself.
Cause: Insufficient heating of the storage tank – Legionella
The bacterium Legionella pneumophila multiplies in water at temperatures of 25 to 45 °C. If the low-temperature boiler operates in economy mode and only heats the storage tank to 40 °C, an ideal environment for bacterial growth is created. For hygienic reasons, the water temperature in the storage tank should be at least 60 °C (at this temperature Legionella dies within 2 minutes). Modern Vaillant control units have a "Thermal disinfection" function – automatically heating the storage tank thermally to 70 °C once a week for disinfection.
Solution: Turn on the thermal disinfection function in the boiler's menu. The minimum operating temperature of the storage tank should be 55 to 60 °C. If the storage tank has been kept at a low temperature for a long time, we recommend heating it once to 70 °C for 1 hour and then flushing all outlet fittings.
Cause: Corrosion of the anode rod
Storage tanks made of enameled steel are protected by an active magnesium anode. When the anode is consumed (usually after 2 to 5 years, depending on water quality and hardness), the storage tank itself begins to corrode. Symptoms: discolored (rusty) water, an increased amount of sediment in the storage tank, and odor. The anode needs to be checked and replaced regularly – it's one of the most important, yet most neglected, service tasks.
Solution: Remove the anode (usually via a screw plug on the top of the storage tank), visually inspect it, and replace it if its diameter has been reduced by more than 50%. Price of a magnesium anode: 8 to 25 € depending on size.
Fault #9 – The boiler displays an error code during storage tank heating (specific Vaillant codes)
Vaillant atmoTEC and turboTEC plus boilers are equipped with a diagnostic system that displays error codes. Here are the most common codes related to the storage tank:
- F.22 – Lack of water / low pressure: System pressure below 0.3 bar. Top up water via the filling valve to a pressure of 1.2–1.5 bar. If pressure drops repeatedly, look for a leak or a faulty expansion vessel.
- F.24 – Temperature rising too fast (overheating): Caused by poor flow through the storage tank heat exchanger – a clogged exchanger, air in the circuit, or a faulty pump.
- F.28 / F.29 – Ignition error / flame failure: Not directly related to the storage tank, but can occur when starting storage tank heating if the boiler does not have stable combustion during a sudden power demand. Cause: contaminated combustion chamber, faulty gas pressure sensor, insufficient air supply (atmoTEC).
- F.75 – Pressure sensor fault (water pressure sensor): The water pressure sensor is defective or the pipe to it is clogged. The boiler will not start up. The sensor needs to be replaced.
- F.36 – Fan fault (turboTEC): During storage tank heating the boiler requires higher output, and if the fan cannot keep up, this error appears. Check the fan and the air supply.
For the atmoTEC plus VU 280/3-5 and atmoTEC plus VU 240/3-5 models, we recommend always having the service manual with the complete list of error codes on hand, or downloading them from the manufacturer's website – the coding varies slightly between boiler generations.
Fault #10 – The boiler does not start at all – electrical power or fuse fault
The storage tank is one of the biggest "consumers" of control energy in the system. When a storage tank with electric backup heating (a combined storage tank) is added, the total power draw of the system increases. It is not uncommon for a combined 3 kW storage tank + boiler to overload the circuit breaker of the household distribution board – especially if the storage tank's electrical circuit was added to an existing branch without calculating the coincidence factor.
Solution: The storage tank's electrical circuit must have its own dedicated circuit breaker sized for the power draw of the electric heating element (for a 3 kW element, at least a B 16A breaker at 230 V). The boiler must be powered from a different circuit with its own B 10A breaker, or as specified in the boiler's technical documentation. This is not a job for a layperson – the electrical installation must be carried out by a qualified electrician.
Preventive maintenance – how to avoid ninety percent of faults
In practice, a well-maintained boiler + storage tank system will last 15 to 20 years without a major fault. Neglected systems report faults after just 5 years. Here is a basic maintenance schedule:
- Annually: Service inspection of the boiler by an authorized technician (checking the burner, heat exchanger, pump, seals, system pressure). Checking the expansion vessel. Checking the storage tank's anode rod.
- Every 2 years (hard water) / 4 years (soft water): Chemical descaling of the storage tank and heat exchanger.
- Every 5 years: Inspection of the storage tank's safety valve (function test – the valve should release water at an overpressure of 6 bar; if no water comes out, it is clogged and needs to be replaced). Replacement of the filter cartridge on the cold water inlet.
- Ongoing (monthly): Monitoring the system pressure on the boiler's pressure gauge. Checking that the storage tank's safety valve is not dripping (it should be dry). Listening for unusual sounds.
We cover the topic of preventive maintenance in great detail in the article Maintenance and servicing of a low-temperature boiler with a storage tank – what and when to check.
When to call a technician and when you can handle it yourself
Based on many years of experience, I can say that roughly 30% of call-outs for storage tank heating faults are unnecessary – the user could have solved the problem themselves if they knew what to look for. Another 30% are routine service tasks (sensor replacement, venting, pressure top-up, boiler reset). The remaining 40% are genuine technical faults requiring a technician.
You can handle yourself: Venting the circuit, topping up the pressure, resetting an error code, adjusting the temperature in the boiler's menu, checking the anode rod, replacing a simple temperature sensor, replacing the circulation check valve.
Call a technician for: Replacing the three-way valve (positions need to be set correctly), repairing or replacing the control board, descaling the entire circuit (if you're not used to working with acids), any work on the boiler's gas circuit, replacing the safety valve at high pressure, diagnosing electrical wiring if a control fault is suspected.
Frequently Asked Questions (FAQ)
Why does the boiler light up, but the storage tank stays cold, even though the storage tank thermostat is calling for heat?
The most common cause is a faulty three-way motorized valve that hasn't switched into the storage tank position. The second possibility is a fault in the control board or a break in the control cable between the storage tank thermostat and the boiler. Check whether the boiler even registers the storage tank's demand – the appropriate indicator (a storage tank symbol or a heating wheel icon) should appear on the display. If the symbol is not there, the problem is in the electrical signal. If the symbol is there, but the storage tank does not heat up, the problem is in the hydraulics or the valve.
How many years does a magnesium anode last in a storage tank?
With average water quality (medium hardness, pH 7 to 8), a standard magnesium anode lasts 2 to 4 years. With soft water or a high chloride content, it wears out faster – sometimes in as little as 12 months. With hard water (over 20 °dH), limescale on the anode slows its corrosion, but the storage tank is at risk from limescale. An alternative is an electrically powered titanium impressed-current anode – its advantage is practically unlimited service life, and its function does not depend on water quality.
What to do if the storage tank's safety valve keeps dripping?
Dripping from the storage tank's safety valve (usually set to 6 bar) occurs due to the thermal expansion of water – this is a normal occurrence in closed circuits if there is no cold water expansion vessel, or if it is faulty. However, continuous dripping is not normal. Solution: install an expansion vessel on the cold water supply to the storage tank (volume 8 to 18 liters depending on the storage tank's volume, pre-charge pressure of 3 bar). If the valve does not open at an overpressure of 6 bar, but drips at a lower pressure, it is faulty and needs to be replaced – never block it!
Is it normal for the boiler to "bang" during storage tank heating?
No, it is not normal. Banging or knocking during storage tank heating is almost always caused by limescale on the heating coil – steam bubbles are suddenly released from the hardened surface. Another cause can be water hammer from the rapid closing of the three-way valve, or high static water pressure. Descaling the storage tank usually solves the problem. If banging occurs on a recently descaled storage tank, have the water pressure checked (it should be up to 3 bar at the storage tank inlet) and the function of the pressure reducing valve checked.
How often does the storage tank circuit need to be vented?
In a properly functioning system with an automatic air vent, air should not be a problem in the circuit at all. If you need to vent it more than once a year, something is wrong – either you have recently topped up water (which is saturated with air from the water mains), or there is a micro-leak in the system causing periodic refilling and, with it, the introduction of new air. Look for the leak before venting again.
Can I connect a storage tank to a low-temperature boiler myself, without a technician?
The hydraulic connection – piping, valves, circulation pump – can be carried out by a skilled person with basic plumbing experience. However: gas work in Slovakia is reserved exclusively for authorized persons (Act No. 251/2012 Coll. and the relevant decree). The first start-up of the boiler after installing the storage tank must be carried out by an authorized service technician of the manufacturer – otherwise the warranty claim is voided. The electrical connection of the storage tank must be carried out by a qualified electrician. Our article Installing a low-temperature boiler with a storage tank connection step by step describes the entire installation process step by step.
Conclusion
Low-temperature boilers with a connected storage tank are a reliable technology – provided they are correctly installed, set up and regularly maintained. Most of the faults described in this article can be prevented with an annual service inspection and by monitoring the basic operating parameters (pressure, sounds, water temperature). When a fault does occur, a systematic diagnostic approach following the steps in this article will save you time and money – and in many cases will show that the solution is simpler than it seemed.
If you're considering replacing your boiler or storage tank and want to know which configuration is best suited for you, check out our other topics: Atmospheric vs. turbo low-temperature boiler – which to choose with a storage tank or Energy savings when combining a low-temperature boiler and a hot water storage tank. You'll find specific models suitable for connecting a storage tank in our category low-temperature boilers with the option of connecting a storage tank.
Do you have a question on this topic?
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