Most Common Condensing Boiler Faults and How to Fix Them
A condensing boiler is today the most reliable way to heat a family house or apartment, but like any technical device, it occasionally needs attention. Most of the faults that condensing boiler owners encounter have a clear symptom and an equally clear cause – the only problem is that without experience it's hard to tell what you can fix yourself in a few minutes and what is strictly a job for an authorized service technician. In this article we go through the eight most common condensing boiler faults in a clear overview, explain what usually lies behind them, and clearly mark when you can intervene yourself and when any DIY intervention is dangerous or prohibited.
Important notice up front: for any fault related to gas combustion, a gas leak, a smell of gas, smoke, soot, a burner fault, the chimney/flue, or suspected carbon monoxide (CO) leakage, never attempt to intervene yourself. Close the gas supply (if you know how to do so safely), ventilate the space, and immediately call an authorized service technician or the gas emergency line. The same applies to any intervention in the boiler's electrical wiring – this is strictly a job for a qualified professional with the relevant authorization.
Four of the eight symptoms covered in the table below.
Quick overview: symptom, cause, can you fix it yourself?
| Symptom | Probable cause | Can you fix it yourself? |
|---|---|---|
| Low pressure, boiler reports a pressure fault | Micro-leak or need to top up water | Partly – topping up water yes, finding the leak no |
| Frequent switching on and off (cycling) | Oversized output, dirty heat exchanger | No – requires a service adjustment |
| Temperature sensor error message | Faulty or dirty sensor (NTC, outdoor sensor) | Partly – replacement yes, diagnosis better left to a technician |
| Bubbling, gurgling, noise from radiators | Air in the system, worn circulation pump | Yes – you can bleed the air yourself, a technician should replace the pump |
| Insufficiently hot water | Dirty heat exchanger, faulty sensor, weak output | No – cleaning and diagnosis is a job for a technician |
| Shutdown with a fault during frost | Frozen or blocked condensate drain | Yes – you can check and thaw it yourself |
| Water dripping under the boiler | Worn gasket, leaking safety valve, heat exchanger | Partly – tightening a connection yes, working on the boiler itself no |
| Boiler won't turn on, no display | Electronics, fuse, or power supply fault | No – always call a service technician |
The eight most common condensing boiler faults in detail
1. Insufficient pressure in the system
Symptom: The boiler shows an error message about low pressure (typically below 0.8–1 bar), or stops heating altogether. The pressure gauge on the boiler or the display shows a value outside the recommended range (usually 1–2 bar when cold).
Probable cause: This is most often a slow micro-leak – a leaking valve, joint, or small bleed valve somewhere in the system through which water is gradually lost. Less often the cause is a one-off drop in pressure after bleeding the radiators.
Can you fix it yourself? Topping up water via the filling valve to the recommended pressure is something practically every owner can manage – the procedure is described in the boiler manual. However, if the pressure repeatedly drops over the course of days or weeks, this points to a hidden leak, and finding and repairing it (for example a cracked joint or a leaking valve) should be left to a service technician. Repeatedly topping up water without identifying the cause can, over time, damage the heat exchanger due to limescale build-up.
2. The boiler cycles often – switching on and off quickly
Symptom: The boiler runs only briefly (a few minutes or less) and then switches off again, even though the heating hasn't yet reached the desired temperature. This phenomenon is called short cycling and puts long-term strain on both the burner and the electronics.
Probable cause: Most often the boiler's output is significantly oversized relative to the house's actual heat loss, so the boiler reaches the set temperature too quickly. Another common cause is a dirty heat exchanger or incorrectly set output modulation.
Can you fix it yourself? No. Correctly setting the modulation range, or cleaning the heat exchanger, requires professional knowledge and specialized tools. If the true cause is a genuinely oversized boiler, the solution is a service readjustment of the minimum output, or in extreme cases, considering replacement with a more suitable model. This is always a job for a service technician.
3. Error message related to the temperature sensor
Symptom: The boiler displays an error code associated with a sensor (typically something like "F.xx" or a text message about an NTC sensor fault), measures the hot water tank temperature incorrectly, or the weather-compensated control responds illogically – heating too little even in frost, or conversely too much in mild weather.
Probable cause: Temperature sensors (the NTC resistive sensor in the tank, or a separate outdoor sensor used with weather-compensated control) are among the parts that wear out or get damaged over time – through moisture, corroded contacts, or mechanical damage. A faulty sensor sends the boiler an incorrect value, and the boiler regulates its output incorrectly based on it.
Can you fix it yourself? Replacing the sensor itself with a new one is mechanically simple – it's a screwed-in or plug-in connector accessible after removing the cover, so a reasonably handy owner can often manage it themselves following the manual for the given boiler model. However, we recommend first leaving the diagnosis (whether the sensor is really faulty, or whether it's another fault on the circuit board) to a service technician, so you don't buy a part that won't solve the problem.
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NTC sensor set for tank, 10 kΩ – a replacement temperature sensor for the hot water tank, a common cause of error messages related to DHW measurement faults. Price from €20.00. |
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Protherm – Outdoor temperature sensor (wired) for boilers with an eBus system – an outdoor sensor whose failure causes weather-compensated control to malfunction. Price from €33.54. |
4. Noisy or bubbling radiators or boiler
Symptom: Bubbling, gurgling, or hissing can be heard from the radiators or directly from the boiler. The boiler may be noisier than usual, especially when the circulation pump starts up.
Probable cause: In the vast majority of cases this is air that has entered the heating system – after filling, a shutdown, or a change to the piping. If the bubbling and noise persist even after thorough bleeding, the cause is usually a worn or undersized circulation pump that isn't running smoothly or is cavitating noisily.
Can you fix it yourself? Bleeding the radiators through the bleed valve is routine maintenance that every owner can manage – all you need is a bleed key and a cloth. If the problem persists, or if the source of the noise is the circulation pump inside the boiler itself, replacing it should be left to a technician, since it involves working on the boiler's hydraulic circuit.
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Threaded set DAB.EVOSTA2 40-70/180 + backup power supply – a replacement circulation pump, a typical source of noise and faults once it becomes worn. Price from €334.84. |
5. The boiler doesn't heat hot water to the required temperature
Symptom: Water from the tap is lukewarm or cools down quickly, even though a higher temperature is set on the boiler. The problem often worsens gradually rather than overnight.
Probable cause: The most common cause is a dirty heat exchanger (limescale or soot deposits reduce heat transfer efficiency), a faulty temperature sensor in the tank that incorrectly reports the water is already hot enough, or simply an undersized boiler output relative to the household's actual consumption.
Can you fix it yourself? Not directly on the boiler – cleaning the heat exchanger and accurately diagnosing the cause is a job for a service technician, who can tell whether it's mechanical fouling, faulty electronics, or genuinely undersized output. Prevention is helped by good-quality water treatment at the inlet, which slows limescale formation.
6. Blocked or frozen condensate drain
Symptom: During frosty days the boiler suddenly shuts down with a fault, often related to the condensate or flue gas discharge. This typically happens with boilers whose condensate drain runs outside through an exterior wall.
Probable cause: Condensing boilers produce acidic condensate, which is drained via a hose into the sewer system. If this hose runs outside or through an unheated part of the house, in severe frost the condensate freezes inside it and blocks the drain – the boiler interprets this as a fault and shuts down as a safety measure.
Can you fix it yourself? Yes, partly. Checking the route of the condensate hose and, if frozen, carefully thawing it (for example with warm water or a hairdryer, never an open flame) is something the owner can manage alone. A long-term solution is to insulate or reroute the condensate line so it isn't exposed to frost – it's worth consulting a technician about this so the problem doesn't recur every winter.
7. Water leaking around the boiler
Symptom: Drips, small puddles, or damp patches appear under or around the boiler. This can be occasional dripping or a constant, slowly growing leak.
Probable cause: There are several possible causes – from a worn gasket on the hydraulic connections, through a leaking or activated safety valve (which safely releases water once pressure is exceeded), to a more serious problem such as a micro-crack in the heat exchanger.
Can you fix it yourself? Partly. If you find that the leaking water comes from a loose external connection (for example to a radiator or the filling valve), carefully tightening it is usually something you can handle yourself. However, any leak from inside the boiler itself – from the safety valve, heat exchanger, or internal joints – requires a service intervention, since unqualified work can damage the appliance or be dangerous given the proximity of gas and electrical components.
8. The boiler won't start at all, the display is blank
Symptom: After pressing any button the boiler doesn't respond, the display stays completely dark, and no indicator lights come on.
Probable cause: This is almost always an electrical fault – no power supply (a tripped circuit breaker, power outage), a blown fuse inside the boiler itself, or a fault on the circuit board.
Can you fix it yourself? The only thing an owner can safely check themselves is whether the boiler is plugged in, whether a breaker has tripped in the distribution box, and whether the boiler's main switch is on. If the boiler still doesn't respond after this basic check, never attempt to open the cover and work on the electronics or fuses yourself – call only an authorized service technician. Working on the boiler's electrical wiring without the appropriate qualification is dangerous and can cause injury or further damage to the appliance.
How to prevent most faults
Many of the faults described above can be prevented with simple, regular care. The quality of the water used to fill the system has a major impact on the lifespan of both the heat exchanger and the pump – hard or dirty water speeds up limescale build-up and is one of the main reasons a boiler gradually stops heating adequately and the heat exchanger needs increasingly frequent cleaning. A filter fitted at the water inlet to the heating system catches mechanical impurities before they reach the boiler's sensitive components.
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NANO HOT 3/4" - 1" 500 mcr water filter for heating systems, brass – a brass filter at the water inlet that helps prevent faults caused by system contamination and micro-leaks through clogged valves. Price from €65.50. |
In addition to water filtration, we recommend having a professional service inspection carried out once a year, during which a technician checks the pressure, the tightness of the joints, the function of the safety valve, the condition of the condensate drain, and the combustion settings. This regular check not only prevents most of the faults described here, but in many cases is also a condition for maintaining the manufacturer's warranty. You can read more about the scope and intervals of servicing in our separate article on condensing boiler maintenance and lifespan.
The first half of the eight faults covered in the article.
The second half of the eight faults covered in the article.
The most effective prevention according to the article above.
Frequently Asked Questions
- How often should I check the pressure in the heating system? We recommend checking the pressure gauge at least once a month, especially at the start of the heating season. If you notice the pressure regularly dropping and needing water top-ups, this is a sign of a micro-leak that a technician should investigate.
- Can I replace the temperature sensor myself? Mechanically it's a simple job – disconnecting the connector and screwing in a new sensor according to the manual for your model. However, we recommend leaving the diagnosis of whether the sensor is really the cause of the fault to a technician, so you don't unnecessarily buy a part that won't solve the problem.
- Is it normal for the boiler to make a bubbling sound the first time it starts up in the season? Yes, slight bubbling after a longer shutdown or after topping up water is normal – it's air in the system. Simply bleed the radiators and the lowest points of the circuit. If the sound persists even after bleeding, it could be a worn pump.
- I smell a slight gas odor near the boiler, what should I do? Immediately close the main gas shut-off valve if you can access it safely, ventilate the space, don't operate any electrical switches or open flames, and call the gas emergency line or an authorized service technician. Never try to handle this yourself.
- Why does my boiler shut down with a fault only during severe frost? The most common cause is a frozen condensate drain if it runs through an exterior wall. Check the route of the hose and, if frozen, carefully thaw it with warm water, never an open flame.
- How long can a condensing boiler run without a major fault? With regular annual servicing and good-quality water, a modern condensing boiler can reliably run for 15 years or more, with minor faults such as replacing a sensor or a pump being common during this time without marking the end of the appliance's life.
- Is it cheaper to repair an old boiler or invest in a new one? It depends on the boiler's age and the extent of the fault – with repeated heat exchanger or electronics faults on a boiler 12 years old or older, it's worth comparing the repair cost with the price of a new condensing boiler, which will also save on gas consumption.