Common Faults in Condensing Boilers and How to Recognise Them

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Common Faults in Condensing Boilers and How to Recognise Them

The condensing boiler is today the most widespread type of gas heating in Slovak households, and rightly so – its efficiency above 100% (measured on the basis of calorific value) gives it a significant energy advantage over older conventional boilers. Nevertheless – or perhaps precisely because it is a more technically sophisticated appliance – condensing boilers experience faults that can catch even an experienced owner off guard. Over years of working with customers, I have seen dozens of cases where a simple problem grew into an unnecessarily expensive repair simply because the boiler owner didn't know what a particular error code meant, or ignored a symptom that had been appearing for weeks before the actual failure.

This article will help you navigate the most common condensing boiler faults, explain their causes, and teach you to distinguish what you can handle yourself from what should be left to a certified service technician. If you're also interested in how to prevent faults through regular maintenance, we recommend the article Servicing and Maintenance of a Condensing Boiler: What and How Often to Check, where we cover this topic in detail.

Why Are Condensing Boilers More Prone to Certain Faults Than Old Boilers?

An old atmospheric boiler was relatively simple: burner, exchanger, thermostat. A condensing boiler contains a lot more – a premix burner with modulated output, a condensing heat exchanger made of corrosion-resistant material (aluminium-silicon alloy or stainless steel), a system for discharging acidic condensate, a closed circuit with an expansion vessel, a circulation pump, several temperature sensors, an electronic controller, and in many cases also a flow-through domestic hot water (DHW) heater. Each of these components is a potential source of failure.

Moreover, condensation, which ensures high efficiency (by utilising the latent heat of water vapour in the flue gases), creates an acidic environment with a pH of around 3 to 5. This condensate must be properly discharged – if the siphon dries out, becomes clogged, or freezes, the boiler immediately detects this and shuts down. This is a specific feature that simply did not exist with old boilers.

Error Codes: The First Signal You Need to Know How to Read

Modern condensing boilers – including the entire BOSCH Condens GC2300iW range – have an error code system on the display. Each manufacturer has its own set of codes, but the logic is similar: the boiler diagnoses the problem itself, stores the error and either attempts an automatic restart (a so-called reset), or remains locked until a technician clears the fault.

It is important to distinguish between these situations:

Lock-out: the boiler has stopped, but starts up again after pressing reset. If the same code repeats several times a day, it's a signal that something is systematically wrong – call the service, don't just keep pressing reset over and over.
Hard lock: the boiler will not switch on until a technician physically fixes the fault. This is typically a safety-related fault – gas leak detection, flame ionisation error, pressure switch fault.
Warning: the boiler is working, but is alerting you to a condition that requires attention – for example, low system pressure, an upcoming service inspection, or a dirty filter.

Specific codes can always be found in the operating manual. If you've lost the manual, most manufacturers offer it in PDF format on their website. For BOSCH boilers, there's also a mobile app that explains the error code directly in plain language.

Low System Water Pressure – The Most Common Cause of Unnecessary Service Call-Outs

This is by far the most common reason customers call for service. The boiler reports a fault, displays a warning symbol or a pressure-related code, and in the worst case simply won't switch on. Yet the solution is often in the owner's own hands.

The correct operating pressure in a closed heating system is usually 1.0 to 1.5 bar when the system is cold (before heating), or 1.5 to 2.0 bar when the system is warm (during heating). You can see the value on the pressure gauge – either mechanical (a manometer with a needle) or digital on the boiler's display. If the pressure drops below 0.8 bar, most boilers will lock out.

Why Does Pressure Drop?

There can be several causes of pressure drop:

Micro-leaks in the system: ageing seals, joints, radiators. The leak may be so small you can't even see it, but you can lose 0.5 bar or more over a season.
Defective expansion vessel: the expansion vessel in the boiler has a membrane and a nitrogen pre-charge pressure (typically 0.75 bar). If the membrane bursts or the nitrogen leaks out, the vessel stops performing its function and system pressure fluctuates extremely – it's normal when the boiler is cold, but shoots up when heated, and the safety valve discharges water, causing repeated refills and cumulative problems with limescale in the system.
Automatic venting: when the system is first filled, air is gradually released and pressure drops – this is normal and it's enough to top up the system.

If pressure drops regularly (for example, water needs topping up every week), don't just look for the fault in the fill valve – have the system inspected by a technician who can identify the source of the leak.

How to Top Up the Pressure Yourself?

Most boilers have a fill valve (filling tap) directly under or near the boiler. Slowly open the valve and watch the manometer – top up to a value of around 1.2 bar. Then close the valve. If you don't know where the fill valve is, check your boiler's manual or the article How Condensing Boiler Installation Works and What to Prepare, where these details are described in the context of installation.

Problems with Condensate Drainage

Condensate – acidic water with a pH of 3–5, formed when water vapour condenses out of the flue gases – must drain continuously from the boiler. The amount of condensate depends on the boiler's output and operating conditions: at full output and low return temperature, a boiler with an output of 24 kW can produce as much as 2–4 litres of condensate per hour.

Condensate drains via a siphon (a plastic U-shaped fitting, which also prevents flue gases from escaping back through the drain) into the sewer. The siphon is precisely the most common site of problems:

Siphon drying out: if the boiler has stood idle for a long time (summer, holidays), the water seal in the siphon evaporates. When the boiler then starts up, it may have trouble venting the condensate path, or the safety switch may detect the siphon's contents as abnormal.
Siphon clogging: impurities, corrosion from the piping, and limescale deposits accumulate in the condensate. The siphon should be removed, cleaned, and flushed once a year.
Freezing of the drain pipe: if the condensate drain runs through an unheated wall or garage, it can freeze in cold weather. The boiler detects this (back pressure) and locks out. In extreme cases, frozen condensate can damage the drain siphon or even the heat exchanger.

The solution is simple, but needs to be thought of preventively. Before winter, check that the condensate drain pipe is not routed through a place where it could freeze, and ideally insulate it thermally or reroute it into a heated space.

Combustion System Faults: Igniter, Ionisation Electrode, Burner

Condensing boilers use premix burners with electronic ignition. The entire ignition process is fully automatic and controlled by electronics. If something goes wrong, the boiler won't ignite and will display an error code.

Igniter Problems

The igniter is an electrode that produces a spark to ignite the gas-air mixture during start-up. If it is dirty (deposits from condensate or combustion products), the spark is weak or irregular. The boiler attempts ignition several times (typically 3 attempts), and if it fails to ignite, it locks out. Symptom: the boiler makes a ticking sound (ignition attempts), but the burner doesn't fire up, after a while it stops and displays an ignition fault code.

Ionisation Electrode: The Boiler's "Eyes"

The ionisation electrode detects the presence of a flame by measuring the ionisation current in the flame (on the order of microamps). If the electrode is dirty, the signal is weak or faulty, and the boiler thinks the burner hasn't ignited – it locks out for safety reasons. In practice, this manifests in the same way as an igniter fault. Cleaning the electrode is a simple task during regular servicing – fine sandpaper or a cleaning cloth.

Dirty Burner or Venturi Tube

The premix burner mixes gas with air in a precise ratio (lambda control). If the venturi tube or burner is contaminated with dust, cobwebs, or corrosion, the mixture ratio is incorrect. The result: unstable combustion, increased CO emissions, reduced output, and in a worse case, boiler lock-out. This is a job for a service technician equipped with a flue gas analyser.

Circulation Pump Faults

The circulation pump ensures the circulation of water in the system. Most modern condensing boilers have the pump integrated directly into the boiler body. Pump faults are fairly common after five or more years of operation, especially if the boiler has stood idle for a long time (e.g., an empty house in summer).

Stuck Rotor

After a summer break, the pump rotor can get stuck due to deposits from the water (limescale, magnetite). The boiler starts up, the burner fires, but the heat isn't carried away – the exchanger's temperature rises quickly, and the boiler's thermostat locks it out for safety reasons. An overheat code or a circulation fault code usually appears on the display. Symptom: the boiler fires up, after a while (30–60 seconds) it locks out with an overheating fault, the outlet pipe is hot to the touch, but the water in the radiators is cold.

Solution: A technician unscrews the drain/vent screw on the pump and mechanically frees the rotor. If the pump is damaged by corrosion or worn out, it needs to be replaced.

Air in the System – A Silent Enemy

Air trapped in the system causes irregular noises (bubbling, knocking), reduces heating efficiency (air pockets in radiators), and in a worse case prevents proper circulation. Radiators are cold at the top and warm at the bottom – a classic symptom of an air pocket. The solution is to bleed the radiators using a bleed valve (a radiator key). After bleeding, check and, if necessary, top up the system pressure.

Electronics and Controls: When the Boiler's Brain Fails

The control board (PCB – printed circuit board) is the brain of the condensing boiler. It processes signals from all sensors, controls modulation of the burner's output, operates the pump, and communicates with the thermostat. Electronics faults are rarer, but when they occur, diagnosis is more difficult and repair more expensive.

NTC Temperature Sensor Faults

A condensing boiler usually has 2–4 NTC temperature sensors: an outlet temperature sensor, a return temperature sensor, a flue gas temperature sensor (on some models), and a DHW temperature sensor (on combi boilers). If any of the sensors shows a faulty value (short circuit, break, drift outside tolerance), the boiler detects this and locks out. The display will show the code for the specific sensor. Replacing a sensor is a relatively cheap and simple job for a technician.

Valve System Faults

Combi boilers with DHW heating (e.g., BOSCH Condens GC2300iW 22/25 C) have a three-way valve that switches between the heating circuit and DHW heating. If this valve gets stuck (most often again due to deposits or mechanical damage), the boiler either doesn't heat DHW at all, or heating doesn't work properly. Symptom of a three-way valve stuck in the DHW position: radiators are cold even though the boiler is running, hot water works normally. Symptom of the opposite position: DHW is cold, heating works.

Flue Gas Discharge and Air Intake Faults

Condensing boilers are usually turbo boilers with forced flue gas discharge by fan, and are also airtight – they draw in outside air through a coaxial pipe (a pipe within a pipe: the inner pipe discharges flue gases, the outer one supplies air). Faults in this system are critical for the boiler, because they directly threaten operating safety. You can find out more on this topic in the article Flue Gas Discharge and Air Supply for a Condensing Boiler: What You Need to Know.

Dirty or Blocked Flue Outlet

In practice, I've come across cases where a bird's nest, cobweb, or ice blocks the flue pipe outlet on the facade. The boiler detects back pressure (differential pressure switch), the fan runs, but the pressure is not within the normal range, and the boiler locks out. Symptom: the boiler switches on, the fan runs for a few seconds, the burner doesn't ignite, fan or pressure switch fault code.

Fan Fault

The fan (the flue gas system's air fan) can suffer from bearing failure, damaged blades, or an electric motor failure. For sound diagnostics: if the boiler makes an unusual noise from the fan area when starting up (squealing, vibration), it's a sign of bearing wear. Have it checked as soon as possible – a damaged fan can also damage the flue pipe.

Boiler Noise: What Different Sounds Mean

A normal condensing boiler operates relatively quietly – at full output it produces a gentle hum from the fan and burner. Any other sound is information about the state of the appliance:

Bubbling and gurgling: air in the system or the boiler. It needs venting.
Knocking in the radiators: water hammer from thermostatic valve heads rapidly opening/closing. Solution: hydraulic balancing of the system, differential pressure control.
A squealing tone from the boiler: could be pump cavitation (pressure too low, air), pump bearing wear, or a problem with the DHW flow switch.
A loud bang when igniting (puffback): delayed ignition – gas accumulates and ignites all at once. Cause: dirty ignition electrode, incorrectly set gas pressure, faulty gas valve. This needs to be dealt with immediately!
Boiling ("kettle sound"): limescale on the heat exchanger. When limescale builds up on the exchanger, thermal conductivity decreases, local temperature rises, and the water near the exchanger starts to boil. Besides the noise, this significantly reduces efficiency and shortens the exchanger's lifespan.

Faults in Domestic Hot Water (DHW) Heating

Combi condensing boilers provide both heating and hot water production. Faults with the DHW function are noticeable and frustrating – cold water from the tap is an unpleasant experience. Let's look at the most common causes:

Clogged DHW Flow Exchanger

With hard water (unfortunately the case in a large part of Slovakia), limescale builds up in the DHW exchanger. The result: DHW temperature drops, flow rate decreases, and in a worse case the exchanger becomes completely blocked. If you're in the vicinity of Bratislava, Nitra, or other areas with hard water, preventively installing a filter or water softener can significantly extend your boiler's lifespan. Symptom of clogging: DHW is only warm at low flow (it's cold at full flow), DHW pressure is reduced compared to cold water.

DHW Flow Sensor Fault

A flow switch or flow meter detects when the hot water tap is opened. If it's dirty (magnetite, limescale), the boiler "doesn't see" the DHW request and doesn't heat it. Or the opposite can occur – a switch stuck in the "flowing water" position causes the boiler to keep heating DHW even when the tap isn't open, which increases gas consumption.

Effect of Water Quality on the Condensing Boiler

This is a topic that's rarely discussed, but from experience I know that the quality of the fill water can determine whether a boiler lasts 10 or 20 years. Condensing boilers have stricter water requirements than old boilers, because they use a condensing exchanger made of corrosion-resistant but sensitive materials.

Ideal fill water parameters for a closed heating circuit:

Hardness: up to 2–3 °dH (German hardness degrees). Softer water is better against deposits, but extremely soft water (demi water without pH adjustment) can be aggressive towards aluminium exchangers.
pH: 7.5–9.0 for aluminium exchangers, 8.0–10.0 for steel systems. Most manufacturers specify the permitted ranges in the installation manual.
Oxygen content: minimal (a closed system limits the intake of oxygen, but if the system is often topped up with fresh water, oxygen corrodes steel radiators and produces magnetite – black sludge).
Chloride content: below 50 mg/l (chlorides corrode stainless steel).

If you suspect that sludge in the system is the source of problems, a technician can flush the system with special agents and install a magnetic filter, which captures magnetite and other ferromagnetic impurities before they reach the boiler.

Faults in the Thermostat System and Controls

Condensing boilers are designed to work with a control system – ideally weather-compensated control (the outlet temperature is set according to the outdoor temperature) or a smart thermostat. Faults in communication between the boiler and thermostat are relatively common and can manifest in various ways: the boiler doesn't heat at all, heats continuously regardless of the set temperature, or switches on and off alternately in short cycles (short cycling).

Short Cycling

Short cycling is when the boiler switches on and off every minute or two. There are various causes: too small a hydraulic capacity of the system (too little water in the circuit), incorrectly set thermostat hysteresis, a boiler poorly sized for actual heat demand, or a stuck three-way valve. Short cycling puts enormous strain on the boiler – the combustion system, pump, and electronics. Every start-up is more taxing on the ignition system than continuous operation. If you have a boiler that switches on and off all the time, don't leave it be – have the system checked.

For optimal control of a modern condensing boiler, a smart thermostat such as the Bosch Easycontrol CT 200 has proven excellent, allowing programming, remote control via smartphone, and adaptive learning of household habits. Proper control not only increases comfort but also extends the boiler's lifespan by minimising unnecessary switching cycles. You can find out more about control options in the article Smart Control of a Condensing Boiler: Regulation and Smart Thermostats.

Faults Related to Boiler Ageing and How to Recognise Them

Condensing boilers typically have a lifespan of 15–20 years with proper maintenance. However, after ten years of operation, faults related to wear and tear begin to appear:

Seals and O-rings: ageing rubber seals lead to micro-leaks. Symptom: water dripping from the boiler, moisture near the boiler, gradual pressure drop without a visible leak.
Heat exchanger: after years, micro-cracks can develop in the aluminium-silicon exchanger (corrosion, temperature cycles). Symptom: the boiler reports a temperature fault, condensate mixing with heating water, contaminated condensate (instead of clear condensate it's cloudy). Replacing the exchanger is expensive – with old boilers, it's worth considering replacing the whole boiler.
Gas valve: wear of the gas valve's membranes causes unstable gas pressure to the burner. Symptom: unstable flame size, fluctuations in outlet water temperature, a louder burner.

If your boiler is older than 12–15 years and faults keep recurring, consider investing in a new appliance. Modern condensing boilers such as the BOSCH Condens GC8700iW 30 P achieve significantly higher efficiency than older models, and the investment payback through gas savings is realistically 5–8 years for a typical family house. You can read more about how to choose a suitable replacement in the article How to Choose a Condensing Boiler: What to Focus on Before Buying.

What You Can Handle Yourself and What Needs a Technician

This is a question I get very often. Clear boundary: anything related to the gas path (gas valve, burner, gas connection) must be handled exclusively by a certified gas fitter with a valid licence. The electrical part (replacing sensors, boards) may only be done by an electrician with the relevant qualification, or a service technician authorised by the manufacturer. Most manufacturers (including Bosch) make warranty repairs conditional on the service intervention being carried out by an authorised technician.

What you can realistically handle yourself without risking loss of warranty:

Topping up water pressure via the fill valve
Bleeding the radiators
Cleaning the outer combustion air grille (cobwebs, dust)
Checking and cleaning the condensate siphon
Pressing reset after verifying the basic states (pressure, condensate)
Checking that the gas shut-off valve before the boiler is open
Setting the thermostat and heating programme

Preventive Maintenance as the Best Protection Against Faults

Experience from practice speaks clearly: boilers that get a regular annual inspection break down much less often, and their faults are less serious. An annual service of a condensing boiler typically costs €80–150 (depending on region, manufacturer, and condition of the appliance) and includes:

Checking and cleaning the burner, ignition and ionisation electrode
Flue gas analysis and adjustment of the combustion ratio
Checking seals and pressure components
Checking and servicing the pump
Checking the expansion vessel and safety valve
Cleaning the siphon and condensate path
Checking the electrical part and all sensors
Testing safety functions

This list can help you when talking to a service technician – ask what specifically was checked and cleaned. You'll find a detailed overview of service work in the article Servicing and Maintenance of a Condensing Boiler: What and How Often to Check.

Frequently Asked Questions (FAQ)

My boiler reports a fault and won't switch on, but the gas and electricity supply are both working. What should I check first?

Check three things in this order: 1) the water pressure on the manometer (should be 1.0–1.5 bar with a cold boiler), 2) the condensate siphon – it should be filled with water and the drain should be clear, 3) the flue outlet on the facade – check it isn't blocked (bird's nest, ice, snow). If all three are fine, check the error code in the manual and call the service.

My boiler keeps starting and stopping every 2 minutes. Is this a fault?

Yes, this is short cycling – an unhealthy condition for the boiler. There are several possible causes: the boiler's output is too large relative to actual heat demand, the thermostat's hysteresis is incorrectly set, a problem with the system's hydraulics (small water volume in the circuit, a stuck three-way valve). Have the system checked by a technician – the solution could be simple (adjusting the controls) or might require hydraulic balancing of the system.

Water is dripping from my boiler. Is it condensate or a leak?

Condensate is a normal occurrence – it forms during the condensation process and must be discharged via the siphon into the sewer. If water is dripping directly from the boiler's body (not from the siphon/drain pipe), it's most likely a leak – a worn seal, a loose joint, or in a worse case a crack in the exchanger. In that case, call the service. You can also distinguish condensate by taste – it's acidic (pH 3–5), while heating water is neutral or slightly alkaline (it contains corrosion inhibitors).

My boiler heats normally, but the hot water is lukewarm. What is the cause?

With combi boilers (with flow-through DHW heating), the most common cause is a plate DHW exchanger clogged with limescale – typical with hard water. Other possibilities include a three-way valve stuck in the heating position (not switching to DHW), a faulty DHW flow sensor, or insufficient cold water flow (low pressure in the household water supply). Start by checking the cold water pressure and call a technician to descale the exchanger.

How do I recognise that the expansion vessel needs replacing?

Typical symptoms of a defective expansion vessel: the system pressure is normal when the boiler is cold (e.g., 1.2 bar), but when heated (above 60 °C) it shoots up to 3 bar or more, the safety valve discharges water, and after cooling the pressure is low again. Another symptom: dripping from the safety valve (the red ring on the valve) after each heating cycle. A technician can check the nitrogen pre-charge pressure in the expansion vessel (should be 0.5–0.75 bar when cold and empty). If the membrane has burst, replacing the vessel is essential.

I have an old boiler that breaks down every season. Is it worth repairing, or replacing?

A rule of thumb: if the repair costs exceed 30–40% of the price of a comparable new boiler, and the appliance is older than 12 years, replacement makes more economic sense. A modern condensing boiler is typically 15–25% more economical than an older model from the same manufacturer, because control and output modulation technology has improved significantly. For example, switching from an older atmospheric boiler to a modern BOSCH Condens GC2300iW 24 P can bring savings of 20–35% on your annual gas bill, which for typical consumption of 1,500–2,500 m³ of gas per year represents real savings of €200–500 per season.

Conclusion

Condensin

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