Common Faults of Condensing Boilers and How to Fix Them

Common Faults in Condensing Boilers and How to Fix Them

Condensing boilers are among the most reliable heat sources on the market – yet even they will sooner or later develop a fault. Experience from dozens of service jobs shows that most problems repeat themselves: error codes, low pressure, burner sooting, condensate drainage issues. The good news is that a large portion of faults do not require an immediate technician visit – an owner with basic technical knowledge can diagnose, and sometimes even repair, the issue themselves. This article systematically covers the most common faults: the cause, the consequence, the diagnostic procedure and the solution. Where a service technician is required, we will clearly say so.

If you are still choosing a boiler or are interested in its proper installation, we also recommend reading the related articles in our Knowledge Center – specifically Installing a Condensing Boiler – Procedure, Space Requirements and Flue Gas Discharge and Maintenance and Servicing of a Condensing Boiler – What and How Often to Check. Understanding proper installation and regular maintenance is, after all, the best prevention against most of the faults described below.

Why Condensing Boilers Fail – Basic Principle and Weak Points

A condensing boiler works on a different principle than an old atmospheric boiler. It cools the flue gases below the dew point (usually below 57 °C), which causes water vapor to condense and release latent heat – this is the source of its high efficiency of 97–109% (relative to fuel calorific value). However, this principle brings specific design elements that can wear out or become clogged:

• Heat exchanger made of aluminum alloy or stainless steel – condensate is mildly acidic (pH 3–5); corrosion appears when it becomes fouled
• Condensate drainage system – siphon, hoses, neutralizer – can become clogged or freeze
• Hermetic burners with ionization electrode – sensitive to gas quality, dirt and wear
• Circulation pump – on cheaper models, usually the first part to need replacement after 5–8 years of operation
• Expansion vessel – loses pre-charge pressure, leading to system pressure fluctuations
• Electronic control unit and sensors – sensitive to power surges and moisture

Understanding these components is key to proper diagnosis. Most error codes displayed by the boiler directly refer to one of these elements.

Fault No. 1 – Boiler Displays an Error Code and Does Not Heat

This is the most common reason owners call a service technician. Error codes are actually a great help – the boiler tells you directly what is happening. The problem is that each manufacturer uses a different coding system. Nevertheless, there are categories common to almost all brands:

• E1 / F1 / A01 – ignition failure: the boiler attempted to ignite, but the ionization electrode did not detect a flame. Causes: closed gas valve, air in the gas pipe after a supply interruption, dirty or worn ionization electrode, low gas pressure.
• E2 / F22 / A02 – low water pressure: system pressure has dropped below the minimum (usually 0.8–1.0 bar). Water needs to be topped up.
• E3 / F28 / A03 – heat exchanger overheating: the NTC sensor detected an excessively high temperature. Causes: blocked circulation pump, clogged filters, closed thermostatic valve.
• E5 / F75 – pump or pressure sensor failure: at startup the boiler did not register a change in pressure, indicating a non-functioning pump or a clogged sensor.
• E9 / STB – safety thermostat: the boiler protected itself from overheating. A manual reset is required after the cause has been resolved.

Procedure for an error code: First, download the service manual for your specific model (available on the manufacturer's website, or we can provide it as a PDF in the manuals and documentation section). Check that the gas valve is open, what the water pressure reads on the gauge, and that no shut-off valve on the heating circuit is closed. Only then reset the boiler using the button (usually for 3 seconds). If the code reappears within 30 minutes, call a service technician – repeatedly resetting without resolving the cause damages the electronics.

Fault No. 2 – Drop in System Water Pressure

Is the pressure gauge showing below 1.0 bar? Is the boiler reporting low pressure or not starting at all? This is one of the most common and, at the same time, one of the easiest faults to fix – but only if you know what to look for.

Causes of pressure drop:

• Micro-leaks in the system – at joints, seals, thermostatic heads. They can be so small that they're invisible to the naked eye, but pressure drops by 0.1–0.2 bar per week.
• Failed diaphragm in the expansion vessel – the vessel loses its ability to compensate for the thermal expansion of water. Pressure fluctuates: low in the morning, extremely high after heating (the safety valve releases water).
• Air venting through the automatic air vent – this is normal during initial commissioning, but if it continues even after years, it indicates a problem with air in the system.
• Loose safety valve – at pressures above 3 bar (or even lower if the valve is worn), it releases water. You can recognize this by moisture or dripping on the discharge pipe.

How to top up the pressure: There is a filling valve on the boiler (usually a grey or red lever near the cold water inlet). Open it, watch the pressure gauge, and top up the water to 1.5–2.0 bar (with the system cold). Never go above 2.5 bar. If you need to top up more than once a month, it's a sign of a leak or a faulty expansion vessel – this needs to be addressed by a technician.

Fault No. 3 – Boiler Does Not Ignite or Goes Out Shortly After Ignition

Burners in condensing boilers are more sensitive than those in old atmospheric boilers because they operate with a precise gas/air ratio (lambda control). If the boiler repeats ignition attempts, you hear a click from the igniter, but no flame appears or it immediately goes out, look for the problem in these areas:

Ionization electrode: This is a small rod in the burner that detects the presence of a flame (ionization current 1–10 µA). After years of operation, it becomes coated with oxide or worn out. Symptom: the boiler ignites, burns briefly (2–5 seconds), then goes out and reports a fault. Solution: clean the electrode with fine sandpaper (grit 400–600) or replace it – the price of the replacement part is usually €15–40, and a technically skilled owner can replace it in 20 minutes (after shutting off the gas!).

Gas pressure: Condensing boilers have precise gas control and do not function properly at low inlet pressures (below 18 mbar for natural gas). If you have problems with insufficient pressure, call the gas distributor – it's their issue, not the boiler's.

Clogged gas filter: There is a mesh filter in front of the gas valve. Over time it can become clogged with debris from the pipeline (scale, dirt from the gas line). Cleaning is part of the annual service.

Defective gas valve: If all other causes have been ruled out, it may be a fault in the magnetic gas valve. This must not be repaired by a layperson – a certified service technician with the appropriate authorization is required.

Air in the gas pipe: After a gas supply interruption (regulator replacement, work on the network), air remains in the pipe. The first 2–3 ignition attempts will fail until the air is pushed out. This is a normal condition, not a fault – just press reset a few times.

Fault No. 4 – Condensate Drainage Problems

Condensation is desired and expected – it is precisely because of it that the boiler achieves high efficiency. In a typical family home with a condensing boiler, 1–3 liters of condensate are produced daily (depending on the return temperature and moisture content of the flue gases). Condensate must drain smoothly – if drainage fails, the boiler protects itself and shuts down.

Clogged siphon: The siphon (water trap) prevents flue gases from entering the room through the drain. In summer, when the boiler stands idle for a long time, the water in the siphon can evaporate or a biological film can build up in it. Symptom: odor from the boiler (flue gases leaking through) or condensate leaking near the boiler. Solution: clean and fill the siphon with clean water (a 5-minute task, once a year).

Frozen drain: In unheated spaces (garage, boiler room against an exterior wall), the condensate drain can freeze in extreme frost. The boiler detects this either itself (via a sensor) or the siphon becomes clogged and condensate leaks near the boiler. Solution: insulate the drain pipe or reroute it to a warmer space.

Condensate neutralizer: Some installations (required by legislation or local regulations) have a neutralizer before the connection to the sewer – a container with magnesite granulate that raises the pH of the condensate from 3–4 to a more neutral 6.5–7.0. The granulate is gradually consumed and needs to be topped up once a year. If you don't do this, the neutralizer acts as a sieve and can clog the drain.

Fault No. 5 – Excessive Boiler Noise: Knocking, Banging, Squeaking

Condensing boilers should operate quietly. If you hear unusual sounds, it's always a signal to pay attention. Different sounds indicate different problems:

Knocking and bubbling in the heat exchanger (kettling): This is a sound similar to water boiling in a kettle. Cause: limescale on the heat exchanger. In water with high hardness (above 14 °dH), calcium carbonate deposits on the hot surfaces of the exchanger and reduces heat transfer. The boiler overheats locally, and water partially turns into steam. Solution: chemical descaling (flushing the system with a descaling agent) or mechanical cleaning of the exchanger. If you know you have hard water, install a water softening device or an inhibitor dosing unit on the boiler inlet.

Loud whistling or squeaking: The most common cause is air in the system or a damaged circulation pump bearing. Air can be removed by manually bleeding the radiators (via the valve on the side of the radiator). A damaged pump bearing must be replaced.

Knocking in the pipes at startup: This is known as water hammer. The cause is usually a rapidly closing thermostatic valve, an incorrectly sized bypass, or an overly powerful circulation pump. Solution: adjust the differential bypass or reduce the pump speed.

Ticking and cracking: Thermal expansion of pipes is normal. But if the pipes are installed without expansion loops or without sufficient floating routing within walls, they can crack loudly when heating up. Solution: free pipe routing with padding to prevent direct contact with masonry.

Fault No. 6 – Boiler Does Not Heat Domestic Hot Water (DHW)

Condensing boilers with an integrated tank or combination (combi) boilers must heat hot water quickly and reliably. If the hot water is cold, lukewarm, or the temperature fluctuates, look here:

Clogged flow sensor: The boiler will not start heating DHW if it doesn't detect sufficient flow. The flow sensor (turbine or Hall-effect type) can become clogged with deposits or mechanically jam. It can be cleaned or replaced (a common part, costing €10–30).

DHW heat exchanger throughput: The internal heat exchanger (coil) or plate DHW heat exchanger becomes coated with limescale. If you have hard water and have never descaled it, after 3–5 years the flow can be reduced by 50–70%. The boiler then protects itself and limits output. Solution: chemical decarbonization.

Insufficient flow on the primary circuit: If the pump is blocked or the inlet filter (Y-strainer before the boiler) is dirty, the boiler cannot transfer enough heat. Regular cleaning of the Y-filter (once a year) is a quick and simple task.

Boiler settings: Check the maximum DHW temperature setting in the boiler's menu. Sometimes, after a power outage or reset, a lower temperature than desired gets set.

Fault No. 7 – Circulation Pump Not Working

The circulation pump is the heart of the system. Without it, the boiler burns gas, but heat doesn't reach the radiators or underfloor heating. Modern condensing boilers have a built-in electronically controlled (EC) pump; older models have a mechanical pump with fixed speed settings.

Pump stuck after summer: The most classic fault when first turning on the heating in autumn. The pump has been idle all summer, and the impeller has "seized" and the motor cannot turn it. A repair anyone can handle: turn off the boiler, find the plastic cover on the side of the pump (a hex screw or plug), open it and use a flat screwdriver to manually turn the pump shaft several times. Warning: there is water under the cover – have a cloth ready. Then turn the boiler back on – it will usually start right away.

Air in the pump: The same symptom – the pump hums but doesn't push water. Bleed it using the same drain plug: open it slightly until water (not air) starts to flow, then tighten it.

Worn bearings: After 8–15 years of operation, pump bearings naturally reach the end of their service life. Symptoms include noisy operation, vibrating pipes, or increased electricity consumption. Replacing the whole pump (including the electronic module on EC versions) is a routine service task, with the replacement part costing €80–250 depending on the model.

Fault No. 8 – Expansion Vessel Has Lost Its Function

The expansion vessel compensates for the volume of water that increases when heated (water expands by about 3% from 10 °C to 80 °C). Without a functioning expansion vessel, pressure in the boiler rises with each heating cycle, the safety valve discharges water, and then the pressure drops and the boiler shuts down.

The expansion vessel has a diaphragm inside (rubber or EPDM) that separates the air side (under a pre-charge pressure of 0.75–1.0 bar) from the water side. After years, the diaphragm can tear or lose its elasticity. Diagnosis: close the filling valve, drain a bit of water from the boiler, check the air valve on the expansion vessel (similar to a bicycle tire valve). If water (not air) comes out of it, the diaphragm is torn.

Solution: if the pre-charge pressure is low (below 0.5 bar) and the diaphragm is fine, simply inflate it with air (a tire pump). If the diaphragm is damaged, the entire expansion vessel needs to be replaced – it's a common part, costing €30–80 for standard sizes (5–18 liters). For larger systems (underfloor heating, larger homes), external expansion vessels with a larger capacity are needed.

Fault No. 9 – Boiler "Short Cycles" (Rapid On/Off Cycles)

Short cycling is a condition where the boiler runs for only a few minutes, shuts down, and then restarts after a short time. Besides discomfort and noise, short cycling is a serious issue in terms of service life – every start puts stress on the electrodes, burners, and heat exchanger.

Causes of short cycling:

• The boiler is oversized for the given heating system – the most common cause. If you have a 24 kW boiler and, after reducing your home's heat loss through renovation, you only need 8 kW, the boiler will short cycle because the system doesn't need that much heat. Solution: properly set the burner modulation to its minimum, or replace the boiler with a smaller one.
• Too small a system water volume – small radiators, short circuits. Water heats up quickly, and the boiler shuts off. Solution: hydraulic separator, buffer tank, or enlarging the system.
• Incorrect thermostat hysteresis setting – a thermostat with too small a differential (0.1–0.2 °C) causes frequent switching. Set the hysteresis to 0.5–1.0 °C.
• Faulty NTC temperature sensor – if the sensor shows a higher temperature than the actual one, the boiler shuts off prematurely.

If you're addressing the correct sizing of a boiler for your building, also read our article What Output Does My Condensing Boiler Need – Calculation Based on Area and Insulation, where you'll find a practical procedure for calculating heat loss.

Fault No. 10 – Flue Gas Discharge Faults (Coaxial or Twin-Pipe Systems)

Condensing boilers in turbo (Type C) configuration have a sealed combustion chamber and discharge flue gases through a coaxial pipe (pipe within a pipe) or separate ducting. Problems in this area are serious and can have safety implications.

Frozen flue outlet: In severe frost, condensate forming directly in the flue gases at the pipe outlet can freeze the opening. The boiler detects back pressure, the sensor interprets this as a fault, and the boiler shuts down. Solution: thaw the outlet (carefully with warm water, never an open flame!), check the direction of the outlet (it should point slightly downward or have an anti-freeze cover).

Leaks in the flue duct: Loose joints, cracked pipe, missing seals – these are first-category safety risks. Symptom: smell of flue gases indoors, soot deposits around joints. Immediately turn off the boiler and call a service technician!

Condensate in the flue duct: In a longer horizontal section, condensate can accumulate if the correct slope (at least 3° toward a drainage point or the outlet) is not maintained. Condensate leaking near the boiler or from the outlet is a sign of incorrect installation.

We cover proper flue gas discharge in more detail in the article Installing a Condensing Boiler – Procedure, Space Requirements and Flue Gas Discharge.

What the Owner Can Do Themselves and What Not

This is a fundamental question we want to answer honestly. Not for commercial reasons, but for safety reasons and warranty reasons.

You can do these yourself (without voiding the warranty, safely):

• Topping up the water pressure (filling valve)
• Bleeding radiators
• Resetting the boiler according to the manual
• Cleaning and filling the condensate siphon
• Refilling the granulate in the neutralizer
• Unblocking (manually spinning) a stuck pump
• Checking thermostat and boiler settings
• Cleaning the combustion air filter (if present)

Requires a certified service technician (gas fitter/inspection technician):

• Any work on the gas pipe, gas valve, or burner
• Replacing the ionization and ignition electrode (we recommend a technician, even though it's technically simpler)
• Cleaning the heat exchanger with chemical agents
• Adjusting the combustion ratio (lambda)
• Repairing flue duct leaks
• Replacing the gas valve, gas pressure sensors
• Any intervention that would break the manufacturer's seal

Keep in mind that most manufacturers' warranty terms require an annual service by a certified technician. If you think you don't need a service inspection, also read our recommendations in the article Maintenance and Servicing of a Condensing Boiler – What and How Often to Check.

Fault Prevention – What to Do Regularly

Most of the faults described above can be prevented, or at least detected early, through regular maintenance. From experience, we know that owners who don't skip their annual service keep their boiler in good condition even after 15 years. Those who "save" on service end up dealing with expensive heat exchanger or gas valve repairs after 5–7 years.

Once a month (2 minutes):

• Check the pressure on the gauge (1.5–2.0 bar with the system cold)
• Check that the boiler isn't registering any active error code
• Check the condensate drain – any leaks?

Once a year (ideally before the heating season, September–October):

• Service inspection by a certified technician – cleaning burners, checking electrodes, measuring flue gases, checking seals
• Cleaning the Y-strainer filter before the boiler
• Cleaning the condensate siphon, refilling the neutralizer granulate
• Checking the expansion vessel pre-charge pressure
• Bleeding the entire system
• Checking thermostats and controls

Once every 3–5 years:

• Chemical decarbonization of the heat exchanger (sooner if water is hard)
• Replacing the DHW tank anode (if a tank is present)
• Checking the condition of the expansion vessel and possibly replacing the diaphragm
• Flushing the system with a corrosion inhibitor

Error Codes by Common Manufacturers – Reference Table

The following table is indicative and simplified. Always follow the specific service manual for your model, which you can find in the manuals and documentation section on atria.sk.

Fault Type	Vaillant	Viessmann	Buderus / Bosch	De Dietrich
Ignition failure	F28	E1	EA	E3
Low water pressure	F22	E9	H2O	E9
Overheating	F75	E3	L0	E2
Pump fault	F74	E7	C6	E6
Temperature sensor fault	F10, F11	E5	S0	E5
Safety thermostat (STB)	F50	E9	Lx	E8

Note: codes may vary depending on the specific model and generation of the boiler. Always check the original manual.

Frequently Asked Questions (FAQ)

Can I reset the boiler myself, or does a technician have to come?

A one-time reset is fine and is usually the first step recommended by the manufacturer. Press the reset button according to the manual (usually for 3–5 seconds) and watch to see if the boiler starts normally. If the same error returns within an hour, another reset without fixing the cause is not advisable – it can damage the electronics and, in some cases, may void the warranty. Three or more resets in a row without a service intervention is a sign that you need to call a technician.

Why doesn't my boiler work after summer, even though it worked fine before the season?

The most common cause is a stuck circulation pump. During the summer, when the boiler is idle, the impeller (the rotating part of the pump) can mechanically "seize up" due to deposits in the water. Solution: manual unblocking via the service opening on the pump (flat screwdriver). Another possibility is a dried-out condensate siphon – fill it with water. In some cases, it's a loss of pre-charge