Most Common Faults of Low-Temperature Boilers and How to Fix Them
Most Common Faults of Low-Temperature Boilers and How to Fix Them
An older low-temperature (atmospheric, classic) gas boiler can reliably serve for 15-20 years, but as it ages, minor and more serious faults become more frequent. Most of them have typical, recurring causes - the boiler won't ignite, "goes into fault" during operation, bubbles, or fails to heat hot water properly. In this article we clearly explain why this happens, what you can fix yourself, and when it's essential to call a service technician. The goal is not to replace professional service, but to help you quickly recognize the nature of the problem before you call a technician - saving you time and money on an unnecessary callout.
1. The boiler won't ignite or start at all
If the boiler doesn't start at all, the first step is to rule out the simplest cause - check that the main gas shut-off valve is open and that none of the taps on the gas supply to the boiler itself are closed. It's also worth checking whether gas works on another appliance in the household (for example, a cooker), which rules out a wider supply outage.
If the gas is fine and the boiler still won't ignite, the most common cause is a dirty or worn ignition electrode - this creates a spark at start-up that ignites the gas at the burner. Deposits, soot, or a shifted electrode position cause the spark to either not form at all or to be too weak. A second typical cause is a fault in the ionization (sensing) electrode - after the flame ignites, this confirms to the boiler that the burner is actually burning; if it's damaged or dirty, the boiler "doesn't see" the flame and shuts down immediately as a safety measure, even though it's actually burning. A third possible cause is insufficient chimney draft - if the boiler tests flue gas flow before ignition (for example, via a pressure switch) and the draft is insufficient, it will refuse to ignite for safety reasons.
Identifying the cause at home without professional measurements is limited - most modern boilers at least roughly indicate the type of fault with an error code on the display (the exact meaning of the code can always be found in the manual for the specific model). However, actual intervention on the electrodes, burner, or gas valve requires a service technician authorized to work on gas appliances.
2. Low pressure in the system
The normal operating pressure of a heating system is roughly 1 to 2 bar (the manufacturer states the exact range in the manual). When pressure drops below approximately 0.8-1 bar, the boiler usually signals this on the display or with an indicator light and shuts down as a safety measure, so it doesn't operate "dry" without sufficient water circulation.
A one-off slight pressure drop is a normal occurrence (for example, after bleeding radiators or after longer operation) and can easily be fixed by topping up water via the filling valve on the boiler or heating circuit, until the pressure returns to the recommended range. Even a layperson can handle this without problems - it's routine maintenance.
However, if pressure keeps dropping repeatedly over days or weeks and water needs to be topped up again and again, it's almost always a leak somewhere in the system - it could be a leaking joint, a radiator, a circulation pump, or very often a worn membrane in the expansion tank. The expansion tank compensates for volume changes in the water as it heats and cools; when its membrane cracks or loses elasticity, the tank stops fulfilling its function and the system behaves exactly as if it had a leak - pressure fluctuates and drops even without a visible water leak.
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We always recommend leaving the replacement of the expansion tank and the diagnosis of the actual cause of repeated pressure drops to a service technician - they can also check other possible sources of leaks in the system.
3. Bubbling and noise from the boiler
Bubbling, gurgling, or increased noise during boiler operation usually has one of two causes. The first and clearly most common is air in the system - air bubbles trapped in radiators or piping cause characteristic bubbling and knocking, especially after refilling water or after a longer shutdown. The solution is simple and any owner can handle it themselves - bleeding the radiators using a bleed key (radiator valve key), starting with the lowest-positioned radiators and working upward, with ongoing pressure checks and topping up as needed.
The second possible cause, especially if the bubbling or rumbling sound comes directly from the boiler body, is deposits (limescale, sludge) in the heat exchanger. Deposits restrict water flow inside the exchanger, water overheats locally in the restricted space and creates a characteristic "boiling" and rumbling sound - similar to boiling water in a pot with a scaled bottom. This is no longer routine DIY maintenance - it requires chemical or mechanical cleaning of the exchanger by a service technician.
4. The boiler shuts off during operation or "goes into fault"
If the boiler stops heating a few minutes after ignition for no apparent reason and throws a fault, there can be several causes. A frequent one is a fault or dirt on the temperature sensor - if the boiler cannot reliably measure the heating water temperature, it shuts down as a safety measure. Another typical cause is a clogged heat exchanger - as with bubbling, deposits restrict water flow, it overheats locally and the boiler reacts with a safety shutdown due to the risk of overheating.
A third group of causes is a problem with flue gas discharge - if the boiler continuously monitors flue gas flow (for example via a chimney pressure switch) and the flow deteriorates during operation (for example due to a gust of wind, partial blockage of the duct, or a change in pressure conditions in the house), the boiler shuts down as a precaution. The exact reason for the fault during operation is usually shown by the error code on the display according to the manual for the specific model - decoding it is a good first step before calling service, speeding up both diagnosis and the repair itself.
5. Weak chimney draft and flue gas backflow
This fault mainly concerns atmospheric boilers connected to a masonry chimney (unlike turbo or condensing boilers, which have their own forced flue gas discharge via a fan and a facade or roof outlet). An atmospheric boiler relies exclusively on natural chimney draft for flue gas discharge - that is, on the difference in temperature and density between flue gases and outside air.
If the chimney is clogged with soot, has an incorrect diameter or height, or if conditions around the house have changed (a new extension, a tree, another chimney nearby), draft can worsen significantly. This shows up as slowed or insufficient flue gas discharge, or in worse cases, partial backflow of flue gases into the boiler room. For atmospheric boilers this is a somewhat more serious safety risk than for turbo or condensing models - precisely because flue gases are not forced out of the building but rely on natural draft, which can fail without visible warning.
Symptoms that should immediately alert every owner: a smell of flue gases in the room with the boiler, condensation or black marks around the chimney neck, frequent shutdowns of the boiler with a fault related to discharge, or (in an extreme case) headaches and fatigue in people near the boiler room, which can indicate a carbon monoxide leak. In such a case, never try to solve the problem yourself - shut down the boiler immediately, ventilate the room and contact a service technician or a chimney sweep, who will check the condition and patency of the chimney.
6. Insufficient hot water heating
If the boiler heats properly but the hot water from the tap is only lukewarm or there's too little of it, the most common cause in both instantaneous and combi boilers is a heat exchanger clogged with limescale. This is especially true in areas with harder water, where lime deposits gradually build up on the exchanger walls and restrict heat transfer to the flowing water - the boiler burns fine, but the heat isn't transferred efficiently further on.
Prevention is clearly cheaper here than subsequent cleaning or replacement of the exchanger. Installing a filter at the water inlet to the system catches mechanical impurities and also partly limits limescale build-up, which extends the life of the exchanger and maintains its efficiency.
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Cleaning an already clogged exchanger itself (chemical flushing or mechanical removal of deposits) is a professional job that you should leave to a service technician. However, installing a filter can be done during a routine service inspection as a preventive measure for the future.
Situations where you should not wait.
When to Call a Service Technician
Some interventions you can handle yourself without risk - topping up water when pressure drops, bleeding radiators, checking the gas tap is open, or checking the error code in the manual. Anything relating to the burner, ignition or ionization electrode, gas valve, exchanger, or any suspicion of a gas leak belongs exclusively in the hands of an authorized service technician.
Especially important: at any suspicion of a gas leak (characteristic smell) or leakage of flue gases into the living space (smell of flue gases, headaches, dizziness, soot-blackened walls around the boiler or chimney), switch off the boiler immediately, shut off the gas supply, ventilate and call a service technician or the gas emergency service without delay. This is not a situation for a DIY fix or for postponing - it is a direct threat to health and life. A regular annual service inspection and chimney inspection by a chimney sweep are the best prevention against most of the faults described above.
The first half of the six faults covered in this article.
The second half of the six faults covered in this article.
Distinguishing routine maintenance from professional intervention.
Frequently Asked Questions
Why does my boiler often go into fault?
The most common causes are a dirty or clogged heat exchanger, a faulty temperature sensor, or a problem with flue gas discharge. The exact reason is shown by the error code on the display according to the manual for the specific model - take this to a service technician.
What should I do if the boiler bubbles?
In the vast majority of cases it's air in the system - bleeding the radiators from the lowest-positioned upward helps. If bubbling persists even after bleeding, it's probably deposits in the exchanger and you need a service technician.
Is it dangerous if the boiler doesn't have sufficient chimney draft?
Yes, for atmospheric boilers this is a more serious safety risk than for turbo or condensing models, because flue gases rely exclusively on natural draft, and if it fails, they can leak into the boiler room. If in doubt, switch off the boiler, ventilate and call a technician or chimney sweep.
How often do I need to top up water in the heating system?
Occasional topping up (for example after seasonal bleeding) is normal. However, if you have to top up pressure repeatedly over the course of weeks, it's a leak in the system or a worn expansion tank membrane - it needs to be checked.
Can I replace the ignition or ionization electrode myself?
It's not recommended - it's an intervention on the gas part of the boiler that should only be carried out by an authorized service technician, both for safety and to preserve the warranty.
Why do I have too little hot water at home even though heating works normally?
The most common cause is a heat exchanger clogged with limescale, especially in harder water areas. Prevention is installing a filter at the water inlet - cleaning an already clogged exchanger requires a service technician.