Saunier Duval F29 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F29 fault code mean?
The F29 code indicates that the boiler's burner successfully ignited but the flame then went out unexpectedly during operation. As soon as the control board detects this loss of flame signal, it triggers an immediate safety lockout — cutting all heating and hot water — to prevent unburned gas from accumulating. This is distinct from the F28 code, which relates to a failure to ignite in the first place. You'll typically notice the boiler attempting to start, briefly firing, then shutting itself down, often repeating this cycle before locking out completely.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Insufficient or interrupted gas supply Common
If the dynamic gas pressure reaching the boiler drops too low — perhaps because the isolation valve is only partially open or there is a wider supply interruption — the burner lights but cannot sustain the flame, leading directly to an F29 lockout. This is the single most common trigger for this fault code.
- Dirty or faulty ionisation electrode Common
The ionisation electrode is a small probe that continuously monitors flame presence by detecting electrical conductivity through the flame. Carbon deposits, corrosion, or physical misalignment mean it sends a weak or absent signal to the PCB, which then treats this as a flame loss — even if combustion is actually occurring — and shuts the boiler down.
- Blocked or restricted flue terminal Sometimes
A partially blocked flue — caused by a bird's nest, windblown debris, or frozen condensate at the terminal — creates back-pressure inside the combustion chamber. This starves the flame of oxygen and extinguishes it during operation, producing an F29 lockout.
- Fan or air pressure switch fault Sometimes
The combustion fan must run at the correct speed to draw fresh air in and expel exhaust gases. If the fan is running slowly, is seized, or the air pressure switch that monitors airflow is defective, the boiler detects a combustion anomaly and cuts the flame as a safety precaution. Unusual noise at start-up or repeated early lockouts can point to this cause.
- Dry or blocked condensate siphon Sometimes
On condensing models such as the Isomax Condens and Thema Condens, a condensate siphon traps exhaust gases. If this siphon dries out or becomes blocked, flue gases can flow back towards the burner, disrupting the flame signal and triggering F29.
- Wiring or connection fault on the flame detection circuit Rare
Oxidised terminals or damaged wiring between the ionisation electrode and the PCB can interrupt the flame detection signal intermittently. The board registers a flame loss even though combustion is taking place normally, causing unpredictable F29 lockouts.
- Faulty PCB Rare
If the main control board itself develops a fault, it may misinterpret signals from the ionisation electrode or send incorrect commands to other components, producing an F29 error that does not correspond to a genuine flame loss. This is generally only suspected after other causes have been ruled out.
How to fix it
- Check your gas supply is live DIY safe
Confirm the gas isolation valve on the pipe feeding the boiler is turned fully open (handle in line with the pipe). Then test another gas appliance such as a hob or cooker. If that appliance is also not working, contact your gas supplier as there may be a network interruption — do not attempt any further boiler work.
- Look for obvious blockages at the flue terminal DIY safe
Go outside and visually inspect the flue terminal — the pipe where exhaust exits the building, typically on an external wall or roof. Look for visible obstructions such as leaves, a bird's nest, or ice. Do not dismantle any flue components. If you can safely remove loose debris without tools, do so; otherwise note it for the engineer.
- Top up the condensate siphon (condensing models only) DIY safe
On condensing Saunier Duval models, the condensate siphon is usually located at the base of the boiler casing. Unscrew the filling cap, rinse the internal chamber with clean tap water, and refill to the level marked on the siphon body. This prevents exhaust gases tracking back to the burner. If you are unsure whether your model has one, check your user manual.
- Reset the boiler DIY safe
Press and hold the reset button (often marked with a flame or reset symbol) for approximately 3 seconds, then release. The boiler will attempt a fresh ignition cycle. If it runs normally and the F29 code does not return within a few minutes, the fault may have been a one-off. Limit yourself to two or three resets in total — repeated resets without resolution risk accumulating unburned gas and indicate a deeper fault requiring professional attention.
- Call a Gas Safe registered engineer Gas Safe engineer
If the F29 fault persists after the checks above, a Gas Safe engineer must attend. They will test and if necessary replace the ionisation electrode, check dynamic gas pressure at the valve, inspect and clean the burner, carry out combustion analysis, check the fan speed and air pressure switch operation, inspect internal flue connections, and test or replace the PCB if required. Do not attempt any of these tasks yourself — it is a legal requirement in the UK that gas-related internal boiler work is carried out by a Gas Safe registered engineer.
Parts you may need
- Ionisation / flame detection electrode · from £25
- Ignition electrode (often replaced as a pair) · from £30
- Combustion fan · from £120
- Gas valve · from £150
- Air pressure switch · from £40
- Condensate siphon · from £20
- PCB (main control board) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£350, depending on the underlying cause.
Frequently asked questions
What is the difference between Saunier Duval F28 and F29?
F28 means the boiler failed to ignite at all — no flame was ever established during the ignition attempt. F29 means ignition was successful but the flame then went out during operation. Both cause a safety lockout, but F29 points more specifically to issues that disrupt an established flame, such as a dirty ionisation electrode, a drop in gas pressure mid-cycle, or a flue obstruction, rather than a failure of the ignition components themselves.
Can I reset my Saunier Duval boiler myself when F29 appears?
Yes — one to three resets are reasonable to try. Press and hold the reset button for around 3 seconds and allow the boiler to attempt a full ignition cycle. If the boiler runs normally and the fault does not return, it was likely a temporary blip. However, if the F29 code comes back after a couple of resets, stop and call a Gas Safe engineer. Repeatedly forcing a boiler to attempt ignition when there is an underlying fault is not safe and will not fix the root cause.
How much does it cost to fix a Saunier Duval F29 fault in the UK?
For the most common causes — a worn ionisation electrode or a gas pressure check — most homeowners pay between £100 and £250 including parts and labour. Fan or air pressure switch faults typically bring the cost up to around £200–£350. A gas valve replacement sits in a similar range. A PCB replacement is more expensive and can reach £350–£500 or more, though it is rarely the first diagnosis. If you are in any doubt whether repair is worthwhile on an older boiler, ask the engineer to assess whether replacement would be more cost-effective.
How do I stop the F29 fault from coming back?
The most effective prevention is an annual boiler service carried out by a Gas Safe engineer. This includes cleaning the burner and checking the ionisation electrode, which are the two most common triggers for F29. It is also worth inspecting the flue terminal each autumn before the heating season to clear any debris, and keeping the area around the condensate siphon clean on condensing models. Proactive replacement of the ionisation electrode every five to seven years — or whenever the engineer notices wear during a service — can significantly reduce the likelihood of repeat lockouts.