What the F29 code means
On most ecoTEC models, F29 means the flame was lost during operation. The difference from F28 is timing. With F28 the boiler never manages to light. With F29 it lights normally, runs for a while, and then the flame sensor stops detecting a flame. The boiler will usually try to relight a few times, and if it cannot, it locks out and shows F29.
Because the boiler has already proved it can light, F29 often points to something that changes while it is running: the gas supply dropping away, exhaust gases getting drawn back in, or an electrical problem with how the flame is sensed. Some causes are easy to check. Others need a Gas Safe registered engineer.
It helps to note when the fault happens. If F29 appears only when a hot tap is running, only on windy days, or only when another gas appliance is in use, write that down. Patterns like these narrow down the cause quickly and can cut the time, and the cost, of an engineer visit.
Common causes of F29
Gas supply interrupted
If gas stops flowing while the boiler is running, the flame goes out. A prepayment meter running out of credit is a classic cause. So is a problem with the supply to the property, or another gas appliance drawing heavily at the same time on a system with low gas pressure.
Flue gas recirculation
If exhaust gases from the flue are drawn back into the air intake, the flame can struggle and go out. This can happen when the flue is partly blocked, damaged or poorly positioned, or in certain wind conditions. Flue checks are an engineer job.
Earthing or electrical problems
The boiler senses the flame through a small electrical current. A poor earth connection, or a problem with the wiring to the boiler, can stop that signal reading correctly, so the boiler thinks the flame has gone.
Worn or dirty electrode
On many ecoTEC models the same electrode handles ignition and flame sensing. If it is worn, dirty or out of position, it may light the boiler but then lose the flame signal.
Condensate problems
A blocked or frozen condensate pipe can cause water to back up inside the boiler, which on some models disturbs the burner and leads to lockouts during operation as well as on start-up.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Gas supply interrupted | Hob also cuts out or will not light | You (credit), or gas supplier |
| Frozen condensate pipe | Freezing weather, gurgling sounds | You, by thawing the pipe |
| Flue gas recirculation | Worse in windy weather, flue damaged | Gas Safe engineer |
| Earthing fault | Random lockouts, sometimes after electrical work | Gas Safe engineer or electrician |
| Worn or dirty electrode | Lights, then cuts out, worsening over time | Gas Safe engineer |
Safe checks you can do yourself
- Check whether your hob or other gas appliances work. If they do not, the problem is the gas supply.
- If you have a prepayment meter, check the credit and top up if needed.
- Check that the gas isolation valve under the boiler is open, with the handle in line with the pipe.
- In cold weather, check the condensate pipe outside for ice and thaw it with warm (not boiling) water. Our frozen condensate pipe guide explains how.
- Check the fused spur and power supply to the boiler, especially if there has been recent electrical work in the house.
- Press reset as your manual describes. Stop after two or three attempts if the code returns.
If you smell gas at any point, do not press reset. Turn off the gas at the meter if it is safe, open doors and windows, leave the property and call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
If your gas supply is fine and F29 keeps returning, call a Gas Safe registered engineer. They can check the gas pressure at the boiler while it runs, inspect the flue and its terminal, test the earth and wiring, and look at the electrode and burner. They can also read the boiler's fault history, which often shows a pattern.
If you have had electrical work done recently, such as a new consumer unit or rewiring, and F29 started soon afterwards, mention this when you book. Earthing problems introduced during other work are a known cause of flame sensing faults, and they are usually quick to put right once found.
Do not remove the casing, adjust the flue or touch the electrode. Flue faults in particular carry a risk of carbon monoxide, so they must be checked properly. Make sure you have a working carbon monoxide alarm in the same room as the boiler.
Typical repair costs in the UK
F29 repairs range from nothing at all to a new gas valve. These typical ranges give a rough idea, but prices vary by region and engineer.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Electrode replacement | Typically £100 to £180 |
| Flue repair or adjustment | Typically £100 to £300 |
| Earthing or wiring fault | Typically £80 to £200 |
| Gas valve replacement | Typically £250 to £450 |
How to stop F29 coming back
Keep prepayment credit topped up so the supply never cuts out mid-use. Keep the area around the flue terminal outside clear of plants, bins and anything else that could affect airflow. An annual service is the best way to catch a worn electrode or a flue problem early, and our boiler service cost guide explains what to expect. If the fault appears alongside no hot water but working heating, our no hot water guide may also help.