This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Vaillant F.29 fault codewhat it means and how to fix it
The boiler lit correctly and then lost the flame while it was running. It has shut the gas off and locked out, and it needs an engineer.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Vaillant F.29
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
How long does the boiler run before it cuts out?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
It fires and dies within a few seconds
The flame is establishing and then being lost almost immediately. Gas supply under load and flame sensing are the usual candidates.
Do not keep resetting. Book a Gas Safe registered engineer and mention that it lights before it fails — that detail narrows the diagnosis considerably.
It runs for a minute or two and then locks out
That timing matches condensate backing up. The boiler starts producing condensate within a minute of firing, and a blocked pipe fills fast.
If the external condensate pipe is white plastic and it is cold outside, thawing it with warm water may clear it. If not, it needs an engineer.
It only happens when the hob or another gas appliance is on
The supply cannot maintain pressure with two appliances drawing at once, which usually means an undersized or partially obstructed gas run.
This needs a working gas pressure test at the appliance. An engineer will measure it in minutes.
Anyone in the house has headaches, nausea or drowsiness
Treat this as a carbon monoxide emergency until proven otherwise. Combustion that keeps failing can produce CO.
Open the windows, turn the boiler off, leave the property, and call the National Gas Emergency Service on 0800 111 999.
What F.29 means
F.29 differs from F.28 in an important way: the boiler did light. The flame established, then went out during the burn, and the flame sensing electrode stopped seeing it. The control board immediately closes the gas valve.
A flame that establishes and then fails points at something that changes once the appliance is running — gas pressure sagging under load, a flue problem developing as the fan comes up to full speed, or condensate backing up as the boiler starts producing it. Because it involves live combustion behaving unpredictably, F.29 is treated as a danger-severity fault. There is nothing safe for a householder to investigate here beyond checking whether the condensate pipe has frozen.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Interrupted or inadequate gas supply | Pressure that holds at rest but drops once the burner is at full rate. Often shows up when the hob is on at the same time. | Common |
| Blocked or frozen condensate pipe | The boiler produces condensate as soon as it fires. If the pipe is blocked, it backs up into the appliance within a minute or two of lighting — which is exactly when F.29 appears. | Common |
| Flue blockage or recirculation | Products of combustion re-entering the air intake starve the flame of oxygen. This is the cause that carries a carbon monoxide risk. | Occasional |
| Contaminated flame sensing electrode | The electrode detects the flame by ionisation current. Carbon deposits reduce that current until the board decides the flame has gone out. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£130–£480
Time on site
1h 30m
Thawing a condensate pipe is free if you can reach it. Electrode or gas valve work runs £130–£420. A flue investigation is £120–£250, and rectifying a badly installed flue can be considerably more.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant ecoTEC ignition and sensing electrode
0020039221£19–£46Authorised Vaillant parts distributor
Price checked 28 September 2026
Condensate trap and siphon
COND-TRAP£15–£38Plumbing merchant
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.28Stop — call an engineerThe boiler tried to light and failed every attempt it is allowed — three on an ecoTEC, five on the 2022 CS/1-5 — so it has shut the gas off and locked out.Related: ignition and flame
- F.27Stop — call an engineerThe boiler is detecting a flame when the burner should be off. Switch it off and get a Gas Safe engineer out rather than resetting it.Related: ignition and flame
- F.68Get it checkedThe flame is lighting but its signal is unstable. Vaillant's causes run through the gas supply, the air ratio, the condensate route and the flue, ending with the electrode and the board.Related: ignition and flame
- F.87Get it checkedThe ignition transformer, or on the ecoFIT pure the electrodes, are not connected as the board expects. A wiring fault on the ignition side; the boiler will not attempt to light.Related: ignition and flame
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: same brand
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: same brand
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.29
What is the difference between F.28 and F.29?
F.28 means the boiler never lit. F.29 means it lit and then lost the flame. That distinction matters: F.29 points at something that changes once the burner is running, such as gas pressure under load, a flue problem, or condensate backing up. Tell the engineer which one you have.
Can I reset F.29?
Once, to confirm the fault is repeatable. After that, leave it. This code involves combustion failing while live, and repeatedly forcing an appliance back into that state is not a risk worth taking to save a call-out.
Could this produce carbon monoxide?
Where the cause is a flue blockage or recirculation, yes. Incomplete combustion is exactly how CO is produced. If anyone in the property has unexplained headaches, drowsiness or nausea that improves when they leave the house, treat it as an emergency.
Does a frozen condensate pipe really cause this?
Frequently, in the first hard frost of the season. The pipe is often the cheapest possible run in 21mm white plastic straight out through the wall, and once it ices up the boiler has nowhere to put the condensate it starts making within a minute of firing.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Trade body guidance
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.