The short answer: Vaillant’s F28 code means the boiler tried to start but could not ignite. F29 means the flame lit and then cut out during operation. Both are gas or ignition faults, so they need a Gas Safe registered engineer. If your boiler also keeps losing pressure, water is escaping somewhere, which is what we find.
Few things are more frustrating than a boiler that refuses to behave. You ask for heating or hot water, the boiler clicks and whirrs, and instead of warmth you get a code on the display. For Vaillant owners, F28 and F29 are two of the most common culprits, and they always seem to appear on the coldest morning of the year.
This guide explains what each code means, the safe checks you can carry out yourself, and who to call. We will be upfront from the start: these are ignition faults, so putting them right is a job for a Gas Safe registered engineer rather than a leak detection company. There is, however, one closely related problem we deal with every week across Devon, and we cover that further down.
What does the F28 fault code mean on a Vaillant boiler?
F28 appears when your Vaillant tries to start but cannot ignite. The boiler runs through its ignition sequence, fails to detect a flame, tries again, and after several failed attempts it locks out and shows F28 on the display. In plain English, the boiler asked for a flame and never got one.
You will usually notice it first thing in the morning. The heating stays off, the radiators stay cold, and the display shows F28 instead of the usual reading. The boiler will not run again until the fault is cleared, either by a successful reset or by an engineer putting the cause right.
The code appears across the Vaillant range, including the popular ecoTEC models. Wording and behaviour vary slightly between models and production years, so always check your boiler’s manual for the explanation that applies to your particular appliance.
Common causes of an F28 fault
Most F28 lockouts trace back to one of these:
- A gas supply problem, such as an interrupted supply, a partly closed emergency control valve or low gas pressure
- A prepayment meter that has run out of credit
- A frozen condensate pipe in cold weather, which forces the boiler to shut down for safety
- Worn, dirty or poorly positioned ignition electrodes
- A blocked or obstructed flue
Some of these are simple supply issues you can rule out yourself. The rest involve gas components, and checking those is strictly a job for a Gas Safe registered engineer.
What does the F29 fault code mean?
F29 is F28’s close relative. Instead of failing to light, the boiler ignites successfully and then loses the flame while it is running. The burner drops out, the boiler tries to relight, and if the flame keeps failing it locks out and displays F29.
The two codes often share causes. An unstable gas supply, a frozen condensate pipe, a flue problem or tired ignition components can cut a flame out mid-operation just as easily as they can stop one being established in the first place.
Here is the difference at a glance:
| Fault code | What happens | What you notice |
|---|---|---|
| F28 | No flame is detected during the ignition phase, so the boiler locks out | The boiler will not start at all |
| F29 | The flame lights but is then lost during operation | Heating or hot water cuts out while the boiler is running |
Safe checks you can make before calling anyone
There is no harm in ruling out the simple explanations first. None of the following involves opening the boiler casing, which you should never do yourself.
Five safe checks for F28 and F29:
- Check whether other gas appliances in the home, such as a gas hob, are working normally
- If you have a prepayment meter, make sure it has credit
- Confirm the gas emergency control valve, usually next to the meter, is fully open
- In freezing weather, check the condensate pipe (the plastic pipe running outside). If it has frozen, thawing it gently with warm water, never boiling, often clears the lockout
- Try one reset, following the procedure in your boiler's manual, and only one
If the code comes straight back after a reset, stop there. Repeated resets will not cure the underlying fault, and forcing a boiler that cannot hold a stable flame to keep trying is not sensible.
One important safety note. If you smell gas at any point, do not use electrical switches or naked flames. Open doors and windows and call the National Gas Emergency Service on 0800 111 999 straight away.
Why F28 and F29 are a job for a Gas Safe engineer, not us
We will be completely straight with you here, because plenty of websites are not. F28 and F29 are gas and ignition faults. Diagnosing and repairing them means working on gas components, and in the UK that work must legally be carried out by a Gas Safe registered engineer.
We are leak detection specialists. We do not repair boilers, we do not work on the gas side of any appliance, and we would be doing you a disservice if we pretended otherwise. If your Vaillant is showing F28 or F29 and the checks above have not cleared it, your next call should be to a Gas Safe registered engineer. The Gas Safe Register lists qualified engineers in your area.
So where do we fit in? There is one boiler complaint that lands with us constantly, and it is often tangled up with fault codes like these.
Boiler keeps losing pressure? That is the part we help with
Alongside fault codes, the classic Vaillant complaint is pressure that will not stay put. The gauge drops, the boiler grumbles, you top it up using the filling loop, and a week or two later you are doing it all over again.
Topping up is straightforward on most sealed systems. You open the filling loop until the gauge sits at around 1 to 1.5 bar with the system cold, then close it fully. Your boiler’s manual gives the exact procedure and the correct pressure for your model, so check it before you start.
Here is the part that matters. A sealed heating system should hold its pressure. If it keeps dropping after you repressurise, the water is going somewhere. Sometimes the cause is a boiler component, such as a tired expansion vessel or a pressure relief valve letting by, which is engineer territory again. Very often, though, it is a hidden leak in the heating circuit itself: a weeping joint under the floorboards, a corroded pipe buried in the screed, or a slow drip inside a wall that never shows on the surface.
That is exactly what we find. Our central heating leak detection service uses non-destructive methods such as thermal imaging, acoustic listening equipment and tracer gas to pinpoint hidden heating leaks without pulling up floors on a hunch. We cover homes and businesses across Devon, with same-week appointments often available.
If your engineer has cleared the ignition fault but the pressure keeps falling, or you have been quietly topping up for months, that is the moment to give us a ring.
The bottom line
F28 means your Vaillant could not ignite. F29 means it lit and then lost the flame. Run through the safe checks, try one reset, and if the code returns, book a Gas Safe registered engineer. Treat regular pressure top-ups as a separate warning sign that water is escaping from the system somewhere, because finding that leak is our job. You can read more plain-English guides on our articles page.
Frequently Asked Questions
Boiler keeps losing pressure?
If you are topping up the system every week, water is escaping somewhere. We trace hidden heating leaks across Devon with minimal disruption. Same-week appointments often available.