Vaillant F.28 and F.29: two ignition faults with one owner check
F.28 is no flame detected during ignition. F.29 is a failed re-ignition after flame loss. Both point first at gas supply and a frozen condensate pipe.
What this means
Vaillant defines F.28 as the flame signal not being detected during the ignition phase, and F.29 as an unsuccessful re-ignition after a flame loss during operation. The codes describe when the flame went missing, not what removed it, and Vaillant lists insufficient gas, a blocked condensate pipe, and a faulty ignition electrode among the causes of both.
Equipment and model context
Vaillant ecoTEC gas boilers covered by the published United Kingdom fault code list
- Vaillant ecoTEC plus
- Vaillant ecoTEC pro
- Vaillant ecoTEC sustain
These meanings come from Vaillant's own United Kingdom fault code list for its gas boilers and are written with a full stop between the letter and the number. Other manufacturers use F28 and F29 for entirely different conditions, and a code read from a non-Vaillant appliance does not carry these definitions.
What you may notice
- The display shows F.28 or F.29 and the boiler stops producing heat and hot water.
- Bubbling or gurgling comes from the boiler or the pipe running outside before the code appears.
- The code appears during a cold spell after the boiler has worked without complaint.
Likely causes
- Vaillant lists insufficient gas, a blocked condensate pipe, and a faulty ignition electrode as causes of F.28.
- Vaillant lists a gas supply interruption, a flue blockage, and earthing issues under F.29.
- Vaillant publishes F.54 as a gas supply fault appearing in combination with F.28 or F.29.
- Vaillant attributes one cause to ice forming in the pipe that carries away the liquid produced by combustion.
| Code | Vaillant's published meaning | What it implies about the burner |
|---|---|---|
| F.28 | No flame signal during the ignition phase | The burner never established a flame |
| F.29 | Re-ignition after flame loss was unsuccessful | The burner was alight and then went out |
| F.54 | Gas supply fault, in combination with F.28 or F.29 | The supply itself is implicated |
| F.27 | Flame signal detected while the burner is off | The opposite condition, and not an ignition fault |
| F.35 | Air or flue pipe blocked | Combustion air or flue path rather than gas |
Diagnostic paths
It is freezing outside and the pipe is gurgling
The code appeared during cold weather, and there is a bubbling or gurgling sound from the boiler or from the plastic pipe running out through the wall.
Vaillant names a frozen condensate pipe as a cause a homeowner can address. Pour warm, not boiling, water along the affected length from a watering can until the ice clears or the pipe drains, then reset the boiler as its manual describes.
Other gas appliances have also stopped
The gas hob will not light, or a prepayment meter is showing no credit.
The fault is upstream of the boiler. Restore the supply or the credit, then reset. Vaillant publishes F.54 for a gas supply fault appearing alongside F.28 or F.29, which is the same conclusion reached from the display.
The condensate pipe is clear and the gas is on
There is no ice, other gas appliances work, and the code returns after a reset.
Stop resetting. Vaillant attributes the remaining causes to the ignition electrode, the gas valve assembly, the flue, or the circuit board, all of which need a Gas Safe registered engineer.
F.29 appears mid-cycle rather than at startup
The boiler starts normally, runs for a period, and then locks out with F.29.
A flame that establishes and is then lost points at supply pressure under load, the flue, or earthing rather than at the ignition attempt. Note how long the boiler ran before the lockout and pass that figure on.
The two codes describe two moments, not two components
Vaillant's wording is precise and worth reading literally. F.28 is the flame signal not being detected during the ignition phase; F.29 is the ignition following a flame loss during operation being unsuccessful. Both are reports about the flame signal, and neither names a part.
That is why the same cause can produce either code. A gas supply that is marginal rather than absent may allow ignition on a mild day and fail under a heavy load, which produces F.29, while the same supply on a colder day fails at the first attempt and produces F.28.
Both codes therefore share a first response. Check whether gas is reaching the appliance and whether the condensate pipe is clear, because those two questions cover the causes an owner is permitted to address on either code.
Why a frozen pipe stops a boiler that has plenty of gas
A condensing boiler recovers heat from its flue gases, and the water vapour in those gases condenses in the process. That liquid has to leave the appliance, which is what the condensate pipe is for, and in United Kingdom installations that pipe frequently runs outside.
When it freezes, the condensate has nowhere to go and backs up inside the appliance. The control responds by shutting the burner down, and Vaillant lists a blocked condensate pipe among the causes of F.28 for exactly that reason.
The gurgling or bubbling that people report before the lockout is the sound of combustion gases passing through standing condensate that cannot drain. It is the most useful early warning the appliance gives, and it appears before the code does.
Thawing the pipe, and the limits Vaillant sets
Vaillant's published method is to pour warm water along the affected length of pipe from a suitable container such as a watering can, continuing until the ice is dislodged or the contents of the pipe have drained. The instruction not to use boiling water is explicit, because thermal shock can crack a plastic pipe.
Once the pipe drains, the boiler is reset according to its own manual. Vaillant then recommends insulating the pipe with commercially available pipe insulation to reduce the chance of it happening again.
The limit is the pipe itself. This procedure applies to accessible pipework at ground level. Nothing about it extends to working at height, to opening the appliance, or to any part of the gas or flue system.
When to stop resetting
A reset that clears the code and produces normal operation for a week has diagnosed a transient condition. A reset that clears the code and is followed by the same lockout within hours has diagnosed nothing, and each further attempt repeats an ignition sequence that has already failed.
Vaillant's remaining causes for these codes are the ignition electrode, the gas valve assembly, the flue system, and the printed circuit board. None of them is accessible without opening the boiler casing, which Vaillant's general guidance tells owners not to do.
The gas smell case sits outside all of this. It is not a fault code question and not a service call. Leave the property and telephone the National Gas Emergency Service on 0800 111 999, which is the number Vaillant publishes alongside its own code list.
The flame signal that F.28 and F.29 report on depends on every item in this list. The code says the signal was absent; it does not say which item was missing.
- Gas at the applianceSupply, meter credit, and isolation valve position
- A clear condensate pathFrozen or blocked, the burner is shut down
- A working ignition electrodeProduces the spark and, on many designs, senses the flame
- A clear flueVaillant publishes F.35 separately for a blocked air or flue pipe
What to give the technician
State the exact code with its full stop, whether it was F.28 or F.29, whether the boiler ran before locking out and for how long, whether other gas appliances work, whether the condensate pipe was frozen and whether thawing it changed anything, and the outdoor conditions at the time. Add the model name printed on the boiler badge and when it was last serviced.
Safe homeowner checks
- Check whether other gas appliances in the property work and whether a prepayment meter has credit.
- Look and listen for a gurgling or bubbling sound at the boiler or the external condensate pipe.
- Thaw an accessible frozen condensate pipe with warm, not boiling, water poured along its length.
- Reset the boiler once according to its own manual after the condensate pipe has drained.
- Insulate the condensate pipe afterwards, as Vaillant recommends, to reduce the chance of a repeat.
Is a reset appropriate?
Vaillant's own F.28 guidance directs owners to the reset instructions in the boiler manual and gives no permitted number of attempts. Treat one reset after removing an identified cause as the whole of the allowance. Repeated resets with the cause still present achieve nothing and remove the record of when the fault appeared.
When to call a technician
- The same code returning after one reset with the gas supply confirmed and the condensate pipe clear.
- Any suspicion of an ignition electrode, gas valve, flue, or circuit board fault, all of which need the casing opened.
- A condensate pipe that is not reachable from ground level.
- F.54 shown alongside F.28 or F.29, which Vaillant attributes to the gas supply.
- The smell of gas, which takes priority over every code on this page and is answered by evacuating and dialling 0800 111 999.
Questions people ask about this
What does F28 mean on a Vaillant boiler?
Vaillant defines F.28 as the flame signal not being detected during the ignition phase. The boiler attempted to light and did not confirm a flame, and the code reports that absence rather than naming the component responsible.
What is the difference between F28 and F29?
F.28 is a failure to establish a flame during ignition. F.29 is an unsuccessful re-ignition after a flame was lost during operation, so the burner had already been alight when the fault occurred.
Can a frozen condensate pipe cause F28?
Yes. Vaillant names a blocked condensate pipe among the causes and publishes a homeowner procedure for thawing it, because condensate that cannot drain backs up inside the appliance and the control shuts the burner down.
Can I pour boiling water on a frozen condensate pipe?
No. Vaillant instructs owners to use warm water and states that boiling water can crack the pipe. A watering can of warm water poured along the affected length is the published method.
How many times should I reset a boiler showing F28?
Once, after removing a cause you have identified such as a frozen pipe or a gas supply interruption. If the code returns with those checks already done, the remaining causes need a Gas Safe registered engineer.
Does F28 mean my boiler needs replacing?
Nothing in Vaillant's published causes implies that. The list runs from a frozen condensate pipe and a gas supply interruption through to the ignition electrode, gas valve, flue, and circuit board, which are diagnoses rather than condemnations.
Evidence recordHow this page was checked
official manufacturer support article · checked 2026-09-08
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Vaillant UK and Baxi technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article
- Scope of the definition
- Vaillant ecoTEC gas boilers covered by the published United Kingdom fault code list
- Last checked
- 2026-09-08