Vaillant F.347 fault codewhat it means and how to fix it
A 2022 ecoTEC plus has detected condensate inside its combustion chamber and stopped the burner. Vaillant's cause is a blocked condensate discharge — the siphon or the pipe — backing up into the boiler.
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What F.347 means
A condensing boiler makes several litres of acidic water a day, and it is supposed to leave through the siphon and the condensate pipe. If it cannot, it rises, and on a CS/1-5 the boiler can tell when it has reached the combustion chamber: F.347 is "condensate has been detected in the combustion chamber; the burner operation is interrupted".
Vaillant give one cause — "condensate in the combustion chamber due to a blocked condensate discharge" — and three steps for the engineer: check and if required clean the condensate discharge pipe including the siphon; check the combustion chamber (electrodes, insulating mats, burner); and if required replace the insulating mats in the combustion chamber. The order tells you what the code protects against. Water sitting around the burner corrodes the electrodes and soaks the insulating mats that line the chamber, and Vaillant's remedy anticipates that the mats may already need changing.
Older Vaillants have no direct detection of this; on a two-digit boiler a blocked condensate route shows up as an unstable flame (F.68) or a failed ignition (F.28), and the CS/1-5 lists it as a cause under those codes too. In winter the blockage is very often ice in an external condensate pipe, and Vaillant's consumer guidance on thawing a frozen pipe applies here as much as to any other code; that is the one householder step this page gives.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Frozen external condensate pipe | The winter cause. A pipe run outside with too small a bore or too little insulation freezes solid and the condensate backs up to the boiler. | Common |
| Siphon or discharge pipe blocked with debris | Vaillant's stated cause. Sludge from the heat exchanger and biological growth in the trap block the route in any season. | Common |
| Condensate pipe run with a rise or a sag | A discharge that does not fall continuously holds water and blocks. Context rather than Vaillant's wording; found when the pipe is inspected. | Occasional |
| Insulating mats and electrodes damaged by condensate | The consequence Vaillant's remedy anticipates. Mats that have been wet may need replacing, and electrodes corroded by condensate may need changing. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
1 hour
Clearing a siphon and pipe is £80 to £150. If the insulating mats and electrodes have to be replaced, £200 to £300. Re-routing or insulating an external pipe is priced on the job.
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
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Related Vaillant fault codes
- F.32Get it checkedThe fan is not running at the speed the boiler expects, so combustion cannot be verified as safe and the boiler has locked out.Related: fan and flue
- F.35Get it checkedThe air intake or flue is blocked. Something is physically obstructing the path combustion products take out of the building.Related: fan and flue
- F.77Get it checkedThe condensate pump or the flue non-return flap has not responded. The boiler cannot clear its condensate or confirm the flue, so it has shut down.Related: fan and flue
- S.32Get it checkedA protective wait, not a lockout: the fan speed has strayed outside tolerance and the boiler is holding off before it tries to start again. If the wait keeps recurring, F.32 is next.Related: fan and flue
- F.33Stop — call an engineerAn air pressure switch fault: the boiler cannot prove the fan is moving air through the flue. Vaillant check the flue route first, so treat it as a flue fault until proved otherwise.Related: fan and flue
- F.334Get it checkedA commissioning code on the 2022 ecoTEC plus: the flue system configuration was never completed in the installer settings. F.336, F.337 and F.338 are its neighbours — a configuration set wrongly.Related: fan and flue
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.347
Is condensate in the combustion chamber dangerous?
It is a fault the boiler has caught early by stopping. The risk is to the boiler — corroded electrodes and soaked insulation — rather than to you. Leave it stopped until the route is clear.
How do I stop it happening every winter?
Ask the engineer about insulating the external run, increasing its bore, or re-routing it to discharge inside. Vaillant's installation instructions set out how a condensate pipe should be run, and a pipe that freezes every year is not meeting them.
Why does my older Vaillant not have this code?
Two-digit ecoTECs have no condensate detection in the chamber. On those boilers a blocked condensate route appears as F.68 or F.28, and the condensate pipe is on Vaillant's cause lists for both.
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
- Manufacturer support portal
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.