Vaillant Gas boiler fault F.25
Safety shutdown: flue gas temperature too high
Gas boilerWhat it means
Safety shutdown due to flue gas temperature exceeding the allowed limit.
Likely causes
- Flue gas safety cut-out plug is loose/not plugged in
- Interruption in the wiring harness
- Leak in flue system
- Flue gas recirculation in extended flue gas installations
- Incorrect flue fall causing condensate to not drain back to boiler
- Condensate pooling in flue leading to blockage or corrosion
- Use of mineral-oil-based grease on flue pipe seals causing seal damage
- Unsuitable or insufficient combustion and room air
- Obstructed air supply to installation room
- Combustion air contaminated with sprays, solvents, chlorinated cleaning agents, paint, adhesives, ammonia compounds, dust or similar substances
- Chemical substances stored at installation site
- Combustion air routed through chimneys previously used with oil-fired boilers causing soot build-up
- Air/flue pipe not completely installed
- Front casing not installed/closed
- Empty condensate trap / siphon allowing flue gas into room air
- Damaged seals due to use of grease instead of water/soft soap during installation
Where to start
- Check the plug and the plug connection.
- Check the wiring harness.
- Check the flue gas installation is intact in accordance with the latest gas safe technical bulletin and installation instructions
The full answer, free
- 13 more checks in order
- The readings and component tests from the manual
- How to put it right, step by step
- The manual page for this fault (page 61), right under the answer
Safety: Isolate gas and power before you open the case. If you have been into the gas side, do a tightness test, and a combustion and flue check before you hand back. Gas Safe (UK) or RGI (Ireland) work only.
From the Vaillant manual for the Gas boiler. Check the model on the data plate before you order parts.
Other Vaillant Gas boiler fault codes
- F.00Flow temperature sensor interruption
- F.01Return temperature sensor interruption
- F.03Cylinder temperature sensor interruption
- F.10Flow temperature sensor short circuit
- F.11Return temperature sensor short circuit
- F.13Cylinder temperature sensor short circuit
- F.22Safety shutdown: water deficiency
- F.23Safety shutdown: temperature spread too great
- F.24Safety shutdown: temperature rise too fast
- F.27Safety shutdown: flame simulation
- F.32Fan fault
- F.33Air pressure switch fault
- F.64Fault: electronics/NTC
- F.65Electronics temperature fault
- F.72Flow and/or return temperature sensor fault
- F.74Water pressure sensor signal outside correct range (too high)
- F.75Pump fault/water deficiency
- F.77Accessory fault (flue non-return flap, condensate pump, etc.)
- F.83Flow and/or return temperature sensor temperature change fault
- F.84Temperature difference fault at the flow and return temperature sensors
- F.85Flow and return temperature sensors installed incorrectly (inverted)
- P.02Minimum load check programme
- S.42Flue non-return flap closed
- S.46Comfort protection: minimum load, loss of flame