Vaillant VU 446/5-5 / VU 606/5-5 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 VU 446/5-5 / VU 606/5-5. Check the model on the data plate before you order parts.
Other Vaillant VU 446/5-5 / VU 606/5-5 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