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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: January 24, 2024. Updated: May 17, 2024

Vaillant F25 Fault Code – How to fix

The Vaillant F25 fault code indicates a rapid rise in flow temperature. This is often due to blockages, airlocks, or component failures. Some steps like bleeding radiators can be done by homeowners. However, resolving the root cause typically requires a professional.

Understanding the Vaillant F25 fault code

The F25 error on Vaillant boilers is triggered when there’s a quick rise in flow temperature, indicating potential issues in the heating system. This will lead to the boiler shutting down as a safety measure.

Common causes of the F25 error

  • Potential blockages that hinder water circulation, leading to uneven heating.
  • Airlocks in the system can also cause this error.
  • Faulty components, such as limiters or thermistors, can contribute to the temperature imbalance.

DIY checks and fixes

Homeowners can perform basic checks, like bleeding radiators, to remove airlocks. But, these measures might not resolve the issue. This is especially true if it’s related to a component failure or a more complex system problem.

Professional repair and costs

  • Addressing the F25 error often requires a Gas Safe registered engineer.
  • The cost of hiring a professional varies based on the complexity of the repair and regional rates. But it is a necessary investment to ensure safe and efficient boiler operation.

Boiler replacement considerations

  • In cases where the boiler is old, or the repair cost is high, it might be more economical to consider a new boiler replacement. Especially considering ongoing repair costs and new boiler warranties.
  • Replacement can be a significant expense, but finance options are available to help manage the cost.

Warranty coverage

  • Vaillant boiler warranties cover defects related to manufacturing. But, they won’t cover issues arising from misuse or external factors.
  • To maintain warranty validity, regular servicing by a Gas Safe engineer is essential. And a qualified professional must have commissioned the boiler.


If you’re facing a Vaillant F25 fault code, understanding the potential causes and remedies is crucial. While some initial steps can be taken at home, most solutions require professional intervention. Regular maintenance and understanding your warranty can help prevent such issues in the future.

Unfortunately, your boiler may be unrepairable. If this is the case, get a boiler replacement quote for a premium boiler with Boilerhut:

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