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Witten by Bernard Morgan (Engineer).
First Published: January 24, 2024. Updated: February 7, 2024

Vaillant F23 Fault Code – How to fix

The Vaillant F23 error code, indicates a temperature difference issue. This can be caused by a faulty pump, blockage, airlock, or thermistor fault. While some fixes like bleeding radiators are DIY, others like pump or thermistor repairs require a professional. Costs vary based on the repair needed.

Understanding the F23 error code

  • The F23 code appears when the temperature difference between flow and return pipes exceeds 35°C, causing the boiler to shut down.
  • This error can be caused by various factors, including a faulty pump, blockage, airlock, or thermistor fault.

Addressing the F23 error:

  • A faulty pump, indicates a circulation issue. It will require a replacement by a Gas Safe Registered engineer.
  • Airlocks can often be resolved by bleeding radiators, something you may be able to do yourself.
  • Physical blockages might be cleared by flushing radiators (close all but 1 radiator at a time when the boiler is running). A professional power flush may be needed for more extensive sludge removal.
  • Thermistor faults, affecting temperature regulation, must be handled by a qualified professional.

Costs of professional repairs:

  • Charges for a Gas Safe engineer vary by region, typically between £40 to £80 per hour.
  • Specific repair costs can range from £80 for bleeding radiators to £400+ for replacing a faulty pump.
  • Full boiler replacement ranges from £1,950 to £5,000 or more depending on your circumstances.

Boiler warranty and F23 faults:

  • Vaillant boiler warranties generally cover manufacturing defects.
  • Regular servicing by a Gas Safe engineer is required to maintain the warranty.
  • Check with your boiler’s manufacturer if your warranty is active and applicable to the F23 fault.

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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