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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: February 7, 2024. Updated: May 17, 2024

F83 Vaillant Fault Code – How to fix

The Vaillant F83 fault code indicates an issue with the boiler flow temperature in the flow and return pipes.

Repair costs vary. Repressurising is potentially a DIY fix, while most causes necessitate a Gas Safe registered engineer.

Understanding F83

The F83 fault code signifies a problem with the temperature regulation in the heating system. This can stem from various issues ranging from insufficient water in the system to thermistor calibration errors. This fault is crucial as it impacts the boiler’s efficiency and safety.

Common Causes of F83

  • Low Water Pressure: Insufficient water can lead to inaccurate thermistor readings, often compounded by leaks or natural pressure decreases over time.
  • Low Heat Input: Adequate heat input is essential for the heat exchanger to function correctly.
  • Thermistor Recalibration: Incorrectly calibrated NTC thermistors (sensors) may provide false resistance readings, affecting temperature regulation.
  • Connection Issues: Faulty wiring between the PCB and thermistors can disrupt communication, leading to the F83 error.

DIY vs Professional Repair

  • Low Pressure: Homeowners might manage to repressurise the boiler or bleed radiators as a quick fix.
  • Other Causes: For issues like leaks, low heat input, or thermistor problems, professional intervention is necessary. Attempting DIY repairs on gas appliances without the proper qualifications poses significant risks to safety. It is illegal to work on gas appliances without being Gas Safe registered.

Costs Involved

The cost of resolving an F83 fault varies based on the specific issue and location within the UK. Labour fees for heating engineers range from £50 to £80 per hour, with potential additional costs for parts and specific repairs. Here’s a breakdown of estimated costs for various fixes:

  • Leak Repairs: £50 to £160
  • Heat Exchanger Repairs: £100 to £500
  • Thermistor Recalibration: £50 to £240
  • Wiring Issues: £70 to £350 (requiring an electrician)
  • Heat Exchanger Replacement: £600 to £1,200
  • Increasing Gas Flow Pressure: £60 to £350
  • PCB Replacement: £300 to £900
  • New Boiler Replacement: £2,100 to £3,400


The F83 error highlights a significant issue within the Vaillant heating system. Understanding the potential causes and associated costs can guide you towards the most appropriate course of action.

Professional advice and services are crucial in ensuring safe and efficient repairs, particularly when dealing with complex systems like modern boilers.

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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