Viessmann F5 Fault

Viessmann F5 Fault

The Viessmann F5 fault code typically indicates an issue with the boiler’s flame signal. Specifically, it suggests that the boiler is experiencing repeated flame failures during operation. This condition can be caused by several factors affecting the ignition process or flame stability, making it crucial to identify and resolve the underlying problem to ensure the boiler operates safely and efficiently.

Causes of the F5 Fault Code

  1. Gas Supply Problems: Inconsistent gas pressure or interruptions in the gas supply can prevent the burner from maintaining a stable flame.
  2. Faulty Ignition Electrodes: Worn, misaligned, or dirty ignition electrodes may struggle to ignite the gas consistently, leading to flame failures.
  3. Blocked Burner: A dirty or blocked burner can impede gas flow, affecting the stability of the flame.
  4. Flame Sensor Issues: If the flame sensor is faulty or covered in debris, it may not detect the flame properly, causing the boiler to shut down repeatedly.
  5. Air Supply or Flue Issues: Adequate airflow is essential for combustion. Blockages or leaks in the air supply or flue system can disrupt the flame.

Troubleshooting and Resolving the F5 Fault Code

  1. Check the Gas Supply: Verify that the gas supply is consistent and the pressure is within the recommended range. You may need a Gas Safe registered engineer to assess and adjust the gas pressure if it’s found to be lacking.
  2. Inspect and Clean Ignition Components: Examine the ignition electrodes for signs of wear or damage. Cleaning or realigning the electrodes, as well as ensuring the burner is clean and unobstructed, can help resolve flame issues.
  3. Clean the Flame Sensor: Remove any buildup on the flame sensor carefully. If the sensor is faulty, it should be replaced by a professional.
  4. Examine the Air Supply and Flue System: Ensure there are no blockages or leaks in the air intake or flue that could affect combustion. This might require inspection by a professional to ensure compliance with safety standards.
  5. Reset the Boiler: After addressing any identifiable issues, reset the boiler to clear the fault code and attempt to restart. If the F5 fault persists, further professional diagnosis is needed.
  6. Professional Assessment: Persistent flame failures indicated by the F5 code should be investigated by a Gas Safe registered engineer. They can perform a comprehensive assessment, addressing complex issues with the gas valve, control board, or other components that might be contributing to the problem.


The F5 fault code on a Viessmann boiler signals a serious concern with flame stability that can impact the safety and functionality of your heating system. While basic troubleshooting can be attempted by the homeowner, most causes of an F5 fault require professional diagnosis and repair to ensure safe, efficient operation. Regular maintenance by qualified professionals can help prevent such issues by keeping the boiler and its components in optimal condition.