What does the Viessmann Vitodens 100-W F78 fault code mean?
The F78 fault code on the Viessmann Vitodens 100-W indicates a problem with the backup power supply, which is essential for the boiler’s operation. This issue can prevent the boiler from functioning correctly, leading to heating and hot water supply disruptions.
Common Causes
- Power Supply Issues: A failure in the main power supply or fluctuations can lead to backup power problems.
- Faulty Backup Battery: The backup battery may be defective or depleted, failing to provide the necessary power during outages.
Can I fix it myself? (DIY)
It is recommended to call a Gas Safe Engineer for this issue, as it involves electrical components that can be hazardous if handled improperly. Attempting DIY repairs could void warranties or lead to further complications.
Estimated Repair Cost (2026 UK Prices)
The estimated repair cost for addressing the F78 fault could range from £150 to £300, depending on whether the issue is a simple power supply check or requires replacement of the backup battery or other components.