What does the Viessmann Vitodens 111-W F78 fault code mean?
The F78 fault code indicates a problem with the domestic hot water (DHW) control signal in the Viessmann Vitodens 111-W boiler. This issue can prevent the boiler from properly heating water for taps and showers, leading to inadequate hot water supply.
Common Causes
- Faulty DHW Sensor: A malfunctioning sensor may fail to send the correct temperature readings to the control system, causing the fault code to trigger.
- Wiring Issues: Damaged or loose wiring connections between the DHW control board and the sensor can disrupt the signal, leading to the F78 error.
Can I fix it myself? (DIY)
It is recommended to call a Gas Safe Engineer for this issue, as it involves electrical components and potential gas safety risks. Attempting to fix it yourself could lead to further complications or safety hazards.
Estimated Repair Cost (2026 UK Prices)
The estimated repair cost for resolving the F78 fault could be around £150 to £250, depending on whether the DHW sensor needs replacement or if wiring repairs are required. This estimate includes parts and labour for a qualified engineer.