Viessmann Vitodens 100-W fault 63

What it means

Burner is placed in a fault state because the flue gas temperature limiter has responded, indicating excessive flue gas temperature.

Likely causes

1. Low heating system fill level
2. Incorrect pre-charge pressure in diaphragm expansion vessel
3. Insufficient flow rate (circulation pump)
4. 3-way diverter valve malfunction
5. Flue system leak causing flue gas recirculation
6. Incorrect gas flow pressure
7. Flue gas leakage
8. Blocked flue system
9. Simultaneous operation of boiler and air-extracting appliances
10. Negative pressure created by extractor hoods, extractors, air conditioning units, or central vacuum cleaning systems
11. Accumulation of condensate blocking flue system
12. External causes sealing the flue system
13. Inadequate supply of combustion air
14. Building modifications affecting cable/pipework routing, cladding or partitions

Where to start

1. Check heating system fill level
2. Check pre-charge pressure in diaphragm expansion vessel and adjust to required system pressure
3. Check whether flow rate is sufficient (circulation pump)