Viessmann Vitodens 100-W fault 347

What it means

Burner in a fault state caused by flue gas recirculation, typically due to leaks or back pressure in the flue system.

Likely causes

1. Flue gas recirculation due to flue system leaks or back pressure
2. Flue system leak causing flue gas recirculation
3. Incorrect gas flow pressure
4. Flue gas leakage
5. Blocked flue system
6. Simultaneous operation of boiler and air-extracting appliances
7. Negative pressure created by extractor hoods, extractors, air conditioning units, or central vacuum cleaning systems
8. Accumulation of condensate blocking flue system
9. External causes sealing the flue system
10. Inadequate supply of combustion air
11. Building modifications affecting cable/pipework routing, cladding or partitions

Where to start

1. Check the flue system for leaks.
2. Check the flue system for flue gas back pressure, e.g. caused by insufficient fall, constrictions or blockages.
3. Check the flue system for tightness (flue gas recirculation).