Short answer
A flue fault means combustion products are not leaving safely. It may show as an air pressure or flue sensor fault, high CO, or a smell of fumes. Flue faults are a carbon monoxide risk and are treated as safety defects.
Common causes
Blocked terminal from birds, debris or ice.
Disconnected or leaking joints and seals.
Incorrect gradient causing condensate pooling.
Plume re-entering air intakes.
Undersized flue for the boiler output or cascade.
What the engineer checks
Visual inspection of the whole flue route.
Flue gas analysis and spillage tests.
Check pressure switch or flue sensor readings.
Repair and prevention
Repair joints, clear blockages, correct gradient and terminal position. Unsafe flues must be made safe and the boiler isolated until repaired.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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