Short answer
Flame loss during operation is the boiler losing flame signal after it has been running normally. Because it is intermittent, it is usually caused by something that changes with time or load: gas pressure falling as other plant starts, flue conditions changing, condensate building up, or a sensor that degrades as it heats.
Common causes
Supply pressure drop when kitchens, other boilers or process plant start.
Wind effects or blockage on the flue and air inlet.
Condensate trap partly blocked, flooding the chamber at high firing rates.
Ionisation probe cracking or shifting position when hot.
Fan speed instability or a failing gas valve.
What the engineer checks
Log gas pressure over a firing cycle with an electronic gauge.
Check the lockout history for time-of-day patterns.
Inspect flue terminal, air inlet and condensate route.
Read the flame signal hot as well as cold.
Repair and prevention
Fix the root cause — commonly the gas supply or condensate route. Where supply pressure is marginal, pipework or meter upgrades may be needed.
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.
Need this done on site?
Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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