Flue Condensate Blocked / Siphon Full
The condensate siphon inside the boiler is full or blocked, preventing the boiler from starting. Common after long periods of inactivity or in freezing weather.
FA in winter often means the condensate pipe or siphon is frozen or blocked with ice. Thaw the external pipe and wait 15 minutes for the boiler to self-clear.
Step-by-Step Fix Guide
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Step 1
The boiler is programmed to run at low burn for up to 15 minutes to fill the condensate siphon — this is normal after being off
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Step 2
If FA persists beyond 15–20 minutes, the condensate trap may need cleaning
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Step 3
In cold weather, check the external condensate pipe for freezing
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Step 4
If you cannot identify the issue, call a Gas Safe engineer
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.