Boiler Flue: What It Does and Signs It's Failing
The flue is the twin-wall pipe that runs from the top of the boiler out through a wall or roof. The inner pipe takes combustion gases outside, and the outer one brings fresh air back in for the burner. It ends in a terminal, which must sit set distances from windows, doors and other buildings.
Who should replace it
Technician job
Related codes
5 error codes on this site point to this part.
How it works
The fan pushes the exhaust out and draws air in through the same assembly, so the boiler's burner is sealed off from the room. Condensing flue gases leave at around 55°C (131°F), and in cold air they make a white plume that Worcester says is normal. Joints must stay sealed all the way along.
Signs it is failing
- Soot or staining around the boiler or the terminal
- Yellow or orange flame where it should be blue
- Carbon monoxide alarm going off
- Terminal blocked by a nest, plants, snow or a new structure
- Flue fault, flame-loss or flue gas temperature codes
Flame-loss and flue gas temperature codes can be caused by a blocked or damaged flue. A steady white plume on a cold day isn't a fault.
Can you replace it yourself?
Looking at the terminal from outside is fine; if something is covering or crowding it, tell your engineer rather than poking at it. Never take apart, cut or seal over flue joints; flue work is Gas Safe engineer work (a Registered Gas Installer, RGI, in Ireland). If a carbon monoxide alarm sounds or anyone has headaches, dizziness or nausea that ease outdoors, switch the boiler off, open windows, get everyone outside and call your gas emergency number.
Buying the right part
Flue kits are made for each boiler range and must be the manufacturer's approved kit for your model, fitted to its length limits.
Error codes linked to the flue
- E110 / E130Overheat lockout (primary water or flue)Baxi
- F.29Flame went out and the boiler couldn't relightVaillant
- EA / EA 227Burner didn't light (no flame detected)Worcester Bosch
- F7Flame signal with the burner offWorcester Bosch
- FAFlame still present after the gas shut offWorcester Bosch
Last reviewed 2026-10-21.