Why it happens
Condensing boilers take extra heat out of the flue gases, so they leave the flue cooler and carry water vapour. In cold air that vapour condenses into a visible white plume, like your breath on a cold day. It looks like steam but is mostly water vapour, and it is most visible in cold, damp weather when the boiler is working hard.
What is normal
A white, wispy plume that fades within a short distance, appears only while the boiler runs and is mainly seen in cold weather is normal. Some dripping of condensation around the flue can also happen in the cold.
What is not normal
Black or brown staining or soot around the flue, a yellow or orange flame in the boiler, a smell, or the boiler repeatedly shutting down are not normal. If a carbon monoxide alarm sounds, leave the property and see our guide to carbon monoxide and gas appliance safety.
Ice, snow and blocked flues
In very cold weather, ice can build up around the flue terminal, and heavy snow can cover it. A blocked flue is a safety risk. Do not poke at it or chip at it, and call a Gas Safe engineer if you think it is blocked.
Checks at your service
The flue is part of what an engineer inspects at an annual service, as listed in what is included in a boiler service. If the plume is a nuisance to a neighbour, an engineer can advise on options.
Not sure your boiler flue is right? Book a Gas Safe boiler service.
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