Why a condensing boiler makes a plume
Burning natural gas produces carbon dioxide and water vapour. A condensing boiler recovers extra heat from its exhaust by cooling it in the heat exchanger, which is why it is more efficient than older boilers. Some of the water vapour condenses inside the boiler and drains away as condensate. The rest leaves through the flue as warm, moist air.
When that air meets cold outside air, the vapour condenses into tiny droplets, just like your breath on a winter morning. The result is a white plume that can look like steam or smoke. It is usually most noticeable:
- on cold, damp or still days, when the plume hangs around instead of dispersing
- when the boiler first fires up, or while it is working hard to heat the house or hot water
- where the flue points into a sheltered corner, a narrow side passage or under a canopy
Condensing flues are designed to slope back towards the boiler, so condensate runs back into it rather than out of the terminal. A persistent drip from the end of the flue can mean it was not fitted at the right angle, which is worth mentioning to an engineer at the next service.
Normal plume or warning sign?
| What you see | Normal or a warning? | What to do |
|---|---|---|
| White, wispy plume that disappears within a metre or two | Normal | Nothing |
| Heavier white plume in cold or damp weather | Normal | Nothing, or consider a plume kit if it is a nuisance |
| Plume drifting onto a neighbour's window, a path or a car port | Normal, but a nuisance | Ask an engineer about a plume management kit |
| Plume blowing back in through your own window or vent | Warning | Keep the opening closed and call a Gas Safe engineer |
| Dark, grey or black smoke | Warning | Turn the boiler off and call a Gas Safe engineer |
| Soot or black staining around the flue terminal | Warning | Turn the boiler off and call a Gas Safe engineer |
| Smell of fumes or burning, indoors or out | Warning | Turn the boiler off, ventilate and call a Gas Safe engineer |
| Steam or water escaping from the boiler casing or indoor flue joints | Warning | Turn the boiler off and call a Gas Safe engineer |
| Damaged, loose or disconnected flue | Warning | Stop using the boiler and call a Gas Safe engineer |
If your carbon monoxide alarm sounds, or anyone has headaches, dizziness or nausea that ease when they leave the house, open windows, turn off the boiler, get outside and call the National Gas Emergency Service on 0800 111 999 (0800 002 001 in Northern Ireland). Our guide on what to do when a carbon monoxide alarm is going off covers the steps in full.
The warning signs in more detail
Smoke, soot and staining
A gas boiler that is burning properly should not produce smoke or soot. Dark exhaust, black marks around the terminal or staining on the wall above it suggest poor combustion, which can also mean carbon monoxide is being produced. Stop using the boiler and book a Gas Safe registered engineer.
Steam or water from the boiler itself
Steam, hissing or water from the casing, or from flue joints inside the house, is not the normal plume. It can mean a leak onto hot parts, a failed seal or a flue section that has come apart. Turn the boiler off and call an engineer. If water is dripping from underneath the boiler, see our guide to a boiler leaking water.
Flue damage or blockage
Look at the flue terminal from the ground or a window without touching it. Birds' nests, ivy, a cracked or loose terminal, or a flue that has been boxed in or had a structure built close to it can stop fumes clearing properly. Many boilers shut down with a flue or overheat code, such as Worcester Bosch E9 224, when something is wrong. Never try to clear, move or repair the flue yourself.
Condensate problems in freezing weather
The water that condenses inside the boiler drains through a separate condensate pipe. In a hard frost that pipe can freeze and cause a lockout, which is a different issue from the plume. Our frozen condensate pipe guide explains how to thaw it safely with warm (not boiling) water.
Plume management kits
If the plume is harmless but annoying, for example because it drifts onto a neighbour's window, a doorway, a path or a patio, a plume management kit can redirect it. The kit is an extension fitted to the flue terminal that carries the exhaust through a separate pipe and releases it higher up or further from the problem area, usually through an elbow that points it upwards.
- Kits are specific to each manufacturer and flue system. Worcester Bosch, Vaillant, Ideal, Baxi, Glow-worm, Viessmann and Vokera all make their own, and the kit must match your boiler and flue.
- The installation has to follow the manufacturer's instructions, including maximum pipe lengths and minimum distances from windows, doors, air bricks, gutters and boundaries.
- Only a Gas Safe registered engineer should fit one, because it becomes part of the flue.
- A terminal guard is usually needed where a flue outlet is low enough for people to touch, commonly below 2 metres above ground that people walk on.
A plume kit can also help when a new boiler has to go in a spot where the terminal would otherwise be too close to an opening window or a boundary. The engineer will check what the manufacturer allows for your model.
When to call an engineer, and typical costs
Call a Gas Safe registered engineer if you see any of the warning signs above, if the plume is getting back into the house, or if the boiler keeps locking out. The flue should also be checked at every annual service, which includes a flue gas analysis to confirm the boiler is burning cleanly. Our boiler service cost guide explains what a service covers.
| Job | Typical UK cost |
|---|---|
| Plume management kit, parts only | Typically £50 to £150 |
| Plume kit supplied and fitted | Typically £150 to £350 |
| Flue terminal guard supplied and fitted | Typically £60 to £150 |
| Engineer visit to check combustion and the flue | Typically £60 to £120 |
| Flue section repaired or replaced | Typically £150 to £400 |
Costs sit at the lower end for a short kit fitted to an easy ground floor flue outside London. They rise for longer kits and extra bends, brand-specific parts, flues that need a tower or scaffolding to reach safely, and London labour rates. If a plume kit is fitted as part of a new boiler installation, the extra cost is usually lower than booking a separate visit later.