What is a boiler flue?
When a gas boiler burns fuel, it produces waste gases, mainly water vapour and carbon dioxide, plus small amounts of carbon monoxide and other gases if combustion is not right. The flue's job is to take all of that outside, away from windows, doors and anywhere people breathe. It is one of the most important safety parts of the whole installation.
Almost all boilers fitted in UK homes today have a room-sealed flue, sometimes called a balanced flue. It is a pipe within a pipe, known as a concentric flue. The inner pipe carries the waste gases out, and the gap between the inner and outer pipes brings fresh air in to the burner. A fan inside the boiler pushes the gases out. Because the boiler takes its air from outside and not from the room, it is sealed off from the living space, which is much safer than the older open-flued boilers that drew air from the room.
Horizontal and vertical flues
Horizontal flues
The most common layout. The flue runs straight out of the back or side of the boiler and through the nearest outside wall, ending in a terminal a short distance from the wall. On condensing boilers, the horizontal flue is fitted rising slightly away from the boiler, at around 3 degrees in most manufacturers' instructions, so the acidic condensate that forms inside drains back into the boiler and out through the condensate pipe instead of dripping from the terminal.
Vertical flues
Where there is no suitable outside wall, the flue goes up through the ceiling and roof, finishing in a roof terminal. Vertical flues are common for boilers in lofts, internal cupboards and flats, and they use special roof flashings to keep the weather out.
Plume kits
A condensing boiler gives off a visible plume of water vapour, especially in cold weather. A plume management kit is an approved accessory that moves the exhaust outlet away from the air intake, for example up the wall or round a corner, so the plume does not drift onto a neighbour's window or a path. It must be the manufacturer's own kit for your boiler.
How far does a boiler flue need to be from a window?
For a typical fan-assisted, room-sealed gas boiler, the terminal must be at least 300 mm from any opening such as a window, door or air brick, whether the opening is directly above, directly below or to the side. The figures come from BS 5440-1, the British Standard for gas appliance flues, which Approved Document J of the Building Regulations refers to in England and Wales. Scotland's Building Standards technical handbook sets out the same kind of table. The main minimum distances for fanned-draught room-sealed terminals in that table are:
| Terminal position | Minimum distance |
|---|---|
| Directly below, above or beside an opening (window, door, air brick) | 300 mm |
| Below gutters or soil pipes | 75 mm |
| Below eaves | 200 mm |
| Below a balcony or carport roof | 200 mm |
| Above ground, roof or balcony level | 300 mm |
| From a vertical drain or soil pipe | 150 mm |
| From an internal or external corner | 300 mm |
| From a surface or boundary facing the terminal | 600 mm |
| From another terminal facing it | 1,200 mm |
| From another terminal on the same wall, horizontally | 300 mm |
Source: Scottish Government Building Standards technical handbook, domestic, Table 3.15. These are minimums. The boiler manufacturer's installation manual always has its own clearance table, which can be stricter for a particular model, and the engineer must follow it. Some figures, such as the distance from a facing boundary, may also change where a plume kit is fitted.
Do I need a flue guard?
Yes, if people can reach the terminal. A flue terminal guard should be fitted where the terminal is less than 2 metres above the ground, a balcony or a flat roof that people can get onto, as Installer Online's guide to BS 5440-1 siting rules explains. The terminal can get hot, and a guard stops people touching it and stops children or pets pushing things into it. Guards are specific to the terminal shape, so they should be the type approved for your boiler. A guard supplied and fitted typically costs £60 to £150.
Boiler flue covers
People often want to hide or protect a flue, and the rules here are strict. Outside, never box in, wrap or cover a flue terminal with anything other than the manufacturer's approved guard or accessory. Anything that restricts the terminal can stop waste gases escaping or let them be drawn back into the air intake, which can make the boiler burn badly and produce carbon monoxide. Keep plants, climbers, bins and stored items well away from it too.
Inside, decorative boxing around the flue pipe is allowed, but the joints must still be accessible for inspection, which leads to the rules on flues in voids below.
Boiler flue extensions
If a boiler is moved, or a new boiler sits in a different spot, the flue often needs extending. Every boiler has a maximum flue length in its installation manual, and each bend counts as an extra length of straight pipe, so there is a limit to how far a flue can run. Extensions must be the same manufacturer's parts, designed for that boiler, and fitted with the correct seals and supports. Mixing brands of flue component is not allowed.
Typical prices for flue work are around £150 to £400 to repair, replace or alter a section, depending on access. Moving a boiler usually means a new flue altogether, which is covered in our guide to new boiler costs.
Can a boiler flue go into a loft?
Yes, a flue can run through a loft or other roof space, usually as part of a vertical flue up through the roof, and some boilers are installed in the loft itself. The key requirement is that the flue can be inspected. Installation standards also expect safe access to a loft boiler, such as a fixed ladder, lighting and boarding to walk on, so that an engineer can service it.
Flues in voids
Where a flue runs through a ceiling void, boxing or roof space where it cannot be seen, the Gas Safe Register's guidance applies. Inspection hatches must be at least 300 mm by 300 mm and within 1.5 metres of any joint in the flue. Since January 2011, engineers have classed a concealed flue they cannot inspect as At Risk. They will recommend not using the boiler and ask permission to turn it off until hatches are fitted. This is most common in flats and newer houses built in the 2000s, where flues often run through ceiling voids.
Signs of a flue problem
- Soot or black staining around the terminal or on the wall.
- A damaged, loose or disconnected flue, inside or out.
- A boiler that shows flame or fan faults in windy weather.
- Water or condensate dripping from inside joints of the flue.
- A carbon monoxide alarm sounding.
A white plume on a cold day is normal for a condensing boiler and is not a fault on its own. A condensate pipe frozen in winter can also stop the boiler, which our guide to a frozen condensate pipe covers.
Only a Gas Safe registered engineer may fit, extend, move or repair a gas boiler flue. Never cover a terminal or cut into a flue. If your carbon monoxide alarm sounds, turn off the boiler, open windows and doors, leave the property and call the National Gas Emergency Service on 0800 111 999. If you smell gas, turn off the gas at the meter if you can and call the same number.