Makes we repair
How far must a boiler flue be from a window?
A modern boiler's flue usually needs at least 300mm from an opening window, whether below, above or beside it. It needs the same from a door or an air vent. That's the figure Viessmann and Ideal publish for the fanned-draught flues modern boilers have, and Baxi gives the same for a window.
Each boiler's own distances are in its maker's installation instructions. They vary with the boiler, its flue and its output. The figures here are typical minimums, not the rule for your boiler.
Placing the flue is the installer's job. The law asks for a flue in a safe position. The Building Regulations' guidance on flues, Approved Document J, covers where a gas boiler's flue can come out. Baxi points installers to the British Standard BS 5440-1, and the maker's instructions, for every distance. Your part is knowing what the rules ask, and ringing us when a flue looks wrong.
An opening window, a door or an air vent, with the flue below, above or beside it
Typical minimum, in the makers' figures
300mm (Viessmann and Ideal, and Baxi for a window)
An opening window or door, from the air intake of a flue with a plume kit
Typical minimum, in the makers' figures
150mm (Baxi)
A gutter, drain pipe or soil pipe above the flue
Typical minimum, in the makers' figures
75mm (Viessmann, Ideal)
A vertical drain or soil pipe, on a boiler over 5kW
Typical minimum, in the makers' figures
150mm (Viessmann)
The eaves above the flue
Typical minimum, in the makers' figures
200mm (Ideal)
The ground, a roof or a balcony below the flue
Typical minimum, in the makers' figures
300mm (Ideal)
The ground under a flue that opens onto a path, walkway or pavement
Typical minimum, in the makers' figures
2,100mm (Viessmann, Ideal)
Another fanned-draught flue facing it
Typical minimum, in the makers' figures
1,200mm (Viessmann)
A roof window above a flue through the roof
Typical minimum, in the makers' figures
2,000mm (Ideal)
The roof a flue goes through, pitched or flat
Typical minimum, in the makers' figures
300mm above it (Ideal)
Older natural-draught flues need more room, the more so the bigger the boiler. Here are Viessmann's figures from an opening window, a door or an air vent:
Up to 7kW
Flue below it
300mm
Flue above it
300mm
Flue beside it
300mm
7 to 14kW
Flue below it
600mm
Flue above it
300mm
Flue beside it
400mm
14 to 32kW
Flue below it
1,500mm
Flue above it
300mm
Flue beside it
600mm
Over 32kW
Flue below it
2,000mm
Flue above it
600mm
Flue beside it
600mm
Viessmann also gives a natural-draught flue 300mm below a gutter or pipe, and 600mm from another flue facing it.
The distances keep the fumes out of your home and your neighbours'. Hot flue gases can also melt plastic gutters and pipes. Ideal adds that a more powerful boiler may need more room. Its figures are minimums for typical home boilers, and the rules are reviewed.
A new window, vent, door or extension near an existing flue can leave the flue too close. It may then need extending, with the boiler's manual giving the distances and a qualified engineer doing the work. A flue can run a long way. Baxi gives up to 10 metres for a 60/100mm flue, and 20 metres for an 80/125mm one, on its own, Main's and Potterton's boilers.
Sources: the makers' figures and advice: Viessmann, Ideal, Baxi and Vaillant, and the Gas Safety (Installation and Use) Regulations 1998, regulation 27.
A flue that looks too close to a window, a door or a vent? Ring us on 020 8064 0067, and we'll ask what you can see from the ground. Having a boiler fitted or moved.
When does a flue need planning permission?
A flue on a house usually needs no planning permission. The planning rules count it as permitted development. Planning permission is not normally needed to install or replace a boiler. A flue on a house does need permission when:
- it would stand 1 metre or more above the highest part of the roof
- the house is in a conservation area, or on other protected land the order sets apart, such as a National Park. Then permission is needed for a flue on a wall or roof slope that faces a road, at the front or a side.
- the council has withdrawn the allowance with an Article 4 direction. That means a planning application, not a ban on the work.
- the house became a home through certain changes of use, or was built under the permitted development rules for new homes.
A flat has no householder permitted development rights, and a listed building needs consent of its own. Each has a section further down this page. The local planning authority decides, and you reach it through your local council. GOV.UK's guide to planning permission.
Building regulations are a separate matter. A new boiler's Gas Safe registered installer notifies it to building control. The Building Regulations certificate a new boiler comes with.
Sources: the General Permitted Development Order 2015, Schedule 2, Part 1, Class G, the Planning Portal on flues and boilers, and GOV.UK.
What are the flue rules for flats and shared buildings?
A flat has no householder permitted development rights. A new or moved flue on a flat's outside wall is a question for the council's planning team. The planning order's definition of a house leaves out flats and the buildings that contain them. The householder allowances are for houses, not flats or maisonettes. A flat's lease, and a flue the whole building shares, can matter as much.
The lease
A flat's lease sets out whether you need permission to make alterations.
Shared flues: SE-ducts and U-ducts
Some blocks of flats share one flue. Around 350,000 UK homes had gas appliances, mostly boilers, on shared flue systems called SE-ducts and U-ducts in 2015. On these, appliances on different floors take their air from, and send their fumes into, a common duct.
Each appliance on such a duct must be one its maker designates as suitable for it. A modern condensing boiler's cooler, wetter flue gases can upset the duct. When a boiler on one needs replacing, there are three ways forward:
- an equivalent appliance approved for the duct
- a balanced-flue boiler in the flat, where that is acceptable
- a new approved chimney system, with every boiler in the building replaced
IGEM's standard UP/17 covers fitting replacement appliances onto these ducts.
Who looks after a shared duct
The landlord or managing agent looks after an SE-duct or U-duct, and each resident their own appliance. Residents who share a flue or chimney should share responsibility for having it checked every year. Carbon monoxide can travel through shared vents.
Renting a flat
A tenant who thinks the flue is in the wrong place should contact the landlord. The landlord must have the flue checked every year. The landlord's yearly gas safety check, and what it covers.
Sources: the General Permitted Development Order 2015, article 2, GOV.UK, the Energy and Utilities Alliance (2015, quoting BS 5440-1), Gas Safe Register and Ideal.
Is a flue hidden in a ceiling void safe?
A flue hidden in a ceiling void isn't unsafe in itself. But a Gas Safe registered engineer who can't see all of it classes the boiler "At Risk". Hidden flues come from room-sealed, fan-assisted boilers whose flue runs behind a ceiling or a wall. They're most often in new-build flats and apartments finished since 2000. The flue matters because it takes the fumes away. A blocked or poor one could put people at risk of carbon monoxide poisoning.
What the engineer does
An engineer who can't see the whole flue classifies the boiler "At Risk", and recommends not using it. With your permission, they turn it off until inspection hatches are fitted. You can refuse, and you'll be asked to sign to accept responsibility. The engineer can still service the boiler.
The inspection hatches
Inspection hatches should be at least 300mm by 300mm and, where possible, within 1.5 metres of each joint in the flue. Some homes need one, and others several. The hatches must keep the ceiling's fire and sound protection. Gas Safe Register recommends at least one hour's fire resistance for hatches added later. A competent builder fits them, told by a gas engineer how many are needed and where.
The law, and the dates
An owner-occupier has no legal duty to fit hatches. But gas engineers have long had a duty to examine the flue whenever they work on the boiler. A landlord must have it checked every year. Engineers' instructions changed in January 2011, and the HSE gave homeowners until 31 December 2012 to fit hatches. That date has passed, so a hidden flue that can't be inspected is classed "At Risk" now.
When hatches can't be fitted
A system that watches the void for carbon monoxide and shuts the boiler down is the industry's alternative where hatches can't go in. Hatches stay the preferred answer, though. A carbon monoxide alarm is a second line of defence, not a substitute.
Who to contact, by the home's age
The right contact for a hidden flue depends on the home's age:
- under 2 years old: the builder
- 2 to 10 years old: the home warranty provider, though the main providers don't cover hatches in homes over two years old
- 10 years old or more: a Gas Safe registered engineer
An engineer is worth contacting whatever the home's age. Don't inspect the flue yourself unless you're a Gas Safe registered engineer. Ring us, and we'll ask about the boiler and where its flue runs.
Sources: Gas Safe Register and its questions on flues in voids, and the HSE's safety bulletin.
Worth knowing
Suspected carbon monoxide comes before any booking. The signs are anyone at home having symptoms such as headaches, dizziness or nausea, or a carbon monoxide alarm sounding. Turn the boiler off if it's safe to, open the doors and windows, and go outside. Then ring the National Gas Emergency Service on 0800 111 999, free, day and night. The full steps for a gas or carbon monoxide emergency.
Can a flue go on a listed building or in a conservation area?
A flue in a conservation area can need planning permission. It does when it goes on a wall or roof slope that faces a road, at the front or a side of the house. Conservation areas are among the protected land the planning order sets apart. Elsewhere on the house the usual allowance holds, unless an Article 4 direction has removed it.
A listed building needs listed building consent for works to alter or extend it in any way that would affect its character. That consent is separate from planning permission. Ask the local planning authority before a flue is fitted to a listed building. Consent is likely to be needed for changes inside as well. Works to a listed building without the permission they need are a criminal offence. The listing protects its features inside and out.
Councils' own conservation guidance often asks for flues away from the street side, and our area pages note the local guidance. The areas we cover, borough by borough.
What does the engineer check on a flue?
A flue is examined whenever a Gas Safe registered engineer works on the boiler. The law asks for it after any work on a gas appliance, and a safety check and a service both cover it. The checks below are the law's, Gas Safe Register's and the makers'.
What the law asks
An engineer who has worked on a gas appliance must then examine four things:
- how well its flue works
- the supply of air for burning the gas
- its operating pressure or heat input
- that it works safely
No boiler may be connected to a flue unless the flue is suitable and in proper condition.
At a yearly safety check
A gas safety check looks at whether the flues, chimneys and air vents are working correctly. It checks the air supply is enough, and that the safety devices work.
At a service
A boiler service covers the same checks and the maker's own tests, which may include analysing the combustion gases. The engineer checks the flue isn't blocked or restricted, and inspects it at the yearly service. What our yearly boiler service includes.
When something is wrong
A fault found on a flue or a boiler is classed under the gas industry's unsafe situations procedure. It's either "Immediately Dangerous", "At Risk" or "Not to Current Standards".
A blocked flue terminal
A flue terminal with leaves or anything else caught in it is a job for an engineer. Look at it from the ground, touch nothing, and ring us. Never try to unblock a flue yourself. A blocked or poor flue could put people at risk of carbon monoxide poisoning.
Sources: the Gas Safety (Installation and Use) Regulations 1998, regulation 26 and regulation 27, Gas Safe Register, Ideal and Vaillant.
Do you cover my part of London?
Yes: we fit, service and repair boilers all over Greater London, in every borough north and south of the Thames. Pick your side of the river to see the areas.
North of the river
Every borough north of the Thames: Barnet and Enfield at the top, Hillingdon in the west, Havering in the east, and central London in between.
South of the river
Every borough south of the Thames: Richmond and Kingston in the west, Bexley in the east, and down through Croydon to Sutton and Bromley.
Questions about boiler flues
Can a flue point at a neighbour's property?
A flue can point towards a neighbour's property within the distances its maker sets. Viessmann gives at least 600mm from the boundary for a flue pointing straight at it. It gives at least 2,100mm from the neighbour's doors and windows, and 300mm for a flue running alongside the boundary. Baxi gives 300mm beside a boundary, less with a plume deflector. It quotes BS 5440-1, which says a flue shouldn't go where its plume is likely to cause a nuisance, such as over a neighbouring driveway. The maker's instructions for your boiler decide the figure, and the installer works to them.
Does moving a boiler mean a new flue?
Moving a boiler gives its flue a new route out of the home, which has to meet the maker's distances in its new place. Makers certify each boiler with its own flue system, so the flue fitted is that maker's. Moving or replacing a boiler, and what the price includes.
Who gives planning permission for a flue?
Planning permission for a flue comes from the local planning authority, which you reach through your local council. Listed building consent comes from the same council. Building regulations are separate. A Gas Safe registered installer notifies a new boiler to building control. What the certificate for a new boiler is.
Can I box a flue in?
A flue can be boxed in where an engineer can still see and reach its whole length. A hidden flue needs inspection hatches of at least 300mm by 300mm, within 1.5 metres of each joint where possible. They're fire-rated to match the ceiling. Ask a Gas Safe registered engineer before anything is boxed in, and ring us if a flue is already hidden with no hatches.
Don't know what has failed? You don't have to.Ring and say what you can see or hear. Finding the fault is our job.
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