Boiler flue rules for flats and terraced houses

The flue is the pipe that takes a boiler’s exhaust gases out of your home. Most modern boilers are room sealed, meaning they take air from outside and send exhaust back outside through the flue, with nothing open to the room. If the flue leaks, is blocked, or ends in the wrong place, exhaust including carbon monoxide can get into a home. That is why flue rules matter so much in flats and terraces, where homes sit close together.

The rules in short

Rule	Where it comes from	What it means for you
Flues in voids need inspection hatches	HSE and Gas Safe industry guidance	Hidden flues must be checkable along their length
Terminal clearances	Approved Document J and the boiler maker’s instructions	Set distances from windows, doors, vents, boundaries and other flues
Notification of a new boiler	Building Regulations, via Gas Safe	Local authority told within 30 days, certificate issued
Carbon monoxide alarm with a new appliance	Approved Document J 2022	Alarm needed when a new or replacement fixed gas appliance is fitted
Work by a Gas Safe registered engineer	Gas Safety (Installation and Use) Regulations 1998	No unregistered fitting or “sign off”

Flues in voids: the flat issue

Many flats built or converted from the 1990s onwards have the boiler on an inside wall, with the flue running across a ceiling void to the outside. You cannot see the flue. If a joint comes apart in that void, exhaust can leak unseen.

HSE published guidance that inspection hatches must be fitted where a flue is concealed and cannot be inspected. Landlords and owners were given until 31 December 2012. From 1 January 2013, a Gas Safe registered engineer who cannot check the flue will tell you the boiler is “At Risk” and, with your permission, turn it off. The HSE flues in voids bulletin and Gas Safe’s flues in voids page explain this.

HSE also says not to fit hatches yourself or check the flue yourself, because hatches in ceilings can affect fire and sound proofing between flats. Use a competent builder, then have a Gas Safe registered engineer check the flue.

If you rent, this falls to your landlord. HSE says landlords must maintain flues serving the home, and where a building has a communal flue they must make sure it is maintained by a Gas Safe registered engineer.

Terminal positions in terraces

The terminal is the end of the flue outside. In a Victorian or Edwardian terrace the boiler often sits in the back addition, the narrow rear wing many London terraces have. The terminal ends up on the side wall facing next door’s side return, or on the back wall near windows.

Approved Document J and every boiler’s installation manual set minimum distances from:

• opening windows and doors
• air bricks and vents
• the boundary line, such as a side fence
• other flue terminals
• gutters, soil pipes and eaves
• the ground, and surfaces people walk past

The exact distances depend on the boiler and the direction the flue faces, so we do not quote them here. You can read Approved Document J for the official guidance.

Plume kits

A condensing boiler gives off a white plume of water vapour. Close to a neighbour’s window or a path, that plume is a nuisance. A plume kit is an extra length of pipe that moves where the vapour comes out, often up and away. It can solve a nuisance problem. It does not replace the need for the main terminal position to comply.

Camden homes and flues

Camden has a lot of variety. There are Victorian terraces in Kentish Town and Gospel Oak, many split into flats. There are mansion blocks with boilers fitted long after they were built. There are council blocks with communal flues. Private renting is high: the Census 2021 shows 35.6% of Camden households rent privately, so landlord duties often apply.

Conversions are where flue problems show up most. A flat formed from part of a house may have its boiler in an inner kitchen, with the flue crossing a bedroom ceiling to reach the back wall. See our Camden area page for more on the area.

Loft conversions and extensions

New windows bring new problems. A rear dormer or a rooflight can end up too close to an existing flue terminal, or a kitchen extension can enclose the wall where the flue used to exit. Our guide on moving a boiler into a loft conversion covers the planning points.

Signs of a flue problem

Call a Gas Safe registered engineer if you notice:

• staining or soot around the boiler or flue
• a flue that has come loose or is damaged outside
• a carbon monoxide alarm sounding
• headaches, dizziness or nausea at home that ease when you go out; the NHS lists these as signs of carbon monoxide poisoning

If a CO alarm sounds or you suspect carbon monoxide, open windows, turn the appliance off if safe, leave and call 0800 111 999. Get medical help, calling 999 if someone is very unwell.

Getting it right with a new boiler

On a boiler installation, the survey decides the flue route before anything else. Gas work in the UK must be done by a Gas Safe registered engineer, so ask to see the ID card. If the best route needs an outside wall in a leasehold flat, check with the freeholder early.