If an extension, loft conversion or outbuilding has been built where the boiler flue terminates, the flue no longer discharges to open air and the boiler should not be used until a Gas Safe registered engineer has inspected it. The flue has to be dealt with before the boiler goes back into service: extended so the terminal sits outside the new structure, converted to a vertical route through the roof, changed to a twin pipe system, or the boiler moved. Which of those is possible depends on the appliance manufacturer's installation instructions and on the terminal positions left after the build. This page sets out the legal position in England, what happens when a loft conversion boxes the flue into a void, and what you can safely check yourself.
What happens when an extension is built over a boiler flue terminal?
A boiler flue that ends inside a new extension, conservatory or outbuilding is discharging combustion products into an enclosed space instead of open air. The boiler should not be used until a Gas Safe registered engineer has assessed the installation. The flue is the pipe that carries combustion products out of the building and, on most modern boilers, draws combustion air back in. Both jobs stop working once a new wall or roof surrounds the terminal.
- Extend the existing flue so the terminal sits in open air beyond the new structure
- Convert to a vertical flue that rises through the roof
- Fit a twin pipe system, where air supply and flue discharge run separately and can terminate in different places
- Move the boiler to a position with a workable flue route
A builder, roofer or planning officer cannot sign off a flue position. Work on a gas appliance, its flue or its gas pipework is for a Gas Safe registered engineer, and the manufacturer's installation instructions for that boiler decide which routes are available.
Which rules apply in England when new building work encloses a boiler flue
Regulation 27 of the Gas Safety (Installation and Use) Regulations 1998 (as amended) states that no person shall install a flue other than in a safe position. It also prevents a gas appliance being connected to a flue that is not suitable and in a proper condition for the safe operation of that appliance. Where a flue runs within an enclosure, the enclosure must be sealed so combustion products cannot pass into other rooms. An extension or outbuilding built around a terminal runs straight into those duties.
Approved Document J is the statutory guidance on combustion appliances in England and covers where a flue terminal may sit. Terminals have to be positioned so products of combustion discharge to outside air, and so a balanced flue can draw its air in. This page covers England; Wales and Scotland have their own building standards. Our guide to boiler flue regulations goes through flue position and sign-off in more detail.
Extending the flue or converting it to vertical: how the two options compare after an extension
Where an extension has built over the terminal, the two routes most often looked at are carrying the existing horizontal flue clear of the new structure, or taking the flue vertically through the roof. Which one works depends on the boiler's installation instructions and on the terminal positions the new building shape still allows.
| Criterion | Extending the horizontal flue | Converting to a vertical flue |
|---|---|---|
| Where the terminal ends up | In open air beyond the new wall or roof, clear of openings into the building | Above the roof line, clear of the new extension or loft room |
| Work inside the home | Usually confined to the boiler area and the wall the flue passes through | A route through ceiling, loft space and a weathered penetration in the roof |
| What sets the limit | Maximum flue length allowed for that boiler, with each bend using part of it | Height and route available above the boiler, plus the same length limit |
| Common reason it is ruled out | No compliant terminal position left outside the new structure | No usable roof route, or the roof is not yours to penetrate in a flat |
A terminal that now sits under a carport, canopy or covered side return is a separate case again, with its own conditions in the appliance instructions. Our guide on a boiler flue under a carport or balcony explains when those positions are allowed.
Does moving a boiler flue after an extension need building regulations approval or planning permission?
Changing a heating appliance or its flue is work that needs building regulations approval in England, unless it is carried out under a competent person scheme. An engineer registered under such a scheme notifies the work, so you do not make a separate application to building control. Building regulations approval and planning permission are two different consents, decided by different routes.
Permitted development rights say nothing about whether a flue terminal is still compliant. A single storey rear extension or a garden outbuilding can be entirely lawful under the householder permitted development rules and still leave the flue in an unusable position. That is why the flue problem is often found after the shell is up, not during the planning stage. Raising the flue route with a Gas Safe registered engineer while the extension is still on paper avoids rework.
A loft conversion that boxes the flue into a void: why inspection hatches belong in the build plan
Boxing a boiler flue into a new ceiling void or stud wall during a loft conversion creates a concealed flue that an engineer cannot inspect. HSE's safety bulletin on flues in voids explains the risk: where the flue cannot be seen, nobody can confirm it is correctly jointed and supported, and a faulty boiler can then release carbon monoxide into living space.
Since 1 January 2013, a registered engineer who finds a concealed flue with no means of inspection classifies the installation as At Risk and, with the owner's agreement, turns off the gas to the boiler. Inspection hatches sized and placed so the flue can be seen along its length belong in the conversion drawings rather than being cut in afterwards. Our guide on a boiler flue in a ceiling void covers what is asked for and where hatches go.
The extension has closed in the gas meter as well: who moves it
An extension or conservatory built over the gas service pipe and meter is a separate safety concern from the flue, and it turns up on the same site visits. Gas Safe Register's Technical Bulletin TB 003, published on 19 November 2010, treats a built-over medium pressure service as At Risk. A built-over low pressure service is recorded as Not to Current Standards.
Where new building work has enclosed the meter and regulator, the meter and service pipework need relocating to an agreed position, normally an external meter box. You can ask your gas supplier or the local gas network operator to arrange that move. It is not work a heating engineer carries out, so it runs on its own timetable alongside the flue work.
Warning signs after building work, and what you can safely check yourself
Four things on a boiler are for you rather than an engineer: reading the pressure gauge, thermostat and programmer settings, the reset button, and bleeding radiators. Everything involving gas, the flue, seals or the inside of the boiler casing is work for a Gas Safe registered engineer. That includes any attempt to assess a terminal that now sits inside new building work.
HSE lists signs of incomplete combustion you can notice without touching the appliance. A yellow or orange flame instead of a crisp blue one, soot or yellow-brown staining on or around the appliance, a pilot light that keeps going out, and heavier condensation on windows all belong in that list.
Early carbon monoxide symptoms look like flu or food poisoning: headaches, dizziness, nausea, breathlessness and tiredness. If you suspect carbon monoxide, HSE advises getting medical advice and calling the National Gas Emergency Service on 0800 111 999. An audible alarm to BS EN 50291 is worth having, and HSE is clear that it is no substitute for a safety check by a registered engineer.
If the flue route only works with the boiler in a different place, or the appliance is near the end of its life anyway, FM Power can look at it during a free home survey and set out a written fixed price for a new boiler installation before any work starts.
Sources
- GSIUR 1998, Regulation 27 — Flues
- The Gas Safety (Installation and Use) Regulations 1998
- GOV.UK — Approved Document J: combustion appliances and fuel storage systems
- GOV.UK — Building regulations approval: when you need approval
- GOV.UK — Permitted development rights for householders: technical guidance
- HSE — Gas boilers: flues in voids
- HSE — Domestic gas: frequently asked questions
- HSE — Carbon monoxide awareness frequently asked questions
- Ofgem — Technical Bulletin 003: built-over gas services (19 November 2010)