A boiler flue can terminate under a carport if the structure is open enough and the outlet is clear of doors and windows. Worcester Bosch sets out at least two open sides where the outlet ends inside the carport footprint, or one fully open side where it ends at least 300 mm beyond it. Under a balcony or overhang, manufacturer manuals ask for at least 200 mm below the lowest point, unless the terminal is extended so it clears the overhang. Both positions are judged by a Gas Safe registered engineer against the terminal position table in your boiler's own installation manual.
Boiler flue under a carport or under a balcony: what differs
A carport and a balcony both sit over a flue outlet, but manufacturers judge them in different ways. A carport is assessed on open sides and on where the outlet ends; a balcony is assessed on the gap below its lowest point.
| Criterion | Flue under a carport | Flue under a balcony or overhang |
|---|---|---|
| What decides the position | How open the carport is, and how far the outlet sits from the carport footprint | Vertical clearance from the lowest point of the balcony or overhang |
| Example figure in manuals | Two open sides if the outlet ends inside the footprint; one fully open side if it ends at least 300 mm beyond it | Not less than 200 mm below the lowest point of the balcony or overhang |
| Distance to doors and windows | Manuals that allow a two-sided carport ask 1,200 mm to an opening in the same wall; air intake at least 150 mm from an opening | Ideal gives 300 mm below, above or beside an opening window, vent or air brick |
| Way to gain clearance | A terminal extended at least 300 mm past the footprint, or a route up through the carport roof ending 300 mm above it | A terminal extended so it clears the overhang; manuals then allow 25 mm in that configuration |
| Added risk to weigh | Plume blowing onto parked vehicles; several manuals state that installations in car ports are not recommended | Plume at head height where the balcony is in use; 300 mm above balcony level is a separate figure |
| Terminal guard | Asked for where the terminal sits less than 2 m above the ground inside the carport | Asked for where the terminal sits less than 2 m above a balcony or an accessible flat roof |
Every figure above is an example from a published manufacturer document, not a universal rule. The terminal position table in the manual for your boiler and flue kit is what applies on your wall.
When a boiler flue under a carport is a workable position
A flue under a carport works where the structure is genuinely open and the outlet sits well clear of doors and windows. Worcester Bosch treats the carport as a special case rather than an automatic refusal. The flue is the pipe that carries combustion gases outside; the terminal is its outlet end.
- A lean-to carport with two fully open sides, where the boiler sits on the side wall and no door leads from the carport into the house.
- A carport where the flue run is long enough for the outlet to end at least 300 mm beyond the roof edge, leaving one fully open side.
- A carport where the parking bay sits at the far end, so the plume of water vapour does not blow across a parked car.
- A carport roof that a flue can pass up through, kept clear of combustible material and ending above the roof, where the manual allows that configuration.
Where a plume management kit is used inside a carport, Worcester Bosch asks for at least 1,200 mm between the terminal and any building opening within the carport footprint. That is a condition, not a workaround.
When a boiler flue under a balcony or overhang is a workable position
A flue under a balcony works where there is enough vertical gap below the lowest point of the structure. Worcester Bosch and Glow-worm flue manuals give not less than 200 mm from that lowest point. Below that gap, the manuals look for a terminal extended out so it clears the overhang altogether.
- A ground-floor flat with a shallow balcony above, where the wall gives more than the clearance the manual asks for.
- A terraced house with a boiler on the rear wall below a modest overhang, with no opening window within the stated distance.
- A property where the flue can be extended past the overhang, if that arrangement appears in the manufacturer's flue kit instructions.
The reduced 25 mm figure in those manuals belongs only to an extended terminal that clears the overhang. It is not permission to sit closer under a balcony. Whether your wall allows it is a judgement for a Gas Safe registered engineer with the manual in hand.
Can a boiler flue terminate in a covered passage or side return?
A flue may terminate in a side return or covered passage only where the position is safe and combustion products cannot enter buildings. A side return is the narrow passage running alongside a house, often roofed over by a neighbouring extension. In England, regulation 27 of the Gas Safety (Installation and Use) Regulations 1998 as amended requires that a flue is not installed other than in a safe position.
Worcester Bosch applies the carport test to any covered, partially enclosed extension. The same questions follow: how many sides are open, how far the outlet sits from openings, and where the plume goes.
Nuisance counts too. Baxi points to BS 5440-1 and gives a plume blowing over a neighbour's driveway as the example to avoid, with a fanned-draught terminal kept not less than 300 mm from the boundary. If a neighbour's outlet is the problem rather than yours, see a neighbour's flue pointing at your house.
How far below deep eaves must a boiler flue terminate?
Manufacturer manuals give 200 mm as the clearance below eaves, and 75 mm below gutters, drain pipes and soil pipes. Ideal lists the same 200 mm figure on its consumer guidance. Deep eaves raise the same question as a balcony: either the gap is there, or the terminal has to reach past the overhang.
Vaillant manuals add two points for eaves. No ventilation opening in the eaves may sit within 300 mm of the terminal, and the terminal is kept 300 mm from a property boundary. Ideal also gives 300 mm from an opening window, vent or air brick, and 2.1 m above ground where a public footpath passes.
These are published examples, and the manual for your appliance governs. Window and vent clearances are set out in more detail in our guide to boiler flue distances from windows.
When is a terminal guard needed on a boiler flue, and what does it not do?
A terminal guard is asked for where the terminal or air inlet sits less than 2 m above the ground, above a balcony, or above an accessible flat roof. Glow-worm, Baxi and Vaillant all describe a purpose-designed guard for those positions. The guard stops people touching a hot terminal and protects it from knocks.
A terminal guard does not make an unacceptable flue position acceptable. It adds protection against contact and damage; it replaces none of the clearances in the terminal position table. Whether a guard is needed, and whether an existing one is correct, is checked by a Gas Safe registered engineer at installation or service.
Manuals also set out how the guard must sit in relation to the terminal and how it is secured. Those are instructions for the engineer, not a job for the householder. Fitting, moving or removing a guard is work on the flue system.
Carport or balcony: which flue position to aim for in a boiler replacement
Aim for the carport position if the carport has the open sides the manual asks for, no door or window of the house sits close to the outlet, and the parked car is clear of the plume. Aim for the balcony or overhang position if the wall gives the full clearance below the lowest point, or if the flue can be extended so the outlet clears the structure in a configuration the manual shows.
Neither position is automatically ruled out, and neither is automatically fine. Where both are possible on the same house, the deciding factors are usually the nearest opening, the boundary and where the plume drifts.
You can look at where your own terminal sits and raise it with your engineer at the next visit. Leave measuring and assessing to them. If you are planning a replacement and the flue position is in doubt, you can arrange a free home survey and a written fixed price before any work starts for a new boiler installation.
Sources
- The Gas Safety (Installation and Use) Regulations 1998, regulation 27 (Flues)
- Combustion appliances and fuel storage systems: Approved Document J — GOV.UK
- Worcester Bosch — TB 0127 a: Flue Configurations and Terminal Positions
- Worcester Bosch — Instruction Manual Flue Kit Installation UK/IE
- Baxi — Key things to consider when sizing and siting a flue
- Glow-worm — Air/flue pipe installation manual (PDF)
- Ideal Heating — What is a boiler flue and how do they work?
- Vaillant — ecoFIT sustain installation and maintenance instructions (PDF)