Approved Document J says flue outlets should be situated externally so that combustion products disperse and, for balanced flues, air can be drawn in, and it sets out minimum separation distances from openings, gutters, corners, boundaries and other terminals for gas appliances.

What the guidance requires

Approved Document J, published on GOV.UK for use in England, says at paragraph 3.23 that outlets from flues should be situated externally so as to allow the dispersal of products of combustion and, if a balanced flue, the intake of air. The document shows a way of meeting this by locating outlets as shown in Diagram 34 and Diagram 35, with a table of minimum separation distances for gas appliances of up to 70kW net input. A further diagram, Diagram 35, covers outlets near roof windows. The guidance concerns building work in England, and nothing in it removes the separate duty under the Gas Safety Regulations that flues be installed in a safe position.

The separation table

The table gives minimum distances, in millimetres, for terminals in a range of locations: below and above an opening, horizontally to an opening, below gutters, soil pipes and drainpipes, below eaves, below a balcony or car port roof, from a corner or boundary alongside the terminal, from a surface or boundary facing the terminal, from another terminal, and above ground, roof or balcony level. The distances depend on whether the flue is balanced or open, whether it is natural draught or fanned, and, for some locations, the appliance’s rated heat input.

Notes to the table explain that an “opening” means an openable element such as a window or a fixed opening such as an air vent, and that smaller separations to a boundary may be acceptable for appliances shown to operate safely with such separations from adjacent or opposite surfaces. Because the figures vary so much with the type of flue, the installer should confirm the applicable distances for the particular appliance against the table and the manufacturer’s instructions.

Terminals and guards

The guidance says flue outlets should be protected where there is significant risk of blockage. Flues serving natural draught open-flued appliances should be fitted with outlet terminals if the flue diameter is no greater than 170mm, and larger flues should be assessed for blockage risk locally; in areas with nesting squirrels or jackdaws a protective cage of specified mesh size may be acceptable. It also says an outlet should be guarded if people could touch it or it could be damaged, and that an outlet in a vulnerable position, such as within reach from the ground or near a balcony, veranda or window, should be designed to prevent entry of anything that could obstruct the flow of flue gases.

Condensing boiler plumes

A note in Approved Document J says that the wet plume from a condensing boiler, positioned according to the safety distances in Diagram 34, can sometimes be a nuisance to neighbours. It states that the nuisance is not within the scope of the Building Regulations but that such installations could be considered a statutory nuisance under the Environmental Protection Act, and suggests installers may adopt the guidance in Chapter 6 of the Guide to Condensing Boiler Installation Assessment Procedure for Dwellings. It also recommends care to locate outlets away from parts of the building that could be damaged by frequent wetting.

Elsewhere the guidance says combustible materials in the building fabric should be protected from heat dissipating from flues, and that a fluepipe or chimney passing through a fire compartment wall or floor must not breach the fire separation requirements of Part B.

If a neighbour builds nearby

The Health and Safety Executive says the Gas Safety (Installation and Use) Regulations 1998 and the Building Regulations require flues to be installed in a safe position at specified distances from the boundary, but that some installations may not comply and still operate safely. It suggests that neighbours, builders and householders consult one another before an extension starts, and that anyone still concerned should ask a Gas Safe registered engineer to examine the flue. Planning permission for an external flue is a separate matter, covered in another guide on this site.

FAQs

Are the distances the same for every boiler?

No. The table in Approved Document J varies by location, flue type, draught type and, for some locations, appliance rating.

Can a neighbour’s extension affect my boiler flue?

HSE says an extension might affect the safety of a flue’s position and advises consultation before work starts and an examination by a Gas Safe registered engineer if concerns remain.

Is a condensing plume covered by the Building Regulations?

Not according to Approved Document J, which says any nuisance is outside their scope but could be a statutory nuisance under the Environmental Protection Act.

Bottom line

A boiler flue outlet must be placed so that combustion products disperse safely, kept clear of openings and boundaries by the applicable distances and protected against blockage and contact. Plume nuisance and neighbours’ building work are separate concerns that are best raised early with the installer or a Gas Safe registered engineer. This is general information for England, not advice on a specific installation.

Sources

This guide draws on the following primary sources, current as of 20 September 2026:

• Ministry of Housing, Communities & Local Government via GOV.UK, “Approved Document J: Combustion appliances and fuel storage systems” (PDF, 2022 amendments)
• GOV.UK, “Combustion appliances and fuel storage systems: Approved Document J”
• Health and Safety Executive, “Gas safety – Home owners”

Related guides

Best Boiler Deals is an independent guide. We may earn a fee from some links; this never affects what we write. This article is general information, not regulated advice on any specific installation — always confirm the details with a Gas Safe registered engineer.