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Part J Flue Sizing & Chimney Requirements
Flue sizes by appliance type, terminal positions, clearances from openings, and hearth specifications, referencing Approved Document J: Combustion Appliances and Fuel Storage Systems (2010 edition incorporating 2013 amendments).
Gas Flue Terminal Positions (AD-J Diagram 25/26)
| Position | Natural Draught (mm) | Fan Assisted (≤ 7 kW, mm) | Fan Assisted (> 7 kW, mm) |
|---|---|---|---|
| Below openable window / air vent | 300 | 300 | 300 |
| To side of openable window / air vent | 300 | 300 | 300 |
| Above openable window / air vent | 300 | 300 | 300 |
| Below gutter / eaves | 300 | 75 | 200 |
| Below balcony or car port | 600 | 200 | 200 |
| From ground level | 300 | 300 | 300 |
| From internal / external corner | 600 | 300 | 300 |
| From adjacent terminal | 300 | 300 | 300 |
Minimum Flue Sizes by Appliance Type
| Appliance | Flue Type | Min. Diameter / Size |
|---|---|---|
| Gas boiler (room-sealed) | Concentric balanced flue | 60/100 mm or 80/125 mm (per manufacturer) |
| Gas fire (open flue) | Conventional flue | 125 mm diameter minimum |
| Oil boiler (room-sealed) | Twin-wall stainless or concentric | 100-150 mm (per manufacturer) |
| Oil boiler (open flue) | Conventional flue or liner | 150 mm diameter minimum |
| Wood stove (≤ 20 kW) | Twin-wall insulated or lined chimney | 150 mm diameter (or 175 mm for > 20 kW) |
| Open fire | Masonry chimney or liner | 200 mm diameter or 185 x 185 mm square min. |
Solid Fuel Hearth Requirements (AD-J Section 2.26-2.33)
| Dimension | Requirement |
|---|---|
| Constructional hearth thickness | ≥ 125 mm non-combustible |
| Minimum overall size | 840 mm x 840 mm |
| Extension in front of appliance | ≥ 300 mm (or 225 mm if raised 100 mm+) |
| Extension to sides and rear | ≥ 150 mm beyond appliance |
| Superimposed hearth | ≥ 12 mm non-combustible on constructional hearth |
Chimney Height Requirements (AD-J Diagram 20)
| Requirement | Dimension |
|---|---|
| Minimum height (appliance top to chimney top) | 4.5 m (solid fuel) |
| Above highest roof intersection | ≥ 1,000 mm |
| Above ridge (if within 600 mm horizontally) | ≥ 600 mm above ridge |
| Oil boiler minimum height | As per manufacturer (typically 3-4.5 m) |
Scotland & Northern Ireland
Scotland follows Technical Handbook Section 3.19-3.22 (Combustion appliances) with broadly similar requirements. Northern Ireland uses Technical Booklet L. Terminal position diagrams and hearth requirements may differ slightly — always check the relevant jurisdiction.
Related Calculators
Use the Chimney Flue Sizing Calculator for appliance-specific sizing, or the Radiator Sizing Calculator for heating system design.
Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.
Next up: related pages
Frequently Asked Questions
Most domestic gas boilers up to 70 kW use a concentric (coaxial) flue of 60/100 mm or 80/125 mm diameter. The exact size depends on the boiler manufacturer's specifications and the flue run length. Room-sealed (balanced flue) appliances are the standard for modern installations. AD-J Section 2 and the manufacturer's installation instructions specify minimum flue sizes and maximum flue lengths.
For room-sealed natural draught gas appliances, the flue terminal must be at least 300 mm from any openable window, air vent, or other opening. For fan-assisted appliances, the distance depends on the appliance input rating — typically 300 mm for appliances up to 7 kW, and 600 mm for those over 7 kW. Full terminal position requirements are shown in AD-J Diagram 25/26.
A constructional hearth for a solid fuel appliance must be at least 125 mm thick (non-combustible), extend at least 150 mm beyond the sides and rear of the appliance, and extend at least 300 mm in front (or 225 mm if the appliance is raised 100 mm+). The hearth must be at least 840 x 840 mm overall. Superimposed hearths (on top of a constructional hearth) must be non-combustible and at least 12 mm thick.
The chimney serving a solid fuel appliance must be at least 4.5 m measured from the top of the appliance to the top of the chimney pot or terminal. The chimney must also extend at least 1 m above the highest point of contact with the roof, and at least 600 mm above the ridge if within 600 mm horizontally of the ridge. See AD-J Diagram 20.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.