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 | Fan Assisted (≤ 7 kW) | Fan Assisted (> 7 kW) |
|---|---|---|---|
| Below openable window / air vent | 300 mm | 300 mm | 300 mm |
| To side of openable window / air vent | 300 mm | 300 mm | 300 mm |
| Above openable window / air vent | 300 mm | 300 mm | 300 mm |
| Below gutter / eaves | 300 mm | 75 mm | 200 mm |
| Below balcony or car port | 600 mm | 200 mm | 200 mm |
| From ground level | 300 mm | 300 mm | 300 mm |
| From internal / external corner | 600 mm | 300 mm | 300 mm |
| From adjacent terminal | 300 mm | 300 mm | 300 mm |
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 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.
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Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.