Short answer: for most modern stoves the minimum flue is 125 mm (5 inch) or 150 mm (6 inch), and which one applies depends on the fuel and whether the appliance is exempt under the Clean Air Act, not on how big the stove is. The 125 mm row is only for closed appliances up to 20 kW that burn smokeless fuel or are exempted appliances for the fuel they burn. Everything else up to 30 kW is 150 mm. And whatever the table says, the flue can never be smaller than the stove’s own outlet or the manufacturer’s stated minimum.
That last sentence is where most flue arguments end, so it is worth reading first.
Three rules that override the table
Before Table 2, Approved Document J sets three rules that sit on top of it:
- Paragraph 2.4. A connecting fluepipe should have the same diameter or equivalent cross-sectional area as the appliance’s flue outlet, and be no smaller than the manufacturer recommends.
- Paragraph 2.5. A flue should be at least the Table 2 size, and not less than the appliance’s flue outlet, and not less than the manufacturer’s recommendation. Whichever is largest wins.
- Paragraph 2.6. A multi-fuel appliance is sized for the fuel that needs the largest flue.
So a stove with a 150 mm outlet needs a 150 mm flue even if it qualifies for the 125 mm row. And a stove whose instructions say “150 mm minimum” needs 150 mm regardless of what Table 2 allows. The table is a floor; the appliance and its maker can raise it.
Table 2: the minimum flue sizes
| Installation | Minimum round flue | Rectangular or square flue of the same area, minimum dimension |
|---|---|---|
| Fireplace with an opening up to 500 mm × 550 mm | 200 mm | 175 mm |
| Fireplace with a larger opening, or exposed on two or more sides | 15 % of the total face area of the opening (2.7) | 200 mm |
| Closed appliance up to 20 kW that burns smokeless or low-volatiles fuel, or is an exempted fireplace under the Clean Air Act for the fuel burned | 125 mm | 100 mm straight; 125 mm with bends or offsets |
| Pellet burner or pellet boiler that is an exempted fireplace | 125 mm | (may be reduced to no less than 100 mm only where the manufacturer permits and a BS EN 13384-1 calculation supports it) |
| Any other closed appliance up to 30 kW, any fuel | 150 mm | 125 mm |
| Closed appliance above 30 kW and up to 50 kW | 175 mm | 150 mm |
Trade sizes map onto these directly: 125 mm is sold as 5 inch, 150 mm as 6 inch, 175 mm as 7 inch and 200 mm as 8 inch.
The 125 mm row, read carefully
This is the row people want to be on, because 5 inch liner is cheaper and fits more chimneys. It has two conditions, and both have to hold:
- The appliance is closed and rated at no more than 20 kW; and
- either it burns smokeless or low-volatiles fuel, or it is an exempted fireplace under the Clean Air Act for the fuel it actually burns.
“Exempted fireplace” is the legal term for what the trade calls a DEFRA-exempt or DEFRA-approved stove: one on the government’s list of appliances that may burn a normally-unauthorised fuel, such as wood, in a smoke control area. That exemption is per appliance and per fuel, and the list changes, so check the stove you are actually buying against the current list on gov.uk rather than a retailer’s page.
Two things people miss:
- Bituminous coal, untreated wood and compressed paper are not smokeless fuels. A stove burning ordinary logs qualifies for 125 mm only through the exemption route, never through the fuel route.
- A non-exempt stove burning wood is on the 150 mm row, however small its output. The 20 kW figure does not help it.
Worked examples
8 kW DEFRA-exempt wood burner, 125 mm outlet, straight chimney. Table 2 minimum 125 mm; appliance outlet 125 mm; the manufacturer says 125 mm. A 125 mm liner is acceptable, subject to the height and outlet provisions. If the liner has to bend or offset, a rectangular flue of the same area would need no dimension under 125 mm.
8 kW non-exempt wood burner, 125 mm outlet. Table 2 minimum is 150 mm because the “other closed appliance up to 30 kW” row applies. The 125 mm outlet does not reduce it; the flue must be 150 mm.
6 kW exempt stove with a 150 mm outlet. Table 2 would allow 125 mm, but paragraph 2.5 says not less than the appliance outlet. The flue must be 150 mm.
Multi-fuel stove, exempt for smokeless fuel only. Burning smokeless fuel it would sit on the 125 mm row. Burning wood it is a non-exempt appliance on the 150 mm row. Paragraph 2.6 says size for the fuel needing the largest flue: 150 mm.
Stove Toolkit takes the appliance class, the outlet size and the proposed flue, applies these rules in that order and cites the paragraph that decided the answer.
Open fireplaces
For an open fire in a fireplace up to 500 mm × 550 mm the flue is 200 mm round, or a rectangular flue of the same area with no dimension under 175 mm.
For a larger opening, or one exposed on two or more sides such as a corner or island fireplace, paragraph 2.7 replaces the fixed size with a proportion: the flue cross-sectional area should be 15 % of the total face area of the fireplace opening(s), with no dimension under 200 mm. Above 120,000 mm² of required area (0.12 m²), or for openings well beyond the standard size, AD J says to seek specialist advice.
Height
Diameter is half the story; a flue also has to draw. Paragraph 2.8 says a minimum flue height of 4.5 m could be satisfactory where the outlet position provisions in paragraphs 2.10 to 2.12 are followed. Where they are not, or where the installation is unusual, the flue is sized by calculation to BS EN 13384-1. That calculation is what the pellet row’s “reduced to 100 mm” option leans on too.
Scope
England, Approved Document J, 2010 edition incorporating the 2010, 2013 and 2022 amendments, Section 2, solid fuel appliances up to 50 kW. Wales publishes its own version of AD J; Scotland and Northern Ireland use different standards. Installing a solid fuel appliance is building work that must be carried out by a competent person, such as a HETAS registered installer, or notified to building control before it starts. Read the source at Approved Document J on gov.uk. For the air supply side of the same installation, see Do I need an air vent for my log burner?
Figures checked against the gov.uk PDF of Approved Document J on 11 September 2026. Contains public sector information licensed under the Open Government Licence v3.0. Plan CD is not affiliated with HETAS, DEFRA or any manufacturer; HETAS is a trade mark of its owner. The exempt appliance list is maintained by DEFRA and is not reproduced here.