Short answer: in England it depends on the stove’s rated output and how airtight the house is. For the common case, a stove with no flue draught stabiliser in a house built before 2008, Approved Document J asks for a permanent air vent only when the rated output is above 5 kW, and then sizes it at 550 mm² for every kilowatt above 5. Newer, tighter houses need 550 mm² for every kilowatt of the whole output. The manufacturer can ask for more, and building control or your HETAS installer will expect to see the working.
The rest of this guide shows where those numbers come from, works three examples, and covers the two mistakes that catch people out: sizing on the hole rather than the vent’s equivalent area, and using the wrong house column.
Where the rule comes from
Section 2 of Approved Document J (the 2010 edition incorporating the 2022 amendments, England) covers solid fuel appliances with a rated output up to 50 kW. Paragraph 2.2 says any room or space containing such an appliance should have a permanent air vent opening of at least the size given in Table 1. Table 1 splits the answer three ways:
- Open fires need permanently open vents of at least 50 % of the throat opening area, or of the flue cross-sectional area if there is no throat. Note 2 gives ready-reckoned figures for simple open fires of 350, 400, 450 and 500 mm nominal size.
- Closed appliances with a flue draught stabiliser (some boilers and cookers) use a higher rate: 300 mm² per kW for the first 5 kW and 850 mm² per kW for the balance in a leakier house, or 850 mm² per kW throughout in a tight one.
- Closed appliances with no draught stabiliser, which is nearly every domestic wood burning or multi-fuel stove, use the figures in the table below.
The “house” split is design air permeability, in cubic metres per hour per square metre of envelope at 50 Pa. Above 5.0 is the leakier column; 5.0 or below is the tighter column.
The figures for a stove with no draught stabiliser
| House | Rule (AD J Table 1) | Permanent vent needed |
|---|---|---|
| Air permeability above 5.0 (typically pre-2008) | 550 mm² per kW of rated output above 5 kW | 5 kW: none · 8 kW: 1,650 mm² · 12 kW: 3,850 mm² |
| Air permeability 5.0 or below (typically 2008 on, or extensively draught-proofed) | 550 mm² per kW of total rated output | 5 kW: 2,750 mm² · 8 kW: 4,400 mm² · 12 kW: 6,600 mm² |
Divide by 100 for cm², the unit most vents are sold in (Table 1 note 1). So an 8 kW stove in a 1930s semi needs 16.5 cm² of equivalent area; the same stove in a 2015 new-build needs 44 cm².
Worked example, 8 kW stove, older house. Output above the 5 kW threshold is 8 − 5 = 3 kW. At 550 mm² per kW that is 3 × 550 = 1,650 mm², or 16.5 cm² of equivalent area. Stove Toolkit shows this working line by line and cites Table 1 next to the result.
For a multi-fuel stove, paragraph 2.2 says to size for the fuel that gives the highest rated output.
Which column is my house?
Table 1 note 4 is the practical guide: a dwelling built before 2008 is unlikely to have an air permeability below 5.0 unless it has been extensively draught-proofed. Appendix F adds that where there is doubt you should assume the house is below 5.0 and fit the larger vent, or seek advice. A house that has had new windows, external wall insulation and a full draught-proofing job may well have moved into the tighter column even if it is Victorian.
Equivalent area is not the size of the hole
This is the mistake that fails the most installations. Table 1 sizes are equivalent area, the figure the vent manufacturer declares under BS EN 13141-1 (Table 1 note 1). A terracotta airbrick or a fly-screened grille can have an equivalent area of half or less of the opening behind it. Paragraph 1.14 says to use the manufacturer’s stated equivalent area; only if there is none should you fall back to the unobstructed cross-sectional area at the vent’s narrowest point, and even then apertures smaller than 5 mm do not count.
Buy the vent by its declared equivalent area in cm², not by the size of the core drill.
Where the vent goes
Paragraph 1.10 sets the placement rules: the vent must be permanently open and non-adjustable, positioned where it is unlikely to be blocked, and not in a fireplace recess. It can be direct to outside or, subject to the provisions in Section 1, via another room or space. The Table 1 sizes are for one appliance only (paragraph 1.3); a room with two open-flued appliances needs separate assessment.
The manufacturer can ask for more
Paragraph 2.3 is explicit that manufacturers may specify larger vents than Table 1, and that their installation instructions should be followed subject to the AD J minimum. The regulation figure is a floor, not a ceiling. Some stoves can also be fitted with a direct external air supply; whether that changes the room vent requirement is a matter for the manufacturer’s instructions and your building control body, and it is outside what this guide and the app calculate.
While you are at it: the carbon monoxide alarm
A new or replacement fixed solid fuel appliance in a dwelling must have a carbon monoxide alarm in the same room (paragraph 2.34). It should be to BS EN 50291-1:2018, and positioned either on the ceiling at least 300 mm from any wall, or on a wall as high as possible above doors and windows but not within 150 mm of the ceiling, and between 1 m and 3 m horizontally from the appliance (paragraphs 2.35 and 2.36). Ventilation and an alarm are complementary; neither replaces correct installation and regular servicing.
Scope and other UK nations
Everything above is the England edition of Approved Document J. Wales publishes its own version with differences; Scotland and Northern Ireland use different technical standards altogether. Installing a solid fuel appliance is notifiable building work: it must be done by a competent person, for example a HETAS registered installer, or notified to your local authority building control before work starts.
Figures in this guide were checked against the gov.uk PDF of Approved Document J (2010 edition incorporating 2010, 2013 and 2022 amendments) on 9 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.