In England, a wood burner or multi-fuel stove with a rated output of 5kW or less does not normally need a permanent air vent in a home with an air permeability above 5.0 m³/(h.m²), which covers most homes built before 2008. Above 5kW, you need 550mm² of permanent vent for every kW over 5kW, and in an airtight home you need 550mm² for every kW of the stove's rated output.
Your installer confirms what applies to your house. Below are the rules in one table, worked examples for common stoves, the cases where a vent is needed even under 5kW, external air kits, and the rules for open fires and gas fires.
The rules in one table
These figures come from Table 1 of Approved Document J, the Building Regulations guidance for England, for a stove without a flue draught stabiliser.
| Your home | Stove 5kW or less | Stove over 5kW |
|---|---|---|
| Air permeability above 5.0 m³/(h.m²) (most pre-2008 homes) | No permanent vent normally needed | 550mm² per kW above 5kW |
| Air permeability 5.0 m³/(h.m²) or less (airtight, most newer homes) | 550mm² per kW of rated output | 550mm² per kW of rated output |
Divide mm² by 100 to get cm². A stove with a flue draught stabiliser follows different, higher rates, shown in the last worked example below.
Worked examples
| Stove and home | Sum | Permanent vent |
|---|---|---|
| 5kW stove in a 1930s semi | 5kW or less in an older home | None normally needed |
| 8kW stove in a 1970s house | (8 - 5) x 550 | 1,650mm² (16.5cm²) |
| 5kW stove in a 2018 new build | 5 x 550 | 2,750mm² (27.5cm²) |
| 8kW stove in a new build | 8 x 550 | 4,400mm² (44cm²) |
| 7kW stove with a draught stabiliser in an older home | (5 x 300) + (2 x 850) | 3,200mm² (32cm²) |
The last example is the one Approved Document J gives itself: with a draught stabiliser in an older home, the rate is 300mm² per kW for the first 5kW and 850mm² per kW for the rest. In an airtight home with a stabiliser, it is 850mm² per kW of the whole rated output.
What counts as the stove's output?
Use the rated (nominal) output on the stove's data plate and manual, not the top of its output range. For multi-fuel stoves, Approved Document J says the air supply should be designed for the fuel that gives the highest output. For example, the Woolly Mammoth 5 gives 4.6kW on wood and 4.9kW on anthracite, so it is assessed at 4.9kW, still within the 5kW line.
For a boiler stove, ask your installer which output figure the vent is based on, because the headline figure includes the heat sent to water as well as the heat given to the room. The manual will list both.
Is your home airtight?
Air permeability is a measure of how much air leaks through the building fabric, found by an air pressure test. Approved Document J notes that a home built before 2008 is unlikely to be as airtight as 5.0 m³/(h.m²) unless extensive measures have been taken to improve its air-tightness, and its Appendix F explains how to assess older houses.
- Built before 2008, little changed: treat it as the older-home column, unless your installer finds otherwise.
- Built from 2008 onwards: assume it is airtight and plan for a vent or a direct air supply.
- Older but heavily upgraded: new windows and doors, sealed floors, insulation and draught-proofing can tighten a house a lot. Ask your installer to assess it.
Why 4.9kW and 5kW stoves are so popular
Because, in an older home, staying at 5kW or under usually avoids cutting a vent through the wall, which means no cold draught and less building work. Many 5kW stoves are also sized right for typical UK living rooms; see what size stove do I need? Popular choices include the Carron ECO 5kW, the FDC5 Eco and, for smaller rooms, the FDC4 Eco.
Browse all small log burners up to 5kW.
When you may need a vent even under 5kW
- Newer, airtight homes, as above.
- Older homes that have been heavily draught-proofed. Your installer may test or assess it.
- Extractor fans. A cooker hood or bathroom fan in or near the room can pull air back down the flue. Installers check for spillage with the fans running.
- The manufacturer says so. Approved Document J says manufacturers may specify larger vents, and their instructions must then be followed.
- A draught stabiliser is fitted. The higher rates above apply from the first kW.
The window test. If your stove smokes or struggles and improves the moment you open a window, the room is short of air, whatever the rules say on paper. Tell your installer; see also why is my log burner smoking?
External air kits: the alternative to a room vent
Some stoves can take their combustion air straight from outside through a duct, so the fire draws little or no air from the room. That avoids a cold draught across the room and helps in airtight homes with mechanical ventilation. The Carron ECO 5kW can take an external air supply; Opus cylindrical stoves such as the Opus Melody LS take the Opus external air kit (100mm stainless duct in 1.5m or 3m lengths); and Woolly Mammoth MK2 stoves convert with a direct air adaptor (ducting not included).
Whether a direct air supply removes the need for a room vent depends on the stove, the house and the installation, so your installer must confirm it meets Building Regulations.
What a stove vent should be like
- Permanently open: no closable louvres.
- Sized by equivalent area, the free area air can pass through, not the size of the grille's outer frame.
- Close to the stove where possible, so incoming air is drawn to the fire rather than across the room.
- Never blocked by furniture, curtains or draught-proofing.
What fitting a vent involves
A stove vent is usually a hole cored through an outside wall, lined, with a grille on each face. Your installer sizes it by the equivalent area the regulations need and picks a position where it will not be blocked. Some grilles are designed to reduce draughts while staying permanently open. If the room has no outside wall, ask your installer about other routes before you buy, or consider a stove with a direct air supply.
Open fires and gas fires need air too
Open fires need far more air than stoves: Approved Document J asks for permanently open vents of at least half the throat opening area, and for simple open fires gives figures such as 16,500mm² for a 400mm opening and 18,500mm² for a 450mm opening.
A decorative fuel effect gas fire in a fireplace with a throat normally needs a vent of at least 100cm², although in a less airtight home one rated no more than 7kW (net) with a flue gas clearance rate of no more than 70 m³ per hour may not. Flueless gas fires need the vent and minimum room size the manufacturer specifies: the Ekofires 5510, for example, needs a room of at least 27 cubic metres and a 100cm² vent. See gas fire types explained.
Who decides
A HETAS registered installer will assess the house, check the stove's instructions and fit any vent needed, then self-certify the installation and notify your council. Every new or replacement stove also needs a carbon monoxide alarm in the same room. These rules are for England; Wales has very similar guidance, and Scotland and Northern Ireland have their own.
Common questions
Do I need an air vent for a 5kW wood burner?
In England, not normally in a home with an air permeability above 5.0 m³/(h.m²), which covers most homes built before 2008. In an airtight newer home you need 550mm² per kW, so 2,750mm² for a 5kW stove.
Do I need a vent for a 4.9kW stove?
Not normally in an older home, as it is under 5kW. In an airtight home built to modern standards, you need 550mm² per kW, about 2,700mm².
How big a vent do I need for an 8kW stove?
In an older home, (8 - 5) x 550 = 1,650mm² (16.5cm²). In an airtight home, 8 x 550 = 4,400mm² (44cm²).
Can an external air kit replace a wall vent?
Sometimes. A stove with a direct outside air supply draws little or no room air, but whether it meets Building Regulations without a room vent depends on the stove and house. Your installer must confirm it.
Can I close my stove air vent when the stove is not in use?
Vents for stoves must be permanently open, so they should not be closable. Do not block them with furniture or draught-proofing.
Where should a stove air vent go?
In the same room, ideally close to the stove, so incoming air goes to the fire rather than across the room. Your installer will choose the position and check it is never likely to be blocked.
Will an air vent make my room cold?
A little cold air comes in, but a vent close to the stove is drawn straight into the fire. A stove with a direct external air supply avoids most of the draught.
Do gas fires need an air vent?
It depends on the type. A decorative fuel effect fire in a fireplace with a throat normally needs 100cm². Flueless fires need the vent and room size the manufacturer specifies. Your Gas Safe engineer will confirm.
Does an open window count as ventilation for a stove?
No. The air supply must be permanently open, and a window can be closed. Opening a window is a useful test of whether the room is short of air, but it is not a substitute for a vent.
Do electric or bioethanol fires need a vent?
Electric fires do not, as nothing is burnt. Bioethanol fires burn fuel in the room, so follow the maker's minimum room size and ventilation advice: the Adam Bellini, for example, needs a room of at least 63 cubic metres.
Choose a stove that suits your home
Stoves of 5kW or less, external air kits and everything in between. Tell us the room size and the age of your house and we will help you choose.
