What Part F asks of your hood, and where recirculating falls short
Approved Document F, under the Building Regulations 2010 for England, sets a minimum rate of air a kitchen extractor has to move. A cooker hood that ducts outside needs to shift 30 litres a second. An extract fan mounted elsewhere in the kitchen, working without a hood above the hob, needs to reach 60 litres a second.
A recirculating hood works on a different principle from a ducted one. It draws air through a grease filter and, on most models, a carbon filter, then blows that same air straight back out into the kitchen. The carbon filter cuts some of the cooking smell, and the grease filter catches airborne fat before it settles on your cabinet doors, but neither filter takes the moisture out of the air.
That is where a recirculating hood parts company with the regulation. Part F compliance needs a kitchen extract that vents to outside, and a recirculating hood never does that: it moves air around the room. Fitted where there is no route to an outside wall or roof, it will still take the edge off cooking smells day to day, but it does not count toward your Part F obligation, and a builder or building control officer checking the kitchen against the regulations will ask where the ducted extract is.
For a new kitchen or a refit that building control has to sign off, that makes the ducting route something to settle before the cabinets go in. Working out whether a hood you're looking at actually clears the 30 or 60 litres a second, once ducting length and bends are allowed for, is easier with numbers in front of you than with the maker's data sheet alone. The cooker hood extraction calculator takes your kitchen's dimensions and the hood's stated airflow and checks the result against the Part F minimum, though the answer it gives is only as good as the figures you put in and your installation instructions and building control still have the final say.
Air changes, duct losses and hood height, explained
Sizing a hood by air changes
One way to size a cooker hood is to work out how many times the air in the kitchen needs replacing each hour, then choose a hood whose extraction rate meets that figure. It gives you a target in cubic metres per hour that scales with the room. A large open-plan kitchen with a low ceiling needs a different extraction rate from a small galley kitchen, even with the same hob, because the volume of air being turned over is different.
Working this out by hand means multiplying the room's length, width and height to get its volume, then applying the sizing rule. The Cooker Hood Extraction Calculator does that arithmetic from the dimensions and hood figures you enter, and sets the result against the Approved Document F minimum at the same time, so you can see both numbers for your own kitchen. It runs in your browser and can only work from the figures you type in, so it is only as accurate as the data sheet and measurements you give it.
Why ducting length and bends matter
The extraction rate printed on a hood's box is measured with the fan running free. Every metre of duct, and every bend the air has to turn through, adds resistance, and that resistance reduces the airflow the hood can actually deliver once it is installed. A hood specified to just meet the Approved Document F minimum on paper can end up below it once its duct run is accounted for, particularly where the route to the outside wall or roof involves more than one bend.
This is why the run from the hood to the outside air matters as much as the model you choose. A short, straight duct keeps most of the stated extraction rate; a longer run with several bends can lose enough that a higher-rated hood, or a shorter route, is needed to still meet the figure your kitchen requires. Where the installation instructions give a stated airflow for the duct run as installed, that figure is the one worth checking against.
Height above the hob when the instructions don't say
Approved Document F sets the mounting height for a cooker hood at whatever distance the manufacturer's own installation instructions give. Where those instructions are silent on the point, the fallback is a distance of 650 to 750mm above the hob.
That range gives you room to work with, but it isn't a free choice. Check the installation instructions that came with the hood first, and only fall back to the 650 to 750mm range if they have nothing to say on the point.
Airflow and noise across the speed range
The table lines up airflow and noise at the lowest and highest normal speed for two Franke hoods against the Approved Document F minimum, converted into the same unit, so you can see how much headroom sits above the regulation's floor. The cooker hood extraction calculator lets you run the same comparison against your own kitchen's dimensions, hob type and duct run.
| Hood | Speed 1 | Top speed | Boost |
|---|---|---|---|
| Franke FDW 908 IB XS (downdraft, 900mm)Energy class A. Speeds 2 and 3 sit at 280 m³/h/49 dBA and 350 m³/h/53 dBA. Read from Franke's own product page, 16 September 2026. | 200 m³/h at 43 dBA | 440 m³/h at 57 dBA (speed 4, top normal speed) | 720 m³/h at 68 dBA |
| Franke Box Premium FBFP XS A70 (700mm)Energy class A. Speed 2 sits at 420 m³/h and 56 dBA. Read from Franke's own product page, 16 September 2026. | 290 m³/h at 47 dBA | 610 m³/h at 64 dBA (speed 3, top normal speed) | 720 m³/h at 68 dBA |
| Approved Document F minimumThe regulation sets a minimum extract rate. It sets no noise limit at any speed. | 108 m³/h (30 litres a second) from a hood that vents outside | 216 m³/h (60 litres a second) from an extract fan elsewhere in the kitchen |
The two hood rows come from Franke's own published specification pages, read 16 September 2026. The Approved Document F row converts the regulation's litres-a-second figures into the cubic-metres-an-hour unit hood makers use. These are the two models published here.
Check the hood height against the rest of the stack before you drill
A hood's mounting height sits on the same worktop and appliance stack covered in the heights and sizes guide, and the cooker hood extraction calculator lets you check your own room dimensions and duct run against the Approved Document F figures first.