Kitchen Extractor Ducting in England: Avoid a Part F Sizing Mistake
Choose kitchen extractor ducting: match hood airflow to diameter, reserve flexible hose for short runs, and check Approved Document F before you buy.
Kitchen Extractor Ducting in England: Avoid a Part F Sizing Mistake
For most UK kitchens, rigid or smooth semi-rigid ducting is the right choice for the main run, with flexible hose reserved for short final connections only. Work out your hood’s extraction rate first, then pick a diameter: 100mm, 125mm or 150mm cover the vast majority of domestic hoods. For a quick shortlist, look at the SIA D3 kit in 125mm, the SIA D7 kit in 150mm, or a Manrose flexible connector for the last stretch to the terminal.
TL;DR:
- Sizing guidance sets 100mm below 450 m³/h, 125mm at 450 to 650, and 150mm at 750 to 1,350; round up near band limits.
- England’s Approved Document F requires 30 litres per second with an externally vented hood, or 60 litres per second when no hood vents outdoors.
- Runs longer than 2.0 metres normally need design calculations for building control, while ducts through unheated spaces require insulation and a vapor sealed wrap.
- Kitchens with an open flued gas appliance need an expert safety assessment, because extraction can draw combustion gases back into the room.
Table of Contents
- Comparing SIA, Manrose and flexible aluminium ducting kits
- How to choose the right diameter and avoid the common mistakes
- What Approved Document F requires for kitchen extraction
- Materials, condensation control and fitting the ducting properly
- Planning ducting routes before you start an extension
- Coordinating ducting with a single-storey rear extension
- FAQ
- Sources
Comparing SIA, Manrose and flexible aluminium ducting kits
Most UK retailers sell a handful of recognisable kit types, and the differences between them come down to bore size, what is bundled in the box, and whether the materials suit a long rigid run or a short flexible connection.
The SIA D3 Universal Kitchen Cooker Hood Extractor Fan Ducting Vent Kit 125mm x 3m is a 125mm bundle aimed at mid-range hoods. It typically includes around 3 metres of ducting plus the fittings needed for a straightforward domestic install, and sits at a budget-friendly price point compared with buying components separately. It suits hoods with moderate airflow output, which covers a large share of standard integrated and chimney hoods fitted in England.
Step up to the SIA D7 Universal Kitchen Cooker Hood Extractor Fan Ducting Vent Kit 150mm x 3m for higher-output hoods, generally those rated between 750 and 1,350 cubic metres per hour. The wider bore reduces resistance at higher airflow, which matters more as extraction rate climbs. Like the D3, it comes as a 3 metre kit, making it a like-for-like upgrade for anyone whose hood spec sheet calls for the larger diameter.
The Manrose Extractor Fan Ducting Back Draught Shutter Internal Fit Wall Kit is a shorter kit, commonly supplied as 150mm in diameter and about 1.5 metres in length, and it is built around the final connection rather than a long run. It bundles a backdraught shutter and wall-fixing hardware, which is useful where the duct run itself is handled in rigid pipe and the flexible section only needs to bridge the last stretch to the wall terminal. UK suppliers market flexible cooker-hood kits such as this one with a terminal grille, hose and fixings included, and they are marketed specifically for domestic extraction systems.
Two canopy options worth naming together are the SIA 52cm Built In Cupboard Canopy Cooker Hood Extractor Fan Ducting UC52SI and the near-identical SIA 52cm Built In Cupboard Canopy Cooker Hood Extractor Fan with 3m Ducting UC52SI. Both are designed to sit under a cupboard or within a 52cm unit opening, with a canopy hood and a small ducting allowance supplied, so they fit kitchens where cabinet space is already fixed and a slimline fan is the only option.
The 70cm equivalents, the SIA 70cm Under Cupboard Canopy Built In Cooker Hood Extractor Fan with 3m Ducting and the SIA 70cm Under Cupboard Canopy Built In Cooker Hood Extractor Fan Ducting, follow the same logic for wider hob setups. They are bundled with 3 metres of ducting, giving an out-of-the-box install for the common 70cm hob width found in many standard English kitchens.
For anything that does not fit a boxed kit, Aluminium Flexible Ventilation Duct sold in multiple diameters fills the gap. It is cheap, bends around obstructions, and is widely stocked, but it carries more airflow resistance than rigid pipe and is better suited to short, awkward connections than to a full run across a kitchen ceiling void.
A few things worth checking before you add a kit to your basket:
- Confirm whether a backdraught shutter or grille is actually included, since some kits list it separately.
- Match the diameter on the box to your hood’s spigot size, not just to what the retailer recommends as a default.
- Check the supplied length against your actual run: a 3 metre kit may need extending or trimming depending on your kitchen layout.
How to choose the right diameter and avoid the common mistakes
Getting the diameter right starts with your hood’s extraction rate, not with whatever ducting happens to be on the shelf. Manufacturer guidance correlates duct diameter with hood extraction capacity: 100mm suits hoods rated below roughly 450 cubic metres per hour, 125mm covers 450 to 650 m³/h, 150mm handles 750 to 1,350 m³/h, and 200mm or larger is reserved for exceptionally high-output systems. Undersizing the duct for a powerful hood chokes airflow and makes the fan work harder for less extraction.
Follow this sequence when you are specifying a kit:
- Check your hood’s spigot diameter and stated extraction rate on the manufacturer’s spec sheet or the unit itself.
- Match that rate to a duct diameter using the ranges above, rounding up if your hood sits near the top of a band.
- Measure the actual run from spigot to external terminal, including any vertical rise through cupboards or a ceiling void.
- Count the bends in that route: every 90-degree bend adds resistance, so fewer, gentler bends outperform several tight turns.
- Choose a reducer only if your hood spigot and your chosen duct diameter do not match, and fit it as close to the hood as possible rather than mid-run.
- Keep flexible sections short, limited to the final connection at either end, with the bulk of the run in rigid or smooth semi-rigid pipe.
Approved Document F reinforces this approach at a regulatory level: rigid or smooth semi-rigid ducts are recommended for intermittent extract systems, with flexible ductwork limited to short final connections rather than full runs.
Pro Tip: Avoid running corrugated flexible duct the full distance to an external wall. It traps grease in the ridges and adds drag that a rigid run simply does not have, so performance drops off noticeably over longer distances.
What Approved Document F requires for kitchen extraction
Building regulations in England set clear minimum figures for kitchen ventilation, and these are the numbers a competent installer or building control officer will actually check.
Approved Document F (2026) sets the minimum intermittent extract rate at 30 litres per second for a cooker hood that extracts to the outside, rising to 60 litres per second if the kitchen has no hood extracting externally, according to the current guidance. These are the figures your hood’s rated output needs to clear once converted from cubic metres per hour.
Beyond the headline extract rate, the same guidance sets practical limits on how ducting is installed:
- Flexible ductwork is intended for short final connections, not full runs between hood and terminal.
- Duct runs longer than 2.0 metres normally need to design calculations submitted to building control to demonstrate the required flow rate can still be met.
- Ducts passing through unheated spaces need insulation to control condensation before they reach the external terminal.
- The external terminal itself needs adequate free area so the fan is not working against a restrictive grille.
If your kitchen has an open-flued gas appliance, there is an added safety dimension: extract rates need to be assessed so the fan does not pull combustion gases back into the room, and Approved Document J advises seeking expert guidance in that scenario rather than treating it as a standard installation. If your duct run will exceed 2.0 metres, it is worth reading our guide to planning drawings for extensions so you know what documentation building control is likely to ask for.
Materials, condensation control and fitting the ducting properly
The material you choose affects cleaning, airflow and how long the system lasts before it needs attention.
Rigid PVC holds a consistent internal bore, which keeps resistance low and makes cleaning straightforward since there are no ridges for grease to catch on. 150mm rigid PVC round ducting is widely used in England for higher-output kitchen extraction, sold as push-fit straight lengths with matching bends and wall plates. Aluminium flexible duct is cheaper and easier to route around obstacles, but its corrugated wall increases resistance and gives grease more surface to collect on. Corrugated foil duct sits at the lower end again: fine for a short connection, poor for anything approaching a full run.
Insulation matters wherever ducting crosses an unheated space, such as a loft void or an unheated utility room. A reasonable specification is insulation equivalent to around 50mm in heated spaces and 25mm in unheated voids, with a vapour-sealed wrap to stop moisture condensing inside the insulation itself and dripping back down the duct.
On fixings, a few habits separate a tidy install from one that leaks air or works loose over time:
- Use jubilee clips at every joint rather than tape alone, which degrades with heat and age.
- Seal rigid joints with an appropriate mastic or sealant rated for ducting, not generic household filler.
- Support long horizontal runs with brackets at regular intervals so the duct does not sag and create a low point where condensation collects.
Pro Tip: Fit the backdraught shutter at the external terminal, not partway along the run. It stops droughts and insects entering when the fan is off, but only works properly at the point where outside air meets the duct.
Planning ducting routes before you start an extension
If a new kitchen is part of a rear extension, it pays to plan the extraction route at design stage rather than after the plasterboard goes up. Retrofitting a duct through a finished stud wall or a new ceiling void is more disruptive and more expensive than allowing for it on the drawings from day one. It also matters where the kitchen sits near any open-flued gas appliance, since the extract rate and duct route both affect how safely that appliance continues to operate. A coordinated design process, where the duct route, the structural openings and the building control evidence are planned together, reduces the risk of a late change forced by an inspector.
Coordinating ducting with a single-storey rear extension
When your kitchen extraction is just one part of a bigger single-storey rear extension, it should be handled as part of the same design and build process rather than treated as an afterthought for your installer to improvise around.
Instant online quoting and live 3D modelling can show where the extractor terminal and duct route sit on your actual layout before any work starts. Coordinated design, planning and construction help ensure duct routing aligns with structural openings, building control evidence required by Approved Document F, and any necessary gas-safety checks for kitchens with an open-flued appliance.
- A fixed price agreed before work starts, rather than variable quotes from separate trades.
- 3D modelling to check duct routes and terminal positions against the finished layout.
- A workmanship warranty covering the completed extension.
If your extension includes a new kitchen, start with our instant extension quote tool to see pricing and a 3D layout for your project, or read more about how booking and staged payments work before you commit. Where duct routing affects a structural opening, our structural partners at Xponexus Engineering and measured-survey partners at SGS Surveys support the calculations and as-built records building control may ask to see.
FAQ
What size ducting do I need for my cooker hood?
Match the duct diameter to your hood’s stated extraction rate: 100mm suits hoods under roughly 450 m³/h, 125mm covers 450 to 650 m³/h, and 150mm handles 750 to 1,350 m³/h, according to manufacturer sizing guidance. Check your hood’s spigot size too, since a mismatch will need a reducer.
Can I use flexible ducting for the whole run?
Flexible ducting is intended for short final connections rather than a full run from hood to terminal. Approved Document F recommends rigid or smooth semi-rigid ducts for intermittent kitchen extract systems, with flexible sections kept to the shortest practical length at each end.
What is the minimum extract rate for a kitchen in England?
Approved Document F sets the minimum intermittent extract rate at 30 litres per second for a cooker hood extracting to the outside, rising to 60 litres per second if there is no hood extracting externally. This is the figure your hood’s output needs to meet once converted from cubic metres per hour.
Do I need insulation on kitchen extractor ducting?
Ducting that passes through an unheated space, such as a loft void, needs insulation to prevent condensation forming inside the duct and dripping back towards the hood. A vapour-sealed wrap around the insulation stops moisture collecting within the lagging itself.
How long can a flexible duct run be before building control needs more evidence?
Runs longer than 2.0 metres normally require design calculations submitted to building control showing the system still meets the required flow rate. Shorter connections generally do not need this extra evidence, but it is worth checking with your installer if your layout is unusual.
Sources
- Approved Document F: Ventilation (2026)
- Approved Document F: Ventilation (Volume 1 dwellings)
- Luxair technical guidance — hood ducting recommendations
- Wickes
- 150mm PVC Ducting for Kitchen Extract Systems – Quality Heating