Kitchen Extractor Ducting: Size, Route and Rules for UK Homes
Good kitchen extractor ducting means a rigid, smooth duct of at least 150mm (or 220 x 90mm rectangular), kept as short and straight as possible, running to a free-flowing terminal on the outside. Get that right and even a mid-range hood performs well. Get it wrong and an expensive hood will be noisy, weak and prone to grease and condensation problems.
This guide explains what size duct you need, how to plan the route, what the regulations say, and where renovations most often go wrong. It is written for homeowners planning a new kitchen, an extension or a full refurbishment, particularly in London homes where walls, lofts and neighbours often limit the options.
TL;DR
Use rigid or smooth semi-rigid duct with a constant cross-section of at least 150mm round, or an equivalent such as 220 x 90mm.
Keep the run short and straight. Every bend reduces performance, and each bend means the maximum duct length must come down.
Insulate and vapour-seal any duct passing through a cold space such as a loft, to stop condensation.
Fit an external terminal with a free area of at least 90% of the duct area, otherwise the grille chokes the hood.
Plan the route before the units go in. Ducting fitted blind behind finished cabinets is where kinks and damage happen.
Table of Contents
Why the ducting matters more than the hood
Most people spend their time choosing the hood and almost none on the duct. In practice the duct is the part that decides whether the hood delivers its rated airflow. Every metre of pipe, every bend and every restrictive grille adds resistance, and the fan has to push against it. The result is lower extraction, more noise and more strain on the motor.
A ducted hood is also the better option for your home. It takes steam, grease and cooking pollutants outside rather than filtering them and returning them to the room. If you are weighing ducted against recirculating, our homeowner’s guide to kitchen ventilation covers the basics in more detail.
What the Building Regulations say
In England, kitchen ventilation is covered by Approved Document F. It requires extract ventilation to the outside in kitchens, utility rooms, bathrooms and sanitary accommodation. That extract can be intermittent or continuous.
The figures you will meet most often:
Situation | Minimum extract rate |
Cooker hood extracting to the outside | 30 l/s |
No cooker hood, or hood that does not extract to the outside | 60 l/s |
The lower figure for a ducted hood is one more reason to duct to the outside. A recirculating hood does not count as extract to the outside, so the kitchen would need a separate fan or other extract route capable of 60 l/s.
The same document also covers how the ducting should be built. It points to rigid ducting for intermittent extract fans, with a maximum length of 2.0 metres where practical. Beyond that, design calculations and drawings should be provided to the building control body to show the intended flow rates will be achieved. For a cooker hood on a longer run, that is the sort of evidence a competent installer should be able to produce.
This is guidance for England. If your property is in Wales, Scotland or Northern Ireland, check the equivalent rules for that nation.
Choosing the right duct size
The size most hoods are designed for is a 150mm round duct. The regulatory guidance says the duct should have a constant cross-sectional area of at least 150mm, or an equivalent rectangular section such as 220 x 90mm. Manufacturers say the same. One extraction guide from Caple, for example, specifies rigid 6 inch (150mm) circular or 90 x 220mm rectangular ducting for its downdraft units.
Round or rectangular?
Both work, provided the cross-section stays constant along the run. Rectangular flat channel is useful when you need to run along a plinth, through a shallow ceiling void or inside a narrow stud wall. Round duct is generally simpler and has fewer joints for a straight run to an outside wall.
Do not reduce down
A very common shortcut is to connect a 150mm hood outlet to smaller, older ducting using an adapter or reducer. It looks harmless, but it drastically reduces extraction capacity, increases noise and can void the manufacturer’s warranty, according to the Westin installation manual. If a 150mm path genuinely cannot be achieved, choose a hood designed for that restriction instead of squeezing a larger one onto a smaller pipe.
If you are replacing an old hood and the existing duct is 100mm or 125mm, treat that duct as something to be replaced, not reused. The cost of replacing it is small beside the cost of a hood that never performs.
Rigid, semi-rigid or flexible?
The material matters as much as the diameter.
Rigid duct (metal or smooth plastic) has a smooth interior, so air flows with the least resistance and grease does not catch on ridges. It is the best choice for the main run.
Smooth semi-rigid duct is acceptable where the guidance allows, and is helpful for awkward connections.
Flexible corrugated hose is the weakest option. Using it for long runs severely restricts airflow, increases noise and can trap grease. Keep it for the short final connection to the hood spigot, and pull it taut so the inner surface stays as smooth as possible.
The cheap flexible kits that dominate online shopping results are tempting, because they are easy to push through a gap. They are the reason so many hoods under-perform. A kit is only as good as the route it is asked to follow.
Planning the route
The best duct route is the boring one: from the hood, straight through an external wall, with no bends. The Westin manual calls a straight run through an external wall the gold standard for performance. Everything else is a compromise, and compromises should be made on paper before building starts.
Step by step
Fix the hob position first. The hood sits above it, and the duct starts at the hood. Moving the hob later moves the duct.
Find the nearest suitable external wall. Check for obstructions such as beams, joists, drainpipes, windows and neighbouring boundaries. In terraced and mansion-block London homes the shortest wall is not always the one you can use.
Measure the run and count the bends. Every bend degrades performance and reduces the length you are allowed. If the run is long or has several turns, ask for calculations rather than hoping.
Decide where the duct is hidden. Plan the boxing, ceiling void or cabinet space so there is room for a full 150mm duct, not just the hood body.
Choose the terminal. Think about where air leaves the building and what it will blow onto.
Install before finishing. Fit the duct while you still have access, then seal joints and test before cabinets and ceilings close it in.
Why not fit it blind?
When duct is pushed through a gap with no clear access, it often ends up with unseen kinks or damage that compromise extraction. Nobody notices until the hood is switched on and the kitchen still smells of last night’s dinner. Planning the route early in the design, ideally at the same time as the kitchen layout, avoids this.
Position of the hood
Fumes spread as they rise, so manufacturers advise placing the extractor as far forward over the hob as practicable, to minimise escape. There is also a minimum and maximum distance between the hob and the bottom of the hood. The minimum is there for safety and to prevent overheating, while exceeding the maximum reduces efficiency, as one Westin manual explains. Always follow the clearances in your model’s own instructions.
Stopping condensation in cold spaces
Kitchen air is warm and full of moisture. When it travels through a cold duct, the moisture condenses on the inside wall and can drip back or leak at joints. In a loft, cold void or external soffit, that can damage ceilings and insulation.
The guidance in Approved Document F is that ducts passing through unheated spaces should be insulated with at least 25mm of material with a thermal conductivity no higher than 0.04 W/m·K, and be fully vapour sealed. Two practical points follow:
Keep the run through cold areas as short as possible.
If you are converting a loft, plan the duct together with insulation and roof build-up. Our loft insulation workflow shows how those layers fit together.
The external terminal
The outside end of the duct is easy to overlook. The regulations require that every external terminal has a free area of at least 90% of the duct’s area. A decorative cover with narrow slots, an insect mesh that clogs, or a grille with a small opening can cancel out the benefit of a powerful hood.
When choosing one:
Check the free area figure, not just the grille size.
Use a terminal with a damper or flap that closes when the fan is off, to limit draughts and backdraught.
Position it away from windows, doors and air bricks so extracted air does not blow straight back in.
On a listed building or in a conservation area, check what is permitted on the external elevation before you drill.
Gas appliances and extractor fans
There is a safety point many renovation guides miss. Extract fans can cause combustion gases to spill from open-flued appliances, such as some gas fires or boilers, back into the room. This can happen even when the appliance is in a different room from the fan, as noted in Approved Document F, which directs readers to Approved Document J.
If you have an open-flued gas appliance and are fitting a powerful hood, ask a Gas Safe registered engineer to check it before the new system goes live. Our summary of UK gas safety regulations explains who can do what. In an open-plan layout, where a kitchen joins a living space with a fire or flue, this question needs answering at design stage.
Common mistakes we see
Reusing a narrow old duct for a new 150mm hood.
Long runs of corrugated hose, slack and kinked.
Too many bends, especially tight 90 degree elbows, with no allowance for the lost length.
Poorly sealed joints, which leak greasy, moist air into voids.
Uninsulated duct in a loft, leading to condensation.
A restrictive external grille.
Ducting fitted after the ceiling and cabinets are finished, so nobody can check it.
Our perspective
We have been refurbishing London homes since 2006, and ducting is a good example of a detail that decides whether a kitchen feels finished. It is not visible and it is rarely on the moodboard, so it gets left to the end. By then the cabinets are ordered, the ceiling is boarded and the only route is the long, awkward one.
Our approach is to settle the duct route during design, at the same time as the layout and the electrical plan. Because we manage projects in-house with director oversight, the person who agrees the hob position is the person who is accountable for the duct behind it, with no chain of subcontractors passing the problem along. In a west London terrace or a Kensington flat, where the ideal external wall may be occupied by a neighbour’s boundary or a lightwell, that early conversation is what keeps the kitchen both beautiful and functional.
Our advice is simple. Spend a little more on the duct and the terminal, and a little less on the hood if you must choose. A well-ducted mid-range hood will outperform a premium one fighting a poor run every time.
— Mateja
When ducting is part of a bigger project
Ducting rarely arrives alone. It is tied to the hob position, the cabinet layout, the ceiling height, the electrics and, in an extension or loft, the structure and insulation. In an extension, a new rear kitchen often gives you the straightest external wall you will ever get. In a flat or terrace, it may mean a carefully boxed run across the ceiling.
If you are planning a new kitchen, our kitchen refurbishment service includes working out ventilation alongside layout, plumbing and electrics, so the duct is designed in, not forced in. For larger changes, a full property refurbishment lets us coordinate ventilation across the whole house. We work with homeowners across Fulham, Chelsea, Kensington, Chiswick, Hammersmith and Notting Hill, and we provide a fixed, itemised quotation after a site survey, so you know what the duct, terminal and making good are costing before work starts.
A quick checklist before you buy a hood
Does the hood outlet match a 150mm duct, or an equivalent rectangular size?
How far is the hob from the nearest usable external wall?
How many bends will the route need?
Will any part of the duct pass through a loft or other cold space?
What is the free area of the terminal you plan to use?
Do you have an open-flued gas appliance?
Who will provide calculations if the run is long?
If you cannot answer those questions, the hood is not ready to be chosen. A site survey is the quickest way to answer them.
FAQ
What size should kitchen extractor ducting be?
The standard answer is 150mm round, or an equivalent rectangular duct such as 220 x 90mm, kept at a constant cross-section along the run. Smaller duct restricts airflow and raises noise. Do not connect a 150mm hood to a narrower pipe with a reducer.
Can I use flexible ducting for a cooker hood?
Only for a short final connection to the hood spigot, pulled taut. For the main run, use rigid or smooth semi-rigid duct. Long flexible runs restrict airflow, make more noise and can trap grease.
How long can the duct run be?
Shorter is always better. Approved Document F gives 2.0 metres as the maximum for rigid ducting on intermittent extract fans where practical, and longer runs need calculations and drawings for building control. Each bend reduces the length you can allow, and your hood manufacturer will state its own maximum, so check both.
Does ducting through a loft need insulating?
Yes. Duct passing through an unheated space should have at least 25mm of insulation with a conductivity of 0.04 W/m·K or better, and be fully vapour sealed, to prevent condensation forming inside the duct.
Is a ducted hood better than a recirculating one?
A ducted hood removes moisture, grease and smells from the house, and it counts as extract to the outside at the 30 l/s rate. A recirculating hood does not, so the kitchen then needs a 60 l/s extract route from another source.
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