A powerful extractor hood can only perform as well as the route it uses to send cooking fumes outside. That is why extractor hood ducting requirements deserve attention before you order a new hood, fit cabinets or tile a splashback. Getting the duct size, route and outlet right helps remove steam, grease and odours efficiently - and can save you from a noisy hood that never quite clears the air.
For many kitchens, an externally vented hood is the most effective choice. It removes air from the room rather than filtering and returning it. But it does need a suitable path to an external wall or roof, so a little planning goes a long way.
Extractor hood ducting requirements: the essentials
Start with the hood manufacturer's installation instructions. They set out the duct diameter, maximum recommended run and compatible ducting type for that specific model. These details matter more than a one-size-fits-all rule, particularly with high-extraction chimney, canopy and ceiling hoods.
As a general guide, many modern extractor hoods are designed for 150mm ducting. Some smaller models use 125mm or 100mm connections. Always match the duct to the hood outlet where possible. Reducing a 150mm outlet to 125mm or 100mm may make fitting easier, but it restricts airflow. The result can be more noise, reduced extraction and grease settling in the ductwork.
The duct should lead outdoors through a correctly fitted external vent. It must not discharge into a loft, ceiling void, wall cavity or garage. Moisture and grease-laden air need to leave the property, not collect where they can cause condensation, staining and lingering smells. Never connect a cooker hood duct to a boiler flue, chimney or any other existing flue system.
For a new kitchen, your installer should also consider the wider ventilation requirements that apply to the property. Building Regulations guidance can affect kitchen extract ventilation, especially during more substantial renovations. A recirculating cooker hood may improve day-to-day cooking comfort, but it is not automatically a replacement for dedicated ventilation where this is required. If you are unsure, speak to a qualified installer or your local building control team before work begins.
Duct size is not just a measurement
It is tempting to choose the smallest duct because it is easier to hide behind cupboards or inside a boxed-in section. In practice, a larger duct often gives the hood the best chance of achieving its stated extraction rate.
Think of it like narrowing a pipe. Air has less space to move, resistance rises and the motor has to work harder. A 150mm round duct has a substantially larger internal area than a 100mm one, so it can move more air with less restriction. That can make a noticeable difference when you are frying, boiling several pans or cooking for a busy family.
Flat channel ducting is useful where space is tight, such as above wall units or in a ceiling void. It needs to have an equivalent airflow capacity to the round outlet it replaces. Do not assume that any rectangular channel will be suitable. Check its dimensions and the hood instructions, as undersized flat ducting creates the same bottleneck as a small round pipe.
Keep the route short and smooth
The shortest, straightest route is usually the best route. Every metre of ducting, bend, reducer and join adds resistance. A hood with a long run and several sharp turns may be noticeably louder and less effective than the same hood fitted directly through an outside wall.
Where bends cannot be avoided, use wide-radius bends rather than tight, abrupt elbows where possible. Keep them to a minimum. Rigid or smooth-wall ducting is generally preferable for longer runs because air passes through it more easily and grease is less likely to build up than in heavily ribbed flexible hose.
Flexible ducting can be helpful for a very short final connection where the manufacturer allows it. It should be pulled taut rather than left crushed or sagging behind a chimney hood. A long, concertina-style flexible run is rarely the best answer for performance or cleaning.
Plan the outlet before cabinets are fitted
An external outlet needs a sensible position. For a wall-mounted hood, this is often through the outside wall behind the chimney section or above the wall units. For an island hood, the duct may travel through the ceiling and out through an external wall or roof, depending on the layout.
The outlet should have a suitable external grille or flap to discourage rain, draughts and pests while allowing air to escape freely. Avoid placing it where discharged air is likely to be drawn straight back through an opening window or cause a nuisance near a neighbouring boundary. Your installer can advise on the most practical location for your property.
If ducting passes through a cold loft or roof space, it may require insulation to reduce condensation forming on or around the duct. It should be supported properly too. Loose ductwork can rattle, sag and collect condensation or grease over time.
For homes with solid walls, unusual roof lines or listed-building restrictions, external ducting may be difficult or may need additional approvals. In that situation, a recirculating extractor hood can be the more realistic option. It uses grease filters and, on many models, charcoal or carbon filters to reduce cooking odours before returning air to the kitchen. It does not remove moisture from the room in the way external extraction does, so good general ventilation is still valuable.
Match the hood, hob and kitchen layout
Ducting requirements should be considered alongside the type of hob and the way you cook. A larger hob, frequent high-heat cooking or an open-plan kitchen may justify a hood with stronger extraction and a well-planned 150mm duct route. A compact kitchen with light cooking needs may work well with a smaller hood, provided the installation matches the manufacturer's guidance.
The hood should normally be at least as wide as the hob, while the mounting height must follow the hood instructions and the hob manufacturer's safety guidance. This is particularly important above gas hobs, where clearance requirements can differ from electric, ceramic and induction models. Do not select a hood based on appearance alone if its required height, duct outlet position or chimney dimensions will not work with your cabinets.
Integrated, canopy and telescopic hoods can be excellent space-saving choices, but the cabinet above them must accommodate the unit and duct connection. A shallow wall unit may leave very little room for a large round duct, while an island installation can need more substantial ceiling space. Checking these dimensions before placing an order prevents costly alterations later.
Common ducting mistakes that cost performance
Most problems come from a few avoidable decisions. These are worth checking with your kitchen fitter before the installation date:
- Reducing the duct diameter below the hood manufacturer's recommendation just to fit a tighter space.
- Using too many bends, particularly tight 90-degree turns, on a long duct run.
- Running ducting into a loft or another enclosed area rather than to a proper external outlet.
- Choosing long lengths of crushed flexible hose instead of smooth, supported ducting.
- Forgetting to allow room for ducting inside a wall cabinet, chimney cover or ceiling void.
Before you buy: measure, then ask
Measure the hob width, available mounting height, cabinet depth and the likely duct route. Note where an outside wall, ceiling void or roof access sits in relation to the cooker. If you are replacing an existing hood, check the current duct diameter rather than assuming a new model will use the same connection.
It is also worth asking your installer whether the planned route is realistic before committing to a particular hood. A discounted appliance is only good value when it suits the kitchen and can deliver the performance you expect. If you need help comparing built-in, chimney or island options, the HomeWise Appliances sales team are here to help you find a hood that fits both your space and your budget.
A well-chosen hood with correctly sized, sensibly routed ducting makes everyday cooking more comfortable. Sort the practical details first, and you can choose the style, brand and price that work for your home with far more confidence.
