Cooker Hoods for Kitchen Extensions: Chimney, Canopy, Downdraft and Recirculating Compared

The single most expensive mistake in kitchen ventilation is decided four months before anyone notices it. The duct route, the hole in the wall, the diameter of the ducting buried behind plasterboard, all of that is set at first fix when the walls are open. The hood itself arrives months later at second fix, when the kitchen units go in. If the duct that was run at first fix is the wrong diameter, in the wrong place, or never went in at all, you find out when the fitter holds the hood up to a wall with no exit behind it. By then the plaster is on, the tiles are up, and the cheapest fix involves a recirculating kit you didn't want. This page tells you how to pick the right hood, how to size and route the duct, what Part F of the Building Regulations actually demands, and what to buy at every price level so the two stages stay in step.

What a cooker hood does

A cooker hood removes the steam, grease, and combustion fumes that cooking throws up before they coat your ceiling and settle on every surface in an open-plan room. There are two fundamentally different ways it can do that, and confusing them is where most ventilation errors begin.

An extracting (ducted) hood pulls the air up through a grease filter and pushes it outside through ductwork that exits a wall, soffit, or roof. The fumes leave the building. A recirculating hood pulls the same air through a grease filter and then an activated carbon filter, strips out odour and grease, and returns the cleaned air back into the room. Nothing leaves the building.

This distinction has a regulatory edge. Part F of the Building Regulations sets the minimum ventilation for a kitchen, and a recirculating hood does not satisfy it on its own. Part F counts air that is removed from the dwelling, and a recirculating hood removes none. If you fit a recirculating hood, you still need a separate extract route, a background ventilator, a trickle vent, or a mechanical extract fan, to meet the kitchen ventilation requirement. Read that twice, because plenty of homeowners assume a recirculating hood ticks the building control box. It does not.

Hood types

The body style of the hood determines where it can go, how it ducts, and how much it costs. The five formats below cover almost every kitchen extension.

A chimney hood is the workhorse: cheapest to duct, widest choice, and it sits where the airflow physics want it, directly over and above the hob. A canopy or integrated hood buys you a cleaner look at the cost of capture efficiency, because it sits tighter to the wall units and usually runs a smaller motor. Island hoods are the most demanding installation: there is no wall to duct through, so the duct climbs through the ceiling void to a soffit or roof outlet, and the open sides mean steam can escape sideways before the hood catches it. Downdraft hoods solve a design problem (an unobstructed island) and create an engineering one, since the duct has to go down and away rather than up, often under the floor. Visor hoods are the budget compromise for small rooms where a chimney would dominate.

Part F extraction rates

The numbers you need are not negotiable, and getting them wrong means a hood that cannot keep up with a busy hob. Part F sets a minimum kitchen extract of 30 litres per second running continuously, or 60 litres per second when the extract is intermittent and positioned over the hob (which a cooker hood is). The 60 l/s figure (216 cubic metres per hour) is the one to design around for a hood directly above the cooking surface.

Where the hob sits changes the rate you actually want. A wall hob with a chimney hood only loses steam on three open sides, so the rated airflow on the hood does most of the work. An island hob is open on all four sides, and rising steam drifts away before the hood can grab it. For an island, specify a hood rated well above the minimum, 60 l/s is the floor, not the target, and look at the manufacturer's extraction rate in cubic metres per hour at the working speed rather than the headline boost figure.

Duct diameter is the quiet killer of airflow. The standard for cooker hood ducting is 150mm rigid. Drop to 120mm to fit a tighter void and you lose roughly a quarter of the airflow, because resistance rises sharply as the bore narrows. Rigid ducting beats flexible: every corrugation in flexi-duct adds turbulence and drag. Run rigid 150mm wherever the route allows.

Recirculating: when it is the only option

Sometimes you genuinely cannot duct to the outside. A listed building where you are not permitted to cut a new opening through a protected wall. A rental flat where the lease forbids external alterations. An island in the centre of a deep-plan room with no logical path to a soffit or roof. A north-facing party wall with a neighbour's property on the other side and no external face to exit through. In those cases a recirculating hood is the only practical answer, and it is a reasonable one as long as you understand its limits.

A recirculating hood works in two stages. A metal mesh grease filter (washable, dishwasher-safe on most models) catches the fat and oil. Behind it, an activated carbon filter absorbs the cooking odours before the cleaned air returns to the room. Those carbon filters are consumable: they saturate and need replacing every three to six months depending on how hard you cook.

The regulatory point bears repeating because it is the one people skip. A recirculating hood does not meet Part F on its own. You need a separate extract route alongside it: either a background ventilator (a trickle vent or passive wall vent) sized to the kitchen requirement, or a mechanical extract ventilation system rated to the equivalent extract rate. Treat the recirculating hood as an odour-and-grease control measure, and provide the ventilation separately. Do not let anyone tell you the hood alone is enough.

What to buy

Hood pricing tracks motor quality, noise, and design more than raw extraction. A cheap hood and an expensive one might quote similar airflow figures, but the expensive one hits that figure quietly and holds it against a long duct run, while the cheap one screams at full speed and chokes on the first bend. The tiers below are for a standard chimney or canopy hood; island and downdraft formats sit higher.

For where to actually buy, the high-street appliance retailers are rarely the cheapest. Screwfix and B&Q carry the Cooke & Lewis own-brand range at the entry end. AO.com and Appliances Direct cover the full mid and premium range with frequent discounting and clear delivery. The best value on mid-range and premium brands often comes from kitchen trade suppliers and appliance specialists, who price below the big retail showrooms because they sell to fitters in volume. If your kitchen is being supplied by a trade kitchen company, ask them to quote the hood through their trade account before you buy it retail.

Common mistakes

Forgetting to duct before first fix is closed. This is the big one. The duct route and the external opening have to go in while the walls and ceiling are open. Decide the hob position, the hood type, and the duct route at first fix, and run rigid 150mm ducting to the outlet before the plasterboard goes on. Retrofitting a duct after plastering and tiling is the most expensive ventilation fix there is.

Undersizing the duct diameter. Squeezing 120mm ducting into a tight void to save space costs you about a quarter of the airflow. Run 150mm rigid unless there is genuinely no room, and if there isn't, choose a hood and route that work with the space rather than crippling the airflow.

Letting the duct run get too long. Airflow falls away over distance and at every bend. Each 90 degree bend adds roughly 1.5 metres of equivalent duct length. Keep the total equivalent run under 3 metres, which usually means no more than one or two bends on a short straight route.

Connecting an external cowl without a non-return valve. A cowl with a backdraught shutter (a non-return flap) stops cold air, wind, and insects blowing back in when the hood is off. Skip it and you get a draught and rattling on a windy day.

Assuming a recirculating hood satisfies Part F. It does not. Provide a separate background ventilator or mechanical extract alongside it, every time.

Where you will need this

• Extractor and ventilation, choose the hood type, extraction rate, and duct route at specification stage
• Kitchen electrical provisions, the hood's fused spur and the duct run both go in at first fix
• Kitchen installation, the hood is mounted, connected, and commissioned at second fix

Safety