150mm Rigid Ducting: The Correct Spec for Cooker Hood Extraction

Your kitchen is finished. The plasterer's gone. The tiles are grouted. You switch on the new 600 m³/h cooker hood and it sounds like a hairdryer while barely shifting the steam off a pan of pasta. The builder fitted a 100mm duct because that was what the local merchant had in stock, or squeezed in a 204x60mm flat channel because he thought it was equivalent to your 150mm spec. Neither works. And now the fix involves lifting the kitchen ceiling, cutting through a cavity wall, and repainting. Ducting is one of those components where the wrong choice stays invisible until it's unfixable.

What it is and what it's for

150mm rigid ducting is smooth-bore round pipe used to carry moist, greasy, smelly air from a kitchen cooker hood to the outside of the building. The bore refers to the internal diameter. Rigid means it holds its shape (unlike corrugated flexible hose) and smooth-bore means the inner surface has no ridges, so air passes through with minimum turbulence.

The material is almost always uPVC, a white or grey plastic that cuts cleanly with a hacksaw, is rated to around 60°C (more than adequate for cooker hood exhaust, which is warm rather than hot), and joins with push-fit connectors. The alternative materials (galvanised steel for fire-rated applications, semi-rigid aluminium for short transitional sections) are covered below.

For a UK kitchen extension, a ducted cooker hood is a legal requirement, not an upgrade. Approved Document F 2021 (the Building Regulations ventilation document) mandates that a new or extended kitchen must extract at a minimum rate to the outside of the building. Recirculating hoods, which filter the air and blow it back into the room, do not meet Part F on their own. A ducted extraction route is required, and since 2022 the installer must test and commission the fan to demonstrate the actual airflow, with documentation provided to the homeowner and to Building Control.

The minimum rates under Approved Document F are:

• 30 l/s (108 m³/h) from a cooker hood positioned directly above the hob
• 60 l/s (216 m³/h) from an extract fan located elsewhere in the kitchen

These are floors, not ceilings. A typical modern chimney hood runs at 600 to 900 m³/h at full speed, comfortably above the regulation minimum. But only if the ducting doesn't strangle it.

Why 150mm specifically, not 125mm or 100mm

This is the single most common installation failure, and it comes from a misunderstanding of how airflow scales with pipe diameter. A 100mm duct does not have "a bit less" area than a 150mm duct. It has less than half.

Run the numbers:

• 150mm internal diameter: cross-sectional area = π × 75² = 17,671 mm²
• 125mm internal diameter: cross-sectional area = π × 62.5² = 12,272 mm² (31% less)
• 100mm internal diameter: cross-sectional area = π × 50² = 7,854 mm² (56% less)

Airflow through a duct scales with cross-sectional area, but noise and back-pressure scale worse than linearly. Forcing the air volume that a 600 m³/h motor wants to push through a duct with 56% less area doesn't halve the output. It makes the air velocity shoot up, which creates turbulence, which creates noise, which creates back-pressure on the motor, which causes the fan to labour and the thermal cut-out to trigger on long cooking sessions. Luxair Hoods (a UK extraction specialist) publish internal data showing a 30% airflow loss from reducing a 150mm system to 100mm, and a warranty clause voiding cover if the duct size is smaller than specified.

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The practical conclusion: if your cooker hood specifies 150mm, use 150mm end to end. A short reducer at the wall terminal is occasionally unavoidable (for example where a legacy 100mm core hole can't be enlarged) but is always a compromise and always voids the manufacturer warranty. Plan the route so you never need one.

The flat-channel equivalency trap

The industry's second most common failure is substituting flat-channel duct for round pipe using the wrong equivalency. This matters because flat channel is often needed where the duct runs above wall units, through a shallow ceiling void, or across a boxed-in run where 150mm round pipe would be visible or obstructive.

The confusion: 204x60mm flat channel is NOT the 150mm equivalent. It is the 125mm equivalent.

Round duct	Flat-channel equivalent	Cross-sectional area (both)
100mm	110x54mm	~5,800 mm²
125mm	204x60mm	~12,240 mm²
150mm	220x90mm	~19,800 mm²
200mm	308x110mm	~33,800 mm²

This confusion is perpetuated by retailers who describe 204x60mm as "slim-line" or "low-profile" ducting and stock it alongside 150mm round fittings. It is not a 150mm equivalent. Using 204x60mm on a 150mm hood strangles airflow by roughly 30%, which is exactly the same penalty as dropping from 150mm to 125mm round.

A BuildHub thread captured a manufacturer's instruction word-for-word: "you must not reduce to 125mm, but you can adapt to 220x90mm flat." If your hood spec says 150mm and you need a flat run, the only acceptable flat channel is 220x90mm. The round-to-flat adapter you need is a 150mm-round-to-220x90mm-flat, not a 150mm-round-to-204x60mm-flat, no matter how prominently the latter is marketed.

Standard sizes and the components you actually need

Domestic 150mm rigid uPVC ducting from Domus EasiPipe 150 (the most widely specified range) and Manrose (the most widely stocked at DIY retailers) is sold in standard lengths of 0.35m, 1m, 1.5m, and 2m. A typical kitchen extension run uses a combination: one or two 2m lengths for the main horizontal, a 1m or 0.35m section to finish to the wall terminal, and 2 to 3 elbows.

The complete component list for a straightforward run:

• Round pipe in the lengths you need. Push-fit connections, so you buy exactly the lengths required and trim with a hacksaw if necessary.
• Straight connector (also called a coupler). Joins two pipe lengths in line. You need one per pipe-to-pipe join.
• 90° elbow. The most common bend. One at the hood outlet if the run goes sideways, one where the horizontal run turns down through the cavity wall.
• 45° elbow. Lower resistance than a 90° (half the pressure drop), but less commonly stocked and notably more expensive. Useful for gentler route changes.
• Round-to-flat adapter (150mm round to 220x90mm flat) if part of the run uses flat channel.
• Wall liner sleeve. Protects the pipe through the masonry core hole and takes the load of the brickwork. Often sold as part of a wall kit with the liner and a through-wall section.
• External terminal. Gravity flap is preferred over fixed louvre because it closes when the hood is off, preventing cold air entering and preventing backdraft from wind. A stainless bullnose terminal is the premium option for rendered or prominent external walls.
• Backdraught shutter at the hood end, if not built into the hood itself.

Prices are widely variable depending on where you buy. As a rough current benchmark: 1m of Manrose or Domus rigid pipe costs £5 to £12 depending on retailer, 2m sections run £18 to £25, a 90° elbow £8 to £35, and a wall terminal £9 to £15. Specialist trade suppliers (Naples UK, BES, MAS Ltd for Domus stock) are significantly cheaper on fittings than generalist retailers, which matters because the fittings are where the cost mounts up.

The sourcing reality

You cannot buy a complete 150mm rigid ducting system from Screwfix. That's not a slight on Screwfix. It's a structural issue with how UK ventilation products are distributed.

Screwfix stocks Manrose 150mm straight pipe (1m and 0.35m lengths), straight connectors, and wall fixing kits. It does not stock 150mm round-to-round 90° or 45° elbows as a regular line. Toolstation is similar. Wickes stocks solid-wall duct kits but not the full range of bends. B&Q stocks very little 150mm round at all.

For the full fitting range you need one of these suppliers:

• Naples UK (naplesuk.com): full 150mm round range including 45° and 90° bends, connectors, terminals, gravity flaps. Often the cheapest on fittings.
• BES (bes.co.uk): trade electrical and ventilation supplier. Full Domus EasiPipe 150 range.
• i-sells.co.uk: ventilation specialist. Good for terminals and unusual fittings.
• TLC-Direct (tlc-direct.co.uk): trade supplier with depth on ventilation components.
• Domus Ventilation (domusventilation.co.uk): direct from manufacturer. EasiPipe 150 is the specification-grade system.
• Luxair Hoods (luxairhoods.com): cooker hood specialist; sells matched duct kits with their hoods.

Plan to order fittings online from a specialist and pick up straight pipe locally if you need it quickly. Homeowners and even builders regularly get caught out assuming the local builder's merchant will stock the full range. It won't.

How to work with it

Cutting. A hacksaw with a fine blade is the right tool. Mark the cut with a pencil around the circumference (wrap a piece of paper around the pipe as a guide for a square cut), then saw slowly to avoid splitting the pipe at the cut edge. Deburr the inside edge with a knife to remove any plastic swarf that would create turbulence.

Joining. Push-fit connections with an integral rubber seal are standard on Domus EasiPipe and Manrose systems. Push the pipe fully home into the fitting until it stops against the internal shoulder. For belt-and-braces airtightness on hidden runs, wrap each joint with aluminium foil tape. Don't use silicone sealant on the joint itself; it prevents future disassembly and can contaminate the seal.

Supporting the pipe. Rigid duct is heavier than it looks once it's spanning a ceiling void with a 90° elbow loaded at one end. Clip at roughly 1.5m intervals on horizontal runs using proprietary pipe clips (the same manufacturers sell matched clips). Unsupported pipe sags, stresses the joints, and can pull apart at push-fit seams over time.

Routing through masonry. A 170mm diamond-core drill is the standard hole size for 150mm pipe with clearance for a wall liner sleeve. The core hole should have a slight external fall (about 1:100) to drain any condensation to the outside rather than back into the wall. The liner sleeve takes the weight of the masonry and keeps the pipe protected from point loads.

Condensation and insulation. In cold ceiling voids or lofts, warm moist air from the hood cools inside the duct and condenses on the inner wall. On longer runs this can produce enough water to drip back down into the hood. Wrap the duct in purpose-made duct insulation (a quilted polyester or mineral-wool sleeve with a foil outer) on any run that passes through an unheated void. Budget around £20 per metre of insulation wrap.

How to calculate the equivalent length

Cooker hood manufacturers specify a maximum "equivalent length" of ducting that the motor is designed to push air through. For most 600 m³/h domestic hoods the practical limit is 5 to 6 metres of total equivalent length. Exceed that and airflow drops below the rated output regardless of the hood's advertised spec.

Equivalent length counts each bend as if it were a longer length of straight pipe, because bends create turbulence and pressure drop:

Component	Equivalent length (treat as)
1m straight rigid pipe	1.0m
90° rigid elbow	1.2m
45° rigid elbow	0.6m
Flexible corrugated hose	1.5m per 1m actual
Round-to-flat adapter	0.5m
Wall terminal	0.5m

A worked example for a typical kitchen extension route:

• 0.5m vertical rise from hood to ceiling void: 0.5m
• 90° elbow at the top: 1.2m
• 2m horizontal through ceiling void: 2.0m
• 90° elbow at the cavity wall: 1.2m
• 0.2m through wall with liner: 0.2m
• Wall terminal: 0.5m
• Total equivalent length: 5.6m

That's right at the practical limit. Adding a third bend, or extending the horizontal run past 2.5m, pushes the system past its effective range. If your route needs to be longer, specify a higher-output hood (900 m³/h or more), upsize to 200mm ducting, or rethink the route to put the hood closer to an external wall.

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Materials: uPVC, galvanised steel, and when each applies

For a standard single-dwelling rear extension, the duct runs from the kitchen through the cavity wall or ceiling void and terminates on an external wall. It does not cross a fire-separating compartment boundary, so standard uPVC rigid (class E to BS EN 13501-1) is permissible and is what the overwhelming majority of domestic installations use. Manrose, Domus EasiPipe, Kair, and Verplas all manufacture uPVC round ducting to equivalent specifications.

Galvanised steel (class A1 non-combustible) is required where the duct passes through a compartment wall or floor that must maintain fire resistance. Typical cases:

• A kitchen in a flat where the duct passes through the compartment wall to a neighbouring dwelling or common area
• An extension with an integral garage where the duct crosses the ceiling of the garage (the garage ceiling is a fire-rated compartment per Approved Document B)
• Any application where Building Control specifically requires a fire-rated duct

Rytons and similar manufacturers supply A1 fire-rated 150mm galvanised kits. These are not normally needed for a standard detached or semi-detached rear extension.

Semi-rigid aluminium (corrugated aluminium hose) is useful for a short transitional section between the hood spigot and the start of the rigid system, typically 200 to 300mm where some flexibility is needed to align the hood with the pipe run. It is not suitable for the main run because the corrugations create turbulence (adding roughly 50% to the equivalent length per metre) and collect grease and condensation over time.

Flexible PVC hose is not recommended for any part of a cooker hood installation. Community reports include cases of partial melting where the hood's motor runs hot, and the corrugated internal surface collects grease. Use rigid throughout, with a short semi-rigid aluminium transition if absolutely required.

Commissioning and Part F documentation

Since the 2022 update to Approved Document F, the installer is required to test the installed system, demonstrate that it achieves the minimum airflow rate, and provide a commissioning record to the homeowner and to Building Control. This is a flow test, typically done with an anemometer at the hood inlet or at the external terminal.

Two practical consequences:

1. The person who installs the ducting and connects the hood is responsible for the test. If your builder installs the duct and your kitchen fitter connects the hood, agree in advance which of them provides the commissioning record. This is a common gap where neither party takes responsibility and the homeowner ends up without the documentation Building Control wants at sign-off.

2. If the flow test fails, the fix is not a larger fan. It's a better duct route. Fan output is fixed by the hood; what varies is how much of that output actually reaches the outside. A failing test almost always means the duct is undersized, the run is too long, or there are too many bends.

Cost and what to budget

For a typical kitchen extension with a 3 to 5 metre straight-ish duct run, budget roughly £80 to £150 for the full ducting system including pipe, elbows, wall liner, and external terminal, buying a mix of Manrose or Domus components from a trade specialist. Higher if the run is longer, if insulation wrap is needed, or if a stainless bullnose terminal is specified instead of a plastic gravity flap.

Labour for the ducting installation (through-wall core drilling plus running the duct) is typically a small line item within the first-fix trade's work, in the order of £260 to £420 if priced separately. In practice it's usually absorbed into the main trade's scope rather than itemised.

Compared to a £52,000 kitchen spec, the ducting cost is almost a rounding error. The reason it gets cut corners on is not cost. It's the sourcing friction, the knowledge gap on sizing, and the fact that the consequences are invisible until the kitchen is finished. Spending an extra half-day at first-fix stage to confirm the specification and the route is the best money you'll spend on the installation.

Alternatives

There are essentially no alternatives to rigid ducting for a compliant cooker hood installation. Flexible and semi-rigid alternatives have their place for short transitional sections but are not substitutes. The real "alternative" is a route decision rather than a material choice:

• 220x90mm flat channel as a drop-in for 150mm round where the space profile demands it. Same effective performance when the adapter and transitions are correctly specified.
• 200mm round for longer runs where 150mm approaches the equivalent-length limit. Requires a matched 200mm-capable hood and an upsized wall terminal.
• Recirculating hood with activated carbon filter is NOT an alternative under Approved Document F. A recirculating hood on its own does not meet Part F. It can be fitted additionally to a ducted route, but the ducted route is the regulatory minimum.

Common mistakes

Builder fits 100mm or 125mm duct because it's what the merchant had. The single most common failure. Confirm the diameter yourself, by eye, before the walls close up. Measure the pipe with a tape measure across the outside. 150mm pipe is 150mm outside diameter. If the pipe looks noticeably smaller than a standard wine bottle (a wine bottle is around 75mm diameter, so 150mm pipe is about two bottles wide), it's not 150mm.

Using 204x60mm flat channel on a 150mm hood. Covered above. The 150mm round equivalent is 220x90mm. Verify with a tape measure on the narrow axis of the channel: 60mm is wrong, 90mm is right.

Flexible corrugated hose used for the main run. Condensation collects in the corrugations. Noise doubles versus rigid. Grease builds up in the ridges. Use rigid throughout.

Wall terminal with no gravity flap. A fixed louvre lets cold air, wind, and occasionally rain drive back through the duct when the hood is off. Insist on a gravity-flap terminal. The cost difference is a few pounds.

No commissioning record provided. Agree in writing who commissions the system and when. Without the record, Building Control can hold up sign-off, and you'll be chasing a tradesperson who's moved on to the next job.

No condensation insulation on a cold-void run. Any duct running through an unheated loft or ceiling void should be insulated, otherwise water condenses inside and drips back to the hood over time.

Where you'll need this

Cooker hood ducting is specified at the kitchen design stage and installed during first fix, long before any appliance arrives on site. Getting the specification right before walls are boarded in is the difference between a working extraction system and a costly rework:

• Extractor and ventilation - hood selection, airflow requirements, and duct route planning before the kitchen is specified
• First fix electrics - the electrical supply to the hood is installed alongside the duct route, and the two must be coordinated before walls are closed

Ducting is the component that every kitchen extension needs and that almost nobody plans properly in advance. The material cost is trivial. The cost of getting it wrong, and discovering the problem after the kitchen is installed, is measured in whole weekends of rework. Specify 150mm, verify it on site before the walls close, and keep the route under 6m equivalent length.