Fault Diagnosis: Cooker Hood Not Extracting

Summary

"The cooker hood's not working" is one of the most common kitchen call-outs, and it covers three genuinely different complaints that need different diagnosis: no extraction at all, weak/reduced extraction, and a hood that runs and sounds normal but doesn't seem to be pulling smoke or steam away. Treating all three the same way wastes time — a saturated grease filter looks identical to a failing motor from the customer's description, but the diagnosis and fix are completely different.

This article is a structured fault tree covering all three symptom groups, split by whether the hood is a ducted (extract to outside) or recirculating (carbon filter, returns air to room) type, because the fault set is materially different between them. It covers the safety isolation procedure, the diagnostic logic, the common physical failure points, and the Part F ventilation context that determines whether a "working" hood is actually compliant.

The most common and cheapest fix — a blocked grease filter or exhausted carbon filter — is also the most commonly skipped diagnostic step, because customers (and some tradespeople) jump straight to "the motor's gone." A five-minute filter check before condemning the unit saves a needless replacement call-out on a large proportion of jobs.

Key Facts

• Ducted hood — extracts air to outside via ducting and an external grille; the only type that meets Part F compliance for new/altered kitchen ventilation
• Recirculating hood — filters air through a carbon/charcoal cartridge and returns it to the room; does not remove moisture or meet Part F; a separate ventilation provision (window, wall fan) is required alongside it
• Grease filter — aluminium mesh or baffle filter fitted beneath the hood canopy; catches airborne grease before it reaches the fan and ducting; should be washed or cleaned every 2-4 weeks of normal use
• Carbon filter — activated charcoal cartridge fitted above the grease filter on recirculating hoods only; adsorbs odour; not washable, needs replacement every 3-6 months; saturation is the single most common cause of "weak" recirculating hood performance
• Part F extraction rate (above hob) — 30 l/s minimum intermittent (Approved Document F, 2021)
• Part F extraction rate (hood located elsewhere in kitchen) — 60 l/s minimum intermittent
• Rated vs delivered airflow — the m³/h figure on the box is free-air flow with no duct attached; real-world delivered flow after ducting and bends is typically only 50-70% of the rated figure
• Duct sizing rule — never neck the ducting down below the hood's spigot diameter; 125mm or 150mm round duct is standard for domestic hoods rated for Part F compliance
• Bend penalty — each 90° bend in ducting costs the equivalent of roughly 1m of additional straight duct in terms of flow resistance
• Maximum recommended duct run — around 3m equivalent length for a standard motor before flow drops significantly below rated performance; longer runs need an uprated motor or larger duct
• Backdraught (non-return) flap — a flap at the external grille or within the ducting that prevents wind and cold air blowing back in when the hood isn't running; can seize shut from grease, paint, or corrosion, blocking extraction even with a fully working motor and clear duct
• Recirculating vs extract mode switch — many hoods have a physical damper or flap (sometimes a manual lever, sometimes motor-driven) that must be correctly set for ducted operation; if left in recirculate position (or the flap is stuck), a ducted hood will run but push almost no air outside
• Fused spur — cooker hoods are typically wired via a 3-amp fused spur, not a 13A plug; isolate here before opening the hood body
• Motor lifespan — typically 8-15 years of normal domestic use before bearing wear or winding failure; not usually field-serviceable, replace the motor/fan assembly or the whole hood
• Grease build-up in ducting — over years of use, especially with infrequent filter cleaning, grease can accumulate inside metal ducting and significantly restrict airflow even with a clean filter and working motor

Quick Reference Table

Detailed Guidance

Safety first — isolation procedure

Before removing filters is fine with the hood powered off at the switch, but before opening the hood body, accessing the motor, or working on wiring:

1. Identify the means of isolation — usually a 3-amp fused spur near the hood, sometimes a socket behind an adjacent unit.
2. Switch off and, where practical, remove the fuse or lock off the spur.
3. Verify dead with an approved voltage indicator before touching any wiring.
4. If the hood is integrated into a canopy housing or extractor unit tied into other kitchen electrics, confirm you've isolated the correct circuit — kitchens often have multiple circuits close together.

Decision tree

Cooker hood not extracting — start here
│
├── Does the fan run at all when switched on?
│   │
│   ├── NO — fan doesn't run, no sound
│   │     ├── Check fused spur / isolator — is it on?
│   │     │     ├── Off/tripped → switch on, test → working? Done.
│   │     │     └── On → continue
│   │     ├── Isolate, voltage-test at hood terminals
│   │     │     ├── No voltage at spur → upstream fault, check fuse/circuit
│   │     │     ├── Voltage present, fan still won't run → switch/control PCB fault
│   │     │     │     → replace control module or whole hood
│   │     │     └── Voltage present, fan hums but won't spin
│   │     │           → capacitor or motor winding fault → replace motor/hood
│   │
│   ├── YES — fan runs, but poor/no extraction
│   │     │
│   │     ├── Is this a RECIRCULATING hood (no external duct)?
│   │     │     ├── Check carbon filter — saturated/discoloured/never replaced?
│   │     │     │     → replace carbon filter (every 3-6 months)
│   │     │     ├── Check grease filter — clogged?
│   │     │     │     → wash/degrease (every 2-4 weeks)
│   │     │     └── Filters clean, still weak → fan/motor underperforming
│   │     │           → check fan speed setting, then consider motor fault
│   │     │
│   │     └── Is this a DUCTED hood?
│   │           ├── Check grease filter first — clogged?
│   │           │     → wash/degrease, re-test
│   │           ├── Check mode selector / damper — is it set to EXTRACT not recirc?
│   │           │     → correct the damper/lever position
│   │           ├── Filter clean, mode correct — check airflow at external grille
│   │           │     ├── No air felt at all outside
│   │           │     │     ├── Backdraught flap stuck shut (paint, grease, corrosion)
│   │           │     │     │     → free or replace flap
│   │           │     │     └── Ducting fully blocked or disconnected
│   │           │     │           → inspect duct run end to end, clear or reconnect
│   │           │     └── Weak air felt outside
│   │           │           ├── Duct kinked, crushed, or has excessive bends
│   │           │           │     → straighten run, replace flexible with rigid, minimise bends
│   │           │           ├── Grease build-up inside duct walls (long-term use)
│   │           │           │     → clean or replace duct sections
│   │           │           └── Duct too long / too narrow for hood's rated output
│   │           │                 → recalculate equivalent duct length, upsize duct/motor
│   │           └── All of the above OK, still weak → motor underperforming
│   │                 → check fan speed setting, then consider motor replacement
│   │
│   └── YES — fan runs, kitchen still smells/steams despite apparently good airflow
│         ├── Check hood is rated adequately for Part F (30 l/s above hob / 60 l/s elsewhere)
│         │     → under-rated hood for room size/kitchen layout
│         ├── Check make-up air — is the kitchen very airtight with no trickle vents?
│         │     → poor make-up air reduces effective extraction even with a good hood
│         └── Recirculating hood being relied on as sole ventilation
│               → does not meet Part F; recommend ducted upgrade or supplementary wall fan

Grease filter and carbon filter — check these first, always

The single most common cause of "the hood's stopped working properly" is a filter, not the motor. Aluminium mesh or baffle grease filters should be washed every 2-4 weeks in normal domestic use (dishwasher-safe on most models, or hand wash in hot soapy water); a filter left for months accumulates a sticky grease layer that significantly restricts airflow through the whole unit, whether ducted or recirculating.

On recirculating hoods specifically, the carbon filter is the far more common culprit for "not removing smells" complaints than the motor. Carbon filters are not washable or reusable — they must be replaced (typically every 3-6 months depending on cooking frequency) and many customers don't realise this or don't know the replacement interval. If a recirculating hood has never had its carbon filter changed since installation, that's the starting diagnosis before considering anything else.

Ducted vs recirculating — confirm which type you're dealing with, and which mode it's set to

Some hoods are dual-purpose — capable of either ducted extraction or recirculation depending on how they're installed and how an internal damper or flap is set. A common fault, especially after a kitchen refit or hood replacement, is a ducted-capable hood left in recirculating mode (or vice versa) — it runs perfectly normally, sounds fine, but the air isn't going where the customer expects. Check the mode selector (a physical lever, a damper flap behind the outlet, or in some cases a DIP switch/settings menu on more modern units) before assuming a mechanical fault.

Ducting — the most-overlooked fault on ducted hoods

Ducting problems frequently masquerade as a "weak motor" complaint. Key checks, working from the hood outward:

• Spigot-to-duct match — the ducting should never be smaller than the hood's spigot diameter (commonly 125mm or 150mm); a common installation error is necking a 150mm spigot down to 100mm ducting, which can more than halve the effective airflow
• Bends — every 90° bend costs roughly 1m of equivalent straight-duct resistance; a run with several bends can fail to deliver the rated flow even with a short physical distance
• Flexible vs rigid duct — corrugated flexible duct loses significantly more flow per metre than rigid or semi-rigid duct; long flexible runs are a common cause of underperformance even when nothing is technically "blocked"
• Physical blockage — bird nests, mortar dropout from the original install, or simply grease accumulation over years of use inside metal ducting; inspect end to end, ideally by disconnecting at the hood and shining a light or passing a rod through
• Kinks and crushing — flexible duct run through tight joist spaces or behind cabinetry is prone to kinking, which can go unnoticed for years while steadily reducing performance

Backdraught (non-return) flap — often the hidden culprit

The external grille or an in-line duct fitting usually includes a backdraught flap — a hinged or spring-loaded flap that opens under the hood's fan pressure to let air out, and closes when the fan is off to stop wind and cold air blowing back in. Over time this flap can seize shut from a combination of cooking grease residue, exterior paint applied without masking it, or simple corrosion. When stuck shut, the hood can run at full power, sound completely normal, and still deliver almost no air outside — because the flap is acting as a solid blockage. This is easy to miss because it's outside, at the far end of the duct run, away from where the customer notices the problem. Always check it as part of a "runs fine but no extraction" diagnosis.

Motor and fan faults

If filters are clean, the mode selector is correctly set, and the duct run (where applicable) is clear and correctly sized, the remaining cause is the fan/motor assembly itself:

• Humming but not spinning — capacitor or motor winding fault; not usually field-serviceable on domestic hoods; replace the motor assembly or the whole unit
• Running but noticeably weaker than when new, all other checks passed — worn bearings reducing fan efficiency, or a partially blocked fan housing (grease and dust accumulation inside the fan itself, not just the filters); worth opening the housing (isolated) and inspecting/cleaning the fan blade
• Noisy or rattling — loose fan blade, worn bearings, or debris (a dislodged screw, filter clip, or similar) inside the housing

Domestic cooker hood motors typically last 8-15 years; a hood of that age with a genuine motor fault is usually more economically replaced as a complete unit than repaired, particularly for lower-cost integrated models.

When the hood "works" but doesn't solve the problem

Occasionally a hood passes every mechanical check — clean filters, correct mode, clear duct, good motor — but the customer still reports steam and smells lingering. This usually isn't a fault at all, but an under-specification or make-up air issue:

• The hood may be under-rated for the room or hob (see Part F rates below) — a hood rated at exactly the minimum, with real-world duct losses, may deliver noticeably less than the 30 l/s / 60 l/s the room actually needs
• In an airtight, well-sealed modern kitchen, extraction can be constrained by lack of make-up air — nowhere for replacement air to enter, so the fan struggles against a partial vacuum; trickle vents or an opening window during cooking resolve this

Part F compliance context for replacement

If the hood is being replaced rather than repaired, the replacement should meet Part F: 30 l/s minimum where sited directly above the hob, 60 l/s minimum where sited elsewhere in the kitchen. A recirculating hood never meets this on its own — see kitchen extraction ventilation for the full regulatory picture, duct sizing tables, and installation detailing. For pricing a repair vs replacement decision, see cooker hood installation pricing guide.

Frequently Asked Questions

My cooker hood runs but doesn't seem to pull any smoke or steam away — what's the most likely cause?

Check the grease filter first — a saturated filter is by far the most common cause and takes two minutes to check. If the filter is clean, check whether the hood is ducted or recirculating, and if ducted, whether the mode selector/damper is actually set to extract. If both of those check out on a ducted hood, the next most likely cause is a blocked or badly restricted duct run, or a stuck backdraught flap at the external grille.

How often should the carbon filter be replaced on a recirculating hood?

Typically every 3-6 months with normal domestic cooking frequency, sooner with heavy or frequent frying. Unlike the grease filter, the carbon filter cannot be washed or reused — it must be replaced. Many customers are unaware of this, and a "not working" recirculating hood complaint is very often simply an exhausted carbon filter that's never been changed.

Is a recirculating hood ever acceptable for Building Regulations compliance?

Not for new installations or where a kitchen is being extended/altered — Approved Document F requires ducted extraction to outside air at the specified rate. Recirculating hoods are sometimes accepted for a genuinely like-for-like replacement in an existing kitchen with no other alteration, but Building Control will often still expect an alternative ventilation provision (window or dedicated wall fan) alongside it.

Why is my brand-new ducted hood not extracting well?

The most common cause on a new install is the ducting itself — either necked down below the hood's spigot size, run with too many bends, using flexible duct for the whole length, or simply too long for the motor's rated capacity. New-install underperformance is much more likely to be a duct sizing/routing error than a faulty hood — check the duct spec against the hood manufacturer's maximum recommended run before assuming the unit is defective.

Can I clean the ducting myself, or does it need a specialist?

For accessible domestic ducting (typically 125mm or 150mm), disconnecting sections at the hood and the external grille to inspect and clear blockages by hand or with a rod/vacuum is straightforward competent-person work. Heavily grease-fouled long runs, or ducting concealed within structure that's difficult to access, may be better handled by a specialist kitchen ventilation or duct-cleaning contractor, particularly in a commercial kitchen context where fire risk from grease-laden ducting is a genuine hazard.

Regulations & Standards

• Building Regulations Approved Document F Volume 1 (2021 edition) — Means of ventilation, domestic; extraction rates for kitchens

• Building Regulations Approved Document P — electrical safety; relevant to fused spur wiring and any new circuit work

• BS 5440-2:2023 — installation and maintenance of ventilation provision for gas appliances (supersedes the withdrawn BS 5440-2:2009; relevant where the hob is gas)

• BS EN 13141 (parts 1-11) — ventilation for buildings, performance testing of components

• BS 7671:2018+A4:2026 (IET Wiring Regulations, 18th Edition, superseding A2:2022 and A3:2024) — electrical installation requirements for the fused spur/isolation. Amendment 4:2026 came into force 15 April 2026 with a transition period to 15 October 2026, after which it becomes the only current version

• GOV.UK — Approved Document F: Ventilation — statutory extraction rates

• GOV.UK — Approved Document P: Electrical safety — isolation and notifiable work

• BEAMA Domestic Ventilation Compliance Guide — industry ventilation guidance

• Gas Safe Register — guidance where hob is gas-fired

• kitchen extraction ventilation — full Part F ventilation rules, duct sizing, hood selection

• cooker hood installation pricing guide — repair vs replacement pricing

• extractor fan not working — companion fault tree for bathroom/utility extractor fans

• condensation on windows — related symptom when kitchen extraction is inadequate