Why Is My Kitchen Exhaust Fan Not Extracting Properly?

First identify what the system is doing

Before deciding that the fan is “not working”, place the symptom into one of four groups:

• The fan is completely stopped: there is no normal operating sound, airflow or motor operation.
• The fan runs, but airflow is weak: the sound or air movement has changed and smoke or heat escapes.
• The fan appears to run normally, but the hood does not capture the cooking plume.
• The problem is intermittent: the fan starts and stops, trips after heating up, changes speed or fails only under certain conditions.

This distinction helps separate electrical and motor faults from drive, duct, make-up-air and hood-capture problems.

Safe checks an operator can make

Venue staff can collect useful information without dismantling or testing the system:

• Confirm the normal wall switch, selector or approved operating control is in its usual position.
• Listen from a safe location for a changed fan sound, belt squeal, grinding, rattling or repeated starting and stopping.
• Check whether grease filters are present, correctly seated and visibly overloaded. Do not reach beyond the filters into the plenum.
• Remember that working canopy lights do not prove the exhaust fan is operating.
• Record any VFD or controller fault code without opening the enclosure, changing parameters or repeatedly clearing the fault.
• Note whether doors have become unusually hard to open or air rushes strongly into the kitchen.
• Record recent changes to appliances, menu, fuel, operating hours, filters, cleaning work, building doors or air-conditioning.
• Locate previous cleaning, maintenance, airflow and commissioning records.

If there is doubt about electrical or mechanical safety, take the equipment out of service and arrange professional assessment.

Why a commercial kitchen exhaust fan can stop

A stopped fan may result from one fault or a chain of faults. Common possibilities include:

• Loss of electrical supply, a local isolator position, damaged control wiring or a failed contactor.
• A protective device, motor overload or circuit breaker operating because of an underlying fault.
• A VFD alarm, missing run command, incorrect control signal or failed interlock.
• A fire-system, make-up-air, ESP or ozone interlock preventing normal operation.
• A failed capacitor on applicable single-phase equipment.
• Motor winding failure, overheating, or a seized motor or bearing.
• A jammed or damaged impeller, or severe contamination preventing rotation.

SafeWork NSW electrical guidance states that the reason a circuit breaker, RCD or other protective device disconnected the electricity should be determined before it is switched back on.

Why the fan can run but extract poorly

When the fan is operating but airflow is low, likely cause groups include:

• Incorrect fan speed or a VFD setpoint that has been reduced or altered.
• Reverse fan rotation after electrical work, motor replacement or a phase change.
• A broken, loose or slipping belt, incorrect tension, pulley wear or pulley misalignment.
• Worn bearings, impeller damage, build-up or imbalance reducing useful performance.
• Grease-loaded filters, hood, plenum, ductwork, fan inlet, impeller or discharge.
• A closed damper, obstructed duct, blocked discharge, damaged flexible connection or leaking access panel.
• Additional duct resistance caused by later alterations or treatment equipment.
• A replacement fan, motor, pulley or control setting that does not match the required duty.

A clean-looking hood does not prove the duct and fan are clean, and a dirty system is not the only possible cause. Grease cleaning and mechanical diagnosis are related but separate services.

Make-up air and negative pressure

An exhaust fan cannot continuously remove air from a room unless air can replace it. The ABCB Indoor Air Quality Verification Methods handbook describes a clear pathway for make-up air as part of effective local exhaust. If replacement air is inadequate, the kitchen may become excessively negative and the exhaust system may move less air than expected.

Useful clues include doors that are difficult to open, strong air movement through door gaps, odours moving through the building, or capture improving when an external door is opened. Opening a door is an observation, not a permanent design solution.

Make-up air can also reduce capture if it is supplied at excessive velocity or directed across the face of the hood. A competent assessment should consider both the quantity and distribution of replacement air.

When airflow exists but smoke still escapes

Good duct airflow does not automatically guarantee good capture at every appliance. Smoke and heat may escape when:

• Appliances have been moved, added or replaced with higher-duty equipment.
• The cooking surface or plume extends outside the effective hood overhang.
• Cross-drafts from doors, fans or air-conditioning disrupt the rising plume.
• Make-up air is discharged across the hood face instead of supporting capture.
• Filters are missing, damaged, incorrectly fitted or unsuitable for the arrangement.
• The cooking process produces a plume the existing hood and system were not designed to handle.

The NCC kitchen local exhaust provisions reference AS 1668 for applicable commercial kitchen exhaust hoods. The edition and compliance pathway for a specific site depend on its approval and design documents; a later published edition does not automatically rewrite an existing approval.

Intermittent extraction problems

A fan that works when cold and stops later may be affected by motor overheating, overload operation, bearing deterioration, a VFD fault, an intermittent control command or increased resistance as operating conditions change. Record the time, displayed fault code, cooking load and whether the fan restarts after cooling, but do not bypass or repeatedly reset the protection.

What a professional fault diagnosis should include

The exact scope depends on the system and access. A structured assessment may include:

• Confirming the reported symptom, operating sequence and recent changes.
• Establishing safe isolation and checking the relevant electrical supply, command and protection.
• Inspecting the motor, bearings, mounts, impeller, belts, pulleys and drive alignment.
• Checking fan rotation, operating speed, VFD state and relevant control or interlock signals.
• Observing filters, accessible contamination, duct condition, access panels and discharge restrictions.
• Taking motor-current, airflow, pressure or speed measurements where included in the agreed scope.
• Assessing hood capture, appliance changes, cross-drafts and make-up-air symptoms.
• Comparing results with available drawings, schedules or previous commissioning information.
• Recording the fault, limitations and practical repair, cleaning, replacement or further-assessment priorities.

An airflow reading applies to the agreed test points, equipment state and conditions on the day. It does not by itself certify the entire building or concealed system.

When to request urgent assistance

• The exhaust fan has stopped while grease-producing or smoke-producing cooking is underway.
• There is a burning smell, visible equipment smoke, sparking or evidence of overheating.
• The fan, motor or duct is shaking, grinding or making severe mechanical noise.
• A breaker, overload or VFD fault repeatedly trips or returns.
• Grease is leaking or dripping from the hood, duct, fan or discharge.
• Smoke or heat is spreading into occupied areas and cannot be controlled.

For an immediate fire or electrical danger, follow the site’s emergency procedure and contact emergency services where required.

How Altruism NSW can help

Altruism NSW provides commercial kitchen exhaust fault diagnosis and repair across Sydney and Wollongong. The mechanical and electrical scope may include fans, motors, impellers, belts, pulleys, bearings, starters, isolators, VFDs, speed controls, make-up-air systems, ESP equipment and relevant interlocks.

We can also undertake airflow testing and written reporting where the question requires measurements or comparison with available design information. Grease cleaning of canopies, filters and ductwork should be completed by a specialist commercial exhaust cleaning contractor.

Frequently asked questions

Why is the canopy light on when the exhaust fan is not running?

The canopy light and exhaust fan may have separate supplies or controls. A working light does not confirm that the fan motor, starter, VFD or interlocks are operating.

Can I reset the breaker or motor overload?

Do not repeatedly reset a protective device. A breaker, overload or drive fault may be responding to overheating, excessive current, a seized component or another electrical fault. The reason for the trip should be identified before return to service.

Will cleaning the filters fix low extraction?

Clean and correctly fitted filters are important, but they may not be the only cause. Low speed, belt slip, impeller build-up, duct restriction, make-up-air problems and control faults can produce similar symptoms.

Why does extraction improve when a door is opened?

That can indicate inadequate replacement air or excessive negative pressure. It is useful diagnostic information, but leaving a door open is not a substitute for a properly assessed make-up-air arrangement.

Does a running fan mean the kitchen exhaust complies?

No. Operation alone does not establish required airflow, hood capture, discharge suitability or compliance with the approved design. Assessment must be tied to the specific site, documents, conditions and scope.

What should I send before booking a diagnosis?

Send the venue address, fan photographs, equipment details, controller or VFD fault codes, a description of what changed, recent cleaning or maintenance reports and any available mechanical drawings or commissioning records.

Sources

• SafeWork NSW: Electrical risks at the workplace
• AIRAH Commercial Kitchen Exhaust Management Best Practice Guide
• NCC: Part F6 Light and ventilation
• ABCB Indoor Air Quality Verification Methods handbook

This article provides general information only and is not electrical, engineering, certification, fire-safety or legal advice. Do not access live electrical equipment, roof-mounted plant, moving fan components or ductwork unless you are appropriately authorised, trained and competent.