A failed extract fan can close the kitchen gas interlock, stop gas appliances and make the cooking process unsafe. The root cause may be a local isolator, emergency circuit, fire-alarm input, contactor, variable-speed drive, overload, belt, impeller, motor or control signal.
Safety first: Stop cooking processes that require the extract system. Do not bypass the gas interlock or continue using gas appliances without proven ventilation. Follow the fire and gas procedure if smoke, fumes, overheating, unusual noise or a burning smell is present.
Important: This is an operator-level diagnostic guide, not a repair instruction. Always follow the appliance manual. Gas work, live electrical testing, refrigerant work and access beyond user-serviceable parts must be carried out by people with the required competence and qualifications.
How the appliance and safety sequence work
A typical control chain begins with an enable signal from a switch, BMS, gas-interlock panel or fire-suppression interface. A contactor or variable-speed drive then energises the motor. Overloads, thermal protection and drive fault logic stop operation if current, temperature or control conditions are abnormal.
Fan proof may be monitored by electrical current or differential air pressure. Current proves that a motor is drawing power, while pressure sensing provides information about airflow or pressure conditions at its sampling points. Grease-blocked tubes, filters, ductwork or an incorrectly calibrated sensor can therefore create interlock faults even if the fan can be heard.
What the symptoms suggest
| Observed symptom | What it may indicate |
|---|---|
| No sound or movement | No enable, isolator, fuse, contactor, drive, motor or supply fault. |
| Fan starts then stops | Overload, drive trip, seized bearings, blocked impeller, phase loss or control input. |
| Motor runs but little extraction | Belt slip, wrong rotation, blocked filters/ducts, failed impeller or closed damper. |
| VSD shows a fault code | Overcurrent, under-voltage, motor, control or drive fault; record before reset. |
| Gas interlock says fan fail while fan runs | Proof sensor, pressure tubes, calibration, airflow or interlock-input issue. |
These observations narrow the investigation; they do not prove a component has failed. The same symptom can have an appliance cause, a utility problem or an installation cause.
Safe checks for kitchen staff
- Stop heat-producing equipment: Use the kitchen shutdown procedure and prevent another shift from restarting appliances.
- Check normal user controls: Confirm fan switch, speed control and any clearly labelled emergency-stop reset are in their operating positions.
- Read displays: Photograph VSD, BMS and interlock messages before resetting anything.
- Check accessible filters: If trained and safe, confirm grease filters are fitted, reasonably clean and not physically collapsed.
- Listen and look externally: Record hum, belt noise, vibration, smell and whether discharge airflow is apparent from a safe location.
- Check related safety systems: Note fire-alarm, suppression, emergency-stop and supply-air status. Do not reset a fire system without authority.
Likely technical causes
| Area | How it creates the fault | Competent engineer's next step |
|---|---|---|
| Control or safety input | Fire alarm, emergency stop, BMS schedule, interlock or suppression interface is withholding enable. | Trace the authorised control logic and identify why the input is open. |
| VSD or contactor | A drive has tripped or a contactor cannot deliver the motor supply. | Use the recorded code, check supply and motor, and correct the cause before reset. |
| Motor overload or phase loss | High current, missing phase, poor cooling or seized bearings operates protection. | Measure phase supply and current and inspect motor and mechanical load. |
| Belt or impeller | The motor runs but the fan wheel is not driven correctly, is dirty or is damaged. | Isolate, guard against restart and inspect mechanical transmission and balance. |
| Blocked system | Grease filters, attenuators, duct or discharge restriction reduces airflow and increases load. | Inspect the full extract path and cleaning record, not just the canopy. |
| Fan-proof device | Blocked pressure tubes, failed switch, poor sampling location or incorrect current threshold prevents proof. | Test and recommission the sensing method; never link the input permanently. |
What not to do
- Do not bypass the gas interlock.
- Do not reset a VSD or overload repeatedly.
- Do not open fan access doors until isolated and secured against restart.
- Do not run the fan with guards, filters or access panels incorrectly fitted.
Protective devices and lockouts are evidence. Bypassing them may temporarily hide the symptom while removing the control intended to prevent fire, shock, gas release, pressure, contamination or equipment damage.
Information to give the service engineer
- Fan make, model, motor rating and drive details
- VSD or interlock fault code
- Whether supply-air fan also runs
- Sound, vibration and smell before failure
- Filter and TR19 cleaning history
- Fire-alarm and suppression status
- Whether motor runs but airflow is low
- Recent fan-speed, BMS or electrical changes
A short video or photograph taken from the normal operating position can help, provided doing so does not delay emergency action or expose anyone to heat, gas, electricity, moving parts or chemicals.
Repair or replace?
Sensors, belts, bearings, contactors and drives are frequently repairable. Replacement should be assessed where the impeller or casing is corroded, the motor and drive are both obsolete, duty no longer matches the cooking load, grease access is poor, or repeated breakdowns show the fan system cannot operate reliably at its required pressure and temperature.
Request a written diagnosis rather than a parts list alone. It should identify the failed function, the evidence, any upstream cause and the checks required before the appliance returns to service.
Preventing the fault returning
- Keep grease filters and the wider extract system on a risk-based cleaning schedule.
- Record drive parameters and fault history.
- Maintain belts, bearings, vibration and motor current.
- Do not change fan speeds without checking capture, balance and interlock proof.
- Test safety shutdown and proof functions during competent maintenance.
Frequently asked questions
Can gas appliances run if the fan sounds as though it is on?
Only when the designed ventilation and interlock conditions are correctly proved. Sound alone does not establish airflow.
Why does the interlock show fan fail when the fan spins?
The pressure or current proof may not meet its calibrated threshold, or actual airflow may be restricted.
Can staff reset the VSD?
Only under the site's authorised procedure and after the code is recorded. A repeated trip requires diagnosis.
Related Kitchen Install Guide reading
- Does a commercial kitchen need extraction?
- Commercial kitchen gas interlocks
- Commercial kitchen make-up air
Planning a repair, replacement or kitchen upgrade? Use the guide to connect fan duty, make-up air, gas interlocking, ductwork and appliance load when diagnosing or replacing a system. KIG 317 commercial kitchen installation guide.
Sources and further reading
Technical information is general because control sequences and service procedures vary by manufacturer and model. Check the current manufacturer documentation and the requirements applying to your site.