Fault guide
Gas Interlock Fan Fault & Airflow Switch Faults
A fan fault on a gas interlock means the panel cannot prove that the kitchen extract is working, so it closes the gas. The fault is either a genuine extract failure or a proving device that can no longer see the airflow that is there.
- Fan fail lamp, buzzer and gas shut-off are the designed response to extract failure
- We separate true fan failures from proving-device faults by measurement
- Fan, belt, inverter, damper and airflow switch repairs all covered
System architecture
Genuine extraction fan failure
- Motor burnt out, bearing seized or thermal overload tripped
- Drive belt snapped, slipped or glazed on belt-driven canopy fans
- Inverter fault or trip code, often on over-current or over-temperature
- Isolator switched off at roof level or in the plant room
- Impeller clogged with grease, or fire damper dropped
- Roof cowl or discharge blocked
Airflow proving device faults
| Device | Typical fault | Action |
|---|---|---|
| Differential air pressure switch | Diaphragm perished, set point drifted, tubes blocked | Test, reset or replace the switch |
| Duct airflow sensor | Grease-coated sensing element, drift, wiring fault | Clean, recalibrate or replace |
| Fan current monitor | Set below actual load, CT clamp loose | Reset the trip band around measured running current |
| Damper end switch | Damper not fully opening, switch misaligned | Adjust actuator and switch |
Why it matters
Under BS 6173, gas appliances in a commercial kitchen must not be able to run without proven mechanical ventilation. An extraction failure that did not shut the gas off would allow combustion products and unburnt gas to build up in an occupied kitchen — so a fan fault stopping the gas is the system working correctly, and it needs a proper repair rather than a workaround.
If you can smell gas
- Turn off the gas at the emergency control valve and press the emergency stop button
- Do not switch anything electrical on or off, and do not use naked flames
- Open doors and windows, evacuate and keep people clear
- Call the National Gas Emergency Service on 0800 111 999
- Only then call us to make the installation safe and return it to service
Frequently asked questions
The fan is definitely running — why a fan fault?
Because the panel does not sense airflow, only the proving device's contact. A drifted pressure switch, blocked sensing tube or dirty airflow sensor will report a fan fault with a perfectly healthy fan.
Can you repair the extract fan as well as the interlock?
Yes — we work on the fan, belts, isolators, dampers and inverters as well as the interlock and gas side, so one visit clears the whole fault.
How fast can you get the kitchen back on gas?
Most fan and airflow proving faults are repaired the same day; we hold air pressure switches and interlock panels in stock.
Should the fan fault be logged?
Yes. Recurring fan faults are evidence for your compliance file and often the first sign of ventilation maintenance being overdue.
Related guides
Air Pressure Switch Repair
Differential switch setting, tube blockage and replacement.
Gas Shuts Off When Extraction Starts
Pressure switch set wrong, ducting or make-up air imbalance.
Make-Up Air Fan Repair
Supply fan failures, balance faults and interlock knock-on effects.
Sensors & Controls
Air pressure switches, airflow sensors, current monitors, e-stops.
Interlock Won't Reset
Reset sequence, latched faults and why the panel keeps dropping out.
Gas Interlock Systems
The pillar guide: how interlocks work, standards and fault-finding.
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Merlin, Medem, Intelligas, S&S Northern
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