Fault guide
Gas Shuts Off When Extraction Starts
Gas dropping out the moment extraction starts — or when the fan changes speed — points to the ventilation proving device, not the gas side. The panel is being told airflow is not proven, or the electrical supply is dipping as the fan starts.
- Almost always an air pressure switch, sensing tube or supply-fan balance issue
- Airflow measured and the proving device set correctly, not just swapped
- Fixed on the first visit in most cases, with switches in stock
Common causes
| Cause | Why it trips on fan start | Fix |
|---|---|---|
| Pressure switch set too high | Actual duct pressure never reaches the set point | Measure duct pressure and reset the switch inside its range |
| Sensing tubes swapped or kinked | Differential reads backwards or not at all | Refit tubes to high and low ports, replace perished tube |
| Blocked filters or dirty canopy | Airflow collapses as the system loads | Clean filters and ductwork, then re-prove |
| Make-up air fan not running | Kitchen goes into depression, extract flow falls away | Supply fan repair and interlocked balance |
| Voltage dip on fan start | Coil or panel relay drops out momentarily | Electrical supply check, soft start or separate supply |
| Inverter ramp too slow | Proof times out before the fan reaches speed | Adjust ramp or proving delay at the panel |
| Damper closed or actuator failed | Fan runs against a shut damper | Damper and actuator repair |
What we check on site
- Measured extract and make-up air volumes against the design and canopy requirement
- Differential pressure across the fan with the switch set point compared side by side
- Condition of sensing tubes, tappings and the switch diaphragm
- Panel proving delay and fan run-up timing
- Stability of the safety circuit through a full service-load simulation
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
Is it dangerous to keep resetting it?
Repeated resets mask a real ventilation fault. If ventilation is not proven the kitchen should not be cooking on gas — and the repeated cycling can damage the solenoid coil.
Could the fan be too powerful?
Yes, in unbalanced kitchens. Excess extract with no make-up air pulls the kitchen into depression, which starves the extract flow and can cause both interlock trips and appliance flueing problems.
Can the pressure switch just be turned down?
Only to a properly measured set point. Winding it down until the fault stops is a bypass in all but name — it can prove airflow that does not exist.
How long does the repair take?
Most airflow proving faults are diagnosed and repaired within a couple of hours, with pressure switches carried in stock.
Related guides
Interlock Fan Fault
Fan fail lamps, airflow switch faults and extract failures.
Air Pressure Switch Repair
Differential switch setting, tube blockage and replacement.
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.
Commercial Kitchen Gas Safety
Ventilation interlock, airflow proving and CP42 compliance.
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