Fire suppression & interlocks
If the extraction fan stops or its airflow drops below the proven level, the gas interlock closes the solenoid valve and cuts the gas to the burners. It is a fail-safe - the safe state is gas off - and it works whether the fan has failed outright or simply lost performance.
The short answer
If the extraction fan stops or its airflow drops below the proven level, the gas interlock closes the solenoid valve and cuts the gas to the burners. It is a fail-safe - the safe state is gas off - and it works whether the fan has failed outright or simply lost performance.
The detail
The interlock continuously watches for proof of airflow. A differential pressure switch or fan current monitor feeds the control panel a live yes or no. While the answer is yes, the panel keeps the gas solenoid valve energised and open. The moment the answer becomes no, the panel de-energises the valve and the gas stops.
That covers a hard failure - a tripped motor, a snapped belt, a power loss to the fan - where airflow disappears and the gas is cut almost immediately. Staff see the burners go out and know something is wrong. The interlock has done exactly what it should: removed the fuel when the extraction that makes it safe to burn is gone.
It also covers the softer case. If the fan is still turning but no longer moving enough air - because the duct and filters are choked, or a bearing is failing - the proven airflow can slip under the switch setpoint and the interlock trips just the same. The system does not care why the airflow fell, only that it did.
Emergency stops on the escape route give staff the same power manually, dropping gas across the kitchen in one action. And because the solenoid is held open only while power and proof of airflow are both present, any loss of either defaults to the safe, gas-off state.
What it means for you
Because the interlock trips on lost airflow, a fan quietly strangled by grease will start cutting the gas as reliably as a broken one. To the staff it looks like a fault; in reality the extract has simply fallen below the line the interlock was set to protect.
Restoring airflow is the real remedy. A TR19 clean of the filters, duct and fan brings the moved air back above the setpoint, so the interlock stops tripping without anyone touching the safety threshold. Lowering the setpoint to stop the trips would leave the kitchen burning gas on inadequate extraction.
So the interlock, doing its job, becomes an early warning that the extract needs attention. A pattern of gas cut-outs that clears after a clean tells you grease, not the interlock, was the problem all along.
The service behind the guide
Sibling guides
Fire dampers: function and testing · Fire damper drop testing and how often · Canopy cleaning and fire risk assessment
Questions
Effectively, yes. The interlock only holds the solenoid valve open while airflow is proven, so when the fan stops and the sensor loses that proof, the panel drops the valve and the gas stops within seconds.
Yes. The interlock trips on inadequate airflow, whatever the cause. A fan choked by grease that no longer moves enough air can fall below the setpoint and cut the gas just like a fan that has failed outright.
It is the fail-safe design. Gas off is the safe state, because burning gas without proven extraction risks a build-up of combustion products. The interlock removing the fuel when airflow is lost is exactly what it is meant to do.
Neither as a first move. If a grease-slowed fan is causing trips, the fix is to clean the system and restore airflow. Adjusting the threshold to mask the trips would leave the kitchen running on inadequate extraction.
A telling sign is trips that stop after a thorough extract clean. If restoring airflow with a TR19 clean clears the pattern, grease-derated extraction, not a faulty interlock, was the cause.
Phoenix Duct Clean · by the numbers
Phoenix Duct Clean restores choked extraction to TR19 airflow, so a grease-slowed fan stops tripping the interlock and your burners stay lit.