Fire suppression & interlocks
A gas interlock stops the burners lighting unless the extraction system is actually moving air. It is a safety device that links your ventilation directly to your gas supply, so a kitchen that is not extracting properly is a kitchen that cannot get gas.
The short answer
A gas interlock stops the burners lighting unless the extraction system is actually moving air. It is a safety device that links your ventilation directly to your gas supply, so a kitchen that is not extracting properly is a kitchen that cannot get gas.
The detail
BS 6173, the standard for installing commercial catering gas appliances, requires gas equipment to be interlocked with any mechanical ventilation. The logic is simple: burning gas indoors without adequate extraction risks a build-up of combustion products, so the rule is that gas only flows while airflow is proven. IGEM/UP/19 covers how that interlock is designed and applied.
Airflow is proven by a sensor - most often a differential air pressure switch across the fan, or a fan current monitor confirming the motor is drawing its running load. Where a mechanical supply fan feeds the kitchen as well, both extract and supply are proven. If the sensor does not see airflow, the interlock will not let the gas through.
The device that holds or cuts the gas is a solenoid valve on the supply, driven by an interlock control panel. Emergency stop buttons on the escape route let staff kill the gas manually. The panel ties the whole chain together: prove airflow, energise the valve, allow gas; lose airflow, drop the valve, cut gas.
This all sits under the Gas Safety (Installation and Use) Regulations 1998, with a commercial gas safety check - the CP42 - carried out annually by a Gas Safe registered engineer. The interlock is not optional dressing; it is part of what makes the gas installation legal to run.
What it means for you
The interlock is a threshold device, and grease moves the threshold. Clogged filters and a greasy duct raise resistance and derate the fan, so the airflow it proves drifts down toward the trip point. A system that ran with comfortable margin when clean can start sitting marginally when dirty.
When that margin is gone, the interlock does its job and cuts the gas - often at the worst moment, mid-service. The temptation is to lower the trip threshold to stop the nuisance trips, but that defeats the safety function. The correct fix is to restore the airflow by cleaning the system.
Recurring interlock trips are therefore a useful signal. They often mean the extract has lost performance to grease, and a TR19 clean of the filters, duct and fan restores the airflow the interlock is checking for - keeping the gas on for the right reason.
The service behind the guide
Sibling guides
What a gas interlock does when the fan fails · Fire dampers: function and testing · The compliance certificate
Questions
It is a safety system that only allows gas to the appliances while the extraction is proven to be moving air. Required under BS 6173, it links ventilation to gas supply through a control panel, an airflow sensor and a solenoid valve.
Through a differential air pressure switch across the fan or a current monitor on the motor. Either confirms the fan is genuinely moving air, not just powered, before the panel lets gas through the solenoid valve.
Often because the extract has lost airflow to grease and is dropping below the interlock threshold. The fix is to clean the system and restore performance, not to lower the trip point, which would defeat the safety function.
Interlocking gas appliances with mechanical ventilation is required under BS 6173 and sits within the Gas Safety Regulations 1998. The commercial gas installation is checked annually on a CP42 by a Gas Safe registered engineer.
Frequently, yes. If trips are caused by a grease-derated fan, a TR19 clean of the filters, duct and fan restores the airflow margin so the interlock proves adequate flow and keeps the gas on.
Phoenix Duct Clean · by the numbers
Phoenix Duct Clean restores grease-derated extraction to TR19 performance, so your gas interlock proves the airflow it was commissioned to see.