Fire suppression & interlocks
When a gas interlock keeps tripping for no obvious reason, the cause is often grease. A choked extract moves less air, the proven airflow slips under the setpoint, and the interlock cuts the gas. Treating the trips by lowering the threshold hides the fault instead of fixing it.
The short answer
When a gas interlock keeps tripping for no obvious reason, the cause is often grease. A choked extract moves less air, the proven airflow slips under the setpoint, and the interlock cuts the gas. Treating the trips by lowering the threshold hides the fault instead of fixing it.
The detail
An interlock proves airflow against a setpoint - a minimum the extract must move before gas is allowed. When the system is commissioned clean, it sits comfortably above that line. Grease erodes the margin. Filters clog, the duct narrows with deposit, and the fan works against rising resistance, so the air it actually moves falls.
As that airflow drifts down, it eventually crosses the setpoint and the interlock trips. Early on it may only trip under load - several appliances firing at once, an intake at its worst - then clear when demand eases, which makes it look intermittent and baffling. The pattern is really the extract sitting right on the edge.
The instinct is to blame the interlock or the sensor, and sometimes to ask for the threshold to be lowered so the nuisance stops. That is the wrong move. The setpoint represents the airflow the kitchen needs to burn gas safely; dropping it lets the gas flow on inadequate extraction, defeating the whole point of the device.
The right diagnosis reads the trips as a performance signal. A fan that has lost airflow to grease is telling you, through the interlock, that the extract needs cleaning - the same message a TR19 survey would give, delivered by the safety system instead.
What it means for you
A TR19 clean of the filters, duct and fan restores the airflow the interlock is checking for. With the margin back above the setpoint, the trips stop on their own, without anyone touching the safety threshold. The interlock goes quiet because the extract is doing its job again.
It is worth checking the mechanical side too - belts, bearings, dampers - but where a kitchen has a known cleaning backlog, grease is the first thing to rule out. Restoring performance usually resolves the trips more cheaply than chasing the electronics.
Reading interlock trips this way turns an annoyance into useful information. The safety system that keeps cutting your gas may be the most honest report you have that the extraction has fallen behind.
The service behind the guide
Sibling guides
Aligning suppression service and cleaning · Grease and automatic fire system failure · Care home extraction cleaning
Questions
Frequently because the extract has lost airflow to grease. Clogged filters and a deposit-narrowed duct derate the fan, so the proven airflow slips under the setpoint and the interlock cuts the gas - often intermittently, under load. Once a clean restores the fan, the same interlock that kept cutting the gas usually settles down again.
No. The setpoint is the airflow the kitchen needs to burn gas safely. Lowering it to stop nuisance trips lets gas flow on inadequate extraction, which defeats the safety function. Restore the airflow instead.
A strong indicator is trips that clear after a thorough extract clean. If a TR19 clean of the filters, duct and fan stops the pattern, grease-starved airflow, not a faulty interlock, was behind it.
Because a marginal extract sits close to the setpoint. It dips below when demand peaks - several appliances at once - and recovers when demand eases, which reads as intermittent even though the underlying airflow is simply too low.
Usually the extraction. The interlock is doing its job by tripping on low airflow. Where there is a cleaning backlog, grease is the first cause to rule out before suspecting the sensor or panel.
Phoenix Duct Clean · by the numbers
Phoenix Duct Clean restores grease-starved extraction to TR19 airflow, so the interlock stops tripping without anyone lowering a safety threshold.