Phoenix Journal · Extraction
If your burners keep dying in the middle of service, the interlock is almost certainly doing its job - it has stopped proving that your extraction is pulling enough air. Here is what is really going on and how to get it steady again.
Gas interlock & extraction
When the hobs die in the middle of a busy service and the gas will not relight, it is tempting to blame the appliance - but nine times out of ten the interlock has simply done exactly what it was fitted to do.
A gas interlock is a safety system that only lets gas reach your cooking line while it can prove the ventilation is running and moving enough air. The moment it loses that proof, it closes the solenoid valve and the gas stops. It is not a fault in the usual sense - it is a deliberate cut-off designed to keep carbon monoxide and unburnt gas out of an occupied kitchen.
So the real question is rarely "why has my interlock broken?" and far more often "what has changed that stopped it proving airflow?" This article walks through what the interlock is checking, why it keeps tripping mid-service, the part a grease-loaded extract system quietly plays, and when to call an engineer versus a cleaning specialist.
Under BS 6173 and the Gas Safety (Installation and Use) Regulations 1998, gas cooking appliances in a commercial kitchen must not be able to operate unless mechanical ventilation is proven to be running. IGEM/UP/19, the industry standard for the design and application of interlock devices, sets out how that proving is meant to be done. In plain terms: no proven airflow, no gas.
The system does this with a gas solenoid valve on the incoming supply and a device that watches the ventilation. Most kitchens use an air differential pressure switch fitted across the extract - typically working somewhere in a 20 to 300 Pa band - which senses the pressure the fan generates. Some newer panels use current monitoring instead, reading the electrical draw of the fan motor to confirm it is actually spinning rather than just switched on.
Here is the sequence every time you start up:
If any of those conditions drops away while you are cooking - the fan slows, the pressure falls, a linked carbon monoxide sensor alarms - the panel closes the valve immediately. That is the "hobs just went out" moment. The interlock has stopped seeing proof that it is safe to keep gas flowing.
An interlock that trips once at start-up is usually a reset or sequence issue. An interlock that trips repeatedly during service is telling you the airflow is sitting right on the edge of the proving threshold and tipping below it under load. A handful of causes account for most of these:
The extract fan is moving less air than the switch needs to stay made. That can be a slipping fan belt, a failing motor, a blocked or collapsed duct, or - very commonly - filters and ductwork so loaded with grease that resistance has climbed and airflow has fallen below the design figure. As the kitchen heats up and every burner is lit, demand peaks and the marginal airflow finally dips under the setpoint.
Air pressure differential switches are sensitive by design. On a windy day, gusts across the discharge point can push the switch to click off and on, cutting the gas intermittently. A well-set-up panel uses a built-in time delay to ride out those brief dips, but if the delay is short or the switch setpoint is too tight, you get repeated trips.
Grease and moisture migrating into the sensing lines or the switch diaphragm can make it stick or read falsely. A stuck switch is the dangerous version of this - it can leave gas permanently proven with no real airflow behind it - so erratic behaviour is worth taking seriously rather than working around.
The pattern matters. Trips that get worse as service builds, or worse in bad weather, point at airflow and switch behaviour rather than the gas train itself.
More often than owners expect, yes. The interlock does not measure how clean your ducting is - it measures airflow - but grease is the thing most likely to quietly strangle that airflow over time.
Grease-laden canopy filters are the second most common cause of interlock trips after outright fan failure. As filters clog, the fan has to pull against more resistance, so the volume of air it actually moves falls. Push that far enough and the pressure across the switch drops below the threshold that keeps gas proven. The fan sounds like it is running normally, which is exactly why the cause gets missed and the appliance gets blamed.
It rarely stops at the filters. Under TR19 Grease - the BESA specification for managing fire risk in kitchen extraction - the whole route from canopy to atmosphere is meant to be cleaned, not just the bits you can lift out and rinse. Grease builds along the plenum, the ductwork and the fan itself, and every layer adds resistance and shaves airflow. That same deposit is the fuel load behind duct fires, which is why insurers increasingly ask for TR19 Grease evidence at renewal.
There is a useful diagnostic here: a heavily fouled system that is choking your interlock is usually the same system that starts to smell and stain around the canopy. If that rings true, a kitchen that fails on smell at inspection is often the same grease problem showing up in a different way. Clean airflow paths tend to fix both symptoms at once.
So before you accept a new fan or a rewired panel, it is worth ruling out the simplest explanation: filters and ductwork that have quietly loaded up to the point where the interlock can no longer prove enough air.
Both have a role, and the trick is matching the trade to the symptom rather than guessing. A quick way to split it:
Call a Gas Safe registered engineer if the panel is throwing gas-proving faults at start-up, if you suspect the solenoid valve or gas train, if the pressure switch is stuck or mis-set, or any time you smell gas. Interlock panels must be installed, tested and commissioned by a qualified engineer, and setpoints and time delays should only be adjusted by one. Never tape over, bypass or wedge a switch to keep the hobs alight - that defeats the one safeguard standing between your team and a carbon monoxide incident.
Call an extraction cleaning specialist if the trips track with time since the system was last cleaned, if filters are visibly loaded, if airflow feels weak or the kitchen is slow to clear heat and steam, or if you need TR19 Grease evidence for insurance. Restoring airflow to its design figure often clears marginal interlock trips without touching the gas side at all.
In practice the two work together. A good approach is to have the extract deep cleaned to restore airflow, then have the engineer re-check the switch state and confirm the interlock proves cleanly with the system running as it should.
Questions
No - and it is genuinely dangerous. The interlock is the safety device required under BS 6173 and IGEM/UP/19 to stop gas flowing when ventilation is not proven, which protects your team from carbon monoxide and unburnt gas. Bypassing, taping or wedging a switch removes that protection entirely and would fail any gas safety inspection. The right fix is to find why airflow has dropped, not to defeat the device that is flagging it.
Look at the pattern. Trips that appear or worsen as service gets busy and the kitchen heats up usually point to airflow falling below the proving threshold, often from grease-loaded filters and ductwork. Faults at start-up, gas-proving errors on the panel, or any smell of gas point to the gas train and need a Gas Safe engineer. If filters look loaded and it has been a while since a TR19 Grease clean, restoring airflow is the sensible first step before assuming an expensive gas-side repair.
Phoenix Duct Clean · by the numbers
The right kit only helps if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.