Phoenix Journal · Extraction
A commercial extract fan rarely fails all at once - it fades, slips or stalls, and a sensor usually notices before anyone in the kitchen does. Here is how the two main sensing methods spot a failing fan, and where each one is blind.
Kitchen extraction monitoring
A commercial extract fan rarely stops with a bang - it fades, stalls or slips, and the first thing that notices is usually a sensor, not a person.
In a busy commercial kitchen the extract canopy has to keep pulling grease-laden air, heat and combustion products away from the cooking line all shift. When the fan behind it starts to fail - a worn bearing, a slack belt, a motor drawing too much current, or a duct so loaded with grease that airflow collapses - you need to know before the gas keeps burning under a canopy that has quietly stopped clearing the air. That is exactly what a proving sensor is for, and under IGEM/UP/19 it is the device that decides whether gas is allowed to flow at all.
There are two sensing methods that dominate UK kitchens, and they detect a failing fan in completely different ways. One watches the air the fan moves; the other watches the electricity the motor draws. Both are accepted for gas interlock duty, both have real strengths, and both have blind spots worth understanding before you trust one to protect your kitchen. This guide compares them plainly so you can tell what your system is actually watching - and where it might be looking the wrong way.
An air differential pressure switch (often shortened to air DP switch, or ADPS) measures the pressure difference created when the fan actually moves air. A small sensing tube sits in the extract duct or across the fan, and the switch compares that reading against still-air room pressure. When the fan is running and pulling properly the pressure difference is present and the switch stays made; when airflow drops below a set threshold the switch opens. The trip point is typically set in the low tens of pascals, tuned to the specific duct on commissioning.
Because it responds to the air itself, an air DP switch is measuring the thing you actually care about - flow at the canopy - rather than a stand-in for it. That is why it is a long-standing choice for BS 6173 and IGEM/UP/19 gas interlocks, and why it doubles neatly as a dirty-filter and blocked-duct warning: as grease loads the filters and duct the pressure signature shifts, and a well-set system can flag the change.
A current sensing device watches the electricity feeding the fan motor instead of the air it moves. A current switch or current transducer clamps around the supply cable at the motor connection or in the control panel, and proves that the motor is drawing current in its normal running band. Devices such as the widely used CS2 current switch operate when current flow is present, and integrated interlocks like VentGuard-style panels build the same principle in for both intake and extract fans. If the motor stops, trips or loses a phase, the current disappears and the interlock reacts.
Current monitoring is explicitly accepted under IGEM/UP/19, with one important caveat: where belt-driven fans are used, the belt system must have its own slippage and failure monitoring, because a motor can happily keep spinning and drawing current while a slipping or snapped belt means the impeller has stopped and no air is moving. Sensible panels also delay the gas shutdown - commonly around 30 seconds of proven failure - to avoid nuisance tripping on brief dips.
The figures that matter
Whichever method sits on your fan, the standards behind it are the same, and the sensor is only ever as good as the cleaning and commissioning behind it. These are the points worth remembering when you look at your own interlock.
The recurring theme is that no sensor is a substitute for a clean, well-maintained system. A grease-loaded duct will fool an air DP switch into nuisance trips and hide itself entirely from a current monitor - so scheduled extract cleaning to DW/172 and TR/19 standards is what keeps either sensor honest. It is also worth being clear-eyed about what these devices can and cannot tell you, which we cover in what air quality sensors can and can't tell you.
If you are unsure whether your interlock is watching airflow or current - and whether grease has quietly blinded it - the safest move is a professional inspection alongside a full extract clean, so the sensor and the system it protects are checked together.
Questions
They warn you about different failures. An air pressure differential switch measures the air the fan actually moves, so it catches a slipping belt, a seized impeller or a blocked duct that a current sensor would miss, and it can hint at gradual grease loading. A current sensor only proves the motor is drawing power, so it reacts instantly to a dead motor or tripped supply but stays blind to a fan that is spinning yet moving no air. Many robust installations use current sensing with the belt-slippage monitoring that IGEM/UP/19 requires, or combine both approaches.
Under IGEM/UP/19 the gas is only allowed to flow while the interlock proves adequate ventilation, so a shutdown usually means the sensor no longer sees enough airflow or current, not that the fan has stopped dead. A grease-fouled air pressure sensing tube, a slipping belt, a heavily loaded duct, or a drift out of calibration can all drop the reading below the trip point. Most panels wait around 30 seconds before isolating the gas to avoid nuisance trips, so a repeated shutdown is a genuine signal to have the fan and interlock inspected.
Yes, and it works in opposite ways for the two methods. On an air pressure differential switch, grease can blind the sensing tube and ports, causing false trips or unreliable readings. A current sensor is largely immune to grease on itself, but a grease-choked duct can strangle airflow while the motor keeps drawing normal current, so the sensor happily proves a fan that is barely extracting. Regular extract cleaning to DW/172 and TR/19 standards is what keeps either sensor honest.
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.