Phoenix Journal · LEV Testing
A local exhaust ventilation system rarely fails with a bang. It drifts, quietly, while the paperwork still says everything is fine - which is exactly why a real thorough examination matters more than a tick in a box.
The system had a certificate on the wall, a green sticker on the hood and a report in the folder that said, in plain type, "satisfactory". Nobody in that kitchen had any reason to think the canopy above the range was doing anything other than its job.
And yet the extraction was quietly losing the argument. The fan still turned. The hood still hummed. But the capture velocity at the edge of the cooking line had crept below the point where it could pull a rising plume of grease-laden vapour up and away, and instead a slow drift of it was settling on walls, on light fittings, on the top run of duct where nobody looks. The people working under it noticed nothing. That is the whole problem with a system that looks fine. It does not announce the moment it stops protecting anyone.
This is a story about how that happens, why a piece of paper can say "pass" while the reality says otherwise, and what a genuine thorough examination and test is supposed to catch that a quick visit never will.
Local exhaust ventilation is designed to capture a contaminant at or near the point where it is released and carry it safely away before anyone breathes it. In a commercial kitchen that contaminant is the hot, greasy, moisture-heavy cloud coming off ranges, fryers and chargrills. The system does this using a set of quiet, invisible numbers - the speed of air being drawn into a hood, the velocity moving through the ductwork, the pressure across a filter. When those numbers are right, you see nothing. When they slip, you still see nothing, at least for a while. That is precisely why silent failure is the norm rather than the exception.
Grease is the enemy of its own extraction. Every hour of cooking lays down a thin film inside the hood, along the baffle filters and through the first bends of duct. That film narrows the airway, loads the filters and makes the fan work harder for less. Airflow falls off gradually - a few per cent here, a little more there - so there is never a single day when someone can say "it broke". A belt slackens and the fan slows. A filter blinds and the pressure climbs. A section of flexible duct sags and collapses its own cross-section. None of these trips an alarm. The extraction simply becomes weaker at capturing the plume, and the difference between adequate control and inadequate control is often a change too small for anyone standing under the canopy to feel.
The most common false comfort is that the system runs. A fan spinning tells you the motor has power; it tells you nothing about whether air is actually being captured where it matters. The relevant measurement is what the HSE guidance HSG258, "Controlling airborne contaminants at work", calls capture velocity - the speed of air at the furthest point the contaminant is released from, drawn back towards the hood. A canopy can be visibly moving air and still be failing at its edges, where the reach is weakest and the cooking line is longest. Under the Control of Substances Hazardous to Health Regulations 2002, regulation 9, the duty is not to own a system that turns on. It is to maintain control that works, and to prove it works, on an ongoing basis.
There is a wide gulf between a visit that produces a certificate and an examination that produces evidence. Both end with paperwork. Only one of them actually interrogates the system. The report that quietly signed off a failing kitchen was not necessarily dishonest - it may simply have recorded that the fan ran and the hood looked clean, without measuring whether anything was being controlled. A thorough examination and test, done properly, is a technical assessment, and it leaves numbers behind that you can defend.
When an LEV system is first installed and commissioned, it should have a commissioning report recording its baseline performance - the airflow figures it achieved when new and working as designed. That baseline is the yardstick. A genuine examination measures the current airflows, face and capture velocities and static pressures, then compares them against those commissioning figures and against the design criteria in HSG258. A reading in isolation means little; a reading that has dropped meaningfully below the day-one benchmark is the early warning that a quick visit will never surface. This is also why so many older kitchens are hard to assess fairly - the original commissioning data was never kept, so there is nothing to measure the decline against, and re-establishing a sensible benchmark becomes part of the job.
Under COSHH, this work must be carried out by a competent person - and competent has a specific meaning here, not a vague one. The recognised benchmark is the British Occupational Hygiene Society P601 qualification, "Thorough Examination and Testing of Local Exhaust Ventilation Systems", supported by the professional framework run through the Institute of Local Exhaust Ventilation Engineers. An examiner holding that competence does three things a certificate-filler does not: they identify what the system is meant to control, they measure whether it is actually controlling it, and they state plainly where it falls short and what to do about it. If you want to understand exactly what that competence framework covers and why it matters, it is worth reading what the ILEVE LEV competence framework sets out. The examination should also be on a suitable interval - for most systems that means at least every 14 months, and more often for higher-risk or heavily loaded processes.
The kitchen at the start of this piece was not unlucky. It was normal. Systems drift, grease accumulates, fans tire, and nobody notices because nothing looks wrong. The only thing that reliably closes the gap between "looks fine" and "is fine" is measurement carried out by someone competent to interpret it - and a report you can trust because it shows its working. A green sticker on a hood is reassuring. Numbers on a page are proof.
Questions
Yes, and this is more common than people expect. A report only means as much as what was actually measured. If it records that the fan runs and the hood looks clean but contains no airflow or capture velocity figures compared against the system's commissioning baseline, it has not really tested whether contaminants are being controlled at the source. A system can run, look tidy and still have drifted below effective capture, so ask what numbers sit behind the word "satisfactory".
Under COSHH regulation 9, LEV must be thoroughly examined and tested at suitable intervals, which for most systems means at least every 14 months. Heavily used or higher-risk installations often need testing more frequently. Because grease loading in a commercial kitchen builds continuously, many operators pair the statutory examination with regular cleaning and interim performance checks rather than waiting the full interval between assessments.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.