Diagnostics & problems
A thorough examination certifies a system on the day it is tested. Between tests, performance drifts as filters load, ducts wear and the work changes. A fail after a pass usually means that drift finally crossed the line.
The short answer
It feels contradictory: the same system, the same examiner's standard, and yet a pass last time and a fail this time. The explanation is almost always that a thorough examination certifies performance on the day it is carried out, and performance drifts continuously in between. A fail after a pass usually means that steady drift finally crossed the line that separates adequate control from inadequate.
The detail
Several things degrade quietly over a test interval. Filters load, so the pressure drop across them climbs and airflow falls. Ducts silt up inside or develop leaks at joints, so the pull that reaches the hood weakens. Fans wear: belts slip and perish, bearings tire, impellers corrode or cake with dust, and the whole system slides down its performance curve. Hoods get knocked, dented or moved. None of this announces itself; each one shaves a little capture off, and together they can take a comfortable pass down to a fail.
The single biggest cause, though, is change that was never fed back to the system. A new machine added onto the same fan, a bench relocated, an extra hood spliced in, a partition or a new roller door creating a cross-draught: any of these can shift the operating point or spoil capture without a single component failing. The system was commissioned to control one arrangement of work, and the work has quietly become a different arrangement. A marginal pass last time makes all of this worse, because a borderline or amber result had little headroom to lose in the first place.
What it means for you
This is exactly what benchmarking against the commissioning datum is for. Because the examiner compares live readings to the figures recorded when the system was proven to control, a fail comes with a direction and a magnitude: which hood, how far down, at what pressure. That turns a fail from a verdict into a diagnosis and points the remedial work straight at the cause, whether that is a filter change, a duct repair, a fan service or a redesign to match how the work is now done.
It also reframes the fourteen-month interval. Under COSHH Regulation 9 that is the legal maximum between examinations, not a service schedule and not a promise that the system holds up until then. Degradation is continuous, so the sensible response to a fail is not surprise but a shorter leash: fix the cause, and where a duty is arduous or a system loads quickly, examine and maintain more often than the law's ceiling. A pass tells you the system was in control on a Tuesday; keeping it in control is a maintenance job, not a certificate.
The service behind the guide
We benchmark every reading against the system's design, so a fail comes with a cause and a fix rather than a bare verdict. You get a report that shows what moved, by how much, and what to do about it.
Questions
Because a thorough examination certifies performance on the day, and performance drifts in between. Filters load, ducts wear and leak, fans decline, and the work often changes. A fail after a pass usually means that drift finally crossed the line.
Change that was never fed back to the system: a new machine on the same fan, a relocated bench, an added hood or a new cross-draught. The system was commissioned for one arrangement of work and the work has quietly changed.
Yes. A borderline or amber result has little headroom, so normal drift is far more likely to take it to a fail before the next test. A marginal pass is a reason to maintain sooner, not to relax.
No. It is the legal maximum between examinations under COSHH Regulation 9, not a promise that the system holds up until then. Where duties are arduous or systems load quickly, examine and maintain more often.
Comparing live readings to the commissioning datum shows which hood has dropped, by how much and at what pressure. That turns a fail into a diagnosis and points the remedial work straight at the cause.
Phoenix Duct Clean · by the numbers
A fail is not bad luck. We show you which readings moved and why, benchmarked against the design, so the remedial work fixes the cause. Call to book the examination and get a clear report.