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LEV testing - a failed test
It is a common and reasonable question: the system passed last time, nothing obvious has changed, so why has it failed this year? The answer is usually that something has quietly declined or shifted in the intervening months. Understanding the common reasons helps make sense of a fail - and shows why a failed test is actually useful information, catching a problem you could not see.
The short answer
A system that passed before and fails now has almost always changed in the interval, usually in ways you cannot see. The common reasons are a loaded or neglected filter choking the airflow; a fan that has lost performance through wear or a slipping belt; a blockage or heavy deposit building up in the ductwork; a leak or a loose joint; a hood damaged, moved, or a process that has drifted away from it; or increased demand on the system. Any of these can pull a previously sound system below the mark. A fail is not a mystery - it is the test detecting a real change, which is exactly what testing is for.
The reasonable question
When a system that passed its last examination fails this one, the natural reaction is puzzlement: nothing seems to have changed, the system is the same one that was fine last time, so why has it failed now? It can even feel like the test result must be wrong. But the question has a straightforward answer in almost every case: something has changed in the months between the two examinations, usually gradually and invisibly, and the test has detected it. The system that passed before and the system being tested now are not quite the same system - time and use have altered it, even if nothing looks different.
This is the key to making sense of a fail. LEV declines quietly - the whole reason periodic testing exists is that systems lose performance in ways that are not apparent from looking or listening. So a fail after a previous pass is not a contradiction; it is the expected way problems surface. The system drifted below the mark at some point in the interval, and the examination is the moment that drift is measured and caught. Understanding the common reasons a system changes between tests turns a puzzling fail into something explicable - and reframes it as the test doing its job rather than an anomaly.
The usual causes
Several common changes account for most fails after a previous pass. A loaded filter is among the most frequent: as a filter fills with the dust it captures, its resistance rises and the airflow falls, so a filter that has not been changed can choke the system below the mark. Fan performance decline is another: a slipping or worn belt, a fouled impeller, or worn bearings reduce the airflow the fan moves, gradually pulling the system down. Both of these are quiet, progressive changes that can easily take a system from a pass to a fail over a testing interval without anything visible happening.
Others involve the ductwork and hoods. A blockage or heavy internal deposit building up in the ducts restricts the flow; a leak or a loose or failed joint lets air in where it should not be, robbing the hoods of airflow. A hood can be damaged, or a process can have drifted away from its hood - or a layout change moved things - so capture is lost at that point. And sometimes the demand on the system has increased - a new process, more use - so a system that was adequate is now stretched too thin. Any one of these, or a combination, can be why a previously sound system no longer makes the mark. The examination identifies which, so the cause can be put right.
A fail is useful information
It is worth reframing what a failed test actually is: not simply bad news, but useful information. The system had, in fact, drifted below the mark - that is a real state of affairs, whether or not the test had caught it. The failure was already happening; the test is what revealed it. So a fail is the test doing exactly what it is for: detecting a genuine problem that had developed invisibly, so it can be fixed. Without the test, the same decline would have continued unnoticed, with the system quietly failing to protect people. The fail is the moment that hidden problem became known and actionable.
Seen this way, a fail is far better than a false pass would have been. A system that had declined but was wrongly passed - or never tested - would go on exposing people while everyone believed it was fine. The fail prevents that: it catches the real problem and forces it to be addressed. So while a fail is not the outcome anyone hopes for, it is the testing regime working correctly - finding the decline that periodic testing exists to find. The useful response is not to doubt the result but to understand its cause and fix it, restoring the system to controlling exposure. A fail is information you needed, delivered at the point it can still be acted on.
What to do with a fail
A failed examination should identify why the system failed - which of the common causes, or combination, brought it below the mark. That diagnosis is what makes the fail actionable: a loaded filter is changed, a declining fan repaired, a blockage cleared, a leak sealed, a hood repositioned, or the system's capacity addressed if demand has outgrown it. The examination does not just say the system failed; it points to the cause, so the fix is targeted rather than guesswork. Acting on that diagnosis restores the system to controlling exposure, which is the whole point.
The fail is also a prompt to consider prevention. If a loaded filter caused it, better filter monitoring and changing between tests would catch the loading before it fails the system next time. If fan decline caused it, more attention to the fan's condition in routine maintenance helps. Many of the common causes are things that a good maintenance regime and everyday vigilance can catch earlier, so that the system is kept above the mark between tests rather than drifting down to fail. So a fail, properly used, does two things: it gets the immediate problem fixed, and it highlights where better upkeep could prevent a repeat. Both come from understanding why the system failed - which is why the diagnosis behind the fail matters as much as the result itself.
The takeaway
A system that passed before and fails now has almost always changed in the interval, usually invisibly. The common reasons are a loaded filter choking the airflow, a fan that has lost performance, a blockage or deposit in the ductwork, a leak or loose joint, a damaged or moved hood or a process drifted away from it, or increased demand outstripping the system. Any of these can pull a previously sound system below the mark without anything looking different - which is why a fail after a pass is not a mystery but the expected way a quiet decline surfaces.
Reframed properly, a fail is useful information, not just bad news: the system really had drifted below the mark, and the test caught it so it can be fixed - far better than a false pass that would let the decline continue unnoticed. The useful response is to understand the cause the examination identifies, fix it to restore control, and consider what upkeep would prevent a repeat. A fail is the testing regime working correctly, finding the decline it exists to find - delivered at the point it can still be acted on.
Questions
Because something has changed in the interval, usually gradually and invisibly - the system tested now is not quite the one that passed before. LEV declines quietly, which is exactly why periodic testing exists. A fail after a pass is the expected way such a hidden decline surfaces, not a contradiction or an error.
A loaded filter choking the airflow; a fan that has lost performance through a slipping belt, fouled impeller or worn bearings; a blockage or deposit in the ductwork; a leak or loose joint robbing the hoods of airflow; a damaged or moved hood, or a process drifted away from it; or increased demand outstripping the system.
Almost never. A fail means the system had genuinely drifted below the mark - a real state of affairs the test detected, whether or not it had been caught. The failure was already happening; the test revealed it. Doubting the result misses the point: the useful response is to understand the cause and fix it.
It is not the outcome anyone hopes for, but it is useful information rather than simply bad news. The system had really declined, and the test caught it so it can be fixed - far better than a false pass that would let the decline continue unnoticed while people believed the system was fine. A fail is the testing regime working correctly.
Understand the cause the examination identifies - the loaded filter, declining fan, blockage, leak or moved hood - and fix it to restore the system to controlling exposure. The diagnosis makes the fail actionable, so the fix is targeted rather than guesswork. Then consider what upkeep would prevent a repeat.
Many common causes can be caught earlier by good maintenance and vigilance - monitoring and changing filters before they choke the system, attending to the fan's condition, watching for a weakening pull. Keeping the system above the mark between tests, rather than letting it drift down, is how repeats are prevented.
When a system fails, our thorough examination identifies why - the loaded filter, declining fan, leak or moved hood behind it - so the cause can be fixed and the system restored to controlling exposure.
