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Duct cleaning - diagnosis
When the airflow from an extract system drops off, and the make-up air is not the issue, a clogged extract run is a likely cause - the ductwork restricted by a build-up that is choking the airflow. But knowing it is a clog is only half the job: you have to find where the blockage is to clear it. Here is how to diagnose a clogged extract run when the airflow falls off.
The short answer
When airflow drops off and make-up air is adequate, a clogged extract run is a common cause: a build-up of grease, dirt or debris inside the ductwork is restricting it, so the system moves less air. Diagnosing it has two parts. First, confirm it is a clog - by ruling out the other causes of low airflow (make-up air shortfall, fan problems) and finding the signs of a duct restriction. Second, and the real work, locate where the clog is - because to clear it you have to find it. This is done by tracing the extract run and looking for where the restriction lies: checking the accessible points along the run, measuring the airflow and pressure at different points to find where it drops, and inspecting inside through the access panels. The blockage is usually where grease and debris concentrate - at bends, low points, the fan, or a long horizontal run. So diagnosing a clogged extract run is about confirming the clog and then locating it, so the cleaning can target the actual restriction.
Confirming it is a clog
When airflow drops off, the first step is to confirm that a clogged extract run is actually the cause, rather than something else - because low airflow has a few possible causes, and treating the wrong one wastes effort. The main alternatives are a make-up air shortfall, where the extraction is starved of replacement air (which produces negative-pressure signs like hard-to-open doors), and a fan problem, where the fan itself is failing or under-performing. Ruling these in or out helps confirm whether the airflow drop is really a duct clog. If the make-up air is adequate and the fan is running properly, but the airflow is still down, a restriction in the ductwork - a clog - becomes the likely cause.
The signs of a duct clog specifically are a gradual drop in airflow over time (as the build-up accumulates), often alongside the general signs of a loaded system - grease at access points, a stale smell, reduced extraction performance. A clog that has built up gradually, in a system that has not been cleaned, fits the picture of accumulated grease and debris restricting the ducts. So confirming a clog means ruling out the make-up air and the fan, and finding the signs of a duct restriction and build-up. Once you are confident the airflow drop is a clogged run rather than a starved or fan-limited system, the diagnosis moves to its real challenge: not whether there is a clog, but where it is.
Why you have to locate it
Knowing there is a clog somewhere in the extract run is not enough to clear it - you have to find where the blockage is, because the cleaning has to reach and remove the actual restriction. An extract run can be long and winding, passing through voids and ceilings, so a clog could be in any of many places along it. Cleaning the wrong part, or cleaning generally without targeting the restriction, may not clear the blockage that is choking the airflow. So the practical goal of diagnosis is to locate the clog - to narrow down where along the run the restriction lies, so the cleaning can be directed there and actually remove it.
This is the part of diagnosing a clogged run that takes real work, because the ductwork is largely hidden and a blockage inside it is not visible from outside. You cannot simply see where the clog is; you have to work it out by tracing the run and investigating along it. This is where the diagnostic effort goes - not into establishing that there is a clog, which the airflow drop and the ruling-out have established, but into pinning down where it is. Locating the clog is what turns a general knowledge that the run is blocked into a targeted cleaning that clears the specific restriction. So the heart of diagnosing a clogged extract run is the location: finding where the blockage is, so it can be reached and removed.
How to locate the clog
Locating a clog in an extract run is done by tracing the run and investigating along it, in a few complementary ways. Tracing the run means following the ductwork's path from the canopy to the discharge, identifying its layout - the runs, bends, and where the fan and discharge are - so you know the extent of what has to be checked and where the likely trouble spots are. Measuring the airflow and pressure at different points along the run can pinpoint where the restriction lies: the airflow or pressure will change across a blockage, so measuring at points before and after suspected spots helps narrow down where the drop occurs, and so where the clog is. This is a systematic way to home in on the restriction.
Inspecting inside the ductwork through the access panels is the most direct method: looking inside the ducts at the access points shows where the build-up is heavy and where it is restricting the passage, revealing the clog directly where the access allows. Combined, these methods - tracing the layout, measuring to narrow down the location, and inspecting inside to confirm it - locate the clog. And they are guided by knowing where clogs tend to form: grease and debris concentrate at bends (where the airflow changes direction and the build-up is heaviest), at low points (where material collects), at the fan (where grease accumulates), and in long horizontal runs (where dirt settles most). So the likely locations narrow the search, and the tracing, measuring and inspecting confirm where the actual restriction is - which is what the cleaning then targets.
From diagnosis to cleaning
Once the clog is located, the diagnosis has done its job: it has turned a dropped airflow into a known restriction in a known place, which the cleaning can then target. The cleaning reaches the located blockage - through the access panels, fitting additional access if needed to reach it - and removes the build-up that was restricting the duct, clearing the clog and restoring the airflow. Because the diagnosis located the clog, the cleaning can address the actual restriction rather than cleaning blindly, which makes it effective: the airflow drop is fixed by removing the specific build-up that caused it.
It is worth noting that a clog in one place often means the system generally is loaded, since the same conditions that produced the worst build-up have been loading the rest of the run too - so while the diagnosis locates the worst restriction, the sensible response is usually to clean the whole run, clearing the located clog and the general build-up together. This restores the airflow fully and deals with the loading that would otherwise produce the next clog. So diagnosing a clogged extract run leads naturally into cleaning it: confirm the clog, locate it, and clean it - targeting the located restriction while addressing the whole loaded run. The diagnosis is what makes the cleaning targeted and effective, turning a mysterious airflow drop into a specific, fixable restriction. Airflow dropping off, diagnosed properly, points to exactly where the cleaning is needed.
The takeaway
When airflow drops off and the make-up air is adequate, a clogged extract run is a common cause - a build-up of grease, dirt or debris restricting the ductwork. Diagnosing it has two parts. First, confirm it is a clog: rule out the make-up air (negative-pressure signs) and the fan, and find the signs of a duct restriction and build-up. Second, and the real work, locate where the clog is - because to clear it, you have to find it, and the ductwork is hidden so the blockage is not visible from outside. You cannot just see the clog; you have to work out where it is.
Locating it is done by tracing the run's layout, measuring the airflow and pressure at points along it to narrow down where the restriction lies, and inspecting inside through the access panels to confirm it - guided by knowing that clogs concentrate at bends, low points, the fan and long horizontal runs. Once located, the clog can be targeted by cleaning that reaches and removes it, usually as part of cleaning the whole loaded run. So diagnosing a clogged extract run is about confirming the clog and then finding it, so the cleaning targets the actual restriction. Airflow dropping off, diagnosed properly, points to exactly where the cleaning is needed - turning a mysterious drop into a specific, fixable blockage.
Questions
A few things: a make-up air shortfall starving the extraction, a fan problem, or a clogged extract run - a build-up of grease, dirt or debris restricting the ductwork. When the make-up air is adequate and the fan is running properly but the airflow is still down, a clog in the ductwork becomes the likely cause, usually one that has built up gradually in a system that has not been cleaned.
Rule out the alternatives: check the make-up air (a shortfall produces negative-pressure signs like hard-to-open doors) and the fan (whether it is running properly). If both are fine but the airflow is still down, a duct restriction is the likely cause - confirmed by the signs of a loaded system, like grease at access points, a stale smell and a gradual airflow drop over time.
Because clearing it means reaching and removing the actual restriction, and an extract run can be long and winding, so the clog could be in many places. Cleaning the wrong part, or cleaning generally without targeting the restriction, may not clear the blockage choking the airflow. So the practical goal is to locate the clog, so the cleaning can be directed there and actually remove it.
By tracing the run's layout (following the ductwork from canopy to discharge to know what to check), measuring the airflow and pressure at points along it (which change across a blockage, narrowing down where the drop occurs), and inspecting inside through the access panels (which shows the build-up directly). These are guided by knowing clogs concentrate at bends, low points, the fan and long horizontal runs.
Where grease and debris concentrate: at bends, where the airflow changes direction and the build-up is heaviest; at low points, where material collects; at the fan, where grease accumulates; and in long horizontal runs, where dirt settles most. Knowing these likely locations narrows the search, and the tracing, measuring and inspecting confirm where the actual restriction is.
The cleaning targets it - reaching the located blockage through the access panels, fitting additional access if needed, and removing the build-up to clear the clog and restore the airflow. Because a clog in one place usually means the system is generally loaded, the sensible response is to clean the whole run, clearing the located clog and the general build-up together, restoring the airflow fully.
A dropped airflow often means a clogged run - our cleaning traces the system, locates the restriction, and removes it, clearing the clog and the general build-up to restore the airflow.
