Signs & diagnostics
If smoke drifts across the pass instead of disappearing into the canopy, the extract has lost the pull it needs to catch the plume at source. That capture depends on airflow, and airflow is exactly what a grease-loaded system quietly gives away over time.
The short answer
If smoke drifts across the pass instead of disappearing into the canopy, the extract has lost the pull it needs to catch the plume at source. That capture depends on airflow, and airflow is exactly what a grease-loaded system quietly gives away over time.
The detail
A canopy works by holding a capture velocity across its face - fast enough that the rising column of hot, greasy air is drawn in before it can escape the edges. DW/172, the BESA specification for kitchen ventilation, sizes the extract rate to hold that velocity for the cooking load beneath it.
Capture velocity is a function of airflow. Lose airflow and the boundary where the canopy can grab the plume shrinks inward, so smoke generated at the front of the range or the edge of the cooking line drifts out into the room instead of up and away.
Grease is what erodes the airflow. It loads the baffle filters first, then coats the plenum and duct, raising resistance. A fixed-speed fan meeting that resistance rides up its curve and moves less air - it does not automatically work harder - so the system that captured cleanly last season now lets smoke linger.
Make-up air matters too. If the replacement air supplied to the kitchen has dropped below roughly 85 percent of the extract rate, the room goes negative, doors pull, and the canopy fights to draw against an under-supplied space, which shows up as sluggish, drifting smoke.
What it means for you
Lingering smoke is a capture problem, and grease is the most common cause - but not the only one. A clogged-filter study found volumetric flow can collapse by around half when a filter loads up, which is more than enough to lose capture. So a clean is usually the first and cheapest thing to try.
If smoke still hangs after a verified clean, the diagnosis shifts: the fan may be undersized, the canopy may not cover the equipment properly, or make-up air may be inadequate. Cleaning removes contamination but cannot fix a design or balancing fault, and knowing which you have saves money.
Phoenix Duct Clean measures grease before and after, cleans the full path back to TR19 Grease condition, and can tell you whether restored airflow fixes the capture or whether a separate mechanical issue is in play.
The service behind the guide
Sibling guides
What a louder fan is telling you · Smells even with extraction on · Care home kitchen extraction
Questions
At peak load the appliances produce the biggest plume, so a system running below its design airflow is exposed most when it is worked hardest. A borderline extract copes at low output and fails to capture once several burners are going at once.
It is most often grease loading the filters and duct and stealing airflow, which is the cheapest thing to rule out first. If a verified clean does not fix it, the cause is likely an undersized fan, a poorly positioned canopy or weak make-up air.
It is the airspeed the canopy needs across its face to draw the rising plume in before it escapes the edges. DW/172 sizes the extract rate to hold it. When airflow falls, capture velocity falls with it and smoke drifts out.
Yes. If replacement air is below around 85 percent of the extract rate the kitchen goes negative and the canopy has to draw against an under-supplied room, which weakens capture. It often works alongside grease rather than instead of it.
Immediately. Once the filters, plenum and duct are back to TR19 condition the fan meets its design airflow on the same day, so capture velocity recovers and the plume is drawn away as soon as the system is switched back on.
Phoenix Duct Clean · by the numbers
Book a clean to restore the capture velocity that pulls the plume away at source.