Signs & diagnostics
When the canopy feels weaker than it used to, something has changed the balance between what the fan can pull and what the system makes it pull against. Most of the time that something is grease - but reading the drop correctly saves you from paying for the wrong fix.
The short answer
When the canopy feels weaker than it used to, something has changed the balance between what the fan can pull and what the system makes it pull against. Most of the time that something is grease - but reading the drop correctly saves you from paying for the wrong fix.
The detail
The airflow a canopy delivers is the result of a balance: the fan pulls, and the system - filters, plenum, duct, bends and discharge - resists. Design that balance well and the canopy holds its capture velocity. Upset it, and the delivered airflow drops even though nothing about the fan has changed.
Grease is the usual thing that tips it. It loads the baffle filters first, then coats the plenum and duct, raising resistance. A fixed-speed fan meeting more resistance rides up its curve and simply moves less air - it does not compensate - so the pull you feel at the canopy weakens.
The scale can be dramatic. A clogged filter can cut volumetric flow by around half in severe cases, and that is before the duct's own contribution. So a canopy that feels markedly weaker is entirely consistent with a grease problem that has been building quietly for months.
Make-up air can play a part too. If replacement air has fallen below roughly 85 percent of the extract rate, the room goes negative and the canopy has to fight to draw against an under-supplied space, which reads as weak suction even when the extract path itself is not badly loaded.
What it means for you
Because grease is the common cause and the cheapest to address, a clean is the right first step. Restoring the filters, plenum and duct to TR19 Grease condition brings resistance back down and lets the fan meet its design airflow, and on most systems that recovers the suction on the same day.
If the pull is still weak after a verified clean, the diagnosis moves on: an undersized or failing fan, a canopy that does not cover the equipment, a restrictive duct route, or inadequate make-up air. Cleaning cannot fix a design or balancing fault, and ruling it in or out first is what stops you buying a new fan you did not need.
Phoenix Duct Clean measures grease before and after and restores the airflow, so you know whether reduced suction was contamination or a deeper issue - and you spend on the fix that actually applies.
The service behind the guide
Sibling guides
Grease spots on the ceiling · What a stale grease smell means · Mobile catering extraction
Questions
No, but grease is the most common and cheapest cause to rule out first. It loads the filters and duct, raises resistance and drops the airflow the fan can move. If a verified clean does not restore the pull, the cause is likely a fan, canopy or make-up air issue.
A lot. A clogged filter alone can cut volumetric flow by around half in severe cases, and the loaded duct adds more resistance on top. That is easily enough to turn a canopy that captured cleanly into one that feels noticeably weak.
By cleaning and measuring. If restoring the system to TR19 condition brings the suction back, it was grease. If the pull stays weak after a verified clean, the fan may be undersized or failing, or the canopy and duct may have a design limitation to assess.
It can. If replacement air is below around 85 percent of the extract rate the kitchen goes negative and the canopy struggles to draw against it, which feels like weak suction. It often compounds a grease problem rather than replacing it.
Not before diagnosing the cause. If grease is stealing the airflow, a bigger fan pushes harder against the same restriction and wastes energy. Clean and measure first - often that recovers the suction, and if not, the right fix becomes clear.
Phoenix Duct Clean · by the numbers
Book an inspection with grease readings to find out whether a clean restores the pull.