Fire suppression & interlocks
The filters in your canopy are the first fire barrier over the cookline, not just a grease trap. A baffle filter in good condition limits how far a flame can project into the plenum; a clogged or wrong-type filter turns into a path that lets fire and fuel move on into the duct.
The short answer
The filters in your canopy are the first fire barrier over the cookline, not just a grease trap. A baffle filter in good condition limits how far a flame can project into the plenum; a clogged or wrong-type filter turns into a path that lets fire and fuel move on into the duct.
The detail
A baffle filter is a set of interlocking metal vanes. Grease-laden air has to change direction sharply as it passes through, dropping grease out onto the vanes where it drains away to a collection point. That same tight, staggered geometry means a flame at the cookline cannot shoot straight through - the baffle limits flame projection into the plenum behind it.
DW/172 specifies the baffle as the primary grease filter for exactly this reason. Mesh or panel filters are permitted only as a secondary stage behind a baffle, because mesh holds grease within itself and can act as a flame medium - the very thing you do not want as your first line at the canopy.
Filter condition decides whether that barrier holds. A baffle caked with grease has lost its clean draining geometry and is carrying fuel; a mesh filter used as a primary is a grease-loaded panel sitting directly in the flame path. Either way the first barrier is compromised and flame and fuel find it easier to pass.
Beyond the filters lies the plenum and the duct - the run that TR19 exists to keep clean. A filter that lets grease and flame through is feeding the part of the system where a fire becomes hardest to reach and most damaging. The filter stage is where a local fire is meant to be kept local.
What it means for you
That means the right filters, fitted correctly, and kept clean. Baffles as primary, any mesh strictly secondary, cleaned or swapped on a schedule that matches your cooking volume so they never run caked. A degraded or missing filter is a fire-spread control that has been removed.
It also means cleaning behind them. Grease that has already passed a tired filter into the plenum and duct is fuel in the worst place, so a TR19 clean addresses the whole run, not just the filters at the face.
Reading filters as fire safety rather than housekeeping changes the priority. A clogged filter is not merely dirty - it is a weakened barrier between a cookline flame and the ductwork, and restoring it is restoring a line of defence.
The service behind the guide
Sibling guides
Interlock failures and dirty extraction · Aligning suppression service and cleaning · School kitchens over the holidays
Questions
Yes. A baffle filter limits how far a flame can project into the plenum, as well as dropping grease out of the air. Its condition directly affects whether a cookline fire stays local or travels into the duct.
DW/172 specifies baffles as primary because their draining geometry limits flame projection. Mesh is allowed only as a secondary stage, since mesh holds grease and can act as a flame medium if used as the first line.
It stops draining and starts carrying fuel, and it no longer limits flame projection effectively. A caked filter is a weakened fire barrier that makes it easier for flame and grease to reach the plenum and duct.
Only behind a baffle as a secondary stage. Using mesh as the primary filter puts a grease-holding, flame-carrying panel in the direct flame path, which is why DW/172 reserves it for the secondary position.
It protects the barrier, but grease that has already passed into the plenum and duct is fuel in the worst place. A full TR19 clean addresses the filters and the run behind them together.
Phoenix Duct Clean · by the numbers
Phoenix Duct Clean cleans the filters and the plenum and duct behind them to TR19 Grease, keeping the barrier that stops a cookline flame reaching your ductwork.