Phoenix Journal · Extraction
Smoke rolling back into the kitchen during service is almost never bad luck - it is your canopy losing the balance between the air it pulls out and the air coming back in. Here is how to tell whether the fix is a deep clean or more make-up air.
The short answer
Smoke drifting out into the kitchen during a busy service almost always means your canopy is no longer winning the tug-of-war between the air it pulls out and the air coming back in.
A commercial kitchen is a balancing act. Your canopy and fan drag heat, steam and grease-laden vapour up and away, and an equal volume of fresh air has to arrive to replace it. When those two sides match, the plume of smoke rising off your ranges is captured neatly at the canopy lip and gone. When they fall out of step - the extraction weakens, or the replacement air cannot get in - the plume spills sideways and rolls back into the room. That is the smoke you are breathing at seven o'clock on a Friday.
The industry benchmark here is DW/172, the specification for kitchen ventilation systems published by BESA (the Building Engineering Services Association). It sets a capture velocity across the face of the canopy of roughly 0.3 to 0.5 m/s, rising to 0.6 m/s or more over heavy-duty equipment such as chargrills and solid-fuel ranges. Lose that velocity and capture fails. The useful thing about a smoke problem is that it points at one of two culprits, and they are fixed in very different ways. Below we set the two side by side so you can tell which one is yours.
The first suspect is the extract side. If the fan cannot shift the volume it was designed for, capture velocity at the canopy collapses and smoke escapes. The usual cause is not a dramatic breakdown but slow, invisible fouling. Grease baffles that have not been degreased clog until the motor is trying to breathe through a solid wall of fat; grease then coats the plenum, the throat and the duct walls, narrowing the bore and adding drag at every metre. A slipping or perished fan belt, a fan wheel caked in deposit, or a damper left half-closed does the same job. The plume that used to be captured cleanly now stalls and folds back on itself.
This is where a proper deep clean earns its keep. The relevant standard is TR19 Grease, the BESA specification that governs cleaning grease-laden extract systems and records the film left behind in microns. Bringing the whole run - baffles, plenum, duct and fan - back to a clean condition restores the cross-section and lets the fan pull its rated volume again. Where the system was always marginal, boosting the fan or adding capture aids such as a lip or a longer canopy overhang can lift velocity back over the DW/172 threshold.
The second suspect is the air that is meant to come back in. Every cubic metre your canopy throws out has to be replaced, and if it cannot get in through a planned route it will drag itself in through any gap it can find - under doors, down flues, around window frames. That is negative pressure, and it is the quiet reason so many smoke complaints survive a fresh clean and a new fan. The tell-tale signs are doors that suddenly feel heavy to open, a whistle around the frames, pilot lights that flutter, and cold draughts chasing across the floor while smoke still hangs at head height.
The answer is make-up air: a supply system, usually mechanical and ideally tempered so you are not blowing cold air across the section, that puts the extracted volume back deliberately. DW/172 practice is to return roughly 80 to 90 per cent of the extract volume as introduced make-up air, with the small remainder transferring naturally from adjoining spaces so the kitchen stays very slightly negative relative to the dining room - enough to keep odours off your customers, not so much that the canopy starves. Get that ratio right and the fan finally has air to move; capture snaps back and the plume goes where it should. This is closely tied to how smoke control and ventilation interact across the building, because a kitchen that is fighting the rest of the premises for air will never settle. It is worth reading up on the role of make-up air in kitchen ventilation before you spend a penny on a bigger extract fan.
Before you choose a fix, three figures tell you whether you are short on pull or short on push. Measure them against your own kitchen and the diagnosis usually makes itself.
One more control worth checking: a gas interlock. Under current practice a gas-fired kitchen should not be able to light unless the ventilation is proven to be running at the correct airflow, sensed by pressure or current monitoring. If your interlock is tripping or has been bypassed, that is both a safety flag and a strong hint that the airflow it is watching has drifted away from design - the same drift that lets smoke into the room.
Questions
At peak service every burner, oven and grill is running, so the plume of heat and grease-laden vapour is at its largest just when doors are opening and closing and the building is fighting for replacement air. If your extraction is marginal or your make-up air is short, that is the moment the balance tips and capture fails. Quiet periods hide the fault because the smaller plume is still within the canopy's reach.
A clean fixes the pull side by restoring airflow through the baffles, plenum and duct, but it does nothing for the push side. If the real fault is starved make-up air, the kitchen is under negative pressure and even a perfectly clean, correctly sized fan cannot reach its design volume. The clue is doors that feel heavy, whistling gaps and cold draughts. In that case you need planned replacement air, not another clean.
Not as a first move. If the kitchen is already short of make-up air, a bigger fan simply deepens the negative pressure and can make the smoke worse, while adding noise and energy cost. The right sequence is to restore airflow with a TR19 Grease clean, confirm capture velocity against the DW/172 figure of 0.3 to 0.5 m/s, and check that 80 to 90 per cent of the extract is being returned as supply air before you touch the fan.
Phoenix Duct Clean · by the numbers
The right kit only helps if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.