Heat, energy & performance
Grease does not just raise fire risk - it steals airflow. As deposits build on filters and duct walls, system resistance rises and the fan moves less air, so the canopy stops catching the heat plume and pushes it back into the room. The kitchen gets measurably hotter.
The short answer
Grease does not just raise fire risk - it steals airflow. As deposits build on filters and duct walls, system resistance rises and the fan moves less air, so the canopy stops catching the heat plume and pushes it back into the room. The kitchen gets measurably hotter.
The detail
Every extract system is commissioned to move a set volume of air against a set resistance. Baffle filters, the plenum behind them and the duct run all add up to that resistance. Grease adds more. It narrows the gaps in the baffles, coats the plenum and lines the duct, and each layer makes the air harder to pull through.
The fan responds the way fan physics dictates, not the way an operator hopes. A fixed-speed fan meeting extra resistance rides up its pressure curve and simply delivers less air - it does not automatically try harder. So the extract rate falls, and with it the capture velocity that holds the plume inside the canopy.
Once capture velocity drops below what the cooking load needs, the plume stops being fully contained. Heat and steam roll out from the canopy edges into the working area instead of leaving through the duct. The appliances are producing the same heat; less of it is being removed, so the room warms up.
This is why a kitchen can feel hotter every summer without any change to the equipment. The extract has quietly derated as grease accumulated. TR19 Grease sets a mean deposit target of around 200 microns precisely because heavy fouling is both a fire risk and a performance failure.
What it means for you
A hot kitchen with clean-looking appliances is often an airflow problem, not an equipment problem. If the space has crept warmer over months, the likeliest cause is a loaded extract that is no longer clearing the heat it used to. Turning the fan up further rarely helps, because the restriction is downstream of the fan.
The fix is to remove the restriction. A full TR19 clean of filters, plenum, duct and fan drops the system resistance back to its commissioned level, and the fan returns to its design airflow on the same day. Capture velocity recovers, the plume is contained again, and the room cools.
Phoenix Duct Clean measures grease across the whole system, not just the visible canopy, so the airflow you get back is the airflow the kitchen was designed to have - not a surface tidy that leaves the duct still choked.
The service behind the guide
Sibling guides
The energy cost of a clogged fan · Clean extraction and lower bills · School kitchen extraction cleaning
Questions
Yes. Grease raises system resistance, so the fan moves less air and the canopy stops fully capturing the heat plume. Heat that used to leave through the duct spills into the room instead, and the space runs warmer.
The restriction is grease in the filters and duct, downstream of a fixed-speed fan. Winding it up cannot pull air through a choked path any faster, and it wastes energy. Removing the grease is what restores airflow.
Often, yes. If the room has gradually warmed with no change to the cooking equipment, a grease-loaded extract that has lost capture velocity is a common cause. A survey and grease measurement will confirm it.
TR19 Grease uses a mean average target of around 200 microns across the system. Beyond that, deposits are both a recognised fire risk and enough to noticeably restrict airflow and reduce heat removal.
Not usually. Grease in the plenum, duct and around the fan is where much of the resistance sits. A canopy-only wipe leaves the airflow restricted, so a full TR19 clean of the whole system is what restores performance.
Phoenix Duct Clean · by the numbers
A greased extract is often the cause. Book a TR19 clean and get the airflow back.