How Grease Builds Up in a Commercial Kitchen Exhaust System

It starts as aerosol, not liquid

Cooking on an open flame, a charbroiler or a fryer throws vaporised fats and fine droplets into hot rising air, along with combustion by-products and, on solid fuel, particulate. The hood captures that stream. As it moves away from the heat it cools, and cooling is what turns airborne grease back into a film on metal.

Where it lands, in order

1. Filters. Baffles force the airflow to change direction; the heavier droplets cannot follow the turn and drain to the trough.
2. Plenum. The space behind the filter bank is the first place unfiltered aerosol slows down. It is also the place kitchen staff almost never look.
3. Horizontal duct runs. Low velocity plus cooler metal equals condensation. Long horizontal runs in converted mill and warehouse buildings are the worst offenders, especially where they were built without access panels.
4. Elbows and transitions. Every direction change causes impingement, so grease deposits heavily on the outside of turns.
5. Vertical riser. Deposits are lighter but drain downward, which is why liquid grease sometimes reappears at the base after a hot service.
6. Fan housing and blades. The last cold surface in the system, and the one most likely to shed grease onto the roof membrane.

Why two kitchens on the same street load up differently

• Cooking type. Charbroiling and wok cooking produce far more aerosol than baking or steaming. Solid fuel adds creosote-like residue that hardens.
• Hours and volume. A kitchen cooking sixteen hours a day loads roughly twice as fast as one cooking eight, all else equal.
• Duct geometry. Long, cold, multi-elbow runs collect more than a short run straight to a rooftop fan.
• Airflow. An under-performing fan lowers duct velocity, so more droplets drop out before they reach the roof.
• Filter discipline. Consistent nightly washing measurably slows what reaches the duct.

Why the build-up matters

Accumulated grease is fuel sitting in a sealed metal path that runs directly over a cooking appliance. It also narrows the duct, which reduces exhaust volume, which makes the kitchen hotter and pushes smoke out over the line — and reduced velocity accelerates further deposition. Left long enough, hardened residue becomes difficult to remove without adding access panels or replacing sections of duct.

What to take from this

Set the interval on what the system actually looks like inside, not on a default date. If you have never had the duct opened at an access panel, that inspection is the most useful hour you can buy. Start with exhaust duct cleaning for the run itself, or a full kitchen exhaust cleaning from hood to fan.