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It doesn't take a lot of fuel load to sustain a fire in a grease exhaust system to the point that it would spread out of the hood system into the rest of the building. In his presentation on Ensuring Fire Safety for Food Service Operations in Commercial Facilities at this year's NFMT, Nelson Dilg said that what it took was more or less the equivalent of rubbing a stick of butter all over the hood -- certainly not an amount that would be visually alarming.
But trying keep the grease exhaust system perpetually grease-free is pretty much impossible, so the goal becomes keeping the fuel load low enough that the fire will burn itself out while it is still in the hood, before it breaches the duct system. What's more, the responsibility and liability for ensuring this level of maintenance lies on the operator, not on the contractor cleaning your hoods. Dilg suggests making the cleaning company show you they've cleaned right on the spot, since they'll have the panels off and the ladders up so you can get close enough to verify.
The heaviest accumulation of grease occurs where the duct changes direction, with two to three times as much accumulation as in the straight runs. With proper access panels in place, there should be no area of the grease exhaust system that is uncleanable.
The exhaust path needing regular maintenance extends all the way to the roof, where a containment system around the fan should be in place to trap grease which would otherwise leak onto the roof and underneath the membrane, where it would create a fuel load difficult to combat in the event of a fire.