Kitchen exhaust fire risk: where grease hides (duct, plenum, fan, roof)
The grease you can see on the filters is the least of it. Behind the filters, up the duct, inside the fan bowl, and on the roof is where the fuel load lives. Real photos from our crews, and what each spot means for fire risk.
Grease in a kitchen exhaust system collects in five places, and only one of them is visible from the line: the filters, the plenum behind the filters, the duct run (horizontal sections worst of all), the rooftop fan bowl and blades, and the roof around the fan. The filters catch a fraction. The rest condenses on cold metal as the vapor moves up and out, which is why a kitchen with spotless filters can still have a duct that would burn like a chimney. Cooking equipment caused over 60 percent of fires in eating and drinking establishments between 2014 and 2018, according to NFPA data, with about $165 million in property damage a year [1]. Most of that fuel was sitting where nobody looked.
This guide walks the system from the hood to the roof, with photos from our own jobs, and explains what each spot means for fire risk and for your inspection.
Why does grease build up past the filters?
Because the filters are not a wall. Baffle filters are designed to knock grease vapor out of the airstream by forcing it to change direction, and they work well on the heavier droplets. The fine vapor, the part that is still a gas when it leaves the fryer, goes straight through. It condenses when it hits metal that is cooler than it is, and everything downstream of the filters is cooler than the filters. The plenum. The duct. The fan. The roof.
The rate depends on what you cook on. A charbroiler or a wok range throws far more vapor than a soup kettle, which is why NFPA 96 tiers inspection by cooking type: monthly for solid fuel, quarterly for high-volume, semiannually for moderate, annually for low-volume [2]. The trigger for cleaning is thin. NFPA 96 section 12.6.1.1 sets it at 0.002 inches on hoods, filters, and ducts, 0.078 inches on other interior surfaces, and 0.125 inches in the fan housing [2]. Two thousandths of an inch is a sheet of paper. A duct that looks "a bit dull" is usually well past it.
Where does grease hide in the hood and plenum?
The plenum is the chamber behind the filters, between the filter track and the duct collar. It is the first place a canopy-only cleaner skips, because you have to pull every filter to see it, and it is the first place a fire inspector looks for the same reason.
That photo is a plenum from one of our jobs before cleaning. The grease is thick enough to hold a thumbprint. The grease trough along the bottom of the hood, the channel that is supposed to carry drips to a cup, fills up too and starts dripping onto the line, which is the sanitation finding the health inspector writes up.
Same plenum after. This is what "cleaned to remove combustible contaminants," the words in NFPA 96 section 12.6.2, looks like [2]. If your last cleaning did not leave the plenum looking like this, the filters went back in to hide it. That is one of the five tells of a bad hood cleaning: filters reinstalled immediately so nobody looks behind them [3].
Where does grease hide in the duct?
Everywhere, but the horizontal runs are the worst. Vapor moving up a vertical riser keeps some speed. Vapor moving sideways through a horizontal duct slows down, cools, and drops out. Every elbow and every transition is a collection point. A long horizontal run over a drop ceiling, which is common in strip-mall restaurants and hotel kitchens, can carry inches of grease that nobody has seen since the building opened.
Our crews cannot clean what they cannot reach. NFPA 96 requires the written report to list inaccessible areas that were not cleaned and the location of every access panel [2]. A report that lists inaccessible duct is not a bad report. It is an honest one. The fix is to cut in panels so the next cleaning reaches the whole run, which is what our duct access panel service does. A cleaner who never mentions the duct, never lists panel locations, and never says "inaccessible" on the report either has a very short duct or is not looking.
Where does grease hide in the exhaust fan?
In the bowl and on the blades. The upblast fan on the roof is the last cold surface before the air leaves the building. Grease drops out of the airstream into the fan bowl and packs onto the blade faces. A fan bowl with an eighth of an inch of grease is past the NFPA 96 threshold on its own [2], and it is also the single hottest place in the system during a fire, because the fire is being pulled toward it.
Grease on the blades also throws the fan out of balance. That is the squeal and the vibration a kitchen manager hears a month before the belt goes. It is the reason our crews check belts, bearings, and the hinge on the same trip through exhaust fan maintenance. A fan that was never tipped back on a hinge kit was never cleaned; NFPA 96 section 8.1.6.3 requires access to the fan for exactly this reason [2].
Want to know what is actually in your duct and fan? Get a free estimate. The first visit includes photos of the plenum, the duct, and the fan bowl, so you see it before we clean it. Or call (800) 973-5573 any hour.
Where does grease hide on the roof?
Around the fan. Whatever the fan does not hold, it slings. Grease runs down the fan curb and pools on the roof membrane, where it does two things. It softens and degrades the membrane, which is the roofer's problem and eventually the landlord's letter. And it sits as a fuel load next to the fan discharge, which is the fire marshal's problem. NFPA 96 section 7.8.2.1 requires rooftop terminations to have a noncombustible, closed, rainproof grease collection container [2]. That is what rooftop grease containment is: a box that catches the runoff so it does not reach the roof.
Our crews steam the fan, the curb, and the roof around it on every full cleaning. If your cleaner never asked for roof access, none of this happened.
How does a grease fire actually travel through the system?
A flare-up on the line is normal. A flare-up that reaches the filters with a plenum full of grease behind them is where it turns. The fire follows the fuel. It climbs into the plenum, gets pulled into the duct by the fan, which is now feeding it air, rides the grease up the riser, and arrives at the fan bowl. A duct fire in a building with a long horizontal run over a drop ceiling can be burning above three tenant spaces before anyone in the kitchen knows it left the hood.
That is why the suppression system covers the plenum and the duct collar as well as the appliances, and why the fire marshal cares more about the duct than the canopy. Suppression buys time. A clean duct means there is nothing for the fire to follow.
What can a manager check without going on the roof?
Three things, none of which need tools.
- Pull one filter. Look at the plenum with a flashlight. If you can write your name in it, you are past the threshold.
- Look at the grease trough and cup. If the cup is overflowing or the trough is dripping on the line, the plenum is worse.
- Stand under the hood with the fan on. Weak draw, smoke rolling out the front of the hood, or a squeal from the roof means the fan is loaded or the belt is slipping.
What you cannot check is the duct and the fan bowl. That is what the photo report is for. A full-system commercial hood cleaning covers all five places and photographs them before and after.
Frequently asked questions
Where does most of the grease in a kitchen exhaust system end up?
Past the filters. The plenum, the duct (horizontal runs especially), and the fan bowl hold most of it, because fine grease vapor passes through baffle filters and condenses on the cooler metal downstream.
How much grease is too much in a hood system?
NFPA 96 section 12.6.1.1 sets the cleaning trigger at 0.002 inches on hoods, filters, and ducts, 0.078 inches on other interior surfaces, and 0.125 inches in the fan housing [2]. Those are thin. If it is visible as a film, it is time.
Does hood cleaning include the exhaust fan?
It should. NFPA 96 section 12.6.2 covers hoods, grease removal devices, fans, ducts, and other appurtenances [2]. A cleaning that skips the fan is a canopy cleaning, whatever the sticker says.
Why is grease on my roof?
The fan slings what it cannot hold. Without a grease containment box around the fan, runoff pools on the membrane. NFPA 96 section 7.8.2.1 requires a closed, rainproof collection container at the rooftop termination [2].
Can a grease fire start in the duct itself?
It rarely starts there, but it travels there. A flare-up on the line reaches the plenum, the fan pulls it into the duct, and the grease in the duct becomes the fuel. A clean duct gives the fire nothing to follow.
See the whole system before you sign anything. Get a free estimate with your address, hood count, and cooking equipment, or call (800) 973-5573.
Sources
- Ansul, "Two NFPA restaurant fire stats," citing NFPA data for 2014 to 2018 (ansul.com)
- NFPA 96 Chapter 12 and related sections summary (Table 12.4, 12.6.1.1, 12.6.2, 12.6.14, 8.1.6.3, 7.8.2.1), Facilitec Southwest (facilitec-sw.com)
- Pye-Barker Fire and Safety, "Five ways to avoid a bad hood cleaning" (pyebarkerfs.com)