Grease Duct Fire Risk

Our post on commercial kitchen fire safety covers prevention across the whole kitchen. This one goes narrower and deeper into a single question: what is actually happening inside that ductwork, and how do you judge how risky yours is right now?

How Grease Gets Into the Duct in the First Place

Cooking sends up an aerosol. Not just steam, but tiny droplets of fat suspended in hot air, along with combustion products from the burner.

The canopy pulls that mixture into the system. Grease filters catch a decent share of the larger droplets, which is exactly what they are for. But filters are not perfect and they were never intended to be. The finer aerosol passes straight through.

Past the filters, the air starts to cool. As it cools, the suspended fat condenses out onto whatever surface it touches. That surface is the inside of your ductwork.

Every service adds another layer. It is not dramatic and you cannot see it happening. It is a slow accumulation that only becomes visible when someone opens an access panel.

Why Old Grease Is Worse Than New Grease

This is the part that surprises people.

Fresh deposit is soft and greasy. Left in a warm duct, exposed to repeated heating cycles, it changes. The volatile components cook off and what remains hardens into a dense, dark, varnish like layer, eventually approaching something closer to carbon.

Two consequences follow:

• It becomes much harder to remove. A system cleaned on schedule wipes down. A system left for three years needs scraping and aggressive chemistry, which takes longer and costs more.
• It stays combustible. Hardened grease deposit still burns readily, and once it is alight it sustains itself along the length of the duct.

So the deposit does not just get bigger with neglect. It gets worse in character.

What Actually Starts the Fire

Duct fires very rarely begin in the duct. Something else ignites first, and the duct is what turns a contained incident into a building fire.

The usual sequence:

1. A fire starts at the cooking line. A pan of oil overheats, a flare up from a grill runs high, a burner is left going under an empty pan.
2. Flame reaches the canopy. If the filters are heavily loaded, they ignite.
3. The extract fan is still running, so it is now feeding the fire oxygen and drawing flame into the duct.
4. The grease lining the duct ignites and the fire travels along it.

That fourth step is the one that matters. A flare up at the hob is a fire extinguisher event. A flare up at the hob with a grease loaded duct above it is a fire that can move through concealed spaces, into ceiling voids, and across fire separations you assumed would hold.

This is also why duct fires are so hard to fight. The fire is inside a sealed steel enclosure running through parts of the building nobody can reach.

The High Risk Points in a System

Deposit is never even. These are the places it concentrates, and where we consistently find the worst of it.

• The canopy plenum. Directly behind the filters. Out of sight, and the first place condensing grease lands.
• The base of vertical risers. Grease runs down and pools. This is frequently the single worst spot in an entire system, and it is directly above the cooking line.
• Bends and transitions. Anywhere the airflow changes direction, momentum throws droplets against the outside of the turn.
• Horizontal runs with a slight fall. Grease creeps to the low point and sits there.
• The fan housing and impeller. Everything the duct did not catch ends up here. It is often the dirtiest single component, and an unbalanced greasy impeller generates heat through bearing wear.
• Anywhere with no access panel. Not because deposit prefers it, but because nobody has ever cleaned or inspected it. Unknown is its own risk category.

Working Out Where You Stand

You do not need instruments to form a reasonable view. Work through these.

What do you cook?

Charbroiling, wok cooking and deep frying produce far more airborne fat than baking or steaming. A yum cha kitchen and a sandwich bar with the same ductwork are not the same risk.

How many hours?

A kitchen running breakfast through to late is loading the system two or three times as fast as one doing a single dinner service.

When was the full run last cleaned?

Not the filters. Not the canopy. The full duct run, to the fan. If the honest answer is that nobody is sure, treat that as overdue.

Can you open a panel and look?

The most useful check available to you. Open an access panel away from the canopy and look at the internal surface. Bare metal is what you want. A visible film is a system heading toward due. A deposit you can scrape with a fingernail is overdue. A hard dark crust is a system that needs attention now.

Are there panels at all?

If a long run has no access, nobody has cleaned it properly, whatever the invoices say.

Has anything changed?

New menu, new equipment, longer hours, a new tenant in the same premises. Any of these can outdate a cleaning frequency that used to be fine. If your use has changed, your compliance schedule may need to change with it.

What Reduces the Risk

• Clean the filters properly and often. Every droplet a filter catches is one that never reaches the duct. This is the cheapest risk reduction available and it is entirely in your staff's hands.
• Clean the full run on a frequency that matches your actual cooking. Not the frequency that suited the previous tenant.
• Do not skip the fan. It is the dirtiest component and it is a heat source.
• Fit enough access panels. A duct you cannot open is a duct you cannot manage.
• Keep your suppression system serviced. Where you have a canopy suppression system, it is your last line of defence at the point of ignition.
• Keep the records. After a fire, your insurer will ask what maintenance was carried out. Photographs and dated reports are a much better answer than a recollection.

The Insurance Point

Worth being direct about this. Commercial policies commonly carry conditions around maintaining the premises and its systems. A kitchen fire that spread through ductwork nobody had cleaned in years is exactly the scenario where a loss adjuster starts asking for maintenance records.

We are not insurance advisers and your policy is your policy. But a documented cleaning history costs very little compared with the alternative of not having one.

If you want to know where your system actually sits, we can tell you. A 12A inspection opens the system up and reports on it honestly, and kitchen extract cleaning fixes what it finds.