Most Sydney homeowners clean the range hood filter when it looks visibly dirty or when the fan feels weaker. That seems reasonable — but it misses the real danger. Grease does not need to be dripping or obviously blocked to become a fire risk. The problem builds slowly inside the ductwork behind the filter, and by the time you notice it from the outside, the hazard has often been there for months.
This guide covers what happens inside your range hood system as grease accumulates, when it becomes dangerous, the warning signs to look for, and what Sydney homeowners need to do to keep their kitchens safe.
How Does Grease Get Inside the Duct?
Every time you cook on a stovetop, heat turns the fats and oils in your pan into a fine mist of grease particles that floats into the air. The range hood pulls this greasy air through the filter and into the exhaust duct, where it travels outside.
The filter captures most of these particles — but not all. A portion passes straight through the filter mesh and coats the internal walls of the duct. Over weeks and months of cooking, this coating thickens from a thin film into a solid, oxidised layer of grease lining every surface inside the duct.
The filter itself loads up even faster. An aluminium mesh baffle filter works by forcing the airstream through sharp direction changes so that grease particles impact and stick to the metal. Once the mesh begins to clog, airflow slows and more greasy air escapes into the duct — accelerating the build-up behind the filter.
When Does Grease Actually Become Dangerous?
Grease becomes a fire risk when it accumulates enough to act as fuel and a heat source is nearby. Two situations trigger this most often in residential kitchens:
- Cooking flare-up — if a flame or burning food particles get pulled into the range hood fan, they enter a duct coated in grease. Grease residue on metal ignites at temperatures a stovetop flare-up can easily reach. A fire at the filter face can travel rapidly through the full duct interior.
- Clogged fan motor heat — a heavily loaded motor generates sustained heat during operation. If grease has built up near the motor housing or in the duct directly above it, this heat can bring the grease layer to ignition temperature over time.
The key factor is how thick the grease layer has become. A thin film has limited fuel. A thick, solidified layer built up over months is a substantial fuel load across every surface of the duct. When a grease duct fire ignites, the fuel is everywhere — which is why these fires spread fast and are so difficult to stop.
How Quickly Does Grease Build Up?
The rate depends on how often you cook and what you cook. Here is a general guide for Sydney residential kitchens:
| Cooking Frequency | Filter Saturation | Duct Risk Level |
| Heavy daily — oils, wok, stir-fry | 1 – 2 months | Dangerous within 6 – 12 months |
| Moderate — most evenings | 2 – 3 months | Concerning within 12 – 18 months |
| Light or occasional | 4 – 6 months | Risk builds slowly over 2+ years |
These timeframes assume a working filter that is still trapping most of the grease. A saturated filter that has not been cleaned for months is trapping nothing — all the grease passes directly into the duct, compressing the timeline significantly.
What Are the Warning Signs?
Your range hood gives clear signals before the situation becomes critical:
- Grease dripping from the filter — the filter is completely saturated. Everything is going straight into the duct.
- Filter is dark brown or black — a new aluminium mesh filter is bright silver. Dark brown or black means it has been overdue for cleaning for a long time.
- Noticeably weaker suction — either the filter is blocking airflow, the duct has a grease restriction, or both.
- Unusual smell when the fan runs — a rancid or old-grease smell indicates oxidised grease sitting inside the duct.
- Staining around the exterior exhaust — grease residue outside the exhaust point means grease is travelling through the full duct run.
- Filter not cleaned for more than 3 months — in an active kitchen, this alone is a warning sign regardless of visible symptoms.
Ducted vs Recirculating Range Hoods
The type of range hood you have changes where the risk sits:
| Type | How It Works | Main Risk Area |
| Ducted | Exhausts air outside through a physical duct | Grease accumulates in filter AND full duct run |
| Recirculating | Returns filtered air back into the kitchen | Grease in filters and fan components only — no external duct run |
Ducted range hoods are the most common type in Sydney homes. For these systems, cleaning both the filter and the duct interior is necessary to properly manage fire risk.
How to Clean a Range Hood Filter at Home
Filter cleaning is a task Sydney homeowners can do themselves. In a household that cooks daily, clean the filter every four to six weeks. With moderate cooking, every six to eight weeks is reasonable.
Steps for aluminium mesh baffle filters:
- Remove the filter from the range hood — most have a release catch or latch.
- Fill a sink with very hot water and add dish soap or a degreaser.
- Submerge the filter and soak for 15 – 30 minutes to loosen the grease.
- Scrub gently with a soft brush, working into the mesh openings.
- Rinse under hot running water until the water runs clear.
- Allow to dry completely before reinstalling — a wet filter can drip water into the fan motor.
Many aluminium mesh filters are also dishwasher-safe. Check your range hood manual. A hot dishwasher cycle with a standard dishwasher tablet is an effective and low-effort option.
Carbon or charcoal filters in recirculating systems cannot be cleaned — they must be replaced every three to six months depending on cooking frequency.
What Happens If You Ignore It?
A grease duct fire does not announce itself. In most cases, the first sign is a sudden flare-up that gets drawn into the range hood fan — something most cooks have experienced at least once. In a clean duct, the risk is minimal. In a duct with months of grease build-up, the same flare-up can ignite a fire that spreads through the full duct run in seconds.
Grease duct fires in residential kitchens account for a meaningful proportion of kitchen fires in Australian homes. The common thread in most cases is the same: a filter that had not been cleaned in months and a duct that had never been professionally cleaned since the range hood was installed.
Once a duct fire starts, it typically moves faster than a standard stovetop fire because the fuel — the grease coating — is already distributed across every surface of the duct. Extinguishing it from outside is difficult. The fire follows the duct wherever it leads, including into wall cavities or ceiling spaces, where it can spread to structural elements before it is detected.
When Does the Duct Need a Professional Clean?
Filter cleaning handles the build-up at the front of the system. The grease that accumulates on the internal duct walls cannot be removed at home — it requires professional equipment including rotary brushes, pressure air tools, and vacuum extraction to clear the full duct run.
Arrange a professional duct clean when:
- The range hood has been in regular use for 12 months or more without a duct clean.
- There is visible grease when looking into the duct from the filter position.
- The range hood is being serviced or replaced and duct access is already available.
- A cooking flare-up produced significant smoke drawn into the range hood.
- The home is being sold or rented and the duct cleaning history is unknown.
Cleaning Schedule Summary
| Task | Who Does It | How Often |
| Range hood filter clean | Homeowner — DIY | Every 4 – 8 weeks |
| Carbon filter replacement | Homeowner — DIY | Every 3 – 6 months |
| Kitchen exhaust duct clean | Professional service | Every 12 months |
| Full system inspection | Professional service | Every 12 months or after any flare-up |
Conclusion
Grease becomes a fire risk when it builds up enough to fuel a flame inside the duct. Clean the filter every four to six weeks and arrange a professional duct clean once a year. If you do not know when the duct was last cleaned, book it now.