· Expert Exhaust Cleaning · Exhaust Fans · 5 min read
How Much Airflow Does a Dirty Exhaust Fan Lose?
A grease-clogged kitchen exhaust fan can lose 30 to 50 percent of its rated airflow, and the drop starts long before the kitchen feels smoky or hot.
A dirty commercial exhaust fan typically loses 30 to 50 percent of its rated airflow once grease coats the impeller and duct walls. Even a 2 to 3 mm grease film measurably reduces extraction, and the loss compounds every week the system runs uncleaned. Exhaust fan airflow loss is gradual, so most kitchens keep cooking well past the point where the fan has stopped doing its job.
How much airflow does a dirty exhaust fan actually lose?
Most neglected commercial fans lose between 30 and 50 percent of design airflow, and heavily fouled systems can drop further. The loss comes from three stacking problems: grease adds weight and imbalance to the impeller, narrows the duct cross-section, and blocks the filters that feed the fan. Each restricts flow, so the effects multiply rather than add.
Airflow is measured in litres per second (L/s) against the design figure set under AS/NZS 1668.1. When a fan rated at 1,000 L/s pulls only 600 L/s, the canopy can no longer capture the plume, and heat, smoke and grease-laden vapour spill back into the kitchen.
Why does grease reduce exhaust fan airflow?
Grease reduces airflow because it changes the shape and balance of every surface the air passes over. On a centrifugal impeller, even a thin, uneven grease layer disrupts the blade profile and unbalances the wheel, so the fan moves less air while drawing more current. That means higher power bills and a shorter motor life.
Inside the ductwork, grease narrows the bore and roughens the walls, increasing static pressure the fan has to fight. Because fan output falls sharply as static pressure rises, a small reduction in duct diameter causes a large reduction in flow. Clogged baffle filters then choke the intake before air even reaches the fan.
What are the signs of exhaust fan airflow loss?
The clearest signs are a kitchen that stays hot and smoky, doors that are hard to open against negative pressure, and grease dripping from the canopy edge. These appear because the fan can no longer clear the cooking plume, so vapour condenses on cooler surfaces instead of leaving through the duct.
Other indicators include a louder or vibrating fan, longer clearing times after a busy service, and staff complaints about odours. If you are seeing these symptoms, our guide on a commercial exhaust fan not pulling air walks through the diagnosis step by step.
How much airflow does cleaning restore?
A full clean typically recovers 90 to 100 percent of rated airflow, because it removes the grease driving every restriction at once. Stripping the impeller rebalances the wheel, degreasing the duct restores the bore, and fresh filters reopen the intake. The fan then meets its AS/NZS 1668.1 design figure again on the same motor.
The table below shows how grease load maps to airflow loss and the usual fix.
| Grease condition | Approx. airflow loss | What causes it | Typical remedy |
|---|---|---|---|
| Clean / recently serviced | 0 to 5% | Normal operating film | Routine cleaning schedule |
| Light build-up (1 to 2 mm) | 10 to 20% | Filter loading, thin impeller coat | Filter clean + canopy degrease |
| Heavy build-up (3 to 6 mm) | 30 to 50% | Impeller imbalance, narrowed duct | Full exhaust fan cleaning |
| Severe / hardened deposits | 50%+ | Blocked duct, seized dampers | Deep degrease, possible parts |
How often should you clean to avoid airflow loss?
Clean most commercial kitchens every 3 to 6 months, because that interval keeps grease below the depth where airflow and fire risk climb sharply. Charcoal, wok and high-volume frying kitchens need monthly filter cleaning and quarterly duct cleaning, since these methods atomise far more oil into the airstream.
AS 1851, the standard for routine service of fire protection systems, ties inspection frequency to cooking load rather than the calendar. The reason regular cleaning matters beyond airflow is direct: grease build-up is the leading cause of commercial kitchen fires, and removing grease removes the primary ignition fuel. For a full breakdown, see how often to service a commercial exhaust fan.
What does restoring exhaust fan airflow cost in Sydney?
A single-canopy clean starts from around $390, which covers filters, canopy and the accessible fan. Combined canopy-and-duct cleaning runs $900 to $2,500 depending on duct length and grease load, and multi-storey riser systems start from $3,000 because of access and rope-work requirements.
The cost of not cleaning is higher: a fan running at half airflow draws more power, fails earlier, and voids insurance cover if a grease fire starts. Restoring airflow is almost always cheaper than replacing a burned-out motor or a failed compliance audit.
| Service | Typical Sydney price (AUD) | What it restores |
|---|---|---|
| Single-canopy clean | From ~$390 | Filter and canopy extraction |
| Canopy + duct clean | $900 to $2,500 | Full duct-run airflow |
| Multi-storey riser system | $3,000+ | High-rise vertical duct flow |
Does airflow loss affect fire compliance?
Yes, because a fan that cannot clear grease-laden vapour lets deposits accumulate faster throughout the system. Reduced airflow and heavy grease are the two conditions AS 1851 inspections are designed to catch, and both point to the same underlying cause. Poor extraction and thick grease raise fire risk together, since the grease is the fuel and the stagnant, hot air is the ignition environment.
Keeping airflow near design specification is therefore both a performance and a safety measure. A well-extracting system stays cooler, cleaner and closer to compliance between services.
Do not wait for the kitchen to fill with smoke before acting. If your fan is loud, weak or overdue, call us on 0468 070 392 or get a same-day quote and we will restore your airflow before it costs you a motor or a failed audit.
