Grease Build-Up in Kitchen Exhaust Ducts: Warning Signs and Fire Risk

Every time a restaurant cooks, a little grease travels past the filters and settles inside the exhaust duct. Layer by layer it builds up out of sight. A flame from the cooking line, a flare-up on a grill, or heat from a fire can ignite that grease, and a duct fire can spread along the run to other floors and the roof.

Why duct grease is dangerous

• Grease is fuel: a coated duct is a long channel of flammable material
• Ducts are hidden: the build-up is not visible from the kitchen
• Fires in ducts are hard to reach and can spread beyond the kitchen
• Clogged ducts also reduce airflow, so smoke and heat stay in the kitchen

Warning signs of grease build-up

• Grease dripping from duct joints or access panels
• A grease film around the exhaust outlet on the roof
• Weaker extraction and smoke that lingers (fix smoke in a restaurant kitchen)
• A strong old-oil smell (why a kitchen smells of old oil)
• No cleaning record for the duct, only for the hood

How often ducts should be cleaned

NFPA 96, the standard most kitchens follow, sets intervals by cooking volume: monthly for solid-fuel cooking, quarterly for high-volume cooking, every six months for moderate volume and yearly for low volume. Your local authority, landlord or insurer may set their own requirements. Use the cleaning frequency calculator to work out your schedule.

What proper duct cleaning involves

Cleaning a duct means opening access panels along the run and removing grease from the inside surfaces with scrapers and rotary brush equipment, not just wiping what is visible at the hood. Read our kitchen hood duct cleaning safety precautions, or see our kitchen exhaust duct cleaning service. Every job ends with a report with before and after photos and a cleaning certificate.

Frequently asked questions