A grease duct is a purpose-designed duct that is used to vent grease-laden vapours from commercial cooking equipment such as a stove, "double-decker" pizza oven, deep fryer or wok to the outside of a building or mobile food preparation trailer. Grease ducts are regulated both in terms of their construction and maintenance, forming part of the building's passive fire protection system. Even the cleaning schedule ially dictated by the fire code and evidence of compliance must be kept on file by the owner.
Kitchen exhaust hood interior view, before and after cleaning.
In North America, grease ducts must be in compliance with NFPA 96 as well as the local building codes and fire codes. Cleaning takes place typically every 3 months, 6 months or annually, depending on the nature of the appliances below the hood. For instance, woks require quarterly grease duct cleaning, whereas normal stoves may necessitate the grease duct to be cleaned only every 6 months. Compliance must be proven through certificates issued by the cleaning and maintenance contractors. Purpose-designed fire suppression systems inside the hoods must also be routinely maintained. Proper cleaning must be enabled through the use of approved, fire-resistant access panels. Grease ducts should be kept as short as possible to minimise grease build-up.
A proprietary duct system that has its own inherent fire-resistance rating can be used, or a metallic duct, typically from 16 gauge sheet metal, all welded, per local codes, which is then externally treated with fireproofing. Typical materials used for fireproofing are:
- ceramic fibres
- rockwool
- calcium silicate
- vermiculite boards pressed and bonded with sodium silicate
- intumescents
- endothermic materials, sometimes on their own, sometimes in combination with ceramic fibre
In North American unionised construction sites, metallic ducts are typically installed by the sheet metal trade, whereas external wraps are usually installed by the insulators. Inherently fire-resistant systems are likewise installed by the sheet metal trade.
