Residential Kitchen Exhaust

By law, kitchen exhaust cleaning is required for virtually every commercial cooking establishment in the United States. Restaurants, hospitals, hotels, employee cafeterias and other food-service locations have a "hood" and ductwork over the stove to exhaust smoke, steam, and fumes out of the building. These exhaust gases leave a residue on the inside of the ductwork. This is usually a grease residue of some sort, depending on the type of cooking. Char broilers commonly leave heavy black grease. Chinese cooking normally deposits a sticky or rubbery residue. When a charcoal or wood-burning stove is in use, soot and ash residue builds up in the ductwork. Dishwashers leave heavy lint deposits.

When the buildup of grease becomes heavy, a fire hazard exists. Approximately one of three restaurant fires is caused by grease. A common scenario of how a kitchen exhaust fire starts is this:

1. A flame flares up on the stove.
   
2. The fire contacts the filters above the stove on the kitchen hood. The filters ignite.
   
3. Since the exhaust fan is on, drawing air into the hood, through the filters, and up the duct, the flame on the filters is pulled into the duct.
   
4. If significant grease residue exists on the duct interior, this can act as a fuel and the fire spreads up the duct, perhaps all the way into the fan. We have seen fire climb up a ten-story duct to the fan on the roof and burn up the fan.

Modern duct construction is designed to hopefully withstand such duct fires. The duct seams are welded to prevent grease or fire from leaking out and the shafts around the duct are made of fire- resistive materials. However, older buildings are still at risk, and even in modern ones the fire may leak out or could come out onto the roof via the fan. When an exhaust system is cleaned regularly, however, the chances of a duct fire are extremely remote. In virtually all the duct fires we have seen in our 25 years of experience, the ducts were extremely laden with grease or other flammable material.