Author(s): Doug Horton. Published on July 1, 2015.

IN MY WORK CONSULTING on commercial kitchen ventilation systems, including restaurant fire investigations, I’ve recently encountered fires involving solid-fuel cooking systems. I served as an expert consultant on one such fire that occurred at a pizza restaurant in Maryland, an event that illustrates the kind of potentially serious problems that can occur with solid fuel, typically wood, briquettes, or charcoal. 

The fire started in a tall, rectangular hearth-type pizza oven during mid-morning bread baking operations for the owner’s multiple restaurants. The oven burned natural gas on one end and wood “bricks” made of compressed wood and sawdust on the other, and was located beneath a high-quality, listed, Type I wall canopy exhaust hood. The hood was equipped with listed grease filters and a conventional fire suppression system with detection by fusible links, a system that includes two small brass plates soldered together and attached to tension cables that separate when heated to 360 degrees F (182 degrees C), or other preselected temperature, activating the conventional fire suppression system. The exhaust duct turned horizontal a few feet above the hood, extending close to offices and other spaces in the multi-tenant, multi-story building, and then it turned upward again toward the exhaust fan.

In his report on the incident, the fire investigator wrote that, in his opinion, “this fire started as a result of too many bricks of wood burning” in the oven. The heat of the over-fueled gas and wood fire ignited creosote and grease deposits in the oven, increasing the intensity of the fire that spread upward from the oven flue. Significantly, the fire suppression system did not activate—the fusible links did not separate—and fire traveled the short distance from the flue to the hood, filters, duct, and eventually to what the report described as “roofing materials.” “The heat became so intense that it burned the buildup of creosote off the inner wall of the oven,” the investigator wrote. “The fire extended to the flue of the oven and then breached the firewall protection. The fire then entered into the void area around the flue.”

Case evidence revealed that periodic duct cleaning was insufficient to remove grease and creosote deposits, in part because there were not enough duct access panels and duct cleaning had not been thorough. Damage to other parts of the building was caused by insufficient clearances to combustible construction. There were no deaths or injuries associated with this incident, though the building sustained damages estimated at several hundred thousand dollars, with responsibility allocated by the legal process to several parties.

All of the significant aspects of this incident, including requirements for installation and maintenance of solid-fuel cooking systems, are addressed in Chapter 14 of NFPA 96, Ventilation Control and Fire Protection of Commercial Cooking Operations. Designers, installers, owners, and authorities having jurisdiction (AHJs) need to familiarize themselves with, and follow, these requirements, since solid-fuel cooking in commercial settings can include its own particular hazards. Wood, especially, can introduce highly flammable creosote deposits into the cooking and ventilation systems, adding to the fuel load of grease deposits. It is important that AHJs help stakeholders understand and address these hazards, whether in new construction or in the maintenance of existing solid-fuel cooking systems.

Anecdotally, at least, reports of fires involving solid-fuel cooking systems seem to be increasing, in part a result of the growing number of establishments that have added solid fuel cooking to their operations in recent years. Live-fire cooking has become a popular restaurant concept that can include wood-fueled char-broilers, pit barbeques, rotisseries, smokers, and hearth and brick ovens, the latter being especially popular for cooking pizza. In many locations, both natural gas and wood are used in the same appliance. According to NFPA data, U.S. fire departments responded to an estimated average of 7,640 structure fires per year in eating and drinking establishments between 2006 and 2010. While it’s difficult to say how many of those fires involved solid-fuel cooking, three out of five of those incidents involved cooking equipment.

The creosote problem

In many systems, especially those that burn wood, the chief hazard is creosote buildup. Creosote is made up of condensed volatile gases created by incomplete combustion of the wood, according the Cornell Cooperative Extension Service. As these gases rise in the chimney, they cool, mix with water vapor, and form a tar-like substance that clings to the chimney walls. The substance is highly combustible and is well known for its fire threat in chimneys above residential wood-burning fireplaces.

According to the Chimney Safety Institute of America (CSIA), certain conditions encourage the buildup of creosote, including restricted air supply, unseasoned wood, and cooler-than-normal chimney temperatures. If creosote builds up in sufficient quantities, the CSIA says, and the internal flue temperature is high enough, the result can be a chimney fire. The Cornell Cooperative Extension Service reports that chimney fires “can start quickly and be very powerful, shooting flames many feet above the chimney cap and producing a loud rumble like a freight train.”

The flash point and auto-ignition temperatures of wood tar creosote are surprisingly low. Research has determined the flash point (which requires an ignition source) of wood tar creosote to be 165 degrees F (74 degrees C). Thus, all it takes to ignite creosote in hoods, filters, and ducts is a spark, burning ember, or flame—all present in solid-fuel cooking—that raises the creosote temperature to 165 degrees F. Creosote’s auto-ignition point, which does not require an ignition source, is 451 degrees F (233 degrees C), or the same as paper—which makes sense, since both substances are derived from wood. This temperature is significantly lower than the auto-ignition temperature of grease, which can increase the potential hazard. The combination of creosote and grease in exhaust hood plenums and ducts can be easier to ignite than creosote alone, and can burn hotter.

A large part of the solution to the creosote problem is regular cleaning and maintenance, and Chapter 14 includes a section devoted to these procedures. For example, the combustion chamber is required to be “scraped clean to its original surface once each week” and inspected for deterioration or defects; if any are found, they are required to be repaired immediately. The flue or chimney must be inspected weekly for “residue that might begin to restrict the vent or create an additional fuel source” such as grease-infused creosote, and for “corrosion or physical damage that might reduce the flue’s capability to contain the effluent.” Spark arrester screens located at the entrance of the flue or in the hood assembly, designed “to minimize the passage of airborne sparks and embers into plenums and ducts,” must be cleaned “prior to their becoming heavily contaminated and restricted.”

Systems exist that provide automated daily hood plenum and lower duct cleaning with hot water and surfactant, delivered with the same system that provides electronic detection and fire suppression with water and surfactant. One prominent kitchen ventilation supplier I know will only undertake jobs with solid-fuel cooking if this type of combined system is specified, along with listed ductwork.

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If the inspection, cleaning, and maintenance requirements in Chapter 11 (and Chapter 14 if specifically for solid fuel) were routinely followed by all establishments utilizing solid-fuel cooking systems, we would have fewer fires involving these systems and far fewer fires that grow large enough to cause significant damage, as illustrated by the Maryland pizza restaurant fire.

The Chapter 14 solution

Besides the inspection, cleaning, and maintenance requirements, AHJs and other stakeholders should be aware of additional requirements in Chapter 14.

The chapter addresses venting application, appliance location, hoods, and exhaust systems, specifically that solid-fuel exhaust systems be separate from all other exhaust systems; gas-operated equipment utilizing solid fuel for flavoring must meet a checklist of 11 conditions that must be met in order to be exempt from this requirement for a separate exhaust system. Grease removal and air movement are also covered.

From my observations of unlisted fabricated-on-site ducts leaking grease and not meeting clearance requirements, I also recommend specifying factory-built listed ducts as a minimum requirement for solid-fuel cooking exhaust systems, if not all exhaust systems. With high standard kitchen ventilation systems including listed hoods (UL 710), grease filters (UL 1046), and exhaust fans (UL 762), it makes sense to specify listed exhaust ducts (UL 1978, among others). The ducts also need to adhere to clearance and cleaning access requirements such as those found in NFPA 96.

The chapter’s section on suppression calls for appliances that produce “grease-laden vapors” to be protected by listed fire-extinguishing equipment. Where acceptable to the AHJ, appliances constructed of “solid masonry or reinforced portland or refractory cement concrete” and vented in accordance with NFPA 211, Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, need not require fixed automatic suppression equipment. Hoods, duct systems, and grease-removal devices, with the possible exception of those associated with masonry or concrete appliances, are required to have listed suppression equipment. The chapter specifies water-based agents for use in suppression, and calls for the suppression equipment to be large enough to “totally extinguish fire in the entire hazard area and prevent reignition of the fuel.”

As part of a section titled “Minimum Safety Requirements: Fuel Storage, Handling, and Ash Removal for Solid Fuel Cooking,” the chapter includes provisions for installation clearances for solid-fuel appliances. The most frequent non-conforming code issue I’ve seen in the restaurant fires I’ve investigated is hoods and ducts installed without required clearances to adjacent wood construction, requirements for which are found in Chapter 7 of NFPA 96. Those clearance issues often cause small fires to spread and increase damage, as in the case of the Maryland pizza restaurant. In addition to customary methods, a simple solution in new construction is specifying metal studs, joists, and trusses within 18 inches of planned locations of cooking appliances, hoods, and ducts.

These are the kinds of details and requirements stakeholders need to consider when installing and maintaining solid-fuel cooking systems. The systems may be new for a lot of commercial users, and may require a level of maintenance unanticipated by those users, but the good news is that all of the relevant requirements are addressed in NFPA 96. AHJs have a valuable tool with which to educate stakeholders and to clearly define compliance.

One outcome of not following NFPA 96 is what recently occurred at a barbeque restaurant in Missouri, when the fire in the barbeque pit ignited a fire in the flue. The cook was able to extinguish the pit fire but was unable to stop the flue fire; staff evacuated the restaurant and called 911. During the course of firefighting operations, the culprit was revealed: chunks of burning creosote and grease tumbled out of the flue and landed on the grill. It certainly made the fire marshal’s job easy. In his fire report, for “item/material first ignited,” the answer was a no-brainer: “grease and creosote.”

Doug Horton, MS, CFSP, is principal consultant at D. J. HORTON and Associates, Inc. in Batavia, Illinois.