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Investigating commercial kitchen exhaust system fires.

By Oliver C. Moore

A circular opening in the roof above a commercial kitchen is burned away by fire. The investigation, collection of evidence and analysis of burn and fall patterns are sure ways to diagnose the cause and responsibilities associated with the event. Once it is identified that the fire has been started by heat from commercial cooking operations, mental red flags should wave.

Here are a few helpful points before and during the fire investigative process:

Fuel Loading

Fuel loading in a kitchen exhaust system is just as damaging as a match or lighter in the hands of a child. With a volume of flammable grease buildup and contaminants, depending on the extent and depth of fuel loading in a commercial kitchen exhaust system, this contained fuel loading can generate extremely high temperatures when burning.

Kitchen exhaust systems are designed specifically to contain fire and to remove heat and products of combustion, grease-laden vapors, and toxic gases up and away from the appliance cooktops.

The exhaust fan draws contaminated air through the ductwork and fan housing continually, thereby cooling the metal, which allows heated grease to condense on the interior walls of the hood, plenum and ductwork and in the fan housing during cooking operations. In a very short period of time, the residual oil, particles and grease will accumulate on the walls of the exhaust system. Oily grease turns to crust laid over grease and then to creosote, creating a very serious fire hazard. If not regularly maintained and cleaned to a bare-metal standard of clean throughout the system, including the fan blades, heated flammable grease vapors will flame over. Many kitchen operators and inspectors are not aware of how combustible these cooking residues are and how important it is to have the exhaust system cleaned thoroughly. Cleaning to bare metal by properly trained, qualified and certified persons should be done at regular intervals according to the degree and frequency of cooking operations is vital.

Top-to-bottom cleaning of all grease, oil, crust or creosote to bare metal removes all combustible, flammable and toxic residues from the system. Important rules to remember include the following: (1) All ductwork should be designed and built to be completely accessible. (2) Crust and grime on duct walls can and should be removed completely during cleaning. (3) Fire cannot exist or extend in a bare-metal environment.

Fire Investigation and Evidence Collection

Fire investigation of a commercial kitchen fire shall include a study of the cooking operation, hood, plenum and ductwork up to and including the fan housing and fan to determine the extent of creosote, crust or grease present at the time of the fire. This can be accomplished by first identifying the cleaning cycle, by date of cleaning or date of next cleaning.1 Did the fire occur at the beginning, middle or end of the 3-month cleaning cycle or was the system cleaned at all? This information can be found by requesting the Completion and Deficiency Report from the owner and checking the date stamps or the cleaning hood label or interviewing the owner and the responsible cleaning technicians.2 Of course, insurance investigators will absolutely be interested in this information; however, consideration of notification to all interested parties — that is, owners, system cleaners, suppression service representatives, installers, insurance investigators, claims examiners, appliance/equipment purveyors, etc. — should be made to prevent potential spoliation accusations or adverse legal motions.

One deficiency that should not be ignored is on the rooftop. Has the roof covering been damaged through neglect or ignored during cleaning? Has the roof been burned out or significantly damage by fire? Has grease accumulation, spillage and grease or water overflowed onto the roof covering, spread and damaged the roof membrane, or saturated into the underlayment of the roof covering? Is grease spillage or saturation concentrated around the duct and fan housing discharge, or has extreme grease collection, seepage or another situation violated the roof below the roof covering? Saturation to this extent is an embedded fuel-acceleration process that should be identified and addressed during evidence collection. Also, have roof structural members been pyrolyzed by heat from the sun or saturated with combustible grease or oil from the absence of grease maintenance? Or have both happened?

Kitchen exhaust system fuel loading may have occurred because of the following reasons:

• Poor maintenance practices on the part of the kitchen staff.

• Inadequate cleaning frequency.

• Incomplete cleaning services.

• Inaccessible areas within the system.

• Poor service accountability.

• Restaurant owners not willing to pay for proper or complete system services.

There are two major reasons kitchen exhaust systems are not cleaned properly:

1. The responsible party in the kitchen did not allow system maintenance or cleaning to take place.

a. Pressures from a higher authority to keep costs down.

b. Attempting to balance cost versus risk.

c. Maintenance costs were not budgeted.

d. They were too busy to notice or believed kitchen help was doing the job.

e. Ignorance to the danger.

2. A non-professional cleaning technician was hired to do the job.

a. The technician is not trained, certified or qualified to do the work.

b. Did not complete the contracted job as agreed.

c. Cut corners to get more jobs done (polishing stainless for show, but ignoring hidden areas of the ductwork to cut time).

d. System inaccessibility (access doors and hinge sets missing and/or inadequate tool stock to reach inaccessible areas).

e. Is inexperienced and does not have the confidence or know-how, equipment or skill to do the job properly or to meet industry standards.

f. Scheduled cleaning dates are missed, put off or overlooked.

Fire Protection and Fire Suppression Systems

Due to the cooking medium used in commercial kitchens today, it has become necessary to adjust fire-suppression standards to meet these high-energy, highly efficient and elevated heat-rated cooking oils. Animal fats and AF oils, such as butter or margarine, will burn at considerably lower heat with auto-ignition temperatures of most animal fat-based cooking oils at around 550°F. However, modern vegetable oils heat release rates were designed to be higher — that is, auto-ignition temperatures are from 685°F to 820°F, depending on the type, concentration, depth and viscosity of the oil. Once vegetable oil reaches its threshold temperature, the oil will self-ignite.3

After the heat source has been removed, these oils retain confined temperatures for longer periods of time. In an enclosed environment — that is, vats, deep fat fryers, containers holding appreciable depths of 1/4 inch or more — oil will hold and retain self-ignition temperatures for 25 to 40 minutes after heat has been removed. Should the oil temperature become compressed or pressure density heightened by draining and removing material from the container, the heat flux within the heated vat will raise to the level of auto-ignition and will reignite. This phenomenon is especially true in insulated deep-fat fryers. Safety tip: Deep fat fryers should not be drained for investigative purposes until residual oil has been cooled at the core.

Observations with photographs during the initial investigative walk-through should address the following:

• Did the automatic fire-suppression system fuse and activate, leaving a residue of wet chemical on the cooking line surface? (Fusible links in commercial kitchen applications will fuse at 350°F to 500°F. Most hood links are set at 450°F, depending on appliance type, coverage and application.)

• Was the suppression system within the hood and plenum clean? Were wet chemical nozzles plugged at the time of the fire? Is the wet chemical cylinder empty or full? Did the gas valve trip?

• Identify if ABC/BC extinguishing agent and/or any other extinguishing agents were used on the fire. The cooking surface and the floor will reveal what was used.

• At the time the system fused, were the make-up air fans shut down, and did the exhaust fan continue working during the suppression sequence?


Access panels may be accessible along the length of vertical and horizontal ductwork. These access panels should have a sign that reads “Access Panel–Do Not Obstruct” to identify the panel and bar any obstructions to the panel and will be of value in determining the following:

• Was cleaning accomplished above the duct orifice or visual portion of the ductwork?

• Identify the extent of heat flex within the duct walls (discoloration, warping, melted weld-flux, etc.)

• The condition of the ductwork — that is, weld failures, visible cracks/leaks, transition openings and separations, must be identified for fire cause and used to identify the extension patterns of the fire.

• Any deficiencies in the ductwork must be repaired or replaced before the cooking line can be used or reopened.

Access to duct discharge through the fan housing/airstream will be necessary to identify the following:

• Type and size of fan installed (up-blast, in-line, utility sets or other). Is a hinge set provided for access?

• Fan operation at time of fire: Was the fan working during the fire event? Is the fan motor underpowered? Is the fan belt in place and in good working condition?

• Is all wiring and electrical equipment in place and is power wiring connected? (Wiring systems of any type shall not be installed in ducts. NFPA

• Were the duct discharge and fan blades cleaned? Was the extent of combustible oils, grease and flammable creosote identified at the fan housing height level?

• Is the fan base screwed to the airstream or has a hinge set been provided for ease of access?

While on or observing the roof, look for kitchen oil and grease spillage, leaks or grease flows on the roof membrane or shingles. Kitchen grease is slightly acidic and in time will cut, eat and seep into roof coverings and the underlayment.

Suppression Methods and Fire Extension

It will be important for the investigator to discern or identify by evidence if suppression methods were used to cool or retard the fire and how the fire was able to extend beyond the cooking surface. If airflow was working properly, wet chemical agent would have drawn up the duct to help cool, retard or extinguish fire in the duct.

Grease baffle filters usually shield the plenum and fusible links from flame impingement but not from the heat.

• With steel baffle filters, fire extension into the duct requires a high-heat fuel of some type on or near the filter itself or behind the filters. Grease, oil or creosote residue will be identifiable as a stain or crust depth in measureable amounts in filter troughs, collection pans, ridges and on the metal of the filters, hood, and plenum and duct walls.

• Was excessive grease found on the filters or the exhaust system, or was a disproportionate amount of grease found in horizontal ducts or grease traps?

• Were the filters separated, removed or burned away?

• Were mesh or cloth filtering materials found on filters or on top of the cooking line?

• Did fire burn through and beyond the filters where grease had collected?

• How did the fusible link melt? Fused by heat or direct fire impingement or was the stainless steel cable burned through to fuse the system?

• What type of filters were found? Aluminum or steel?

If high heat flux to this extent has been identified within the exhaust system, the duct will have to be evaluated for damage — that is, weld failures, leaks and separations. These must be identified before the cooking line can be used or reopened; otherwise grease will leak and or saturate structural members.

In Summary

Kitchen exhaust systems are often difficult to understand. Their design is easy to see at the appliance, hood or plenum level, but within the ductwork, the design may be obscured and complicated. Final questions to ask: Is the duct straight up, or does it tie to a horizontal and 45° turn to vertical? Are there two ducts and one duct discharge with one fan or is there one duct that transitions to two duct discharges? And what is the adjustment for different types and sizes of ducts and fans? 4

When questions present themselves during your investigation, feel free to call upon trusted kitchen exhaust system professionals for answers. “Trusted” means they are trained, qualified, certified and experienced to do this kind of work. They must be familiar with the system in question. Can they quickly and accurately recognize the types, features, equipment and code requirements that apply to unfamiliar or new system types? Remember, suppression system professionals may not hold the same credentials as do exhaust system cleaning professionals. They are two separate industries and may require two separate professionals.

Normal combustible fires in the kitchen, freezers/coolers, storage rooms, and offices may require normal investigative principles and procedures to be applied. This is also true with commercial kitchen exhaust system fires. With the high cost of commercial kitchen appliances, hood and duct systems, notification, suppression systems and exhaust system equipment, these fires can be daunting. Considering the downtime, loss of revenue, employee costs and emotional loss, these fires can be devastating to the owner and to the community. Commercial kitchen fire investigations require a great deal of coordination and communication with staff/employees, cleaners, installers and kitchen and building owners to identify cause and responsibility. With this coordination, witness and participant concerns, interference, and media and community pressures, are all considerably perplexing; however, bear in mind that investigations of this kind of fire can be exhilarating, most challenging, and professionally rewarding.



1  NFPA 96 “Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations, 2011 Edition.” Chapter 11.4 and Table 11.4

2 Office of the State Fire Marshal, “Kitchen Exhaust Duct Cleaning Systems.” Completion and Deficiency Report and Hood Service Label with Instructions

3 Dr. Sunderland and Krystyna Buda-Ortins, B.S. Student, “University of Maryland Department of Fire Protection Engineering, May 19, 2010,” Auto-Ignition of Cooking Oils, Page 2 Abstract

4 “Solutions for Air,” “Ventilation Rules of Thumb” Type of Cooking Equipment, CFM/Ft² of Hood

5 Moore, Oliver C., Deputy Utah State Fire Marshal, “Mutualistic Fire Protection.” UFRA Straight Tip, January/March, Volume 14, Issue 1, Page 24




Fires that involve commercial exhaust systems and cooking equipment can be a significant concern for the fire or insurance investigators. More than two-thirds (69%) of structure fires in eating or drinking establishments were caused unintentionally. One in five (22%) were caused by the failure of equipment or a heat source, and one in five fires (21%) in eating or drinking establishments had a failure to clean as a factor contributing to its ignition. An electrical failure or malfunction was a factor in 16%, and a mechanical failure or malfunction was involved in 13% of the fires. Radiated or conducted heat from operating equipment was the leading heat source in these fires (20%). Sparks, embers or flame from operating equipment was a factor in 12%, and 5% were set intentionally. Fires and property damage are significant factors in cost loss or dollar loss in commercial kitchens. (U.S. Fire Admin, NFIRS and NFPA) — Oliver C. Moore


— Oliver C. Moore is an authority on public fire safety and education and community preparedness planning. Chief Moore most recently served the Utah State Fire Marshal for the past 10 years in the role of fire prevention specialist. He retired as Deputy State Fire Marshal in December 2016. Email

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