Class K fire extinguisher: what it is, where it is required and how to service it

What Class K means

Class K is the fire class for cooking appliances that use combustible cooking media, meaning vegetable or animal oils and fats. Amerex, a major extinguisher manufacturer, says the class was introduced in the early 1990s. The trigger was fire testing on a commercially available deep fat fryer: Amerex explains that UL 300, the fire test standard for commercial cooking suppression systems, launched in November 1994 using such a fryer, and that lessons from it were carried into UL 711, the rating test for portable extinguishers. None of the 40-B:C dry chemical extinguishers in use at the time could pass the full test, which Amerex says drove the creation of the K rating.

Grinnell Mutual's loss control bulletin notes that vegetable oils burn hotter than animal fats, which makes the fires harder to put out, and that a 1998 revision of NFPA 10 required extinguishers for cooking appliances using combustible cooking media to be listed and labeled for Class K fires.

How wet chemical agent works: saponification

The agent reacts with the burning fat in a process called saponification, which Amerex likens to making soap. The result is a foam blanket on the surface of the oil that cuts off oxygen. Amerex says wet chemical makes a much stronger and thicker blanket than dry chemical and also cools the oil, which dry chemical does not. Grinnell Mutual warns that dry chemical alone carries a high risk of re-flash in a vegetable oil fire.

• Lower splash hazard when the agent hits the oil (Grinnell Mutual).
• A foam layer that smothers the fire (Amerex, Grinnell Mutual).
• Cooling of the appliance and the oil, which reduces the chance of re-flash (Grinnell Mutual).
• Better visibility and easier cleanup (Grinnell Mutual).

Where a Class K extinguisher is required

NFPA 10 requires Class K extinguishers where there is a potential for fires in combustible cooking media (section 6.6.1), and section 6.6.2 reads "Maximum travel distance shall not exceed 30 ft (9.1 m) from the hazard to the extinguishers." Both are quoted by Amerex, which does not name the edition, so confirm the section numbers against the edition your AHJ enforces. Grinnell Mutual gives the same 30 foot limit measured from the cooking appliances. Our kitchen hood suppression inspection checklist page covers how these items fit into the semiannual hood service.

The fire code adds detail on top of NFPA 10. The 2020 Fire Code of New York State, an International Fire Code based code, is a useful example because its text is public. Other IFC based state and local codes may number these sections differently.

Amerex separately quotes NFPA 10 section 6.7 for solid fuel appliances with fireboxes of 5 cubic feet or less as calling for at least a listed 2-A rated water type extinguisher or a 1.6 gallon (6 L) wet chemical extinguisher listed for Class K fires. That differs from the New York figure above, which is why the governing document matters: check which code and edition your AHJ applies to wood fired ovens and charbroilers before you size the unit.

The same New York code sets mounting rules that apply to the K unit like any other: the top no more than 5 feet above the floor for extinguishers of 40 pounds or less (906.9.1), at least 4 inches of floor clearance (906.9.3), along normal paths of travel (906.5), and not obstructed or obscured from view (906.6).

The extinguisher is the backup to the hood system

A Class K extinguisher does not replace the fixed wet chemical system over the cooking line. Amerex quotes NFPA 96 section 10.2.1 as treating automatic fire-extinguishing systems as primary protection and portable extinguishers as secondary backup. Grinnell Mutual puts the order plainly: the extinguisher is used after the fixed system has operated and the energy to the cooking appliance is off, and if the system has not operated, staff activate it manually first.

That order is printed on the wall. Where a hazard is protected by an automatic system, NFPA 10 section 5.5.4.5.3 (quoted by Amerex, edition not named) requires a placard near the extinguisher stating "that the fire protection system shall be actuated prior to using the fire extinguisher." Amerex also cites NFPA 96 section 12.1.4 for training staff on manual actuation of the system, for new hires and annually for all employees, which is a natural thing to mention to a kitchen manager while you are on site.

Compatibility is a code requirement, not just good practice. The 2020 Fire Code of New York State (906.4) requires an extinguisher protecting cooking equipment to be of an approved type compatible with the automatic system agent. Grinnell Mutual, an insurer, goes further and recommends removing dry chemical extinguishers from the cooking area altogether.

Service cycle: why wet chemical units are on a 5 year clock

Monthly inspection and annual maintenance work the same way for a K unit as for any other extinguisher (see our fire extinguisher inspection requirements page). The difference is the longer cycle. A typical stored pressure dry chemical unit is emptied for internal examination every 6 years and hydrostatically tested every 12. A wet chemical unit gets both every 5 years, so the two line up on the same visit.

The references move between editions. A University of Colorado table built from the 2007 edition lists wet chemical at 5 years in its recharging column under section 6.4.3.11 and the 5 year hydrostatic test under Table 7.2. The Minnesota Department of Health, citing the 2010 edition, puts the internal examination in Table 7.3.1.1.2 and the hydrostatic test in Table 8.3.1. Wet chemical cylinders are low pressure cylinders, so a passed hydrostatic test is recorded on a durable label showing the month and year, the test pressure and who did the work (2010 edition, 8.7.2, per the Minnesota guide). High pressure cylinders such as carbon dioxide are stamped instead (8.7.3).

California writes the alignment into its own regulations. Title 19, section 575.16 requires internal maintenance of wet chemical and water mist extinguishers every 5 years, at the time of the hydrostatic test, following the manufacturer's recommendations. Section 578.12 is short enough to quote in full: "Wet chemical agents shall not be re-used. If a wet chemical extinguisher is partially discharged, all remaining wet chemical shall be discarded."

Electronic monitoring is another place K units differ. The 2020 Fire Code of New York State (906.2, exception 2) lets extinguishers supervised by a listed electronic monitoring device skip the 30 day inspection and stretch maintenance to every 3 years, when the code's listed conditions are met, but only for dry chemical or halogenated agent units. A wet chemical unit on the same wall stays on the normal schedule under that code.

Common mistakes in restaurant kitchens

• An ABC dry chemical unit hanging by the fryer as the only kitchen extinguisher. It is not a Class K extinguisher and, under codes like New York's, it must also be compatible with the hood system agent.
• The K unit mounted more than 30 feet of travel from the cooking hazard, for example after the line is rearranged.
• No placard, or a placard that has fallen off, so staff reach for the extinguisher before pulling the hood system.
• Carts or stacked supplies blocking the extinguisher. The Minnesota Department of Health says it is not uncommon to find kitchen extinguishers blocked by carts during its surveys and suggests adding the item to the monthly inspection checklist.
• Added fryers without added extinguishers. New York's 906.4.2 calls for one more K unit for every additional group of four fryers beyond the first four.
• A partly discharged unit topped off instead of emptied and recharged. California prohibits re-using wet chemical agent.
• The 5 year internal examination and hydrostatic test missed because the unit was tracked on the same 6 and 12 year clock as the dry chemical units in the building.