Issues and improvements in commercial kitchen exhaust ducts

Applicable codes and standards provide requirements for exhaust ducts connected to Type I exhaust hoods. Welded-on-site ducts that are not tested and listed to national consensus standards are hereafter referred to as unlisted ducts. The Minnesota Mechanical Code, in section 506.4.2, requires evaluation of duct leakage by means of an air pressure test. Ducts must hold positive air pressure of 1.0 inch water column for a minimum of 20 minutes unless an equivalent alternate test is specified by the building official. A largely unknown fire risk with leaking kitchen exhaust ducts was discovered by a listing agency conducting a UL Standard 300 test of a prototype fire suppression system. The equipment included a UL 710 listed hood over a natural gas-fueled deep fat fryer. With the exhaust duct pre-heated as required, the deep fat fryer was ignited, and after the specified pre-burn time, the water and surfactant based fire suppression system was activated.