Fire suppression systems used in commercial kitchen hoods are UL listed to operate w/the exhaust fans on or off. Leaving the exhaust fan on helps to distribute the suppression chemical throught the hood and duct. This also helps to move the combustion gases out of the fire area, improving visiblity for the responding fire department. The intake or make-up air is to shut down.
There is an IBC requirement to shut down air handling ducts moving 2000 CFM or more. This requirement may be modified by the type of occupancy, building construction and the whim of the building inspector.
Fireguy
