I stole this from
http://www.eng-tips.com/threadminder.cfm?pid=184
The reason that kitchen exhaust ducts are prohibited from passing through fire walls is:
1) You cannot provide a damper in a kitchen exhaust system due to the operating temperatures. Therefore, if the duct is compromised for whatever reason, a passage for fire extension from one side of the wall to the other is created.
2) I'm not sure what building code is in effect on your project. If the project is in the US, it is most likely subject to compliance with the International Building Code. A major concern for any through-penetration is that the penetration may pull down the fire wall in the event the structure collapses. If this project is subject compliance with the International Mechanical Code, it prohibits kitchen exhaust duct penetration of fire wall
3) From a practical standpoint, I've seen horizontal ducts on kitchen hood system and while they are allowed, they are a PITA (pain in the *****) from a fire safety perspective because they are more difficult to clean. NFPA 96 has extensive ITM requirements, including established cleaning and inspection frequencies. Over the service life of the duct, if it begins to sag, ponding of proteins and oils will occur in the low point(s), and these are commonly missed by cleaning technicians.
Finally, site coordination is a pretty crappy basis for deciding that compliance with a nationally recognized standard for fire protection is not important. I sincerely doubt the jurisdiction would agree with this interpretation but I don't know the knowledge base of the enforcing jurisdiction.
The author said it better than I could. The only thing I could add is concerning horizontal ducts. Not only are they difficult to clean, but normally the access panels for horizontal duct are not accessable themselves. If they are accessable, the way the panels are insulated, the insulation comes off, and gets thrown away. One brand of panel has a problem with the glue that holds the gasket in place. The glue deteriorates and allows the gasket to leak. We install
http://www.flamegard.com/products/subcategory/?category=4 Easy to install, the gasket withstands the temps and the grease.
If you see this
http://www.ductmate.com/product.aspx?id=13 acess door with the red knobs, it is not listed for use in a grease duct. the gasket is foam rubber. These are often installed by HVAC contractors who do not know grease ducts. My solution, remove the panel, get the hose welded and install a new panel that is listed for the grease ducts.
Back to the access issue. During the inspection process, ask a couple of hood cleaners to come in and ask them if the panels are accessable. Remember, after the hoods & ducts are installed, someone will put cabinets, and other appliances in the kitchen that will make the access panels not accessable.