mark: not being smartas here but the manual should give you a maximum length for the vent run. Just remember any bend adds more than the physical length of the pc. For instance I believe a 6" elbow actually adds about 3' as far as the airflow knows. That said If you r using 6" or larger it sounds like you should be ok. I also think I would re pipe the whole run up to the existing roof vent, you may be able to "shorten" the run just by going 45 up and toward the vent then 45 to it.
Mark, as long as you have a 100cfm intermittent or 25cfm continuous ventilation rate when you finish the installation you should be o.k. I do not know of any codes (IRC) that limit the length of a run for for a kitchen exhaust fan. The manufacture installation instructions should list information regarding how the length will affect the performance, if not you can always call the support number listed in the instructions. A clothes dryer duct does have a maximum of 25' run with a deduction of 2.5' for each 45 and 5' for each 90 not including the transition duct. You may also want to check your local codes to make sure there are not any local codes that are more stringent then the IRC codes. Remember though that manufacture installation instructions are allways the final say on how you do the install not the codes because the product is "engineered".
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