Thanks for the replies! my total duct length is the one elbow (90) with 6" of straight attached and about 12 to 14" between the fan and the elbow. So I'm well below the 15'. Thanks a lot for mentioning it though. Yes, the elbow I'm considering is the standard ones made up of 4 sections that swivel. One of those will do it no problem. I'm going to make one more attempt at fitting a striaght section in there otherwise I'll go with the elbow section. Thanks again for the replies. Good to know (most likely) that I'm not going to have any issues (hopefully) if I use the elbow section.

I don't know what San Diego's particular requirements might be, but suggest the following. Screws can ensure that ducting doesn't separate. Rubber lined clamps such as go with the Fantech parts don't need screws. The choice of having the duct section female ends at higher elevation than the male ends will help keep any grease accumulation from leaking out. (If this were a commercial application, the ducting would have to be steel, welded at the connections, and provided with clean-outs. kas

One would have expected Prestige to provide some advice, but without it, I recommend you match six-inch vents with six-inch ducts. Larger ducting reduces resistance but also slows down the air velocity. Commercial ducting requires 2000 ft per min air velocity to limit the amount of grease that will condense. According to some sources I've read, residential velocities should aim for the 1000 to 2000 ft per min range. The two six-inch vents can be merged into a duct with the same cross-section, around 8-inches, but only if both power packs are powered at the same time. Otherwise, keeping the velocity up requires individual ducts. A call to Prestige may be desirable. kas

First, study the VaH ARS system to gain insight into what is possible. Lesser cost recirculating systems likely exist, but haven't been reported on here for how well they approach external ventilation in capture and containment performance. https://www.houzz.com/discussions/2347276/vent-a-hood-ductless-ars-range-hood-update#n=65