The Faber is unusual, since it can work on two 6" ducts in parallel.

If you can only have one round 6" duct, you are up against a serious constraint. I'd favor Kobe, Best by Broan, Broan, Sirius, Miele, or any other Insert in the 600 to 900 CFM range that has one or two large diameter centrifuge motors in it and that calls for a 6" duct in its product PDF.

Round to rectangle transitions are commonplace.

If your available height is 6" you can run a large rectangular duct to the outside and have a rectangular opening on your outside wall. Then you no longer are limited in your choices. To convert rectangular to round equivalents, calculate the are of the cross-section (circle or rectangle) and cut off 10% of the rectangle's area since round ducts are more efficient.

Noise is a big concern for anybody. Physical limits to moving fluid in a cylinder apply to air just as in plumbing (water) ; plumbing vents have prescribed minimum sizes to get a certain amount of makeup air. Moving it with a blower means you hit the physical limit of the tube and 6" is the smallest possible kitchen vent diameter.

Don't put an 1100 CFM blower on a 6" duct. Even with two 6" ducts, be aware that I haven't found Faber to be a noise-reduction oriented company. Like other Italian companies, they disregard noise, considering it as normal part of life. Japanese and Northern European companies regard noise as the thing to design around (to reduce).

With a 6 in. duct you have a single important factor that you cannot ignore. A machine designed for a 7 inch duct will make more noise when reduced to 6 inches. although I do give credit to Kobe for designing an extremely quiet and fairly high output fan (when in quiet mode at about 385 CFM).

I'm assuming low-noise and operating strength are your two main criteria. At lower speeds at least.

David

Thanks so much for the reply. My contractor confirmed that the 2 runs of the Faber will be too much to squeeze in anyways.

We're currently between 2 models: Zephyr Tornado II (AK8200AS) or the Kobe one (IN2636SQB). Both would be transitioned to a rectangular duct so we would have enough area to be equivalent to the area recommendations. Are there any clear winners between these too?

The Zephyr needs an 8" round duct. A custom duct in rectangular shape would have to be 6" by 11½" to be equivalent to an 8" round duct. So if you have enough ducting for this one, you haven't eliminated the Faber based on ducting. Two six-round are about equal to an eight-round duct.

I think you are looking too hard at product solutions before you know your duct size, your needs, and your other constraints.

David

I think I know my duct size limitations - my duct work is being run behind my crown molding on top of my upper cabinets. My crown molding is 6" high and my uppers are 12" deep so that would make my duct work limits 6" x 12" - right?

I'm not trying to rush into a product but my KD wants to know my blower and insert choice before the cabinet order goes in so we can double check the specs. This feels a little bit like the chicken + egg problem - do I set the specs on the wood hood first or pick the blower/insert first. I'm trying to do it all simultaneously instead so that I don't mess it up.

I know the 2 6" duct works would technically fit in my space but my contractor feels that they would be "squashed" and would prefer I just go with a rectangular duct. That's why I thought I was left with the Kobe or the Zephyr options. Yes? No?

I do not have an insert but I do have a Kobe CH176 that can be stand alone or used as an insert and I am very pleased with it's performance and noise level.