an inline damper can be installed. More than one damper can be installed.

a 6" space can give you room to have rectangular duct. This is easy and inexpensive not an upcharge not unusual. The transition back to round can be made at any place in the duct run. When a duct is generally larger than a 6" round, you get more flow.

Here is a link that might be useful: backdraft dampers. Some inline

This install involved ducting through a handfull of TJI's and a drag beam. Engineering was required and we had no choice but to use 3x9 oval metal duct running through "radiused slots". On both ends of this oval duct run smooth tapered sheet-metal transitions were made into 6" round terminations at the hood and rim joist before turning down through enclosed eve. There is no room for a typicall boot atop the hood as the 6" adjustable elbow was barely able to be made flush at that interface. Sometimes comprises just simply have to be made and it was major to even get what we have but is maximized for it sizing.

I would certainly have liked more cfm but the 600 was decided on as likely exceeding the duct run with-out more noise. You can see my desire to not muck-up the flow with an all to typical flow robbing damper. Not to mention the upside down orientation of the application neccesary wall hood mounting.

I am starting to think I might be better off using a quality "in-line" slim damper assy inserted in the 6" round eve termination. It will be nearly 1" proud but can be oriented to work facing down-ward. Then cover that up with an over sized plain wall dood for say an 8" termination just simply mounted to the eves wood finished surface. The thought being the oversize wall hood would offer a minimal-to-no backpressure and be easily accesable for possible playing with the damper until satisified.

I am certainly interested in other opinions. I have learned these kinds of things often do not fallow perscribed logic.

TIA