I purchased the Broan QS230 Allure II unit for above my cooktop. The sheetmetal top has a cutout that is 7" round. I plan on running 6" duct. I plan on inserting a 7" start collar, a 7" to 6" reducer with a backdraft damper in that fitting and then going on with 6". Is this the best way to do this install?
lets say your duct work is only 3 feet long. Then yes, a 6 inch duct would be fine. If your duct is 40 feet long, has a bunch of twists and turns in it, then no a 6 inch would not work.
I would suggest that you research on duct work equivalency. The problem is that every turn or linear foot of duct causes flow restriction and there is a fair amount of difference in the CFM that a 7 inch duct, compaired to a 6 inch duct can move. There are a lot of rules of thumb for ducts and air flow and most will work for you, but you really want to do the reading and make the decision.
7" duct and fittings are hard to find. Also, I have to fit the start collar, backdraft damper, reducer, and two elbows inside of a 24" high cabinet the duct is going through, even 6" may not fit. I only have two elbows and about 6' to the roofline, I don't think getting enough static is going to be a problem.
I'm thinking that I will probably just put a flat sheetmetal plate with a 6" opening on top of the 7" opening and get started right off the bat with 6", it will save me about 4" in the transition piece.