Thank you very much for your time!

I want to install 900 CFM under cabinet hood with 8 inch exhaust going up.

I have existing OTR ductwork that goes down to crawlspace and outside to the right. It is only 6 inch, and is also flattened into 3.25 oval shape inside the wall. It travels about 12 ft and goes through 3-4 90 degree turns. The new hood instructions say ductwork should not exceed 12 ft regardless of turns which would make existing route perfect except for size.

I really want to have high CFM hood. I am also tolerant to eye sores, but want to keep the house marketable and appealing in case I want to sell it in 5-7 years.

Option 1

Drill 8 inch holes through inside cabinets straight to the right.

Pros: Nice looks from outside

Cons: Weakening cabinets with massive holes, and losing cabinet space for future owners (although I don't care that much for it myself);

Option 2

Install V-shape connector to split air into existing ductwork, and adding second arm to connect to over the cabinet rectangular duct.

Pros: It will be low enough to hide with cabinet molding trim.

Cons: No idea about turbulence and noise created by two different air paths.

Option 3

Vent 8 inch through existing 6 inch (squished pipe). Air has to turn 180 degrees, then 3-4 90 degrees, then out.

Pros: No crazy and unsightly ductwork that will spook out new buyers

Cons: Deal with extra noise and reduced efficiency. Does it affect my fan longevity?

Option 4

Somehow find / make my own non-standard 12 in x 4 in (48 sq. in. ~ 50.12 sq. in) rectangular duct to place above cabinets and replace narrow molding with 5.6 (4 height) molding.

Pros: no loss in CFM

Cons: No idea where I would find duct connector, turn, and end parts to outside wall.

Ruled out options and reasons:

- Redo existing ductwork to fit 14 x 3.25 duct - there is a range outlet in the same stud space below, that I believe, I cannot move without an electrician (extra cost, opening up the wall, unexpected hidden problems)

- Stack extra cabinets - too expensive and overkill with high 9 ft ceilings

- Build soffit - with 9 ft ceilings the construction will be massive comparing to the hiding object

- Downgrade CFM to fit into 6 inch - We cook a lot, I really hope to keep high CFM hood

- Vent up, left, back - this is an internal wall in two story house with a living room behind