I am replacing the original kitchen exhaust fan on my 1950s house. The old one vented sideways to an immediate 90-degree bend (to 8-inch rigid ducting) and then straight up about 24~36 inches to the roof cap.
I purchased a new unit with the same sideways exhaust but after reading some interwebs resources (for example) it sounds like avoiding 90-degree bends is best practice. I didn't see anything in the new unit's installation instructions about 90-degree bends or really any kind of ducting recommendations.
If I were to switch to a unit with a vertical vent I would need to make up the displacement by using flexible ducting instead of rigid, which I hear is also non-ideal. (Unless there are bends available at less than 90 degrees for 8-inch ducting; didn't see any at HomeD.)
Which would be the best solution? Or is there a better one yet?