I tried to replace my parents' range hood with one they purchased from Lowes. Turns out the vertical exhaust opening center point is 5" from the back of the unit while the existing was 6". Old one was a Kenmore and new one is Broan.
So the hole in the cabinet floor obviously doesn't match. And it isn't oversized enough to get the exhaust pipe in the collar. I know I can always take a jigsaw and open the hole in the cabinet up... but that would leave a big gap on one side. Also I am wondering why this dimension isn't standard and being that it isn't why can't I find the spec for that measurement on any range hood box or product info web page? Are all Broans 6" and Kenmores 5"?
Usually the size of the ducting is relational to the output of the fan. Perhaps you purchased a unit that had a slightly lower CFM rating than the old one. I'm curious as to what model you purchased from Lowes and what model the old Kenmore was. You can find a copy of duct to CFM ratios here.
We bought probably the same vent hood from Lowes over memorial day weekend. Our old (30 years) Ventahood vent pipe was on a 14 inch center while the new Braun was on a 16 inch center. I was not going to cut a smily face in the bottom of the cabinet so back to Lowes for the carbon filter. Yes the old vent pipe is still in place covering the hole. We just plan on changing the filter.