My 1950s ranch had a loud, ineffective kitchen fan that I decided to fix up this weekend. Turns out that when a previous owner had vinyl siding added, they simply put the vinyl over the vent.
I suspect this greatly lowered airflow and contributed to the lack of venting. But looking at modern options, I can't find any replacements for the 3 1/4"x10" ducting that was run out to under the eaves, only round ones. Will simply drilling more venting holes in the vinyl sheeting help?