My old Lennox 75,000 BTU furnace has an input duct about 30% smaller than the mfr marking on the side panel. The marking suggests a duct of "24 x 15", but my duct is only 24" x 10". The furnace was replaced about 20 years ago and the original ducts were used without modification.

It seems to me that the mfr markings would be important to follow and I'm curious what the experts on this board think.

I'm thinking about a new furnace and I am guessing that some installers may think the input duct needs replacing and that will probably add significant cost.