u/Heroic_Fighter

I'm in the process of trying to replace my 1 inch filter housing with a 4 inch one. Both the old and the new are 20x25. I just noticed that the hole cut into the side of the furnace is only about 14 inches tall, but the return is the full height. I could trim maybe another inch from the top of the furnace side before it reaches the bottom where the blower is mounted.

Should this furnace have been installed with a 16x25 housing instead? Seems like part of the filter is wasted, or is this normal?