The attached pictures are of the community return duct and filter box, installed in a new house last August.
The filter box is 20X25, and I use 5" MERV 16 filters.
From an airflow perspective, the widening of the duct above the filter, and abrupt narrowing after, seems incorrect. Specifically, the manufacturer of the filter box specifies a maximum angle of 20 degrees downstream of the filter, and this sheet metal far exceeds that. The other issues are that the duct the filter box is in is much smaller than the return trunk above it, and the fact that there are no turning vanes in the duct where it makes the 90 degree turn to the blower inlet.
I would propose that, for starters, this sheet metel should be replaced with a straight 20X25 duct all the way up to the return trunk, and that turning vanes be installed at to ninety to the furnace. I'd appreciate any thoughts.