Return Air Filter Sizing for Manual D, 300-fpm velocity
A 14X20 "cheap low resistance filter," according to Hart & Cooley Engineering, will only flow 401-cfm @ 300-fpm velocity through it. The "free-air-area," or (Ak) is only 1.335-sf. Therefore, it would require two 14X20 filter areas to flow 800-cfm @ 300-fpm through a news clean filter.
My furnace lists the filter requirement as 20 x 20, but the filter is 14 x 20. I'm planning on having someone come in, but I'd like to become a little better educated beforehand.
Okay, the 20X20, only flows 575-cfm @ 300-fpm; the "free-air-area," (Ak) is only 1.917-sf; not nearly enough.
Excessive Debris is blown through that type of filter above 500-fpm velocity through it.
The RA filter area is extremely important to efficient airflow.
When you opened the blower door, you bypassed the RA duct system & the restrictive RA filter area, resulting in much improved airflow to the rooms.
Depending on the Btuh Output of the furnace, the temperature rise above the RA temp, may be too high!
Additionally with filters in the RA duct with a 90-ell too close to the filter down stream & near the blower chamber, there ought to be turning vanes in the 90-ell, or the lower part of the filter will get much more air through it at a higher velocity than the upper portions of the filter.
Few airflow systems are designed for optimal efficiency performance...most systems underperform...which causes heat & air equipment to underperform...