Quote Originally Posted by 54regcab View Post
I'm a bit confused on how the location of the filter affects the total static pressure drop. If the filter has a drop of .2" @ 1400CFM and the return duct has a drop of .1" @ 1400CFM doesn't that equal .3" no matter if the return duct or filter comes first?
For total SP, yes, but not for velocity across the face of the filter. We have to also account for filter grills having 40% of the open surface area blocked by louvers.

Add in the ability for unfiltered air to infiltrate the return ducting between the filter grill and the blower assembly, and there are more reasons to put the filter close to the blower than there are reasons to have the convenience of a filter grill.