On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

Who made that determination? Sure, the cheap 1" fiberglass crap filters
that are the minimum were likely put there back in the early days to
protect the eqpt. But today there are advanced filters available,
from 5" thick high MERV to electrostatic that go way beyond what is
needed to protect the eqpt. And they are sold and markteted to clean
the air for humans.




It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.

Any filter, as it clogs, will start to impede air flow, raising the
pressure drop. I think we agree that it won't affect efficiency until
it's substantial enough to cause a significant pressure drop.