I presume this refers to a furnace air filter. Every furnace should have
a filter in its intake path, but in some cases the filter is not
installed at the furnace air handler itself. Often a larger filter is
used at the return air intake grill since the larger filter surface area
allows the use of better filtering but more flow restrictive air
filters. My setup is like this, with the air handler being 20x20 and the
return intake where the filter is 24x24 which is a 44% increase in
filter area vs. one at the air handler.
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