Describe "main" If it is main because it is one of several, then your
only good choice is at the furnace. You want to filter ALL the air entering
That filter is there to protect the furnace, not to provide you with
clean air. It may do both, but the primary purpose is to keep the dust out
of the furnance/AC and keep it running longer. Putting just one filter on
one of more than one air returns will allow un-filtered air though to the
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