What I can't fathom though is how detritus from the bottom of the plenum gets in to the HVAC system at all. Water can only seep in through the various cracks and seams in the airbox - the only direct open path is in the top, and it never (!) gets that high. There are (as far as I know) no large open holes in the bottom of the box so I really don't get how the schmutz gets in there.
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