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The odd thing is, that return grille is a filter grille. It's designed to have a filter in it. From the looks of the pic, about a 2" thick 16x24 filter. If that 24" is oval shaped to fit that grille, it's part of the restriction as well. Then add all the kinks from going through floor joist spaces and you've reduced the effective size of the duct to a point that the static pressure can't be calculated.
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Nick
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