re: Shop Air Filter

BTW, I bought my filters from filterbuy.com.  Comparable prices to what George linked to but significant discounts if you buy in bulk (shipping is free).  One 20x20x2" Merv 13 filter is $35 but 6 of them is are $15 each.   Similarly, 4" filters are $36 for 1 but $17 each if you buy 6.  With over 3 times the filter surface area of a 1" filter.  $17 each is a pretty good deal for a Merv 13 filter.