www.grainger.com/.../kh-what-is-merv-rating-air-filter-rating-chart
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I managed to find a MERV 13 air filter
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The pores are smaller. Doubling up halves the pressure required.
I've got a 1/3 HP fan blowing directly into a MERV 11 14" square filter. It bows out in the center as it clogs up.
The first filter, from March lasted 6 months, the second one lasted 3 days.