I made a box with a fan duct hole on the other side, and this HEPA filter as shown. It's easy to replace, just slide a razor blade along the edge. And I only change every 6 months or so as needed.
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Large surface area let's airflow pass through with hardly any load on the fan. A 6 inch duct has about 19 sq inches of area, and the 16"x25" filter has 400 sq inches of area. Same ones as my furnace takes so I buy them in contractor packs of the HEPA filters. Removes even smoke from the air. Lol.
That was the intake of the room. At the exhaust of the room is the sealed in diy rechargeable carbon scrubber.
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Theres another filter layer before the carbon, an electrostatic fine dust filter.
Then the 8" scrubber tube with a 6" deep bed of carbon in it. Removes all smell completely.
All runs from one 6 inch fan with the large passive intake for two rooms, or 435 cubic ft of space.
Under the load the fans moving about 60 to 70 cfm. Gives me about 8 to 10 full air changes an hour which is plenty for the plants.