Box fan with furnace filter

Air cleaners — or “air purifiers” — are expensive for what they are, which is typically a fan with a filter or series of filters inside a vented plastic box.  Beyond the initial investment, you need to either buy unreasonably expensive replacement filters designed for a specific unit (think printer ink cartridges) or wash a reusable filter that’s not as effective.

It’s allergy season here in Seattle, and double-dosing on Claritin (actually, generic Loratidine, which is identical but an order of magnitude cheaper) doesn’t seem like something I should make a habit of.  So, after spending far too much time reading reviews and pros/cons of different units ranging from $50 to $300, it occurred to me that I could easily build something that would get the job done with household items I already had and a commodity furnace filter.

With just a few minutes of work and a $15 investment on top of the $15-$20 I’d already spent on a 20″ box fan, I had a very effective and reasonably quiet air purifier for a fraction of what I’d have had to spend on a commercial product to move a similar volume of air (e.g., this model by Whirlpool).  A nice savings, not even counting the fact that my filter could be swapped out for about $15 vs. $100 or so for essentially the same thing on the commercial purifier.

The build is very simple:

  1. Locate your 20″ x 20″ box fan, or purchase a new one
  2. Buy a package of 14″ (or longer) zip ties
  3. Buy a 20″ x 20″ furnace filter with a MERV rating that suits your needs and price range
  4. Place the filter on the back of the fan, ensuring the airflow arrow on the filter faces the back of the fan
  5. Poke a zip tie through each corner of the filter and through the fan, and zip tight at the back of the fan
  6. Enjoy your clean air!