There is a new product "Box Fan Filter Grip" that consists of 2 silicone bands that are sized to hold a 1" x 20" x 20" furnace filter to a typical 20" x 20" box fan. 20" x 20" Box fans are available online and at many different retailers. I bought mine at Walmart for under $20 including sales tax.
The grips work best on a box fan that has the electrical cord coming out of the bottom of the fan. This way the cord will not interfere with the filter insuring a more secure fit.
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Step 1: Place a 1" X 20" X 20" Furnace Filter on the Back Side of a 20" X 20" Box Fan
When selecting a filter, it is best to use a 1” x 20” x 20”. This will give the best results. Filters are manufactured with different levels of filtration. If you are wanting to filter out the most common allergens, purchase a filter with a MERV 8 rating or higher. If you only want to reduce dust in a workshop for instance a lower rating would work.
MERV stands for Minimum Efficiency Reporting Value. This rating system was developed by the American Society of Heating, Refrigeration and Air Conditioner Engineers. MERV values vary from 1 to 16. The higher the MERV value the more efficient the filter will be in trapping airborne particles.
Filters like these are available online and at many different retailers. I buy my filters online by the case with a quantity of 6 or 12. Buying this way I can get Merv 8 rated filters for under $6 each including shipping.
Step 2: Stretch the Box Fan Filter Grips Around the Fan and Filter
Place one band on the left side and one on the right side approximately 4" from the side of the fan.
On the bottom side of the fan, the Box Fan Filter Grips should be next to the plastic feet of the fan.
Box Fan Filter Grip can be purchased online.