Step 1: Purchase or Acquire the Following:
- One very good DISPOSABLE PLEATED 20" furnace filter (I like the 3M ones) ($8-$15)
- Good quality duct tape ($2-$4)
1) The brand of fan doesn't really matter. A cheap one works as well as a good one, and price is no indication of how long it will last either.
2) DO NOT buy some goofy expensive electrostatic permanent filter or a filter that isn't pleated. Good quality Disposable Pleated filters pull far more out of the air. If you don't believe me, prove me wrong.
3) Don't skimp by buying cheap duct tape. You'll end up using twice as much and not get as good of a seal.
Step 2: Assemble the Fan According to the Directions That Came With It, Take the Filter Out of Its Packaging.
Step 3: Align the Filter on the Fan
You want to align the furnace filter on the fan so that air exiting the fan will go the right way through the filter.
Step 4: Attach the Filter to the Fan
Step 5: Shroud the Back of the Fan.
Tape over the corners of the fan on the inlet side, covering the outermost 6" or so of each corner. This will prevent air from exiting the back of the fan, allowing the filter unit to work far more efficiently.