BD-3 DIY Air Purifier: a Quiet, Compact, Portable Universal C-R Box

This room air cleaner uses a cheap, widely-available fan and standard filters, and it performs as well or better than comparably-sized consumer HEPA room air cleaner products. Note that true performance isn't clean air delivery rate (CADR) alone but rather CADR at a comfortable noise level. The BD-3 is straightforward to build, the most challenging part being perhaps drawing the pattern for cutting out the cardboard piece. It's intentionally not a full 4-filter "Corsi-Rosenthal box" (C-R box): due to the BD-3 having one blank side, you can orient it to blow either horizontally or vertically (hence why I'm calling it "universal"). It's great for cleaning the air in a small room at home or at work or when going to a hotel, a hospital room, etc.

The "secret sauce" of a project like this is in designing the cut-out pattern such the cardboard piece adapts to the irregular shape of the back of the fan and fits neatly to the edges of the filters. I've done that tricky stuff for you. Call it thinking outside the box, in a box. :)

Materials

1. BLACK+DECKER 9 in. frameless tabletop box fan #BFB09W, and the box it comes in
2. 10 x 10" MERV 13 furnace filter, 1" or 2" thick, quantity 3, for example from Tex-Air Filters
3. Masking tape

Tools

1. Metal ruler or yardstick
2. Pen or pencil
3. Utility knife or razor knife
4. Cutting mat or a couple layers of scrap cardboard
5. Scissors

Since you'll be repurposing the cardboard box, open it carefully:

1. Cut or peel off the tape on the top and bottom (first photo).
2. Empty the contents.
3. Find the box's seam, where the edge of the cardboard is exposed, and cut along it (second photo).
4. Unfold it flat.

1. Put the printed side of the cardboard facing up. This will be hidden inside; the outside will be blank. Orient the cardboard as shown, noting where the box's folds and flaps are.
2. Download the pattern and familiarize yourself with it. It's a sub-scale pattern, the idea being not to print it out and trace around it but instead to refer to it and draw it full-size yourself with a pen and ruler. Starting from the spot on the cardboard indicated by the arrow, which is in the middle of one of the folds, measure and draw the pattern. (We're going to re-use that fold later, bending it the opposite way.)
3. Note that some of the lines are guide marks, not meant to be cut. Using the knife and ruler with the cutting mat underneath, cut the perimeter, including the wide notch at right as well as the narrow notch for the power cord.

Now for the fun assembly stuff. I recommend cutting the tape to length with scissors rather than tearing it, and it's also worth taking a little care to align the tape parallel to the edges of the filters when applying it. It'll be nice for it to look decent when it's sitting in your living space.

1. Lay the filters edge-to-edge in a line, with the airflow direction pointing up and the pleats oriented crosswise (first photo). With the edges of the filters aligned and pressed snugly against each other, stick a piece of tape along each joint. These will act as half of a tape hinge.
2. Fold one of the end filters over, hold it aligned and snug face-to-face with the middle filter, and tape the now-exposed side of the joint to complete the hinge (second photo).
3. Unfold the first end filter and then repeat the previous step with the filter on the other end.

The third photo shows the results.

Things get a little wiggly here before everything is secured, but please bear with the process, it'll go fine!

1. Put the cardboard blank side up. Cut and apply five pieces of tape as shown (first photo), with at least half of the width of the tape hanging off of the edge.
2. Flip the cardboard printed side up again. Pick up the filters and place them onto the tapered half of the cardboard, with the inside edges following the guide lines (second photo). Don't press the tape down just yet. Lift the free half of the cardboard and make sure that the corners of the end filters nestle into the fold line (third photo). Check that the filter inner edges are still following the guide lines, and then go ahead and press the tape onto the edges of the filters all around.

Now it all comes together!

1. Put the fan face-down, with its bottom toward you. Seat the filter box onto the back of the fan, with the cardboard side facing you (first photo). Feed the power cord through its slot as you go. Note how the wide notch in the cardboard fits around the middle foot of the fan. The filters should seat flush against the back of the fan's flange.
2. Seal up the joint between the cardboard and the fan (second photo). It actually doesn't have to make a perfect seal; pretty much all of the air is going to go through the filters anyway. Try to make it look neat, because when you have the air purifier blowing upward, this side will be visible. Seal off the power cord slot too.
3. Flip the BD-3 over and tape the three joints between the filters and the fan (third photo).
4. Add bits of tape as desired to seal the corners of the box and to neaten up the edges of the tape (fourth and fifth photos).

Ta da! Situate the air purifier in the room so that it has plenty of intake space around the filters and so that the exhaust stirs up as much of the room's air as possible. Run it on as high of a fan setting as comfort allows. It's pretty quiet; on low, it's really quiet! For specific usage suggestions, see the last step of my 20" box fan universal C-R box Instructable.

DIY air purifier filters tend to last six months to a couple years, depending upon usage. The electrostatic filter material stays effective for quite a while, well beyond when the filters start to look dirty. If they're visibly thick with gross gray-brown dust, then it's definitely time to change them. To replace the filters: peel off the tape (try to go gently on the cardboard), throw away the used filters, and repeat steps 3-5.