Introduction: Efficient Box Fan Air Cleaner: a Universal Corsi-Rosenthal Box

By cohesivecontrivances.net

You can make an efficient, powerful, quiet, and decent-looking room air cleaner for about $80 and an hour or two of effort. You don't have to be "crafty" to be able to make one, it's easy! Plus, these directions are thorough.

This design accommodates any MERV 13 furnace filter size and thickness, so long as one side of the filter measures 20", and it uses a minimum of materials. You can situate the unit horizontally to blow across a room or vertically to blow upward at the ceiling.

Details:

This type of air cleaner is quite good at removing smoke from the air, and people also commonly deploy it to reduce airborne COVID-19. I've heard it can be effective against pet dander as well. The "Universal C-R Box's" substantial air throughput means it can compare well in terms of room air cleaning performance against typical commercial HEPA room air cleaners. Generally, for the same air flow rate, a larger fan diameter running at lower speed is quieter than a smaller diameter fan that's blasting away. Note that a HEPA filter catches almost everything on the first pass, whereas a small particle may float through a furnace filter once or twice before it's caught. That said, commercial air cleaners are often loud and may move less air, factors which as a practical matter could render their finer HEPA filter material less important for removing particles from the air overall (a measure known as Clean Air Delivery Rate [CADR]). And that's before considering cost. (Regardless, the effectiveness of any room air cleaner depends upon the size of the room and the type and amount of stuff in the air, among other factors. I'm not guaranteeing anything here.)

Indoor air quality academic Richard Corsi, air filter manufacturer Jim Rosenthal, and others popularized the idea of a box fan with furnace filters taped together to form a cube, hence the name "Corsi-Rosenthal cube" or "C-R box."Compared to using just one 20 x 20" filter on the back of a box fan, the C-R box's large filter area reduces the fan's workload and therefore increases airflow. The "plenum" distance between the filter material and the fan lets the airflow arrange itself efficiently, in contrast to a single filter placed closely behind the fan blades. And although furnace filters are flimsy on their own, once they're taped together to make a box, it's pretty sturdy.

This design differs from typical C-R boxes in that it has filters on three sides rather than four. This allows for use in both vertical and horizontal positions, rather than solely one or the other, and it keeps the cost down. Notably, while it always helps, there are diminishing returns as you add filter area: going from one to two filters is a big jump in efficiency, from two to three a lesser jump, and from three to four a lesser jump than that. (Though some people even go for a full five-sided box that sits up on on tall feet or a frame.)

If you're tight on space or budget, then a one-filter setup with a 20 x 20" MERV 13 furnace filter may be best; it'll work fine. Or with two filters it's straightforward enough to assemble them into a wedge configuration and cap the open triangles at top and bottom with cardboard. But if you have the space and can afford it, this three-sided C-R box makes for a good "sweet spot." Beyond making it for your own use, it can also be a great charity project for schools or people in need.

Supplies

A 20" box fan, and the box it came in

Lasko, Utilitech, etc. For this example unit, I used a Utilitech fan, which draws a bit less electrical power than a Lasko fan does and has a large, easy-to-grip power setting knob. The Utilitech housing is plastic, whereas Lasko fans have a steel housing. Also, Lasko fans have a handle for easy carrying.

Three furnace filters

Sized depending upon how much space you have, cost, and what's available. For the example unit shown here, I used Honeywell Home 20 x 25 x 1" FPR 10 filters.

  • Size 20" x anything: 16, 20, 25, 30" etc. Typically the price is the same regardless, so the best bang for your buck is 20 x 30". Think about how much space you have, whether you want to stow it under a table or in a closet, etc. When upright, the top of a unit with 20 x 25" filters sits about level with a tabletop.
  • Any thickness: 1, 2, 4, 5". 1" of course has the least filter area but is easy to find, cheap, and compact.
  • MERV 13 or better: Tex-Air MERV 13, Honeywell Home FPR 10, 3M Filtrete MPR 1900, etc. If you're not using it against COVID-19, you could go a step lower in filter quality; it'll just be less effective. Don't go two steps lower, since a lot more small particles could get through.
  • Consider getting a second set of filters at the same time, to have on hand for when the first set is used up.

Extra cardboard

Enough to make at least a 19 x 19" square.

Long-lasting tape

For this example unit, I used masking tape and duct tape, in order to keep it neutral and because I didn't want to buy extra stuff for it. Don't make it so nice that you're reluctant to replace the filters!

  • Some combination of packing tape, duct tape, masking tape, gaffer's tape, etc. (not cellophane tape or low-tack "blue" painter's tape). If you want to spend extra on looks, the dull cloth finish of gaffer's tape is really nice, and/or get colors of masking tape, etc. Fair warning that the shininess of duct tape does make flaws stand out.
  • Ideally 1½" wide; other widths OK

Decorating materials (optional, not shown)

Markers, washi tape, wrapping paper, etc.

Tools

  • Utility knife, ideally one with snap-off blades. It dulls quickly cutting cardboard, and you want it sharp. A hobby knife like X-ACTO works OK as well.
  • Cutting mat or a couple layers of scrap cardboard
  • Scissors
  • Metal yardstick or a wood yardstick plus a shorter metal ruler
  • Carpenter square (optional)
  • Compass or circular household items for tracing: a CD, a plastic cup, etc. (optional; see "Exhaust Shroud" step)
  • You could get by with just scissors and a 12" ruler instead of all of the above; it'll be kind of a slog though.
  • Two pencils and/or pens

Step 1: Unbox the Fan... and Its Box

If you can peel off the tape holding the box closed and not tear up the cardboard, go for it. Otherwise, just cut the tape along the seams, without cutting through the cardboard.

Take the fan out. If it's Utilitech, leave the feet stowed as-is. If it's Lasko, you won't need the plastic feet.

On the box, find the side where the cardboard overlaps, and cut it apart alongside the exposed edge, as seen in the second photo. Try to cut cleanly; don't let the knife ride around the corner.

Cardboard cutting tip: On the first pass, make a light cut into the first layer, so that the knife doesn't wander. Then make another pass to cut through the first layer and into the middle. Then make one or two more passes to go all the way through.

Essential safety tip: Always cut away from yourself! Hold the cardboard such that if the knife slips, it won't run into any fingers, your stomach, etc. For longer cuts, and especially for curved cuts, you'll end up repositioning a lot, and that's just the way it goes. Don't get lazy about safety.

Step 2: Get Oriented

Throughout these instructions, we'll refer to orientation as if the fan is in its normal position, as seen in the first image.

We're going to turn the cardboard box inside out, with the product labeling on the inside, so that you can have a plain or decorated exterior. The second picture shows what areas will become which panels of the box.

Step 3: Trim the Back Panel

Reverse-bend the box along the existing crease, as indicated by the green line in the previous step and as shown in the first picture here. Note that the area that will become the back face of the unit is shown at right in the photo. Lay that area on the table, and let the rest drape down off the edge of the table, as shown in the second picture.

Measure starting from the outside of the bend, as seen at left in the second picture. The distance you're looking to mark is:

Actual width of a filter + ¾" extra + ⅛" cardboard thickness = Distance

The "width" of the filter we're talking about is the side that's supposed to be 20". Note that the actual filter dimension is less than 20", so you need to measure a filter to find out. (For example, in my case the distance came out to 19¾" + ¾" + ⅛" = 20⅝". Yours may differ.)

Mark it in two places, then cut across the marks to remove the excess strip of cardboard, as seen in the third picture.

Cardboard cutting tip: If it starts to pull or tear, don't wait to change the knife blade to see if you can goose it along for a few more cuts. Just change it already, yo.

Step 4: Trim the Bottom Panel

Now lay the area that's going to become the bottom panel on the table, and let the back panel drape down toward the floor.

Measure starting from the outside of the bend, as seen at left in the first picture. The distance you're looking to mark is:

Actual length of a filter + ½" extra + ⅛" cardboard thickness = Distance

The "length" of the filter we're talking about here is the side that isn't necessarily 20". Again, measure a filter to find its real size. (For example, in my case the distance was 24¾" + ½" + ⅛" = 25⅜".)

If you are using 20 x 30" filters (not shown): You need to extend the edge of the cardboard that'll be toward the front of the unit. Trim off the box's overlap area, then get some extra cardboard and make a butt joint there with tape on both sides. You can go to the "Trim The Side Edges" step first to obtain the scrap cardboard for this step, though if you do that it may take cobbling a few pieces together. (This has nothing to do with the 19 x 19" square of extra cardboard mentioned in the materials list. That's for later, during the exhaust shroud step.)

Mark the distance in two places, then cut across the marks to remove the excess cardboard, as seen in the second picture.

Step 5: Trim the Side Edges

Figure the distance for the cardboard width:

Actual width of a filter + [2 × ¾" extra] = Distance

(For example, in my case the distance was 19¾" + [2 × ¾"] = 21¼".) Center this distance across the cardboard, and mark each side, as shown in the first picture. It'll take up close to all of the flat area of the cardboard. Repeat these marks in a few more places all along the cardboard. Then start cutting, as shown in the second and third pictures. The result should look like the fourth picture. The last picture shows it folded correctly, as seen from the inside.

Step 6: Corner Notches

Notch the two corners of the back panel by ¾" square, as seen in the first picture.

Notch the two corners of the bottom panel by ½" x 2½", per the second picture. (The picture shows the filters sitting where they'll be assembled later, but you don't need to worry about that at this stage).

Decoration (optional, not shown): You could wait until the unit is done, but now, while the cardboard is flat, is a good time to decorate it if you want to. Since these are the back and bottom panels, note that the more important part to decorate is the exhaust shroud that'll go on the front of the fan later on.

Step 7: Assemble the Filters

Now for the part where the magic starts. Note that for this step the photos show 16 x 20" filters and blue tape, for illustrative purposes. (Don't use blue tape, as it doesn't have enough tack to hold together well with use.)

Put two filters face-to-face, as seen in the first picture. Make sure that the flow direction arrow on each filter is pointed inward, toward the other filter.

Look at the edge that's not necessarily 20". (In this example, it's the 16" edge.) Cut a piece of tape to length, and stick it on the edge as seen in the second picture, making sure that the two filters are flush to each other. I found it easier to do this if I stuck the tape to one filter first and then rolled it onto the other filter, from the middle outward.

Open up the filters along the tape hinge, as seen in the third picture, all the way out 180° and lay it flat. Cut another piece of tape and apply it to the inside of the hinge line. Again, make sure the filter edges are flush to each other as you do this, as seen in the fourth picture.

Try bending the hinge back to 90°, per the fifth picture. If there is any looseness, make sure the tape is stuck down well, as seen in the sixth picture. If necessary, you could try to reposition the tape... although if the filter frame starts to tear, stop fussing with it and just leave it.

Repeat the above process with the third filter (not shown).

(You did remember to check the filter flow direction arrows, right?)

The result should be as seen in the last picture. It's back to my example 20 x 25" filters and masking tape for that one.

Step 8: Attach the Bottom

Now for the most awkward part, while things are still floppy. Arrange the filters and cardboard together as if the unit is upside down, as shown in the first two pictures. Prop it up and try to get it as close to squared up as you can without it falling over. Per the first image, make sure that the corner of each side filter is nestled into the bend in the cardboard. For 1" thick filters—which in reality are about ¾" thick—the filter outer edges should match up with the cardboard edges.

(For the eagle-eyed: the bottom panel corner notches that you've already cut aren't in these pictures yet. They'll show up later.)

Look at one of the "bottom" edges, the nearest edge as seen in the third image. Cut a piece of tape to length, and tape around the edge, per the fourth picture.

Repeat on the other "bottom" edge, per the last picture.

If you are using filters thicker than 1" (not shown): This will go a little differently. ¾" of the filter's edge will overlap the cardboard, and the rest of the filter will hang out beyond the cardboard. You can make alignment easer by marking a line beforehand in the appropriate place on the edge of the filter.

Rather than the tape wrapping around 90° from the face of the filter to stick to the cardboard, instead you'll stick the tape flat along the overlap edge. As you go, be sure to tuck the tape over the thickness of the cardboard edge that you're covering, so that there isn't a big transition gap under the tape (which could peel apart later).

Step 9: Attach the Back

Flip the filter box assembly right side up. Look at the back, as seen in the first image. Cut a piece of tape to length for the top edge. Center the cardboard upper edge on the top filter edge, so that any misalignment is evenly distributed on both sides, and tape along the edge.

Also tape both side filter edges to the back.

If you are using filters thicker than 1" (not shown): Same story as in the previous step with the ¾" cardboard overlap onto the filter edge as well as the way to tape them together.

As seen in the last image, use some small pieces of tape to seal up the two upper corners.

Step 10: Power Cord Notch

If the power cord came separately from the fan, plug it into the back of the fan now. To make sure it doesn't wiggle loose over time, use string to tie the power cord to the fan grill (or use some other method of securing it). Of course make sure nothing will come loose later and get caught in the fan blades.

If the power control knob on your fan is on the back, now is the time to set it to the power setting that you want to use! You'll have to rely on plugging and unplugging the fan to turn it on and off. If you want to be able to change the power setting later, you'll need to cut an access hatch in the bottom panel (not shown).

Put the fan face-down on your workbench. Drape the power cord straight over the bottom of the fan. Test-fit the filter box onto the back of the fan, per the orientation in the first picture, while centering it side-to-side as best as you can. Note that the bottom panel is going to overlap the bottom of the fan, but the power cord is in the way. Mark the position of the power cord on the bottom panel. Then cut a notch about ⅝" into the bottom panel, enough so that the power cord isn't pinched anymore.

Step 11: Corner Gussets

These are not actually for strength, they're for sealing the corners where the fan is rounded.

Cut four right-angle gussets, measuring 2½" on the short sides. Tape them securely to the insides of the front corners of the filter box.

Step 12: Attach the Fan

Now for the big one. Set the fan on top of the filter box, with everything pointing up and with the power cord coming out through its notch. Nestle the bottom of the fan against the cardboard bottom panel overlap, and center the fan on the filters side-to-side as best you can. Tape the flat area of the bottom of the fan to the cardboard. (We'll get to the rounded corners of the fan at the end of this step.) Notch the tape for the power cord. Optional: add some pieces of tape around the power cord exit hole to seal it (it doesn't have to be perfect). If you have a Lasko fan, make sure the tape covers the slots where the feet would've gone.

Next, re-center the top of the fan on the filters, and tape the flat area of the top of the fan to the top filter, as seen in the fourth picture. Try to avoid any unsightly wrinkles.

Next, tape the flat areas on both sides of the fan to the side filters. As shown in the fifth picture, try to get it into the seam so that there's almost no gap under the tape.

If you are using filters thicker than 1" (not shown): Again, this will go a little differently than for 1" thick filters, but I'm sure you get the idea.

Finally, use pieces of tape to seal and dress up the corners.

Step 13: Mark the Exhaust Shroud

We're not done yet! The shroud is important for preventing back-flow around the tips of the fan blades. There's a pretty big airflow rate and efficiency boost from this.

Measure the width and height of the fan grill face, where the shroud will sit. (On the Utilitech fan example, it was 19⅛" square.)

If you'll be using a compass (not shown), match it to the fan grill's inside corner radius. Otherwise, find a circular object that's the same radius or a slightly larger radius, as seen in the first picture.

If your fan has its control knob on the front, find another object that's the outer diameter of the control knob area or slightly bigger. Also measure the distance from the center of the fan to the center of the knob.

Get your extra piece of cardboard. Decide which side is going to be the front. (In my example, I had some nice double-layer presentation board on hand, black on the front and white on the back.) On the back, mark off the rectangle or square that you measured to cover the grill, as seen in the second picture. It's OK to undersize it slightly, so that you can be sure it'll fit flush. Then use the compass or object to draw the round corners. Draw an "X" across the corners to locate the center.

If your fan has its control knob on the front, mark where its center should be, and then center up the compass or object and draw the circle. If the fan grill height and width aren't equal, keep track of the orientation and which corner the knob is supposed to be on; it's easy to get it flipped.

Now for the exhaust hole itself: make a simple oversized compass with some scrap cardboard and the two pens/pencils. The exhaust hole radius for peak performance will depend upon the type of box fan: about 8" for a Lasko; about 7" for a Utilitech. When you center the compass pivot, try to do it without punching all the way through the cardboard, since you'll be saving the middle piece for later. Then draw the exhaust hole. (In the example shown in the pictures, I instead used a yardstick as a makeshift compass, but a scrap cardboard compass works better.)

Step 14: Cut Out the Exhaust Shroud

Cut the exhaust hole, taking your time to make it neat. It's OK if the back face edge turns out a little bit choppy as you cut around the circle; the front face edge should turn out crisp. Then cut the round corners, as well as the control knob hole if you have one, and then cut the straight sides.

Flip it over, and the result is as seen in the first picture. Save the circular cut-out! You can use it as a cover for when you're not using the unit, so that dust doesn't get into the grill and fan.

Airflow smoothing bevel (optional): There's a tiny performance boost to be gained if you round off the "upwind" edge of the exhaust hole. To do this, cut a bevel at about 45° into the back surface edge. Make sure to not let the knife cut into the front surface edge, per the arrow in the second picture; wouldn't want to ugly it up on the outside. Then cut segments of tape and wrap them around the exposed edge as neatly as you can.

Cover edge bevel (optional): If you cut a bevel into the back edge of the cover, as seen in the last picture, it can sit gently centered in the exhaust hole. Alternately, you could attach some bits of cardboard around the perimeter of the back surface of the cover to make it align neatly into the exhaust hole.

Decoration (optional, not shown): It may be easiest to decorate the shroud now. Or you can wait until it's assembled.

Step 15: Attach the Exhaust Shroud

Whew, the finishing touch! Tape the shroud to the front of the fan. You can do this as simply or as ornately as you want. Making a seal actually isn't too important here, since the most important part is the exhaust hole and the cardboard area around it.

Lifter feet (optional, not shown): You could add taller feet to the bottom of the fan, so that when situated horizontally it's angled up a bit. This can be nice for not kicking up dust off the floor and for circulating more air higher up in the room.

Decoration (optional, not shown):Entirely up to you.

Step 16: Clean Your Air

Run your Universal Corsi-Rosenthal Box on as high a power setting as your comfort with the noise allows.

When burning candles, you may be better off just opening some windows, as candles produce quite a bit of soot that can fill up the filters quickly.

If your box fan is from before 2012, don't leave it running unattended. In rare instances, some fans from that era can pose a fire risk—they didn't have some of the safety features that modern fans do to prevent the motor from overheating etc.

Use case suggestions:

  • The air outside is smoky from wildfires, and you don't have central air with good filtration: In the room you're using most, open the window a tiny bit for new air to come in, so that it doesn't get stuffy. Place the Universal C-R Box near the window, situated horizontally so that it blows across the room away from the window. You may want to slightly open another window on the downwind side of your home, to keep a steady one-direction flow of clean air going through.
  • A visitor with mild pet allergies is coming: Brush your pet, thoroughly vacuum using a vacuum cleaner with filtered exhaust, wash anything relevant, and start the Universal C-R Box running in the living/dining room a while before the visitor arrives.
  • COVID-19 risk reduction in a classroom: In addition to other COVID-19 safety measures, make enough Universal C-R Boxes so that there's something like one for every normal-room-equivalent volume of air (or use a calculation model to make a more accurate estimate). Place them situated vertically in the middle of the classroom and distributed around.

Replace the filters after six months of use or so, depending upon how often you run the unit. If they start to look a little bit dirty, don't worry, they can still be effective for a while. If the filters are noticeably filling up, or if you can tell that the airflow seems weaker than when the filters were new, it's definitely time to replace them.