Air Cleaner That's Quiet & Space-Conscious: the "V C-R Box"

A "Corsi-Rosenthal box," consisting of a box fan with a cardboard-shrouded exhaust pointed upward, 4 MERV 13 furnace filters on the sides, and a bottom cover, is the quintessential DIY air purifier for medium-sized indoor spaces: it's cheap, made of common components, and easy to assemble. (See my prior Instructable for a version with 3 filters.) It is therefore justifiably a popular way to address matters of indoor air quality: wildfire smoke, exhaled breath mist potentially carrying contagious disease, allergens, and all kinds of harmful particulates.*

C-R boxes can actually perform better than HEPA air purifiers when one considers the true limiting factor on throughput: noise. A HEPA air filter captures pretty much all particulates in one pass, which is nice but means that it creates a large pressure drop for the fan to work against. MERV 13 furnace filters instead capture about 70% of fine particulate matter per pass, but they create much less of a pressure drop, especially when several filters are used in parallel, as in a C-R box. At the same noise level, the air in the room can pass through a C-R box more than once in the same amount of time, such that overall the "clean air delivery rate" (CADR) can actually be higher than that of a HEPA air purifier. (MERV 13 filters are in a "sweet spot" for this—you wouldn't want to go much coarser in filter rating, or else the filtration of fine particulates drops off.) Note that it's a problem to turn a HEPA air purifier down too much, especially in larger spaces, because if it's not stirring up and cycling through all of the air in the room, it's not really that effective. That's not to disparage HEPA air purifiers, of course! It's just a question of relative performance given noise and cost constraints.

All of that said, C-R boxes do have their drawbacks:

1. Noise: A box fan in an elevated position, even on low, makes a fair amount of noise. On medium, it's not really tolerable in an otherwise quiet space, maybe OK in a noisy space.
2. Efficiency: The fan rotor isn't really designed to work against the pressure drop of a filter, so the blades are operating off of their optimum conditions.
3. Bulk: The 20 x 20" footprint of a box fan neatly matches a common furnace filter dimension of 20", which is very convenient for assembly, but in one's living space, a C-R box takes up a lot of room. (It actually ends up at 20 1/2" square, due to the filters' corners overlapping.) That goes especially because the filters need substantial clearance away from walls and solid furniture for them to be able to draw in air effectively. Also, for transport, it's awkward to carry, and it's tough to fit one through the door of a typical car without having to disassemble it.
4. Aesthetics: Box fans, being extremely economical, may not exactly be the finest in home decor.

So, if you're willing to spend somewhat more on a higher-quality, quieter, more compact fan, plus use a few woodworking tools, you can optimize the configuration! Hence this Vornado fan C-R box, the V C-R Box:

1. Quiet: More tolerable on medium, whisper-quiet on low.
2. Efficiency: Deeper blades that may not be as sensitive to having a pressure drop imposed on them as with a box fan's blades.
3. Compact: The 14 1/2" square footprint fits into a room much more comfortably, while the taller height means that the filter area is actually increased. Also the heavy fan with its stock handle travels separately from the lightweight filter box, very convenient.
4. Aesthetics: Slender, tidied-up look!

* Electrostatic filters such as furnace filters are not effective against gases, as gases are molecular and would take an activated carbon filter with substantial mass to address.

Materials:

1. 14 x 30 x 1" MERV 13 furnace filter, quantity 4: If preferred, you could alternatively use a shorter height like 24" instead of 30"; generally the price is the same though. Some stores offer discounts for buying several at a time like this.* Note that for assembly purposes, compared to the listed size, the actual filter is 1/4" undersized in all dimensions.
2. Vornado 279T tilting panel fan: This fan is about as good as it gets off-the-shelf in terms of ease of conversion, size, quality, ability to handle some pressure drop from the filters (due to its deep blades), "throw" (the persistence of the outflow stream across a room, to stir up more air), and looks, at a not-too-outrageous price, $80 as listed by the manufacturer. The one big problem is availability, as this model may have been discontinued, so you might have to resort to eBay or something. Note that there's also an identically-sized 279TR model with timer and remote control features, which is the one shown in these directions. Conversions of other fans like the Vornado 753 are also feasible but can require more effort.
3. 14 1/2" x 14 1/2" plywood square piece, quantity 2: 3/8" thick is about right, or you can go thicker. If you go thinner, or if it's poor quality wood that ends up being fragile once the top piece is cut to shape, you'll likely need to reinforce it with a frame made of 3/8" thick ~1" wide strip wood plus flat head wood screws and glue. For the bottom piece, you could substitute cardboard and 2" wide packing tape (or duct tape).
4. Foam rubber window seal/weatherstripping: Anything similar to the profile shown in the photo should work OK. You'll need a bit over 3' of it.
5. Masking tape: Regular or painter's tape, for assembling the filters. Or use something decorative!

Tools:

1. Cross-head screwdriver with a long shaft, also optionally an electric screwdriver
2. Jigsaw, also optionally blocks or sawhorses and/or clamps to support the work piece, plus safety glasses, hearing protection, and a dust mask
3. Drill and a drill bit big enough for the jigsaw blade to fit
4. Chisel ~1/4" wide and a hammer
5. Sanding block and sandpaper
6. Ruler, also a carpenter square if the plywood pieces aren't cut out yet
7. Pencil
8. Large compass, or scrap cardboard plus another pencil or an awl to make a cardboard compass
9. Knife for cutting cardboard

* As a modification to potentially save money, you could just get 3 filters and make a cardboard blank to replace the 4th side, especially if you intend to put the V C-R box against a wall. Compared to an air purifier that just uses 1 filter, going to 2 and then 3 filters improves throughput a lot, but when it comes to that 4th filter, you start getting diminishing returns.

Unscrew the base, both to separate its halves and at the pivots. Dump all of the pivot pieces out. (You can save that stuff if you want to be able to convert it back later.)

Mark the center of one of the plywood squares. From there, draw a circle 5 15/16" in radius (11 7/8" diameter), using either a compass or a cardboard compass. Parallel to one of the sides of the square (note the wood grain direction), draw a flat 3 1/2" across.

Taking all appropriate safety precautions, drill a hole in the interior to make a place for the jigsaw to start, and then cut out the middle as shown.

Check that the back of the fan fits (it won't be able to seat all the way at this stage). Sand down any irregularities in the jigsaw cut, and sand all edges to prevent splinters.

(Ignore the filters and weatherstripping in the images for now; those are for later.) Again check the fit of the fan, and mark where its two pivot lobes hit the plywood. Chisel out divots to clear the pivot lobes. When done, the fan should rest on its handle and the opposite two corners, as shown.

Chisel a notch in the edge of the circle for the power cord to fit through. Sand down rough corners.

If the plywood top piece is flimsy, you'll want to reinforce it. Cut 3/8" thick strip wood into 4 pieces 12 7/16" long each. Using "pinwheel symmetry," glue and screw them into a frame, making sure that it's evenly square, as there's not much room for error here. Pilot holes will help with the screws, of course.

Sand the corners and lower edges round as shown in the second image.

Carefully center the frame on the back of the plywood top piece (opposite side from the fan pivot lobe divots), making sure that it won't later interfere with the fan housing on the inside and the 3/4" wide filters around the outside. Then glue and screw the frame into place.

Again check the fit of the top piece to the fan, noting the clearance to the circle and flat as well as the slope of the fan housing. Clean off any remaining sawdust from the circle and flat. Starting and ending at the power cord notch as seen in the image, gently apply the weatherstripping around the circle and flat, taking care to lay down the weatherstripping without stretching it.

Then check the fit of the fan once more. Ideally the weatherstripping should just graze the fan housing without rolling over the plywood edge. That said, a perfect seal is not required. Remove the fan, and squeeze the weatherstripping's adhesive into place permanently.

If you're using a cardboard square for the bottom rather than plywood, apply packing tape all around the cardboard piece's edges. This will act as a reusable surface for the masking tape to land on during assembly.

Use masking tape to assemble the 4 filters to each other, pinwheel symmetry-style as seen in the first image. Make sure that the airflow direction arrows on the edges of the filters all point inward, per the second image.

Then tape on the bottom.

And then tape on the top. You can just tack it in place as shown and rely on the weight of the fan to make a seal, or else go ahead and fully tape around the top edges.

Place the filter box where you want it to go. Drape the fan power cord into its notch, and drop the fan into place—it simply sits there under its own weight. Now it's all set to plug in and run!

Put it on speed 2 for lots of throughput if you can handle the noise level, or else put it on 1 to make it whisper-quiet.

Filters under normal usage typically last 6 months to a year. If they start to look kind of dirty, don't worry, they'll still be effective for a while. If the filters are noticeably filling up and turning a gross gray-brown color, or if you can tell that the airflow seems weaker than when the filters were new, then it's definitely time to replace them.