Sleeker and Even Quieter: PC Case Fan Filtration Box

It's like a Corsi-Rosenthal box, but instead of being a square with a 20" box fan, it can be built with six PC case fans (typically used to cool a computer), so it's just 7" wide! Even more important for classrooms where teachers understandably unplug the HEPA unit because it's SOOOO loud, this unit is super quiet. AND, because it's smaller and quieter, it's easier to sneak into a class if your board won't let you have them!! (In one study, these were quieter (42 vs 52 decibels) and used less energy (10 vs 39 watts) than the original CR box. They both outperformed a HEPA unit.)

Rob Wissmann did a bunch of tests on different configurations and the best bang for your buck is six fans, three on one side and three on the adjacent / perpendicular side. (CADR = clean air delivery rate)

You can buy a kit, but they're over $300, some assembly required, and not the best configuration (five on one side). In Canada, North Box Systems sells a kit without the fans and filters for $179 or fully assembled for 279 (but with only five fans in that bottom configuration).

Or you can build your own for about $180 in total (based on costs found online today). 

Unlike the Corsi-Rosenthal box, which just requires a box fan, four filters, and some duct tape (all for under $150) and NO tools except some scissors, this is a bit more extensive, but it's still pretty easy to do yourself if you have a drill and a bit of know-how with tools. 

Warning - lots of amazon links below. It's the cheapest place to find all this (because of all the exploitation along the supply chain!). 

1. You need six PC case fans -- look for fans that are "daisy chained" to avoid having to connect them together. You can get sets of three 12cm for $26.77 (x 2 = $53.54). If you have children in the house, you might also want grills on the fans.
2. Two filters - 1x20x20 - MERV 13 or Filtrete 1900. Because there's only two instead of four, they should be changed every three months instead of every six months - so there's no savings in filters. You can get 12 for $147.60 (/6 = $24.50 for two). 
3. The fans have to be screwed to something - four pieces cut to 20.5" x 6.75" (length of filter + thickness of panel x width of fan + filters). The cheapest option would be a 1x8x8 piece of rough pine barn board, but possibly an easier DIY is melamine wood or MDF panel - a 2'x4'x1/4" is $12.99 at Home Hardware. Many hardware store will make the cuts for you on a table saw for $1/cut (x 5 = $5). To cut the holes, it helps to get a circular cutting tool for $16.99 OR a hole saw for $18.88.
4. Corner pieces of wood to join it all together (if not using barn board) might be available for free at a hardware store. You'll need 4 pieces, 1.5x1.5x6.75, one for each corner - a 2x2x8 will be plenty.
5. Fan connectors: A fan splitter cable - $9.29, and adapter - $18.88 - to connect the fans to a plug. 
6. Fasteners: You need 24 (4 x 6 fans) bolts, nuts, and washers, which you might be able to get individually at a small hardware store. Otherwise you'll end up getting them in a box of 100 for $28.29. Share with friends! Then some 3/4" screws and duct tape to put it together for $12.96.

Here are very general instructions so you can get the sense of how it all goes together. There are much more detailed instructions from Matthew Azevedo or  Joey Fox or NB_Covid_Info, but I find they're too detailed and have too many tips and other bits of info along the way that makes it all a bit convoluted for me to follow and makes it seem way too hard to do myself! Here are the main steps, short and sweet:

1. 
   Cut holes - three down the middle of two of the panels, evenly spaced apart. Most of the links to directions spend a lot of time on how to do this. Use a compass or template. Measure twice; cut once.
2. Screw the fans to the panels: drill some pilot holes in the four corners for each fan, and bolt them in with the bolt heads on the outside where the fans blow out. If you're adding grills to the fans, add them now on the outside of the panels using the same bolt through both the fan and grill.
3. Screw the four panels together: screw corner blocks to two of the panels, one with fans and one without. Then attach the other two panels to them.
4. Drill a cord-sized hole near the bottom of the vertical non-fan panel (that is, not the bottom panel) for the cord to go through.
5. With the four panels on their side, plug the fans together to the splitter cable and adaptor, and run the cord out through the hole. Plug it all in to make sure they work!
6. Pop the filters on either side, tape them in with duct tape, and Bob's yer uncle!!

You can also sign up for a FREE workshop on building them tomorrow, online, from 11am-2:30pm. Here's their blurb:

"Join our experiential learning and do-it-yourself air purifier workshop, where you'll learn about indoor air quality and filtration! Dr. Amy Li, Assistant Professor in Civil and Environmental Engineering, will start us off with a presentation and speak to indoor air quality issues in schools and school buses, and air cleaning technology including upper room UV. Ryan Tennant, Ph.D. candidate [in Systems Design Engineering], will lead the workshop to build a PC fan air purifier. After a break for lunch, Ontario School Safety's chair, Kate Laing will present and speak about Corsi-Rosenthal boxes and their use in air filtration in indoor spaces. Special guest, grade 11 student Shiven Taneja will lead a community CR Box build - with material for 12 boxes donated by Donate A Mask."