By Martin Harris.

DIY Shop Air Filter: The Box Fan Build

The best DIY shop air filter is a Corsi-Rosenthal box: four 20 by 20 inch MERV-13 furnace filters taped into a cube, a cardboard bottom, and a 20 inch box fan on top blowing air out, with a cardboard shroud around the fan. In EPA testing, that four-filter design cleaned air at a rate of about 401 cubic feet per minute. It slowly pulls fine dust out of your shop air after you make it. It does not replace a dust collector at your tools or a respirator on your face.

What a Corsi-Rosenthal box is

Engineers Richard Corsi and Jim Rosenthal came up with this design in 2020 as a cheap way to clean indoor air. Since then, the EPA, UC Davis, and other research groups have tested it. Most of that research looked at smoke and small airborne particles, which are the same size range as the fine wood dust that hangs in a shop.

Two terms help here. CADR (clean air delivery rate) is how many cubic feet of particle-free air a cleaner delivers per minute. It accounts for both airflow and how well the filter catches particles, so it is more useful than a plain fan CFM number. MERV (Minimum Efficiency Reporting Value) is a filter rating from the ASHRAE 52.2 test. MERV 13 filters are in the group rated for particles from 0.3 to 1 micron. The EPA recommends MERV 13 filters for these builds to catch very small particles.

Here is what the EPA measured when it tested different versions of a box fan filter:

Source: US EPA Office of Research and Development, Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors. Values are averages with test variation of plus or minus 1 to 31 cfm.

Two things stand out. The cardboard shroud alone raised CADR by about 40% on the single filter design, at no extra cost. And adding filter area pushed the numbers way up. A 2022 UC Davis study by Dal Porto and colleagues, published in Aerosol Science and Technology, tested the original five-filter version (filters on four sides plus the bottom) and reported a CADR of roughly 600 to 850 cfm, depending on fan speed. The four-filter cube in this guide is easier to build and still moves a lot of clean air.

Materials

Filter and fan guidance from the US EPA DIY air cleaner research page. Original filter thickness per published descriptions of the Corsi-Rosenthal design.

Build steps

1. Unbox the filters and find the airflow arrow printed on each frame.
2. Stand the four filters on edge to form a square tube, with every airflow arrow pointing into the middle of the tube.
3. Tape the four vertical corner seams, inside and out, so no air can sneak between the frames.
4. Lay the 20 x 20 cardboard on one open end of the tube and tape it down on all four edges. This is the bottom.
5. Set the box fan on the open top end, with the fan's airflow direction pointing up and away from the filters. The fan pulls air in through the filters and pushes clean air out the top.
6. Tape the fan housing to the filter frames on all four sides, sealing any gaps.
7. Place the 22 x 22 cardboard over the fan's outlet side. Trace a circle just a bit larger than the fan blade diameter, centered on the fan hub.
8. Cut out the circle and tape the cardboard to the fan so it covers the four corners. This is the shroud. It stops air from looping back around the blades at the corners.
9. Plug the fan straight into a wall outlet and turn it on. Hold a tissue against each filter face. It should pull in on all four sides.
10. Write the build date on a filter frame with the marker, so you know how old the filters are.

Hanging vs floor placement

On the floor or a bench, the box is easy to move to wherever you are working. Parking it a few feet from your sanding station or assembly table lets it pull fine dust near where you make it. Keep it out of the chip stream from your table saw, planer, and bandsaw, because big chips load MERV 13 filters fast. Also keep it out of your walking path and away from the saw's kickback zone.

Hung from the ceiling, the box saves floor space and stays clear of chips, and it keeps working on the whole room while you move around. The tradeoff is that it is harder to reach for filter changes, and it has to be secure. Anchor into ceiling joists, not drywall, and support the cube from underneath with a strap cradle rather than hanging it by the tape or the fan handle. Hang it so a person cannot walk into it, and keep the fan controls reachable or plug it into a switched outlet.

Either way, give the filters room to breathe. Keep at least a foot of open space on each filter face, and do not push the box against a wall.

What it can and cannot do

A room air filter works on dust that is already floating in the air. It does nothing for the plume of dust coming off a sander or saw blade at the moment you cut. OSHA treats capture at the source as the main control for wood dust, and room filtration only as a supplement. Your first dollars should go to collection at the tools. My page on how to reduce dust in a woodworking shop lays out the order I would tackle things in.

Room size tells you how fast it can work. Say your shop is a 20 x 20 foot garage with an 8 foot ceiling. That is 3,200 cubic feet. At the EPA's measured 401 cfm, the box delivers the equivalent of that room's volume in clean air about 7.5 times per hour (401 times 60, divided by 3,200). Real dust removal is slower than that math suggests, because clean air mixes with dusty air and dust keeps getting stirred up.

Keep in mind that the EPA and UC Davis tests used smoke and lab particles, not wood dust. Wood dust has more large particles, which will clog the filters sooner. Expect to change filters more often than smoke-season guides suggest.

DIY CADR from US EPA testing. Ceiling cleaner features are general for the category.

A ceiling unit like the JET AFS-1000B is rated by JET at 1,000 CFM of airflow. That is a fan airflow figure, so it is a different measurement than the EPA's CADR for the box fan filter, and you cannot compare the two numbers directly. What the ceiling unit gives you is a cleaner install, a timer so it keeps running after you leave, and a cabinet that holds up to years of shop life. If you want to compare commercial units, see my best air filtration systems for woodworking. For most garage shops, I think a box fan filter is a smart first step, and many people end up running both.

Fan safety

Pulling air through filters makes a fan work harder, so people reasonably worry about overheating. Underwriters Laboratories tested five box fan models with this kind of filter setup for the EPA, including loaded filters and fully blocked airflow for seven hours. None caught fire, and all measured temperatures stayed below safety limits. The EPA still gives clear rules:

• Use a fan made in 2012 or newer. The EPA says those models added fused plugs and thermal cutoffs.
• Look for a UL or ETL safety mark on the fan.
• Do not use an extension cord. Plug straight into a wall outlet.
• Do not run an older fan unattended. If you must use a pre-2012 fan, turn it off when you leave the shop.
• Keep the motor clean. In a wood shop, dust settles on the fan housing and motor vents. Unplug it and vacuum it off regularly.
• Unplug before changing filters and never reach near the blades while the fan is on.
• Stop using a damaged fan, including one with a cracked housing, a bent guard, or a nicked cord.
• Keep working smoke detectors in the shop and the house.

Filter changes

When the filter faces turn dark and the tissue test barely pulls, the filters are loaded and airflow has dropped. Change all four at once so the box pulls evenly. Wear a respirator when you pull the old filters, since they hold the same fine dust you built this to catch, and bag them before you carry them out. Even with the air cleaner running, wear a respirator for sanding and other dusty work. My guide to respirators for woodworking dust explains what to buy, and is sawdust dangerous explains why fine dust is worth the trouble. For the whole system, start with my complete guide to small shop dust collection.

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