Build a Corsi-Rosenthal Box Air Cleaner for the Maker Shop: Fine Dust Filtration on a Budget

Fine dust from sanding, MDF cutting, CNC routing and laser work stays airborne for hours after the machines stop, and dust collectors at the tool do not catch all of it. A Corsi-Rosenthal box is a cheap, low-tech air cleaner that pulls room air through MERV 13 filters using an ordinary box fan. It was originally developed for indoor air quality, and it is widely used in workshops as a way to clean up the fine particles that escape the main dust collection. This guide covers how to build one and where its limits are.

What It Does and Does Not Do

DoesDoes not Capture airborne fine dust such as sanding dust and wood dust in the airReplace at-source dust collection on a router, saw or CNC Run cheaply and quietly at a low fan speedRemove fumes, smoke odours or VOCs (there is no activated carbon) Use inexpensive standard filtersMake combustible dust safe or prevent dust explosions

For laser smoke and resin fumes, you need a fume extractor with activated carbon, vented outside where possible. This box handles particles, not gases.

Safety

• Use a modern box fan that carries a current safety listing and a thermal fuse, and plug it in only when it is in good condition. Cheap or old fans are a fire hazard when run for long periods.
• Do not run it unattended in a dusty, hot area for days on end unless you are comfortable with the fan's condition. Check the motor for heat and dust build-up periodically.
• Do not use it for collecting metal or combustible powders, or in an atmosphere with flammable vapours, because the fan motor is not rated for hazardous locations.
• Wear gloves and a dust mask when changing dirty filters, and dispose of them in a sealed bag.

Assembly

1. Prepare the filters. Four filters form the sides of the box and the fifth is the base. Check the airflow arrow on each filter: air should flow from outside to inside.
2. Cut the cardboard. Trim a piece of cardboard the size of the fan (about 20 by 20 inches) to use as the lid or base. Some builds use it as a shroud on the fan side to prevent air recirculating.
3. Tape three filters into a U shape. Line up the edges, arrow pointing inward, and tape the joints on the outside.
4. Add the fourth filter to close the sides, forming a square tube.
5. Place the fifth filter as the top or base. In the classic build, the fifth filter is the base, and the fan sits on top blowing air upward out of the box. Lay the fan on top, with the airflow arrow pointing away from the box.
6. Seal the fan to the filters. Tape all around the perimeter of the fan and the top edge of the filters, using a cardboard shroud if the fan's outline is smaller than the filters. Gaps let air bypass the filters and reduce performance.
7. Test it. Turn it on and check that air is being drawn through the filters, not leaking at the seams. Hold a thin piece of paper to a filter face to confirm suction.

Placement and Use

SituationRecommendation General shop cleanupPlace near the middle of the room, raised off the floor on a stand or bench Near a sanding stationPlace near the work, slightly behind, to catch dust that escapes the tool extraction After a dusty jobRun on high for 30 to 60 minutes after work finishes Long-termRun on low with a timer for fixed periods

Maintenance

• Fit a cheap MERV 8 pre-filter on the outside of the box to catch the coarser dust. Replace it more often than the main filters.
• Replace the MERV 13 filters when they are visibly dirty or when airflow drops. In a dusty shop that may be every few months.
• Vacuum the fan's blades and grille when you change filters.

Troubleshooting

ProblemCauseFix Weak airflowFilters clogged or fan on lowReplace filters, increase fan speed NoisyFan or filter resonanceSet on rubber feet, lower the speed Dust visible around the fanAir leakingRe-tape seams and the fan edge Motor hotBlocked or dirty fanClean, replace filters, stop if it stays hot

Closing

The Corsi-Rosenthal box is a low-cost supplement to good dust collection, not a substitute. Build it in under an hour, keep the seams sealed, and change the filters regularly. It will not fix fumes or hazardous dust, but for keeping the fine, hanging dust in a maker shop under control, it is one of the best value additions you can make.