DIY build · About 40 minutes
The Corsi-Rosenthal box: a build guide that cites its numbers
A box fan, four furnace filters, and tape — invented in August 2020 by engineering dean Richard Corsi and filter manufacturer Jim Rosenthal, and since validated in peer-reviewed testing. Here's the correct build, the data, the safety facts, and the honest reasons you might buy a purifier instead.
Smoke-particle CADR, novice buildsPeer-reviewed
184–233 CFM (313–396 m³/h)
Seven first-time builders, same instructions, fresh-smoke challenge — within 8% of each other, ahead of many commercial units.
Parts list (~$60–$100 total)
| Part | Spec that matters | Typical cost |
|---|---|---|
| Box fan, 20″ | UL or ETL listed; 2012-or-newer design (fused plug). Any major brand. | $20–30 |
| 4 × pleated filters, 20x20x2, MERV 13 | MERV 13 is the spec — it's what catches smoke-size particles. 2″ depth balances surface area and price; 1″ works, 4″ works better and costs more. | $40–70 |
| Tape | Painter's tape or duct tape — anything that seals seams. One roll. | $4–8 |
| Cardboard | The fan's own shipping box is exactly the right size for the base and shroud. Free. | $0 |
Disclosure: the links below are affiliate links — we earn a commission if you buy through them, at no extra cost to you. It never changes what we recommend. Search listings: 20″ box fans · 20x20x2 MERV 13 filters. Filter-subscription services (Filterbuy, FilterTime and similar) sell the same sizes with auto-delivery.
The build, in seven steps
- Check the airflow arrows first. Every filter frame has an arrow. All four must point inward, toward the center of the cube — the fan pulls air through the filters and exhausts it up. Backwards filters are the #1 build error and quietly halve performance.
- Tape the filter walls. Stand the four filters on edge in a square, arrows in, and tape each vertical corner seam airtight. Air is lazy; any gap becomes the path of least resistance.
- Add the cardboard base. Trace the cube on cardboard, cut, tape across the bottom, seal all four edges.
- Mount the fan on top, blowing up. Tape the fan's edges to the filter tops and seal the corner gaps. Exhaust goes out the top; intake is through all four filter walls.
- Cut the shroud — the free performance upgrade. A cardboard square the size of the fan face, with a round hole matching the blade sweep (about 14–15″ for most 20″ fans), taped over the exhaust side. It stops air short-circuiting back through the fan's corners; community testing that optimized shroud diameter found a meaningful airflow gain for zero dollars.
- Seal and date it. Final tape pass on every seam, then write the build date on the side — future-you will not remember when the filters went in.
- Place and run. Floor placement, a few feet of clearance on all sides, highest tolerable speed. Cleaning rate scales with fan speed; on high it's loud — measured up to ~67 dB on some fans — so medium overnight is the usual compromise.
What the testing actually shows
Three independent lines of evidence, which is more than most commercial purifiers offer publicly. The original peer-reviewed characterization (Dal Porto et al., Aerosol Science and Technology, 2022) measured clean-air delivery beyond the 450 CFM ceiling of the standard appliance test for larger particles, from a build whose filters and fan cost under $60. An Illinois Institute of Technology chamber test of a five-filter MERV 13 box measured CADR of 166 CFM for smoke-size particles, 321 for dust-size, 464 for pollen-size — efficiency rises with particle size, exactly as filter physics predicts. And the 2024 seven-novice study answered the "sure, but can normal people build one" question: yes, within 8% of each other, at 184–233 CFM against real smoke.
For context, that smoke-range performance is competitive with name-brand HEPA purifiers in the $200–$500 class. The box wins on physics: MERV 13 captures a smaller fraction per pass than HEPA, but four large filters and a big fan move several times more air, and total cleaning rate — efficiency × airflow — is what your lungs experience.
Safety, plainly
The reasonable worry — "is strapping filters to a fan a fire hazard?" — has been tested: independent safety research ran box fans with attached filters and found motor temperatures stayed within safety limits. The practical rules are the same as for owning a fan at all: use a modern UL- or ETL-listed unit (2012-and-newer designs include a thermal-fused plug), don't run damaged cords, keep it off deep carpet that blocks the base, and retire anything that smells hot. Two non-fire notes: the cube is light and tippable, so place it where kids and dogs won't ride it; and it filters particles, not gases — carbon monoxide and VOCs sail through, so it is never a substitute for CO alarms or ventilation.
Maintenance: when to rebuild
Filters load up and airflow falls. Practical cadence: rebuild every 3–6 months of regular use, sooner after a heavy smoke event or when the filters look uniformly dark. (Real-world testing of boxes run continuously found useful electret-media performance over months of service — the media doesn't fall off a cliff, it fades.) The fan is reusable indefinitely; a rebuild is just $40–70 of filters and twenty minutes. That recurring filter purchase is also where a subscription service can make sense if you'd otherwise forget.
Variants worth knowing
Five-filter box: a filter replaces the cardboard base — more media area, slightly higher cost, the version several lab tests used. Mini CR box: a 9″ box fan plus 10x10x2 filters, recently tested by Rosenthal's own shop with respectable per-speed numbers — right-sized for nurseries, offices and hotel rooms. PC-fan builds: quiet computer fans on a filter frame; much lower noise, more cost and fiddliness — the enthusiast's path. When to just buy a purifier: you need quiet above all, the room is small, aesthetics are non-negotiable, or you want gas/odor removal (look for a unit with a real carbon bed, measured in pounds not "pellets").
Common questions
Can I use MERV 11 filters instead?
It'll work, with reduced capture of the smallest particles — smoke is exactly where MERV 13 earns its keep. If MERV 13 is sold out (common during smoke events, which is why building before the season is smart), MERV 11 is a legitimate stopgap. See the MERV guide for what each rating captures.
What room size does one box cover?
Using the standard sizing convention (CADR × ~1.55 ≈ square feet at normal ceiling height), the measured smoke CADRs put one box around 285–360 sq ft for solid performance — a master bedroom or living room. Bigger space or worse air: run it higher, or build two. They're $75.
Filters point in or out?
Arrows on the filter frames point in, toward the fan. The fan exhausts up and out. If you've built it backwards, the fix is re-taping, not buying new filters.
Does it help with viruses?
It removes particles in the size range that carries respiratory viruses, and that's why CR boxes spread through classrooms from 2020 on. Filtration is one layer — it supplements ventilation, it doesn't replace it, and no air cleaner makes a crowded room risk-free.
Sources
- Dal Porto et al., Characterizing the performance of a DIY box fan air filter, Aerosol Science and Technology (2022)
- Evaluating DIY air cleaner variability… during wildfire events, Indoor Environments (2024)
- IIT chamber test report — C-R box with MERV 13 filters (2021)
- Tex-Air Filters field testing of CR boxes in continuous service; mini CR box test data (2026)
DIY at your own judgment: this is general information, not professional advice. Electrical products should be listed, undamaged, and used per their instructions; people with health conditions affected by air quality should follow their clinician's guidance.