Corsi-Rosenthal Box Air Purifier

DIY HEPA-grade air filtration using a box fan and furnace filters

Total Cost

$65

Retail Equivalent

$300+

Build Time

30 min

CADR Rating

800+

Video Tutorial

What Is This?

The Corsi-Rosenthal Box (CR Box) is a DIY air purifier invented during COVID-19. It's named after Richard Corsi (UC Davis) and Jim Rosenthal (CEO of Tex-Air Filters) who popularized the design.

⚠️ This Actually Works

Independent testing shows CR Boxes achieve CADR ratings of 600-800+ CFM - better than $300 commercial HEPA purifiers. Universities, schools, and hospitals use these. The science is solid.

Why It Works So Well:

Massive filter surface area - Four 20x20 filters = 1,600 square inches vs typical purifier's 100-200 sq in
High airflow - Box fans move 1,500-2,000 CFM
MERV-13 filtration - Captures 90%+ of particles down to 0.3 microns (including smoke, pollen, viruses)
Low noise - On medium speed, quieter than most purifiers
Cheap to run - Box fans use 50-100 watts vs 200+ for commercial units

Materials & Cost

Item	Qty	Est. Cost	Where to Buy
20" box fan (any brand)	1	$25	Amazon Lowe's
MERV-13 furnace filters (20x20x1)	4	$40	Amazon Lowe's
Duct tape or cardboard + tape	1 roll	$5	Lowe's Amazon

Total: ~$65-70 (vs $300+ retail HEPA purifier)

💡 Filter Replacement

Replace filters every 3-6 months depending on use. Replacement cost: ~$40 vs $50-80 for commercial HEPA filters. You're still saving money long-term.

Step-by-Step Assembly

Check Filter Arrow Direction

Each filter has an arrow showing airflow direction. They ALL must point inward (toward the center of the box). This is critical - wrong direction = doesn't work.

Create the Box

Stand four filters on their edges in a square. Tape the corners together with duct tape, making sure arrows point inward. You should have a box shape with open top and bottom.

Attach the Fan

Place box fan on top of the filter box. Fan should blow air UP (pulling through filters). Tape fan securely to top of filter box. Seal any gaps with tape.

Optional: Add Cardboard Shroud

Cut cardboard to create a shroud around the bottom, directing airflow through filters. Not required but increases efficiency by 10-15%.

Test It

Turn on fan to medium speed. Hold tissue paper near filter edges - should be pulled toward filters. No tissue movement = air leak, add more tape. Run on medium 24/7 for best results.

Performance Tips

Use MERV-13, not higher - MERV-14/15 restrict airflow too much for box fans. Stick with MERV-13
Run on medium speed - High is loud and uses more power without much benefit
Size your room - One CR Box handles 500-700 sq ft effectively
Place in center of room - Not against walls. Needs air circulation around it
Multiple units - Large rooms benefit from 2+ units
Quieter option - Use a variable speed box fan on low for bedrooms

When to Use This

Wildfire smoke (proven during CA/OR fires)
Allergies and asthma
COVID or flu season (reduces airborne virus)
Pet dander
Cooking odors and smoke
New carpet/paint off-gassing
Workshop dust (sawdust, drywall)

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