How to build a Corsi-Rosenthal box for wildfire smoke

scienceJul 16, 202627884

The Corsi-Rosenthal box is a do-it-yourself air cleaner built from a box fan and MERV13 furnace filters to reduce indoor wildfire smoke, virus-laden aerosols, pollen, and dust. Build guidance from Clean Air Crew and the Corsi-Rosenthal Foundation describes using 20x20x1 or 20x25x1 MERV13 filters and a standard square box fan; deeper 2-inch filters (three 20x20x2 plus two 16x20x2) improve surface area, airflow, noise, and filter life. Practical tips include adding a fan shroud or covered fan corners with cardboard or duct tape to close gaps and boost efficiency; tests found a 13.5, 14.25 inch shroud opening worked best with a Utilitech fan and 15 inches for Lasko. Some filter brands have tested poorly (Filterbuy MERV-13), while Filtrete 1900 and other recommended filters are commonly suggested; sources link to suppliers and full instruction pages. Modifications such as a fiberglass screen can keep pet hair off filters with minimal flow loss, and an alternate round-fan design exists for places without square box fans. Researchers have found the CR box performs very well on clean-air delivery charts, Johns Hopkins lists the design in its ventilation guidance, and experts note that well-constructed DIY cleaners with MERV13 or better are consistent with ASHRAE recommendations for improving indoor air delivery. Community posts emphasize the box as a low-cost, quick build (about 15 minutes) that people are using again during current smoke events and recommend it where commercial purifiers are unavailable.

Chris Person

@papapishu.bsky.social

Reupping this and the reminder that just a furnace filter on a standard Corsi-Rosenthal box is very capable if you can’t find get an air purifier for your house. aftermath.site/3d-printing-...

6146h ago

Chris Person52

Also, I know people I trust that swear by the Airfanta 3Pro. Collapses down to fit when you don’t need it, super energy efficient.

Hobo Joe5

Seconding cleanairkit for those who don't have a printer or just want something quick and cheap! It gets the job done very well. www.cleanairkits.com

Matatoes Lasagna1

Recently got a K2+ with CFS and the CFS just stopped turning on. They sent me a replacement motherboard and I had to spend 3 hours taking apart the thing to swap it out only for it still fail. Sighhh... probably should have just got the bambu.. This has nothing to do with filters I'm just upset

Joshua Dysart1

This is how they monetize air, by making it unbreathable without the need for their products.

Quoting this

Matt Novak53

This is what my friend did and it brought the AQI inside her house from 250 down to 19.

lydia3

i have built one of these for california wildfire season and it’s shockingly effective. enormously powerful tool that breaks down and stores flat.

Avery (they/them) 🔞 DISABILITY PRIDE ♿ | 🇵🇸 🇸🇩 🇨🇩

@averystorm.bsky.social

🔥 ⚠️ 🔥 ⚠️ 🔥 for folks dealing with wildfire (or other) air quality issues!

screenshot of FB post from Jodi LaCroix

Wildfire People!!
Friendly reminder that four MERV-13 furnace filters taped into a square with a box fan on top is a legitimate air purifier. It's called a Corsi-Rosenthal box. It moves more air per hour than a $300
consumer purifier and costs about $50 to build. The filters are $8-12 each at any hardware store and the fan is $20. When the filters get dark you tape on new ones.

406h ago

Ralf W

@raflw.bsky.social

For anyone in the huge smoke plume (MN to the East Coast), I'm intrigued by this DIY Corsi Rosenthal "wedge". I'd seen the CR boxes since the SF Bay area fires, but this seems more space efficient, uses the fan in the expected vertical orientation, and needs 2 v. 4 of the moderately spendy MERV 13s.

Simple pictogram illustration of how to assemble two Merc 13 filters, some cardboard and duct tape into a DIY home filtration system.

396h ago

MaxFunHQ

@maximumfun.org

If you're one of the unlucky folks currently breathing the air my dad described this morning as "hot, thick, and tan," a C-R box could help. It's (relatively) cheap and easy and the materials should all be at your local hardware store. Made one during the SoCal fires last year and it helped a ton.

233h ago

Goth Capo

@tinalope.bsky.social

Hey east coast friends. If the smoke is getting inside, you can build a corsi-rosenthal box for fairly cheap to help filter You can find other options here: corsirosenthalfoundation.org/instructions/ Love, A west coast seasonal wildfire veteran

Instructions to build a corsi-rosenthal box
More instructions on https://corsirosenthalfoundation.org/instructions/

195h ago

Wiredferret!

@wiredferret.bsky.social

Building a corsi-rosenthal box, installing an AC, masking up. We know how to do this. But we can still have grief about it.

262h ago

skáld

@danglinghemmie.bsky.social

another summer, another reason to post the corsi-rosenthal box fan air filter cleanaircrew.org/box-fan-filt...

185h ago

Nella

@nellachronism.bsky.social

In other news this bitch 100% works if you need powerful cheaper air purification asap corsirosenthalfoundation.org/instructions/

107h ago

Fun

@funkelly.bsky.social

For US folks, I got mine from @cleanairkits.bsky.social. It’s so quiet. www.cleanairkits.com

101h ago

Cara Martin (C. K. Kelly Martin)

@caramartin.ca

Jesus the air quality index for Windsor, Ontario is 614 right now. Please use a respirator if you need to be outdoors. Nobody should be rawdogging Ontario air. Indoors use an air purifier or build yourself a Corsi-Rosenthal box. Here's how: www.youtube.com/watch?v=RTRD...

Air Quality Index (AQI) Windsor, Ontario, July 16th at 11.30 AM. 614. Hazardous. https://www.iqair.com/ca/air-quality/canada/ontario/windsor

77h ago

Jacob Englander

@jacobenglander.bsky.social

Time to whip out your Corsi-Rosenthal kits! These are great air cleaners that are very easy to put together with filters that are very easy to replace and get! www.cleanairkits.com?srsltid=AfmB...

57h ago

are you not infotained

@alopex-ii.bsky.social

“Corsi-Rosenthal box” should be the name of a delicate scientific instrument for finding novel space particles, or an eldritch baseball stat for determining the perfect strike zone in four dimensions. It should not be the name for a bunch of taped-together Hone Depot merchandise

106h ago

Xavelrie | The Walker, Nasz'mara 🏳️‍⚧️

@xavelrie.bsky.social

For everyone affected by the wildfire smoke right now, this is a good thing to make on a budget from hardware store parts. In case you don't have an Air Purifier already, this could really help out. corsirosenthalfoundation.org/instructions/

48h ago

✨ Halley Starbun ΘΔ ✨

@halley.systems

For anyone in the smoke enveloping parts of the US/Canada and without access to air filtration devices, there's one you can make yourself for as little as $50 provided you can find some spare cartridges. It's called a Corsi-Rosenthal Box, you can make it in as little as 15 minutes. Stay safe. 💙

This image is of the Corsi-Rosenthal Box air filter, an inexpensive unit designed during the COVID-19 pandemic, to improve ventilation in indoor spaces.

51h ago

Mike Wiser

@drmikewiser.bsky.social

If you don't have a separate air filter, and don't have the supplies to make a Corsi-Rosenthal box, even just using a good filter on your HVAC system and running the fan will help. Don't let the perfect be the enemy of the good. Doing things poorly is better than not doing them at all.

51h ago

3 sources

corsirosenthalfoundation.orgHow to build your own Corsi-Rosenthal Box - Corsi-Rosenthal Foundation engineering.ucdavis.eduScience in Action: How to Build a Corsi-Rosenthal Box cleanaircrew.orgDIY box fan filters – Corsi-Rosenthal box - Clean Air Crew

How to build a Corsi-Rosenthal box for wildfire smokescience National Academies back extreme‑event attribution sciencescience

How to build a Corsi-Rosenthal box for wildfire smoke

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