With the recent wildfires and the gestures broadly at the world I just wanted to share this link to the Corsi-Rosenthal Box. A simple homemade air filter that can be made with a box fan or several computer fans using off the shelf merv 13 or higher filters from your local store.
Yes, these have existed since before COVID. I had someone elsewhere get made at me because “it shouldn’t be named that! Those two didn’t invent it!” but like, I didn’t choose the name lol. Anyways, I’ve made two for my house (former hoarder house we inherited and we haven’t had the ducts cleaned yet but everything else is clean) and I can say, we both sleep better at night and they worked great through the wild fires.
Anyways, just wanted to share this as a quick little easy project. Or you know, not so quick if you wanna be like the people on Reddit’s r/CRBoxes who 3D print fancy housings and do elaborate computer fan builds. Whichever you prefer.
Edit to add: Someone posted a Build Guide (non Reddit link) on Reddit for making boxes with PC fans.
I know we don’t love Reddit links but
- Here’s a Reddit post for a DIY/buy guide
- An Information post
- A tall computer fan box
- Triangle box fan
- MERV explained by the EPA (not a Reddit link) than you OatScoop
Sorry for the Reddit links, they just have a lot of conversations going on there currently.
Having built several of these for friends and family along with a lot of reading on these and air filtration in general, there’s a few things I want to bring up.
First: scoff all you want that this goofy looking thing was designed by “experts”, but the design choices they made resulted in something that’s cheap, fast to build, and works exceptionally well. They knew what they were doing, and there’s good reasoning behind the choices they made.
You need to use MERV 13 rated filters, and ideally at least 4 of them.
Air filters have ratings, the common one in the USA being MERV, explained here on the EPA website.. Short version: bigger number catches more (and smaller) particles, but is more restrictive to airflow and more expensive.
MERV 13 is a good compromise, as each time air passes through it will capture particles:
Far cheaper than HEPA (MERV 17) and commonly available in a size that works well for this setup. With 4 or more of them a box fan has sufficient airflow to recirculate the air and capture even more particles as time passes. You can use 1 filter, but the consequence is greatly reduced airflow as MERV 13 is fairly restrictive. The better the airflow, the more effective the system is.
The “shroud” (cardboard with a hole for the fan) is essential. It prevents unfiltered air from bypassing the filters through the corners of the fan enclosure. It creates higher negative pressure which increases flow through the filters. It also makes the whole thing noticeably quieter. The actual experts have already figured out the best size hole for a couple popular box fan models so you don’t have to experiment with what works best.
As for the fact it’s taped together … it was designed during COVID to be cheap and fast to build. You can absolutely make improvements to the design – I’m doing so myself. But I also built one per the directions, only deviating by using painter’s tape so it’s easy to tear down once I’ve built a proper enclosure.
More on filters:
Most HVAC filters are designed to keep the system clean enough to avoid problems – they aren’t for purifying air, they’re to keep gunk from building up on equipment and ducts. Using a higher rated filter in a system that wasn’t designed for it will cause expensive problems down the road. The restricted airflow (which only gets worse as the filter gets clogged) is hell on the equipment: it’s less efficient and can’t exchange heat at the rate it was designed to. E.g.
Unless your HVAC system was purposefully designed to act as an air purified (and you’re following the manufacture’s directions) you’re better off using a “low efficiency” filter for it and purifying the air with purpose build devices … like a Corsi-Rosenthal box.
That’s great information thanks for sharing!