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
Sorry for the Reddit links, they just have a lot of conversations going on there currently.
Is it really worth building a cube out of the things or would slapping a filter on each side of the box fan work about as well?
Edit: Thanks everybody! Got a lot more responses than I expected 😅️
It does not work well. The dramatically reduced airflow means the fan is working hard for basically nothing.
However, if you are gunna go through the trouble of making a cube anyway, you can make one out of thin strips of wood or multi-layer laminated cardboard, and attach washable air filter material all around it with Velcro dots or magnets, and you never have to rebuild it or replace the filters. Just wash it with a sprayer of some sort (hose, shower, sink) once every few months.
My bad, deleted my comment because I realized I was saying basically what you were (because I can’t read today lol)
deleted by creator
There are alternate builds https://cleanaircrew.org/box-fan-filters/.
In past years, I’ve built one with only one filter that the fan pulls through. You need to offset the filter from the face of the fan still by 8" or so with a cardboard spacer to help it be more efficient. It worked.
I’ve also built one with two filters in a triangle configuration, that one worked well too.
This is the first year that I’ve built one that is actually a cube with 4 filters. They all work, the 4 filter one works the best, but it also takes up the most space.
I also built one, and I attached charcoal filters to it, which has significantly reduced the amount of dust in my house, and the air generally smells not at all.
The previous owners of my house were dog owners, so if I don’t run it for a while, then there’s like a hint of dog odor that even replacing all of the carpet and flooring in the entire house and running an ozone machine on workdays for a week has not gotten rid of.
But as long as the fan is running, that odor is taken care of.
One filter taped flat to the fan is an option that works but it is not as good. More filters means more air can flow through them and the fan is less strained by restricted airflow.
No, it wouldn’t work about as well for several reason. The biggest reason is you’re halving the surface area and reducing the airflow.
The biggest reason is that such fans are designed for low pressure and high volume operation, when you blow air around the room you really don’t need very much pressure.
So the fan blades already are already no longer operating at their optimum pressure when they have 5 filters to suck air through (each filter only has to let 1/5 of the air through, so the differential pressure over the filters is much lower), but it’s much closer to the ideal operating pressure than two filters behind each other.
If the pressure gets too high the fan stops being able to push any air through the filters and the efficiency reaches zero.
And I guess the crap wouldn’t be neatly enclosed in a box too.
I use one of these and they are great. The stuff that it filters gets stuck to the outside of the filters, not the inside of the box.
If you turned the fan the other direction it would trap it inside the box.
I did a filter on the back of a box fan until my multi pack arrived from Costco. It does something but, not as much. You want more surface area for more filtering from what I understand.
A box fan with a filter slapped on each side would probably work nearly as well for half as long. This is assuming that the filter doesn’t starve the fan of air. Filters are more or less all about surface area. Two filters instead of four means half the surface area doing the same amount of work.
Tldr; the cube is better, but even a single filter on a fan is better than nothing.