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.
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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:
- 0.30-1.0µm greater than or equal to 50%
- 1.0-3.0µm greater than or equal to 85%
- 3.0-10.0µm greater than or equal to 90%
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.
- Blower motor.
- AC evaporator coil.
- Heating elements or burners.
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!
You might post this over on https://slrpnk.net/c/disasterresponse as well!!
I forgot that was a place! Thanks
It looks like peak efficiency.
This youtuber I follow created a fully repairable one with out of the self components and 3d printed parts:
https://www.kickstarter.com/projects/joelcreates/oddcube-air-purifier-kit
Variations of this idea are genuienly vital health/survival info, right now, and will continue to be going forward.
Yes, one particular design may or may not be more effective than the other, but the basic concept of ‘duct tape a HEPA filter onto a box fan’ is absolutely a critical life hack to know if you live near anywhere that does or can experience signifcantly degraded air quality.
It isn’t perfect, but it immensely better than doing nothing, and most importantly, it is not horrendously cost prohibitive (at least currently, jfc).
Walking around for 30 minutes in wildfire smoke is roughly equivalent to smoking a couple of packs of cigarrettes.
You probably do not want to be inhaling that shit all day long, for many days at a time, if you live in a home/building that does not have its own air purification system.
Chances are you probably don’t have air filtration. Having AC often does not mean you also have filtration.
So have these things on hand, or ready to assemble, when a wildfire smoke plume or something decides your city is especially beautiful to it.
Fires and smoke can come fast, suddenly, with relatively little warning.
Be prepared.
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.
Personally I use a single 4" thick HVAC filter taped to the back of a box fan, which has similar advantages to the Corsi box, but in a smaller footprint, and allows it to still be pointed at things besides the ceiling.
The 4" thick filter has far more surface area than a single 1", giving it much less flow resistance, so no danger of potentially overheating/straining the motor.
This is also a great idea
I tried the single-filter design some years ago, and it worked pretty poorly. It didn’t seem to do a good job of cleaning the air, and it stressed out the motor a lot. I’d consider this 4-filter design for the future.
Was it a 1" thick filter, or 4" thick?
In my area, we have annual fires that get pretty bad. Add in ag related pollution, allergies, etc…and air quality is almost always unhealthy.
Our local makerspace has partnered up with our local asthma collaborative to make custom PM sensors here: https://www.sjvair.com/ (code is here: https://github.com/sjvair)
We have tried out a couple of designs and had actual science done on which custom air purifiers did the best. One of the members rigged up a airtight area and pushed smoke in. Got a couple of readings, then did the same test with the air purifier designs. I think we tested close to 10 different designs including some air purifiers you get from local stores.
We found a couple that work well.https://github.com/opennukit/Nukit-Open-Air-Purifier/ is a good one.
The local makerspace did some work and found a design that worked well. It was 2nd best at air purification but was super cheap.
One that we improvised was the following:
3d printed brackets, a filter, and some activated carbon and you have a good air purifier. Family uses these for white noise and air purification at night. It works wonders! The fan is 20$ at any wallmart, the filters are dirt cheap. And you can get huge rolls of activated carbon on amazon. No need to over complicate things, just bracket everything on, let it sit for a good 3-4 months, then look and see if anything needs replacing.
One note of caution, some of the huge box designs like the one OP is suggesting was having trouble with the engine overheating. Most of these box fans use the air to cool off the internals. If it doesn’t get that for many hours, it can start overheating. Just check after running whatever solution you come up with and make sure the engine isn’t getting hot.
I tried that single-filter design in the past, and it seemed to do a poor job of filtering the air, and it stressed out the fan motor a lot.
Im not saying your wrong but that design and that spefic fan is over a decade old now.
It might just be the brand/specific motor that people get on the market.
I haven’t heard anything about box fans overheating with the boxes on the back and I’ve been following these builds on Reddit for 2 years before I built one. The general consensus is there’s MORE air flow with the box.
Get a heat gun. You can see it happening in real time. Or not if its not.
Its good not to trust me, as I didnt put any real data on what we did here. But it should be self evident if you make the build,. run the system for a while, then check the temperature. You dont want to strap a ton of flammable material near an engine that could get hot. Also if we were to do real science, I bet its because certain fans are older/have less oil/etc…which causes a buildup of heat.
No fires here, how much traffic pollution do they remove and how much goes through?
Have wondered about making one, not sure where I would get HEPA sheets like that though, or a fan like that tbh. Don’t seem to be that common in the UK.
Check the particulate size of what you want to filter out and buy a filter that will filter that size or smaller. HEPA will go down to 0.1-0.3 microns.
They are forced-air/furnace filters.
I don’t think we have those here
They are also known as HVAC or MVHR filters, which appear to be available in the UK
Based on this chart, you’d want an M5 or M6 rated filter to remove car exhaust particulate, something like this (though I have no idea if that’s the best deal to be had, that site was just the first one that popped up from searching UK HVAC Filter).
Edit: Hm, I guess the bigger problem is that you guys don’t really have box fans over there, and the ones available online there need a 120v step down transformer. Bugger :\
I guess you could still attach a 4" or 3" thick filter to the back of a large round fan, then tape cardboard cutouts on the corners that the fan doesn’t cover, and seal the sides of the fan with tape so it doesn’t try to draw air from the sides, and exclusively pulls from the filter… That should still be pretty effective.
I use two thicker filters in a trangle config with just cardboard on the top an bottom. This way my birds don’t stand on the grill and poop in the fan.
Same! With four 120mm PC fans, it works amazingly and actually fits in my room!
I have to assume that this has happened before? Lol. Birds are fun.
so pigeons outside only have wet splat poops because of some sort of microbe, and normally they should poop nugs. but a pigeon laying eggs will hold it in longer and poop out cigars. The smaller birds like cockatiel or budgie will poop out little pellets that cleanly fit between the grills. Sickly old retired bird from the shelter on daily meds will poop out gross slimey puddles. The senegal parrot will attack the robot mop and flip it…
Well that sounds like a lot of… Fun. Lol. But I’m sure that are delightful in other ways.
I’ve been wondering if you could make a hybrid of this and an air washer, replacing the filters with a frame and cotton sheets and then putting it in a shallow tub of water.
I know some air filters are made of cotton to begin with, if they’re already in a plastic frame you could use that as-is.
I don’t think I’ve heard of an air washer before. Is it like an air scrubber? I don’t actually know how those work but I know theyre a thing lol.
I think air scrubbers are for more intensive situations, like cleaning a flood damaged home, or painting things, since they’ve got chemical filters and UV sterilizers in them. Air washers are mostly just air filters that rely on water to trap stuff first and foremost, one I saw was just a series of spinning disk’s half submerged in water. Surface tension caused water to stick to the disks, and a fan blew air between them, so anything in the air would get caught and pulled down into the water tray.
The other function they serve is passive humidifiers, as air passes over the water it evaporates, but only if the air is capable of holding the water. If it’s already humid no evaporation occurs, so you don’t get things like condensation that you’ll see with active humidifiers that aerosolise water.
You can diy one by hanging up a towel, having the bottom touching a container of water, and pointing a fan at it, and capillary action will draw the water up into the towel. So with one of these boxes with cotton filters, would serve the same purpose.
I’m not sure I would call the humidification passive given the action of the fan, but yes, in the limit it will push to 100% humidity. One thing to note with that kind of setup is that mildew/mold can quickly build up in the unit and, if it keeps the ambient humidity high enough, in the dwelling. So, it can just trade one air quality issue for another. Buildup in the unit can be prevented by adding a small amount of biocide to the water (formulations for humidifiers are usually alkonium salts). However, it well eventually encrust the wick and force replacement or washing (wear gloves).
Ohhh ok that’s fascinating
Nuka tempest seemed decent for a diy idea. The shark 5 yr purifier is good for set it and forget it and low cost of ownership. Definitely look up cost of filters before you buy. Housefresh has detailed reviews and you can watch the videos of them testing the device
I love how some engineers “had the idea” and slapped their name on something people have been making for literal decades.
Until you test it and document it, it’s just a guess that it’s actually doing something.
Is actually showing that something is useful worth getting your name on it? Well, now it’s more published and more people are aware of it, so maybe?
