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Experiments on a $50 DIY air purifier that takes 30s to assemble

292 points by dyno-might 6y ago

nkurz 6y ago
Thanks for the article! Using incense for testing seems like a good approach. And yes, the lack of difference between "taped" and "non-taped" is definitely interesting. I'm wondering if this might be in part because of your choice to use thin and small HEPA filters instead of the more common thicker and full size MERV filters. Intuitively, what matters is the product of the efficiency of the filter and the amount of air passing through the filter. It's possible that because the resistance of the small HEPA filters is so high, the unfiltered air passing around the filters generates a lower pressure in front of the filter than the non-moving air after the back is taped. I recently built up a couple similar box fan filters, but using 20"x20"x5" MERV 15 filters bought "used" off Amazon for about $20 each. Note that the total surface area of the folded filter matters for air resistance, not just the height and width of the filter. That is, depth matters, because all else equal the folds increase the amount of filter area. I sealed well, because I thought it would matter. I removed the back plastic grill to reduce resistance a bit more, and also added a cardboard cowling on the front of the fan, based on the research here: https://www.texairfilters.com/how-to-improve-the-efficiency-of-the-box-fan-and-merv-13-filter-air-cleaner/ https://www.texairfilters.com/how-to-improve-the-efficiency-.... The idea is that with a high resistance filter on the back, in the absence of cowling you get unhelpful recirculation from the front corners of the fan, although maybe this recirculation might actually be helping reduce pressure in front of the filter in your taped example? I haven't done any air quality measurements, but I think the system is working the way I'd expect. Noise at low speed is tolerable when in another room, but I only use higher speeds when leaving the house. I'd be interested in more details about your particulate monitor, especially if you are satisfied with it and if it's available in the US.
artvark11 6y ago
Are diy filters effective at removing smells or only removing the measurable particles?
- ethanwillis 6y ago
  Not sure, but I know my commercial air filter has an optional activated charcoal filter screen in order to do smell removal. I also know that one trick to remove the smoke smell from a used car is to use an ozone generator in the car.
  - culopatin 6y ago
    The problem with ozone generators in cars is that they often react with some plastics and do more harm than good. Unfortunately they are the only way of getting in the nooks and crannies and inside the ac vents to clean mold and that kind of stuff. I’ve had steering wheels turn to gel using ozone and they smell funkier than what was in there.
- cyounkins 6y ago
  Most things that smell are very very small particles not captured by even HEPA filters and require a different kind of filtration. Most commonly activated carbon is used, either as granule or deposited on fibers.
  - rckoepke 6y ago
    HEPA filters are actually more efficient at capturing very very small particles (<0.1 micron) than they are at capturing simply small particles (≈0.3 micron). The tiniest particles are light enough to be captured using electrostatic forces rather than the more typical mechanical filtration that is effective at capturing large particles. More precisely, many odors are not particles at all but rather gases/vapors. Activated charcoal can capture larger organic molecules because it has an incredibly jagged/porous surface - just 100 grams of activated charcoal provides over 1,000,000 square feet of "effective" filtration surface area.
    - cyounkins 6y ago
      Yes, that's true that HEPA is very efficient at particles in the 0.01 to 0.1 micron range. It may also be below that, but I personally haven't seen that data. But because HEPA fails to filter gases/vapors, its performance must fall off before 1 angstrom = 0.0001 micron. > odors are not particles at all but rather gases/vapors I think the confusion comes from people referring to "particulate" as particles of >= 0.01 micron, whereas in physics we say that gases/vapors are also composed of particles. They may be single atoms floating (can't actually come up with an example though) or small molecules like O^2, CO, etc, but I'd still call those particles.
spthorn60 6y ago
As most home woodworkers could confirm, a filter-on-a-box-fan works wonders in a (saw)dusty garage. Even after sanding or sawing MDF, the air is clean after about 10-15 minutes. From personal observation, I'd suggest that non-taped is a good idea, as any non-filtered air is routed through the fan to help overall circulation; ideally you want to displace all the air in the room quickly.
- StavrosK 6y ago
  How long does it take to clear the air without the filter? I'd imagine sawdust is heavy enough to fall down on its own after a while.
  - bluGill 6y ago
    Sawdust is a mix of particle sizes. http://billpentz.com/woodworking/cyclone/beginnnerscorner.php http://billpentz.com/woodworking/cyclone/beginnnerscorner.ph... has a ton of information on sawdust.
  - spthorn60 6y ago
    Right the heavy stuff falls pretty quickly. The light particles will take quite a long time, and in the meantime, you're potentially breathing all that. I use a large cyclone for the machinery, a filter-on-a-fan, and a mask. My shop is a lot cleaner than it used to be, and I'm sure my lungs are too.
cyounkins 6y ago
Love the testing. I think the taped/non-taped results may diverge as time goes on and the filter becomes loaded. The static pressure of air filters approximately doubles over the lifetime of the filter, and in an unconstrained system more and more air will be going around the filter instead of through it.
sal9000 6y ago
Nice article! Which EPA filters you ended up using? Thinking of building one myself.
totalZero 6y ago
This is nutty. I love it. The limitation for me would simply be the appearance factor; I want my filter to blend in quietly, not to draw attention. I think appearance and noise are part of why people pay so much for fancy purifiers. Also, I imagine a centrifugal fan would draw less electricity per CFM under load (air resistance through the filter).
- chopin 6y ago
  With the same throughput radial fans are much bigger than an axial fan. But they can deliver differential pressures in the range of several 100 Pa, which axial fans cannot.
- peteretep 6y ago
  https://smartairfilters.com/en/sqair/ https://smartairfilters.com/en/sqair/ It’s a fan + a HEPA filter and it’s kinda pretty
  - ionwake 6y ago
    Do you know how long the filters last?
    - peteretep 6y ago
      Depends how long you run it and on what speed. I seem to remember we were working on the basis of changing it every three months if we ran it continuously during that time
Rebelgecko 6y ago
I wish there were multiple reps of each experiment, especially with how noisy the results were and the difficulties of reproducing smoke Regardless, this project actually makes me feel better about buying my fancy air filter Instead of buying 2 filters (both HEPA), a fan, and a poorly calibrated air quality sensor for $150 like the author, I bought an air purifier that contains 2 filters (1 HEPA, 1 activated charcoal) a fan and a poorly calibrated air sensor for $153. However since my air quality sensor is integrated with my purifier, it only pulls ~60 watts at the times when I'm cooking or if something is on fire. That probably saves me $50/year on electricity
- thomas 6y ago
  Yeah, same thought. Something like the Coway air filter ( https://www.youtube.com/watch?v=zxiDrEnjYEo https://www.youtube.com/watch?v=zxiDrEnjYEo ) regularly sells for under $200, and it is going to be much quieter than this option. It also has been tested fairly extensively from what I've seen, it's EnergyStar approved, etc, and it has a built in sensor. It doesn't look too bad either. But I understand the point, especially in times when you can't find a purifier. Also, good video on all this: https://www.youtube.com/watch?v=kH5APw_SLUU https://www.youtube.com/watch?v=kH5APw_SLUU
  - Rebelgecko 6y ago
    Exactly! I have the uglier but slightly cheaper version of that air purifier (the Coway Airmega 200M). I think the faceplate is the only difference. I forgot to mention in my original comment that it also has a prefilter for larger dust particles, which I imagine probably improves the lifetime of the other filters.
- riverdroid 6y ago
  Be aware that the air quality sensor on most units is a cheap VOC sensor. It detects gasses, not particles. It'll trigger if you fart near it or cook something odorous, but not for dust or smoke. I have the Winix 5300 and 5500 purifiers (which I highly recommend), and typically run on mode 3 out of 4 because it pulls about 12 watts (and is quiet) but is not massively different airflow than mode 4 which draws about 70 watts (and is noisy).
kylebenzle 6y ago
With Covid-19 Rates Rising Adding an Air Purifier To Your Home is the Single Most Important Thing You Can Do to Improve Your Health [0] [0] https://kylebenzle.medium.com/adding-an-air-purifier-to-your-home-is-the-single-most-important-thing-you-can-do-to-improve-your-4b9568c8056e https://kylebenzle.medium.com/adding-an-air-purifier-to-your...
WalterBright 6y ago
Another way to do it is to bubble the air through water. As a kid, we did an experiment where a cigarette was "smoked" by attaching a tube to the cigarette filter, the tube exited at the bottom of a flask filled with water, and a pump would suck the air from the top of the flask. It didn't take long before the water would turn black. Quite a turnoff for wannabee smokers, which was the obvious point of the experiment. It's the same idea as the Rainbow vacuum cleaner, which uses a tank of water as its filter.
- archi42 6y ago
  That's essentially a Shisha you built there, so smoking that way is a thing ;) The tobacco is different, as is the heat source, but I can confirm that the water still turns rather unpleasant.
- fencepost 6y ago
  Another way to do it is to bubble the air through water. As a kid, we did an experiment where a cigarette was "smoked" by attaching a tube to the cigarette filter, the tube exited at the bottom of a flask filled with water, and a pump would suck the air from the top of the flask. "We built a bong and discovered that bong water is really nasty."
  - WalterBright 6y ago
    Thanks for the great laugh. I never thought of that! Just think of that nasty bong water being what the smoke does to the fluids in your lungs.
chopin 6y ago
What I am wondering about: I worked with and designed clean room equipment in the nineties. I remember that HEPA requires a considerable pressure difference which cannot be delivered by an axial fan. You'd need a radial one which can deliver a considerable differential pressure, at cost of throughput (where the axial fan shine). If you strap a HEPA to an axial fan and seal it, the fan runs effectively "empty", i.e. it displaces only little air which most likely even doesn't pass the filter. That's why HEPA filters work in vacuum cleaners since they have radial fans. I am astounded by the results of the post. I tested simply with a laser. In a normal room you can see the laser beam in dark. If you have a good purifier, you can't.
- Traubenfuchs 6y ago
  > In a normal room you can see the laser beam in dark. Dear god, do you live next to a coal plant? I would argue that in a normal room one should NOT be able to see the laser beam!
  - joncrane 6y ago
    You'd be surprised how "dirty" (dusty) the air inside houses is.
    - metiscus 6y ago
      Especially given the relatively wide range for age and consequently construction type. I've found that newer drywall construction tends to be more dusty in my limited experience than well maintained plaster and lath.
      - yborg 6y ago
        Wall-wall carpet, especially old carpet, is a dust factory.
        metiscus 6y ago
        My house has tile floors which get visibly filthy really quickly. I have a roomba and I personally clean the floors quite often manually. In my parents house which had wall to wall carpet, they would vacuum relatively infrequently but the same amount of dirt was likely being tracked in and around. Their house was more dusty than mine seems to be because of the fact that carpet tends to hide how "dirty" it is.
ahaucnx 6y ago
We have open source, open hardware build instructions for a very reliable Air quality sensor that can be used to measure PM2.5 and also CO2. https://www.airgradient.com/diy/ https://www.airgradient.com/diy/ We have tested it extensively and it is very accurate for the fraction of the costs of commercial sensors. You can build it for less than USD 20 and send the data to our cloud server or any other backend. We also measure CO2 because if you have purifiers running you often do not have enough fresh air ventilation and your CO2 often gets very high (often >1500ppm) causing headaches and reduce cognitive performance. We measured this in classrooms some time ago and the results were not very good. Details. https://www.airgradient.com/blog/2020/02/07/we-measured-the-co2-level-in-a-classrooms-this-is-what-we-found-out/ https://www.airgradient.com/blog/2020/02/07/we-measured-the-...
- azmodeus 6y ago
  It would be nice if preassembled kits could be purchased from you. I would be happy to pay USD 40 for a good preassembled kit.
  - headmelted 6y ago
    This. I no longer have access to a soldering iron and want a bunch of these, not enough that it's worth the cost of buying equipment and the time spent making them. Help us give you money! :-)
    - joshmlewis 6y ago
      You can get good soldering irons for less than $100.
      - headmelted 6y ago
        Yeah but I have to buy the soldering iron then spend my time figuring the kit out and doing the job. I’d be happy to pay OP for their work and everyone wins.
  - cinntaile 6y ago
jansan 6y ago
This is certainly done with COVID-19 in mind. Is there any source that shows how effective HEPA filters are for filtering SARS-CoV-2, given that its average size is just 130nm, which is twenty times smaller than PM2.5.
- ddeck 6y ago
  HEPA is a bit of a loose term. It generally means 99.97% efficiency for 0.3 micron sized particles at some particular airflow. 0.3 micron was chosen because as noted by another commenter below, high efficiency filters typically filter better at smaller sizes than this. A more stringent certification is EN1822, which first tests which particle size penetrates the filter the most, and then rates the efficiency based on that particle size. With respect to viruses, a minimum of EN1822 H14 is required for use in European biological safety cabinets. An H14 filter is 99.995% efficient as it's most penetrating particle size (MPPS). That said, it's important to realize the difference between using a filter to pass air into/out of a clean environment (e.g positive/negative pressure clean room) versus just using an air purifier in a room. In the latter case, the machine is only going to filter the air going through it, and will take a significant amount of time to turn over the air in the room, during which time it's just blowing things around.
- mkskm 6y ago
  The respiratory aerosols containing viruses such as COVID are typically ~3 um. See section 10.4 on this page: https://tinyurl.com/FAQ-aerosols https://tinyurl.com/FAQ-aerosols
deleted 6y ago
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NietTim 6y ago
Interesting. I only have central ventilation but my mom has central heated air in her house, in theory she could chuck a HEPA filter in front of it to get cleaner air in all of her house.
- dsr_ 6y ago
  Odds are good that she has a MERV filter... and that it hasn't been changed in years.
  - NietTim 6y ago
    She probably has, but the heating module gives her a periodic reminder to clean it (which I then have to do because she can't reach it). It gets replaced every few years, I think she has a service contract :)
- nkurz 6y ago
  Note that this can be a really bad idea. While it might temporarily improve the air quality, extreme filtration will reduce the air flow through the system. This will in turn reduce the efficiency of the heater, possibly in a way that causes its internals to overheat and destroy themselves. Rather than "upgrading" to higher filtration, you are safer replacing whatever filter is there with a new clean one that matches the correct specs and then adding higher quality filtration separately.
  - exhilaration 6y ago
    I've read the same thing, that if you put a heavy duty filter on your furnace you put a heavier strain on the motor and risk burning it out. I'm not an HVAC professional so research this yourself.
  - NietTim 6y ago
    This is a very good point, thanks
infofarmer 6y ago
Smart Air is a whole company built around a super cheap HEPA-on-a-fan, started by foreigners in China. They still sell their old DIY1.1 model for $35 shipped (in Mainland). I own and love their filters, although I miss the «smart» features of their competitors. https://smartairfilters.com/en/about/our-story/ https://smartairfilters.com/en/about/our-story/
jsingleton 6y ago
There is some interesting info on building a cheap ~$20 purifier here: https://airpollutionguide.net/ https://airpollutionguide.net/ I haven't tried as cheap HEPA filters appear to be harder to get hold of in the UK. Maybe due to different heating systems. Clearing the Air: The Beginning and the End of Air Pollution by Tim Smedley is an excellent book if you are interested in this area.
JohnJamesRambo 6y ago
It would be neat to see results of a good furnace filter on the fan. Saves the cost of the bungee because the air flow holds it on pretty much, and the filters are cheaper. It is what I use in my grow house to filter the air.
mleonhard 6y ago
Any MERV8 or higher filter will work. You don't need a HEPA filter. HEPA is only useful when you can only pass the air through the filter once, like in hospitals. A home air filter can push the air through the filter several times, so an MERV8 is just as good as a HEPA and lasts longer. I used a DIY filter to remove wildfire smoke from my apartment. It's just a 12"x12" MERV8 filter taped to the front of a small fan. It worked well. I periodically opened my windows to exchange the stuffy air with fresh+smokey air. After closing the windows, the filter took about 30 minutes to remove all of the smoke inside.
konjin 6y ago
I build one during the forest fires earlier in the year, I bought a box fan and 6 Merv 14 furnace filters. My best idea was cutting out the front and back wall of the box the filters came with and squishing the fan between two. It worked well enough with the 50cm fan that it could make a 10x8m room pleasant in 10 minutes. The only improvement I would make would be buying a lower rated filter for the intake since it got clogged up with a lot of large particles that were kicked up by the fan. I was planning a big build with an impeller fan, but there is something to be said for a solution so stupid that I can rebuild it in 5 minutes with $50 in parts.
totetsu 6y ago
water filters also have the incompatible proprietary filters problem. I wanted to try filtering some water so I could make coffee that wasn't bitter but every water filter seemed to want me to buy a bunch of plastic to go around a replaceable filter. In the end I settled on pyrolysized cherry wood branches, (activated carbon, or biochar). It seems them carbonated bamboo is also used in tea making. It worked but it certainly took a long time leaving the sticks in the water.
- dole 6y ago
  In the spirit of DIY/OSS here, I think ideally you’d want to emulate something like a Berkey water filter, these are basically just steel tanks with large threaded cylindrical charcoal filters (2” diameter, ~10” long) that use gravity to pull the water from the top to the bottom. A simple 3D printed housing filled with some charcoal/carbon media to trickle filter like a Brita would be interesting to see its effectiveness.
  - tk75x 6y ago
    At my parents house, we set up a whole-house water filtration system by repurposing water softeners. empty the guts of one, refill with aquarium supply activated carbon, then adjust the electronics to your liking and you're good to go. Those systems are designed to run 24/7 and soften/filter all the water going into your house.
- bluGill 6y ago
  There are standard sizes and housings for water filters. No fridge uses anything standard, but you can find them. you need some sort of plastic (or at least something food safe) around the filter though.
westurner 6y ago
From "Better Box Fan Air Purifier" https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-air-purifier.html https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-a... : > Air purifiers can be expensive and you've probably seen articles recommending to just put a 20" x 20" x 1" furnace filter on a cheap 20" box fan and POOF! instant cleaner air for not a lot of money. It really does clean the air pretty cheap. > There's a problem with this though. These fans weren't designed to be run with a filter. The filter will restrict air flow which will put a higher strain on the motor causing it to use more electricity and in worse cases could be a fire hazard. The higher the MERV rating (cleaning efficiency) of the filter the more stress it will put on the fan. > Don't worry! You can still have your cheap air purifier as long as the filter area is increased to decrease the effect of air resistance. Instead of using one 20x20x1 filter we'll use two 20x25x1 filters which increases the filter surface area over 250%. It's a little more expensive because you're using two filters instead of one but the increased filter surface area also helps the filter last longer before it gets clogged up and we're saving on energy use compared to a single filter.
- rootusrootus 6y ago
  > we'll use two 20x25x1 filters which increases the filter surface area over 250%. That seems like an overly complicated first attempt, why not just switch to a 20x20x4?
  - post_break 6y ago
    Those can be harder to source I've found. Cheaper to use two filters than one as well since you can replaced the prefilter more often.
    - bluGill 6y ago
      I used a 25x20x4 which was easy to find. Even though I didn't do anything to stop air from flowing around the backside of the filter to the fan it still gets noticeably black fast.
    - rootusrootus 6y ago
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Mister_Snuggles 6y ago
The only downside of the DIY route is the noise. Most box fans that I've seen are very noisy relative to "real" air cleaners/purifiers. I've used a box fan plus furnace filter for years to deal with cat litter dust. Since the particles I want to filter are quite large, I can get away with using a fairly cheap furnace filter. The nice thing about this is that it's really easy to scale it to your requirements. To deal with the noise, I used a motion sensor and switched outlet to only run it for 10 minutes after the litter has been used. This runs through my existing home automation stuff (Home Assistant+Node-RED), but it could also be done with an Arduino (you don't even something that powerful - two I/O lines is enough), a PIR sensor, and a solid state relay for the fan control. I've tried a number of low-dust/no-dust litters, but most of have lead to the cats finding other places to do their business or have disappeared from store shelves when I've needed to replenish it. Everything about this is a compromise between litter acceptance, litter availability, litter cost, filter cost, filter life, noise, and dust production.
- batter 6y ago
  I'm using Vornado fan with 2 16x20x1 merv 11 filters from lowes (2 to make triangle). And in lowest speed it's very quiet. I have commercial air cleaner and my diy is way cheaper to run and as quiet. https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-air-purifier.html?m=1 https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-a... - like this.
  - Mister_Snuggles 6y ago
    I swear I came across that post at one point - it looks very familiar! I really want to do this because 20x25x1 filters are a LOT easier to find (where I am anyway) than 20x20x1 filters. The only obstacle is that the resulting unit ends up taking a lot more space, and that's at a bit of a premium in the place I need it. If I move the litter boxes to somewhere with more space I will definitely switch to this design.
- deleted 6y ago
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yoyman 6y ago
Did this with a small fan (around 6" in diameter) and it died out within a week of use. To be fair, the HEPA filter I used was really restrictive but anyone who plans to do this should probably keep that in mind.
nether 6y ago
The downside is that these filters sell out in places where there are fires. So the time to buy them is now.
- rootusrootus 6y ago
  I agree. When we had wildfire smoke descend on our area, the stores sold out within hours. If you live in an area which routinely experiences wildfire smoke, it's worth stocking a few filters, both 20x20x4 for box fans and whatever size your furnace uses. It made a huge difference to have fresh filters to swap out every few days.
textech 6y ago
Without an activated carbon filter on the outside to collect dust and other bigger particles your HEPA filters will be utterly useless in only a few weeks. If you keep your windows closed then I guess it could last longer but then you may have CO2 issues.
causality0 6y ago
Reading the graph, it looks like it took both purifiers a full twenty minutes to make the air as clean as it was before the fans kicked all the particles off surfaces and into the air.
- nkurz 6y ago
  Are you making a joke, or did you perhaps miss the description of the expiriment that involves burning a piece of incense when he starts the test? I agree though that in a more formal test, a "null" version without the incense would be a good comparison.
abyssin 6y ago
I tried building a similar air purifier in Belgium last year and I've never been able to source the parts at a similar cost.
- boredpudding 6y ago
  Same problem for The Netherlands here. I just want to put a hepa filter on a fan, but unfortunately I can only find hepa filters for vacuum cleaners or existing air purifiers. If anybody has a link for where to buy a regular square/rectangle hepa filter, that would be great.
  - bluGill 6y ago
    This is a hepa replacement for one of the air purifiers you can buy. Just go to the store, find an air purifier, read the back of the box to see what filter it takes, then look up/down/over a shelf - they replacement filters are generally carried in the same aisle. You will probably be surprised at just how small the filters for most of those commercial air purifiers are.
Scene_Cast2 6y ago
I did a bit of experimentation with different filters for a DIY build like that. Setup: I tried out a bunch of different air filters in a medium-sized apartment. Two objectives: how fast PM2.5 is removed, and down to what level can it bring down PM2.5. From worst to best: * Nordic Pure 20x20x4M15+C MERV 15 - the carbon layer absolutely kills airflow, doesn't work * Filtrete 20x20x1, AC Furnace Air Filter, MPR 2800 / MERV 14 - works okay * Nordic Pure 20x20x4 - works well * Lennox Corporation X7935 20X20X5 MERV 16 - works the best. This thing is an absolute beast in terms of filtration performance; significantly lower steady-state PM2.5 readings during the California fires. Test equipment: Awair and Plume Flow v2.
- esaym 6y ago
  I've been using the Nordic Pure filters for many years now. Way better (and cheaper) than the stuff you get at home depot, etc.
ocdtrekkie 6y ago
I used a 20x20x1 taped to a box fan for over a year as an air purifier because I had ferrets, and ferrets leave a particular odor floating about most of the time. I didn't want to use an odor-removing filter with charcoal on my furnace because they're super thick and put strain on the motor, and I don't care about the box fan. It worked really well, probably the biggest challenge was securing the filter to the fan: I put my filter on the "out" side of the fan, and it was really hard to keep it attached. Duct tape and the like actually often would fail, and air would just start moving around the filter. For good airflow through the filter I really had to give the air little other choice. I'm curious if it would've worked just as well had I filtered the air on the "in" side of the box fan... keeping it attached would've been much easier, but I'm not sure if there's any forces that would make "pushing" air through the filter work better than "pulling" air through the filter.
- rootusrootus 6y ago
  Personally I'd do the filter on the input side of the fan. They're designed to be pretty free flowing to begin with, since they're intended to be on the input side of the furnace blower. It would also keep the fan itself a bit cleaner. Suction is almost sufficient by itself to keep the filter in place. FWIW, Alton Brown made a jerky dryer using a similar technique, and he used bungee cords to hold the filters to the box fan. That's probably less hassle than duct tape.
- turtlebits 6y ago
  They sell silicon bands designed to keep the filter attached to 20" box fans. They're <$10 and work well.
roflchoppa3 6y ago
These boxfan + hepa filters seem pretty affordable, but seem to have a really strange physical footprint. Do SUV cabin fans not push enough CFM to be effective? https://www.rockauto.com/en/moreinfo.php?pk=10863244&cc=1502557&jsn=822 https://www.rockauto.com/en/moreinfo.php?pk=10863244&cc=1502... This paired with a filter seems like it would do the job, and based on the enclosure you might be able to have a smaller footprint. No CFM data though.
- UncleOxidant 6y ago
  I think box fans are used because that's what people already have around. When wildfires rendered our air here in the PNW unbreathable last September a lot of folks were scrambling for something that would work quickly. Waiting around for an SUV cabin fan to arrive and then trying to figure out an enclosure made that a non-starter.
- bluGill 6y ago
  They should, and probably stay closer to the rated cfm with a filter there. However now you need a 12 volt power supply instead of plugging into a normal outlet. Plus you need to design a housing for it. Neither of these are hard problems for DIY, but they are not nearly as easy or quick as a cheap fan and some tape.
dbsmith83 6y ago
Love the data on this experiment. I made one of these during the wildfires, and it was apparent that it was working after just a few days. The once colorless filter had become noticeably grey/yellowed.
acvny 6y ago
Wait. I did the same thing and after running the filter for several months, when I disassembled it, the filter was clean as new, but the bulk of the dust was laying somewhere between the blades and the motor and on the edges of the fan. It is very hard to actually get the right type of fan and assemble it correctly because simple fan will not work (the air will flow over the blades, thus not providing the required suction so that the air passes through the filters). The filters have pretty high resistance so you need a very efficient sucking / blowing fan to make the air pass through.
neilv 6y ago
When I made one of these with a cheap box fan and a good 3M filter that covered one face completely, I ran into the airflow problem. For reasons (cheap fan with the cable and switch going in the center of the rear), I had to have the filter on the outflow side of the fan, and I was getting much more airflow back out the corners of inflow side of the fan than through the filter. I'm frugally-inclined, but I was desperate to clean up some nasty dust from construction work, so I ended up buying 2 Levoit air purifier appliances. They have better airflow, and are much quieter. They also exhaust upwards, which I guessed might be less likely to disrupt a whole lot of dust-at-rest at once. A problem now is not so much that the consumable filter cartridges are expensive, which I could live with if they are motivated to be high-quality, but that counterfeits are inevitable. A related problem is that I don't have a great way to measure filter effectiveness, whether I'm buying high-quality genuine, third-party, counterfeit, or genuine facing pressure from counterfeits and seeing less value in remaining high-quality.
- EricE 6y ago
  Just cut down a furnace filter to fit inside. I’ve been doing that for years with a few Sharp units I can’t get filters for any more (discontinued) and some newer Levoits. If you leave them running all the time on low they are more effective than running them part of the day with the “official” filters.
  - neilv 6y ago
    Good idea -- that would work for my larger Levoit unit, which takes a pair of flat rectangular filters, though not for the smaller cylindrical unit. For now, I'm going to try to keep buying the Levoit filters, since I think they have more "layers" than the furnace filters, and I don't have a good sense of what all I should to be filtering in this location. But it's good to have a backup plan.
pengaru 6y ago
I thought an "air purifier" included VOC removal, as in had activated charcoal in the filter medium. Isn't this just an air filter if it only removes particulates?
batushka3 6y ago
TOP indor polution sources from my 2 year experience with concentration detector: - frying pan with just a bit of smoke, pm 2.5/10 goes hundres easy - parafin candles, HCHO formaldehyde goes over 1mg/m3 (can't tell if other are better though) - presurized cans (sunscreen, desodorant, shoe care etc) pm goes hundreds easy - spray surface sanitizer alcohol based, registers high folmaldehyde too And it's incredible how well air mixes in flat, spray in one room - few minutes later register in another. No fans needed.
ionwake 6y ago
I don't mean to go against the spirit of the thread but does anyone know how these DIY compare to actual commercial products - and can anyone recommend a good air purifier? Thank you ( I will remove this comment if it is offtopic )
- hundchenkatze 6y ago
  > how these DIY compare to actual commercial products That's actually what this article discusses: "If you’re in a hurry, this post says that if you strap two HEPA filters to a box fan, it will clear the air of basically all the particles we can measure, and it will do it faster than a commercial filter that costs twice as much."
  - ionwake 6y ago
    Thank you
- nkurz 6y ago
  I think a reasonable summary is that the homemade versions are cheaper and actually more effective than most commercial versions, with the downside that they are usually larger and often noisier. The largest factor for effective air purification is airflow, and the homemade versions offer higher airflow but at the cost of greater noise and looking janky. There are somewhat reasonable fears that the homemade versions might be fire hazards, but in practice they are probably no more dangerous than the bare fans on which they are based. Really, both solutions work just fine. If looking commercial, be sure to consider the cost and availability of replacement filters. If considering DIY, realize that you don't need to get it perfect, almost anything is better than nothing.
- EricE 6y ago
  Radial fans (also known as squirrel cage blower, or visualize a water wheel from an old fashioned water driven flour mill) will move air more quietly than an axial fan in a box fan. Levoit on Amazon are some great, relatively inexpensive units and when the HEPA filter it comes with needs replacing I use use furnace filters cut down to size. If you want it to last a bit longer, you can use quilt batting as a pre-filter before the furnace filter to catch the big stuff and extend the life of the furnace filter.
igor47 6y ago
i am also a "fan" of the home-made air purifier. we had a bunch of them around our house over the fire season this summer. it's helpful to compare the numbers: https://igor.moomers.org/minimal-viable-air-quality https://igor.moomers.org/minimal-viable-air-quality if you want to build your own monitor, i still have a bunch of PMS7003 sensors around. ping me if you want to grab one in berkeley
giantg2 6y ago
It could be more efficient if you use a old cardboard boxes and duct tape to build a custom fit shroud connecting/ mounting the filter to the fan. I use a similar setup on a homemade glove box / flow hood type device for culturing mushrooms. I have an 8" duct booster fan pushing air into a cardboard box. Between that box and the flow hood I have a HEPA filter sandwiched between them using bolts and wing nuts. The only thing I'd change is the fan. The booster fan was cheap, but has trouble with the back pressure (lower air flow). I would probably look at using squirrel cage or maybe a radon fan. They should be able to handle the pressure.
kingnothing 6y ago
Wirecutter also has some info on this setup compared to commercially available options. tldr: a good HEPA filter will remove 99% of impurities, their box fan setup removed 87%. https://www.nytimes.com/wirecutter/reviews/best-air-purifier/#what-if-you-just-put-a-furnace-filter-on-a-box-fan https://www.nytimes.com/wirecutter/reviews/best-air-purifier...
leafmeal 6y ago
I'd like to see a control run where you don't attach a filter to the fan. My hypothesis is that it won't look very different compared to when the filter is attached (I hope I'm wrong though).
lurkertech 6y ago
The OP design might see a big jump in effectiveness if they add an air guide one one or both sides of the box fan (just make a simple square cardboard tube with some tape) so that by the time the air reaches the filter, it has laminar flow. This helped enormously in a DIY design I kludged together in Thailand (where box fans are NOT available, so I had to get something to work with regular round desk fans, which everyone already has): https://slice-of-thai.com/air/ https://slice-of-thai.com/air/ "cannon" design (ugly but effective): https://slice-of-thai.com/air/#p3 https://slice-of-thai.com/air/#p3 This may also explain why the OP heard that the tape "helps" but was surprised when it didn't. The key issue is likely not sealing with tape but rather forming an air guide.

292 points by dyno-might 6y ago

Thanks for the article! Using incense for testing seems like a good approach. And yes, the lack of difference between "taped" and "non-taped" is definitely interesting. I'm wondering if this might be in part because of your choice to use thin and small HEPA filters instead of the more common thicker and full size MERV filters. Intuitively, what matters is the product of the efficiency of the filter and the amount of air passing through the filter. It's possible that because the resistance of the small HEPA filters is so high, the unfiltered air passing around the filters generates a lower pressure in front of the filter than the non-moving air after the back is taped. I recently built up a couple similar box fan filters, but using 20"x20"x5" MERV 15 filters bought "used" off Amazon for about $20 each. Note that the total surface area of the folded filter matters for air resistance, not just the height and width of the filter. That is, depth matters, because all else equal the folds increase the amount of filter area. I sealed well, because I thought it would matter. I removed the back plastic grill to reduce resistance a bit more, and also added a cardboard cowling on the front of the fan, based on the research here: https://www.texairfilters.com/how-to-improve-the-efficiency-of-the-box-fan-and-merv-13-filter-air-cleaner/ https://www.texairfilters.com/how-to-improve-the-efficiency-.... The idea is that with a high resistance filter on the back, in the absence of cowling you get unhelpful recirculation from the front corners of the fan, although maybe this recirculation might actually be helping reduce pressure in front of the filter in your taped example? I haven't done any air quality measurements, but I think the system is working the way I'd expect. Noise at low speed is tolerable when in another room, but I only use higher speeds when leaving the house. I'd be interested in more details about your particulate monitor, especially if you are satisfied with it and if it's available in the US.

artvark11 6y ago

Are diy filters effective at removing smells or only removing the measurable particles?

ethanwillis 6y ago
Not sure, but I know my commercial air filter has an optional activated charcoal filter screen in order to do smell removal. I also know that one trick to remove the smoke smell from a used car is to use an ozone generator in the car.
- culopatin 6y ago
  The problem with ozone generators in cars is that they often react with some plastics and do more harm than good. Unfortunately they are the only way of getting in the nooks and crannies and inside the ac vents to clean mold and that kind of stuff. I’ve had steering wheels turn to gel using ozone and they smell funkier than what was in there.
cyounkins 6y ago
Most things that smell are very very small particles not captured by even HEPA filters and require a different kind of filtration. Most commonly activated carbon is used, either as granule or deposited on fibers.
- rckoepke 6y ago
  HEPA filters are actually more efficient at capturing very very small particles (<0.1 micron) than they are at capturing simply small particles (≈0.3 micron). The tiniest particles are light enough to be captured using electrostatic forces rather than the more typical mechanical filtration that is effective at capturing large particles. More precisely, many odors are not particles at all but rather gases/vapors. Activated charcoal can capture larger organic molecules because it has an incredibly jagged/porous surface - just 100 grams of activated charcoal provides over 1,000,000 square feet of "effective" filtration surface area.
  - cyounkins 6y ago
    Yes, that's true that HEPA is very efficient at particles in the 0.01 to 0.1 micron range. It may also be below that, but I personally haven't seen that data. But because HEPA fails to filter gases/vapors, its performance must fall off before 1 angstrom = 0.0001 micron. > odors are not particles at all but rather gases/vapors I think the confusion comes from people referring to "particulate" as particles of >= 0.01 micron, whereas in physics we say that gases/vapors are also composed of particles. They may be single atoms floating (can't actually come up with an example though) or small molecules like O^2, CO, etc, but I'd still call those particles.

spthorn60 6y ago

As most home woodworkers could confirm, a filter-on-a-box-fan works wonders in a (saw)dusty garage. Even after sanding or sawing MDF, the air is clean after about 10-15 minutes. From personal observation, I'd suggest that non-taped is a good idea, as any non-filtered air is routed through the fan to help overall circulation; ideally you want to displace all the air in the room quickly.

StavrosK 6y ago
How long does it take to clear the air without the filter? I'd imagine sawdust is heavy enough to fall down on its own after a while.
- bluGill 6y ago
  Sawdust is a mix of particle sizes. http://billpentz.com/woodworking/cyclone/beginnnerscorner.php http://billpentz.com/woodworking/cyclone/beginnnerscorner.ph... has a ton of information on sawdust.
- spthorn60 6y ago
  Right the heavy stuff falls pretty quickly. The light particles will take quite a long time, and in the meantime, you're potentially breathing all that. I use a large cyclone for the machinery, a filter-on-a-fan, and a mask. My shop is a lot cleaner than it used to be, and I'm sure my lungs are too.

cyounkins 6y ago

Love the testing. I think the taped/non-taped results may diverge as time goes on and the filter becomes loaded. The static pressure of air filters approximately doubles over the lifetime of the filter, and in an unconstrained system more and more air will be going around the filter instead of through it.

sal9000 6y ago

Nice article! Which EPA filters you ended up using? Thinking of building one myself.

totalZero 6y ago

This is nutty. I love it. The limitation for me would simply be the appearance factor; I want my filter to blend in quietly, not to draw attention. I think appearance and noise are part of why people pay so much for fancy purifiers. Also, I imagine a centrifugal fan would draw less electricity per CFM under load (air resistance through the filter).

chopin 6y ago
With the same throughput radial fans are much bigger than an axial fan. But they can deliver differential pressures in the range of several 100 Pa, which axial fans cannot.
peteretep 6y ago
https://smartairfilters.com/en/sqair/ https://smartairfilters.com/en/sqair/ It’s a fan + a HEPA filter and it’s kinda pretty
- ionwake 6y ago
  Do you know how long the filters last?
  - peteretep 6y ago
    Depends how long you run it and on what speed. I seem to remember we were working on the basis of changing it every three months if we ran it continuously during that time

Rebelgecko 6y ago

I wish there were multiple reps of each experiment, especially with how noisy the results were and the difficulties of reproducing smoke Regardless, this project actually makes me feel better about buying my fancy air filter Instead of buying 2 filters (both HEPA), a fan, and a poorly calibrated air quality sensor for $150 like the author, I bought an air purifier that contains 2 filters (1 HEPA, 1 activated charcoal) a fan and a poorly calibrated air sensor for $153. However since my air quality sensor is integrated with my purifier, it only pulls ~60 watts at the times when I'm cooking or if something is on fire. That probably saves me $50/year on electricity

thomas 6y ago
Yeah, same thought. Something like the Coway air filter ( https://www.youtube.com/watch?v=zxiDrEnjYEo https://www.youtube.com/watch?v=zxiDrEnjYEo ) regularly sells for under $200, and it is going to be much quieter than this option. It also has been tested fairly extensively from what I've seen, it's EnergyStar approved, etc, and it has a built in sensor. It doesn't look too bad either. But I understand the point, especially in times when you can't find a purifier. Also, good video on all this: https://www.youtube.com/watch?v=kH5APw_SLUU https://www.youtube.com/watch?v=kH5APw_SLUU
- Rebelgecko 6y ago
  Exactly! I have the uglier but slightly cheaper version of that air purifier (the Coway Airmega 200M). I think the faceplate is the only difference. I forgot to mention in my original comment that it also has a prefilter for larger dust particles, which I imagine probably improves the lifetime of the other filters.
riverdroid 6y ago
Be aware that the air quality sensor on most units is a cheap VOC sensor. It detects gasses, not particles. It'll trigger if you fart near it or cook something odorous, but not for dust or smoke. I have the Winix 5300 and 5500 purifiers (which I highly recommend), and typically run on mode 3 out of 4 because it pulls about 12 watts (and is quiet) but is not massively different airflow than mode 4 which draws about 70 watts (and is noisy).

kylebenzle 6y ago

With Covid-19 Rates Rising Adding an Air Purifier To Your Home is the Single Most Important Thing You Can Do to Improve Your Health [0] [0] https://kylebenzle.medium.com/adding-an-air-purifier-to-your-home-is-the-single-most-important-thing-you-can-do-to-improve-your-4b9568c8056e https://kylebenzle.medium.com/adding-an-air-purifier-to-your...

WalterBright 6y ago

Another way to do it is to bubble the air through water. As a kid, we did an experiment where a cigarette was "smoked" by attaching a tube to the cigarette filter, the tube exited at the bottom of a flask filled with water, and a pump would suck the air from the top of the flask. It didn't take long before the water would turn black. Quite a turnoff for wannabee smokers, which was the obvious point of the experiment. It's the same idea as the Rainbow vacuum cleaner, which uses a tank of water as its filter.

archi42 6y ago
That's essentially a Shisha you built there, so smoking that way is a thing ;) The tobacco is different, as is the heat source, but I can confirm that the water still turns rather unpleasant.
fencepost 6y ago
Another way to do it is to bubble the air through water. As a kid, we did an experiment where a cigarette was "smoked" by attaching a tube to the cigarette filter, the tube exited at the bottom of a flask filled with water, and a pump would suck the air from the top of the flask. "We built a bong and discovered that bong water is really nasty."
- WalterBright 6y ago
  Thanks for the great laugh. I never thought of that! Just think of that nasty bong water being what the smoke does to the fluids in your lungs.

chopin 6y ago

What I am wondering about: I worked with and designed clean room equipment in the nineties. I remember that HEPA requires a considerable pressure difference which cannot be delivered by an axial fan. You'd need a radial one which can deliver a considerable differential pressure, at cost of throughput (where the axial fan shine). If you strap a HEPA to an axial fan and seal it, the fan runs effectively "empty", i.e. it displaces only little air which most likely even doesn't pass the filter. That's why HEPA filters work in vacuum cleaners since they have radial fans. I am astounded by the results of the post. I tested simply with a laser. In a normal room you can see the laser beam in dark. If you have a good purifier, you can't.

Traubenfuchs 6y ago
> In a normal room you can see the laser beam in dark. Dear god, do you live next to a coal plant? I would argue that in a normal room one should NOT be able to see the laser beam!
- joncrane 6y ago
  You'd be surprised how "dirty" (dusty) the air inside houses is.
  - metiscus 6y ago
    Especially given the relatively wide range for age and consequently construction type. I've found that newer drywall construction tends to be more dusty in my limited experience than well maintained plaster and lath.
    - yborg 6y ago
      Wall-wall carpet, especially old carpet, is a dust factory.
      - metiscus 6y ago
        My house has tile floors which get visibly filthy really quickly. I have a roomba and I personally clean the floors quite often manually. In my parents house which had wall to wall carpet, they would vacuum relatively infrequently but the same amount of dirt was likely being tracked in and around. Their house was more dusty than mine seems to be because of the fact that carpet tends to hide how "dirty" it is.

ahaucnx 6y ago

We have open source, open hardware build instructions for a very reliable Air quality sensor that can be used to measure PM2.5 and also CO2. https://www.airgradient.com/diy/ https://www.airgradient.com/diy/ We have tested it extensively and it is very accurate for the fraction of the costs of commercial sensors. You can build it for less than USD 20 and send the data to our cloud server or any other backend. We also measure CO2 because if you have purifiers running you often do not have enough fresh air ventilation and your CO2 often gets very high (often >1500ppm) causing headaches and reduce cognitive performance. We measured this in classrooms some time ago and the results were not very good. Details. https://www.airgradient.com/blog/2020/02/07/we-measured-the-co2-level-in-a-classrooms-this-is-what-we-found-out/ https://www.airgradient.com/blog/2020/02/07/we-measured-the-...

azmodeus 6y ago
It would be nice if preassembled kits could be purchased from you. I would be happy to pay USD 40 for a good preassembled kit.
- headmelted 6y ago
  This. I no longer have access to a soldering iron and want a bunch of these, not enough that it's worth the cost of buying equipment and the time spent making them. Help us give you money! :-)
  - joshmlewis 6y ago
    You can get good soldering irons for less than $100.
    - headmelted 6y ago
      Yeah but I have to buy the soldering iron then spend my time figuring the kit out and doing the job. I’d be happy to pay OP for their work and everyone wins.
- cinntaile 6y ago

jansan 6y ago

This is certainly done with COVID-19 in mind. Is there any source that shows how effective HEPA filters are for filtering SARS-CoV-2, given that its average size is just 130nm, which is twenty times smaller than PM2.5.

ddeck 6y ago
HEPA is a bit of a loose term. It generally means 99.97% efficiency for 0.3 micron sized particles at some particular airflow. 0.3 micron was chosen because as noted by another commenter below, high efficiency filters typically filter better at smaller sizes than this. A more stringent certification is EN1822, which first tests which particle size penetrates the filter the most, and then rates the efficiency based on that particle size. With respect to viruses, a minimum of EN1822 H14 is required for use in European biological safety cabinets. An H14 filter is 99.995% efficient as it's most penetrating particle size (MPPS). That said, it's important to realize the difference between using a filter to pass air into/out of a clean environment (e.g positive/negative pressure clean room) versus just using an air purifier in a room. In the latter case, the machine is only going to filter the air going through it, and will take a significant amount of time to turn over the air in the room, during which time it's just blowing things around.
mkskm 6y ago
The respiratory aerosols containing viruses such as COVID are typically ~3 um. See section 10.4 on this page: https://tinyurl.com/FAQ-aerosols https://tinyurl.com/FAQ-aerosols

deleted 6y ago

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NietTim 6y ago

Interesting. I only have central ventilation but my mom has central heated air in her house, in theory she could chuck a HEPA filter in front of it to get cleaner air in all of her house.

dsr_ 6y ago
Odds are good that she has a MERV filter... and that it hasn't been changed in years.
- NietTim 6y ago
  She probably has, but the heating module gives her a periodic reminder to clean it (which I then have to do because she can't reach it). It gets replaced every few years, I think she has a service contract :)
nkurz 6y ago
Note that this can be a really bad idea. While it might temporarily improve the air quality, extreme filtration will reduce the air flow through the system. This will in turn reduce the efficiency of the heater, possibly in a way that causes its internals to overheat and destroy themselves. Rather than "upgrading" to higher filtration, you are safer replacing whatever filter is there with a new clean one that matches the correct specs and then adding higher quality filtration separately.
- exhilaration 6y ago
  I've read the same thing, that if you put a heavy duty filter on your furnace you put a heavier strain on the motor and risk burning it out. I'm not an HVAC professional so research this yourself.
- NietTim 6y ago
  This is a very good point, thanks

infofarmer 6y ago

Smart Air is a whole company built around a super cheap HEPA-on-a-fan, started by foreigners in China. They still sell their old DIY1.1 model for $35 shipped (in Mainland). I own and love their filters, although I miss the «smart» features of their competitors. https://smartairfilters.com/en/about/our-story/ https://smartairfilters.com/en/about/our-story/

jsingleton 6y ago

There is some interesting info on building a cheap ~$20 purifier here: https://airpollutionguide.net/ https://airpollutionguide.net/ I haven't tried as cheap HEPA filters appear to be harder to get hold of in the UK. Maybe due to different heating systems. Clearing the Air: The Beginning and the End of Air Pollution by Tim Smedley is an excellent book if you are interested in this area.

JohnJamesRambo 6y ago

It would be neat to see results of a good furnace filter on the fan. Saves the cost of the bungee because the air flow holds it on pretty much, and the filters are cheaper. It is what I use in my grow house to filter the air.

mleonhard 6y ago

Any MERV8 or higher filter will work. You don't need a HEPA filter. HEPA is only useful when you can only pass the air through the filter once, like in hospitals. A home air filter can push the air through the filter several times, so an MERV8 is just as good as a HEPA and lasts longer. I used a DIY filter to remove wildfire smoke from my apartment. It's just a 12"x12" MERV8 filter taped to the front of a small fan. It worked well. I periodically opened my windows to exchange the stuffy air with fresh+smokey air. After closing the windows, the filter took about 30 minutes to remove all of the smoke inside.

konjin 6y ago

I build one during the forest fires earlier in the year, I bought a box fan and 6 Merv 14 furnace filters. My best idea was cutting out the front and back wall of the box the filters came with and squishing the fan between two. It worked well enough with the 50cm fan that it could make a 10x8m room pleasant in 10 minutes. The only improvement I would make would be buying a lower rated filter for the intake since it got clogged up with a lot of large particles that were kicked up by the fan. I was planning a big build with an impeller fan, but there is something to be said for a solution so stupid that I can rebuild it in 5 minutes with $50 in parts.

totetsu 6y ago

water filters also have the incompatible proprietary filters problem. I wanted to try filtering some water so I could make coffee that wasn't bitter but every water filter seemed to want me to buy a bunch of plastic to go around a replaceable filter. In the end I settled on pyrolysized cherry wood branches, (activated carbon, or biochar). It seems them carbonated bamboo is also used in tea making. It worked but it certainly took a long time leaving the sticks in the water.

dole 6y ago
In the spirit of DIY/OSS here, I think ideally you’d want to emulate something like a Berkey water filter, these are basically just steel tanks with large threaded cylindrical charcoal filters (2” diameter, ~10” long) that use gravity to pull the water from the top to the bottom. A simple 3D printed housing filled with some charcoal/carbon media to trickle filter like a Brita would be interesting to see its effectiveness.
- tk75x 6y ago
  At my parents house, we set up a whole-house water filtration system by repurposing water softeners. empty the guts of one, refill with aquarium supply activated carbon, then adjust the electronics to your liking and you're good to go. Those systems are designed to run 24/7 and soften/filter all the water going into your house.
bluGill 6y ago
There are standard sizes and housings for water filters. No fridge uses anything standard, but you can find them. you need some sort of plastic (or at least something food safe) around the filter though.

westurner 6y ago

From "Better Box Fan Air Purifier" https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-air-purifier.html https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-a... : > Air purifiers can be expensive and you've probably seen articles recommending to just put a 20" x 20" x 1" furnace filter on a cheap 20" box fan and POOF! instant cleaner air for not a lot of money. It really does clean the air pretty cheap. > There's a problem with this though. These fans weren't designed to be run with a filter. The filter will restrict air flow which will put a higher strain on the motor causing it to use more electricity and in worse cases could be a fire hazard. The higher the MERV rating (cleaning efficiency) of the filter the more stress it will put on the fan. > Don't worry! You can still have your cheap air purifier as long as the filter area is increased to decrease the effect of air resistance. Instead of using one 20x20x1 filter we'll use two 20x25x1 filters which increases the filter surface area over 250%. It's a little more expensive because you're using two filters instead of one but the increased filter surface area also helps the filter last longer before it gets clogged up and we're saving on energy use compared to a single filter.

rootusrootus 6y ago
> we'll use two 20x25x1 filters which increases the filter surface area over 250%. That seems like an overly complicated first attempt, why not just switch to a 20x20x4?
- post_break 6y ago
  Those can be harder to source I've found. Cheaper to use two filters than one as well since you can replaced the prefilter more often.
  - bluGill 6y ago
    I used a 25x20x4 which was easy to find. Even though I didn't do anything to stop air from flowing around the backside of the filter to the fan it still gets noticeably black fast.
  - rootusrootus 6y ago

deleted 6y ago

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Mister_Snuggles 6y ago

The only downside of the DIY route is the noise. Most box fans that I've seen are very noisy relative to "real" air cleaners/purifiers. I've used a box fan plus furnace filter for years to deal with cat litter dust. Since the particles I want to filter are quite large, I can get away with using a fairly cheap furnace filter. The nice thing about this is that it's really easy to scale it to your requirements. To deal with the noise, I used a motion sensor and switched outlet to only run it for 10 minutes after the litter has been used. This runs through my existing home automation stuff (Home Assistant+Node-RED), but it could also be done with an Arduino (you don't even something that powerful - two I/O lines is enough), a PIR sensor, and a solid state relay for the fan control. I've tried a number of low-dust/no-dust litters, but most of have lead to the cats finding other places to do their business or have disappeared from store shelves when I've needed to replenish it. Everything about this is a compromise between litter acceptance, litter availability, litter cost, filter cost, filter life, noise, and dust production.

batter 6y ago
I'm using Vornado fan with 2 16x20x1 merv 11 filters from lowes (2 to make triangle). And in lowest speed it's very quiet. I have commercial air cleaner and my diy is way cheaper to run and as quiet. https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-air-purifier.html?m=1 https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-a... - like this.
- Mister_Snuggles 6y ago
  I swear I came across that post at one point - it looks very familiar! I really want to do this because 20x25x1 filters are a LOT easier to find (where I am anyway) than 20x20x1 filters. The only obstacle is that the resulting unit ends up taking a lot more space, and that's at a bit of a premium in the place I need it. If I move the litter boxes to somewhere with more space I will definitely switch to this design.
deleted 6y ago
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yoyman 6y ago

Did this with a small fan (around 6" in diameter) and it died out within a week of use. To be fair, the HEPA filter I used was really restrictive but anyone who plans to do this should probably keep that in mind.

nether 6y ago

The downside is that these filters sell out in places where there are fires. So the time to buy them is now.

rootusrootus 6y ago
I agree. When we had wildfire smoke descend on our area, the stores sold out within hours. If you live in an area which routinely experiences wildfire smoke, it's worth stocking a few filters, both 20x20x4 for box fans and whatever size your furnace uses. It made a huge difference to have fresh filters to swap out every few days.

textech 6y ago

Without an activated carbon filter on the outside to collect dust and other bigger particles your HEPA filters will be utterly useless in only a few weeks. If you keep your windows closed then I guess it could last longer but then you may have CO2 issues.

causality0 6y ago

Reading the graph, it looks like it took both purifiers a full twenty minutes to make the air as clean as it was before the fans kicked all the particles off surfaces and into the air.

nkurz 6y ago
Are you making a joke, or did you perhaps miss the description of the expiriment that involves burning a piece of incense when he starts the test? I agree though that in a more formal test, a "null" version without the incense would be a good comparison.

abyssin 6y ago

I tried building a similar air purifier in Belgium last year and I've never been able to source the parts at a similar cost.

boredpudding 6y ago
Same problem for The Netherlands here. I just want to put a hepa filter on a fan, but unfortunately I can only find hepa filters for vacuum cleaners or existing air purifiers. If anybody has a link for where to buy a regular square/rectangle hepa filter, that would be great.
- bluGill 6y ago
  This is a hepa replacement for one of the air purifiers you can buy. Just go to the store, find an air purifier, read the back of the box to see what filter it takes, then look up/down/over a shelf - they replacement filters are generally carried in the same aisle. You will probably be surprised at just how small the filters for most of those commercial air purifiers are.

Scene_Cast2 6y ago

I did a bit of experimentation with different filters for a DIY build like that. Setup: I tried out a bunch of different air filters in a medium-sized apartment. Two objectives: how fast PM2.5 is removed, and down to what level can it bring down PM2.5. From worst to best: * Nordic Pure 20x20x4M15+C MERV 15 - the carbon layer absolutely kills airflow, doesn't work * Filtrete 20x20x1, AC Furnace Air Filter, MPR 2800 / MERV 14 - works okay * Nordic Pure 20x20x4 - works well * Lennox Corporation X7935 20X20X5 MERV 16 - works the best. This thing is an absolute beast in terms of filtration performance; significantly lower steady-state PM2.5 readings during the California fires. Test equipment: Awair and Plume Flow v2.

esaym 6y ago
I've been using the Nordic Pure filters for many years now. Way better (and cheaper) than the stuff you get at home depot, etc.

ocdtrekkie 6y ago

I used a 20x20x1 taped to a box fan for over a year as an air purifier because I had ferrets, and ferrets leave a particular odor floating about most of the time. I didn't want to use an odor-removing filter with charcoal on my furnace because they're super thick and put strain on the motor, and I don't care about the box fan. It worked really well, probably the biggest challenge was securing the filter to the fan: I put my filter on the "out" side of the fan, and it was really hard to keep it attached. Duct tape and the like actually often would fail, and air would just start moving around the filter. For good airflow through the filter I really had to give the air little other choice. I'm curious if it would've worked just as well had I filtered the air on the "in" side of the box fan... keeping it attached would've been much easier, but I'm not sure if there's any forces that would make "pushing" air through the filter work better than "pulling" air through the filter.

rootusrootus 6y ago
Personally I'd do the filter on the input side of the fan. They're designed to be pretty free flowing to begin with, since they're intended to be on the input side of the furnace blower. It would also keep the fan itself a bit cleaner. Suction is almost sufficient by itself to keep the filter in place. FWIW, Alton Brown made a jerky dryer using a similar technique, and he used bungee cords to hold the filters to the box fan. That's probably less hassle than duct tape.
turtlebits 6y ago
They sell silicon bands designed to keep the filter attached to 20" box fans. They're <$10 and work well.

roflchoppa3 6y ago

These boxfan + hepa filters seem pretty affordable, but seem to have a really strange physical footprint. Do SUV cabin fans not push enough CFM to be effective? https://www.rockauto.com/en/moreinfo.php?pk=10863244&cc=1502557&jsn=822 https://www.rockauto.com/en/moreinfo.php?pk=10863244&cc=1502... This paired with a filter seems like it would do the job, and based on the enclosure you might be able to have a smaller footprint. No CFM data though.

UncleOxidant 6y ago
I think box fans are used because that's what people already have around. When wildfires rendered our air here in the PNW unbreathable last September a lot of folks were scrambling for something that would work quickly. Waiting around for an SUV cabin fan to arrive and then trying to figure out an enclosure made that a non-starter.
bluGill 6y ago
They should, and probably stay closer to the rated cfm with a filter there. However now you need a 12 volt power supply instead of plugging into a normal outlet. Plus you need to design a housing for it. Neither of these are hard problems for DIY, but they are not nearly as easy or quick as a cheap fan and some tape.

dbsmith83 6y ago

Love the data on this experiment. I made one of these during the wildfires, and it was apparent that it was working after just a few days. The once colorless filter had become noticeably grey/yellowed.

acvny 6y ago

Wait. I did the same thing and after running the filter for several months, when I disassembled it, the filter was clean as new, but the bulk of the dust was laying somewhere between the blades and the motor and on the edges of the fan. It is very hard to actually get the right type of fan and assemble it correctly because simple fan will not work (the air will flow over the blades, thus not providing the required suction so that the air passes through the filters). The filters have pretty high resistance so you need a very efficient sucking / blowing fan to make the air pass through.

neilv 6y ago

When I made one of these with a cheap box fan and a good 3M filter that covered one face completely, I ran into the airflow problem. For reasons (cheap fan with the cable and switch going in the center of the rear), I had to have the filter on the outflow side of the fan, and I was getting much more airflow back out the corners of inflow side of the fan than through the filter. I'm frugally-inclined, but I was desperate to clean up some nasty dust from construction work, so I ended up buying 2 Levoit air purifier appliances. They have better airflow, and are much quieter. They also exhaust upwards, which I guessed might be less likely to disrupt a whole lot of dust-at-rest at once. A problem now is not so much that the consumable filter cartridges are expensive, which I could live with if they are motivated to be high-quality, but that counterfeits are inevitable. A related problem is that I don't have a great way to measure filter effectiveness, whether I'm buying high-quality genuine, third-party, counterfeit, or genuine facing pressure from counterfeits and seeing less value in remaining high-quality.

EricE 6y ago
Just cut down a furnace filter to fit inside. I’ve been doing that for years with a few Sharp units I can’t get filters for any more (discontinued) and some newer Levoits. If you leave them running all the time on low they are more effective than running them part of the day with the “official” filters.
- neilv 6y ago
  Good idea -- that would work for my larger Levoit unit, which takes a pair of flat rectangular filters, though not for the smaller cylindrical unit. For now, I'm going to try to keep buying the Levoit filters, since I think they have more "layers" than the furnace filters, and I don't have a good sense of what all I should to be filtering in this location. But it's good to have a backup plan.

pengaru 6y ago

I thought an "air purifier" included VOC removal, as in had activated charcoal in the filter medium. Isn't this just an air filter if it only removes particulates?

batushka3 6y ago

TOP indor polution sources from my 2 year experience with concentration detector: - frying pan with just a bit of smoke, pm 2.5/10 goes hundres easy - parafin candles, HCHO formaldehyde goes over 1mg/m3 (can't tell if other are better though) - presurized cans (sunscreen, desodorant, shoe care etc) pm goes hundreds easy - spray surface sanitizer alcohol based, registers high folmaldehyde too And it's incredible how well air mixes in flat, spray in one room - few minutes later register in another. No fans needed.

ionwake 6y ago

I don't mean to go against the spirit of the thread but does anyone know how these DIY compare to actual commercial products - and can anyone recommend a good air purifier? Thank you ( I will remove this comment if it is offtopic )

hundchenkatze 6y ago
> how these DIY compare to actual commercial products That's actually what this article discusses: "If you’re in a hurry, this post says that if you strap two HEPA filters to a box fan, it will clear the air of basically all the particles we can measure, and it will do it faster than a commercial filter that costs twice as much."
- ionwake 6y ago
  Thank you
nkurz 6y ago
I think a reasonable summary is that the homemade versions are cheaper and actually more effective than most commercial versions, with the downside that they are usually larger and often noisier. The largest factor for effective air purification is airflow, and the homemade versions offer higher airflow but at the cost of greater noise and looking janky. There are somewhat reasonable fears that the homemade versions might be fire hazards, but in practice they are probably no more dangerous than the bare fans on which they are based. Really, both solutions work just fine. If looking commercial, be sure to consider the cost and availability of replacement filters. If considering DIY, realize that you don't need to get it perfect, almost anything is better than nothing.
EricE 6y ago
Radial fans (also known as squirrel cage blower, or visualize a water wheel from an old fashioned water driven flour mill) will move air more quietly than an axial fan in a box fan. Levoit on Amazon are some great, relatively inexpensive units and when the HEPA filter it comes with needs replacing I use use furnace filters cut down to size. If you want it to last a bit longer, you can use quilt batting as a pre-filter before the furnace filter to catch the big stuff and extend the life of the furnace filter.

igor47 6y ago

i am also a "fan" of the home-made air purifier. we had a bunch of them around our house over the fire season this summer. it's helpful to compare the numbers: https://igor.moomers.org/minimal-viable-air-quality https://igor.moomers.org/minimal-viable-air-quality if you want to build your own monitor, i still have a bunch of PMS7003 sensors around. ping me if you want to grab one in berkeley

giantg2 6y ago

It could be more efficient if you use a old cardboard boxes and duct tape to build a custom fit shroud connecting/ mounting the filter to the fan. I use a similar setup on a homemade glove box / flow hood type device for culturing mushrooms. I have an 8" duct booster fan pushing air into a cardboard box. Between that box and the flow hood I have a HEPA filter sandwiched between them using bolts and wing nuts. The only thing I'd change is the fan. The booster fan was cheap, but has trouble with the back pressure (lower air flow). I would probably look at using squirrel cage or maybe a radon fan. They should be able to handle the pressure.

kingnothing 6y ago

Wirecutter also has some info on this setup compared to commercially available options. tldr: a good HEPA filter will remove 99% of impurities, their box fan setup removed 87%. https://www.nytimes.com/wirecutter/reviews/best-air-purifier/#what-if-you-just-put-a-furnace-filter-on-a-box-fan https://www.nytimes.com/wirecutter/reviews/best-air-purifier...

leafmeal 6y ago

I'd like to see a control run where you don't attach a filter to the fan. My hypothesis is that it won't look very different compared to when the filter is attached (I hope I'm wrong though).

lurkertech 6y ago

The OP design might see a big jump in effectiveness if they add an air guide one one or both sides of the box fan (just make a simple square cardboard tube with some tape) so that by the time the air reaches the filter, it has laminar flow. This helped enormously in a DIY design I kludged together in Thailand (where box fans are NOT available, so I had to get something to work with regular round desk fans, which everyone already has): https://slice-of-thai.com/air/ https://slice-of-thai.com/air/ "cannon" design (ugly but effective): https://slice-of-thai.com/air/#p3 https://slice-of-thai.com/air/#p3 This may also explain why the OP heard that the tape "helps" but was surprised when it didn't. The key issue is likely not sealing with tape but rather forming an air guide.