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Experiments on a $50 DIY air purifier (2020)

409 points by ZainRiz 5y ago

hourislate 5y ago
It was sub $50 a few weeks ago and comes with a Merv 10 filter. You can always buy the heavy duty Lasko 20 x 20 for sub $30 and stick a MERV 13 Filter behind it when it's on. The suction keeps the filter pinned to the back and you get HEPA with VOC removal. https://www.amazon.com/Lasko-FF305-20-inch-Purifier-Purifying/dp/B089P1Z38J/ref=zg_bs_3303865011_5?_encoding=UTF8&psc=1&refRID=SH1869KWFX1TB5RJQ73P https://www.amazon.com/Lasko-FF305-20-inch-Purifier-Purifyin...
- fighterpilot 5y ago
  The MERV 13 is neither a HEPA nor VOC filter. It is almost as good as a HEPA filter (it performs the same function albeit it's very slightly less effective), but it doesn't filter out VOCs. For VOCs you need an activated carbon layer.
  - hourislate 5y ago
    I do recallusing a 3M filter that removed VOC's but perhaps it was a higher MERV Rating. Here is a substitute. FPR 12 HomeDepot filter. https://www.homedepot.com/p/Honeywell-16-x-25-x-1-FPR-12-Air-Filter-HW12FPR16251/304984285 https://www.homedepot.com/p/Honeywell-16-x-25-x-1-FPR-12-Air... The Honeywell Ultimate Allergen filter is made of a reinforced pleated media designed to capture particles down to 0.3 microns, 300x smaller than the diameter of a human hair. This filter incorporates the power of Arm and Hammer odor reduction technology into the media in the form of performance enhancing dots. Its large surface area improves the efficiency, providing a greater dust-holding capacity and lower energy costs. The filters electrostatic charge helps attract particles that would otherwise pass through. The Honeywell Ultimate Allergen Filter is effective against the following contaminants: dust/lint, airborne dust mite debris, pollen, pet dander, mold spores, bacteria, microscopic allergens, virus carriers, odor, most smoke and smog particles, as well as VOCs.
    - ncmncm 5y ago
      Even a MERV 17 will not clear VOCs. You need a chemical reaction, and MERV describes a purely mechanical property. It is smart to combine an activated-carbon filter with a particulate filter, particularly (heh) if you are operating a 3d printer. Those really foul the air, in ways hard to detect without instruments.
deleted 5y ago
[deleted]
second--shift 5y ago
Great study & article. Perhaps incense is not the most scientific test, but it's effective enough for this setup. Does anyone know if there is raspberry pi add-on or device that acts as an "air quality" sensor, or similar? It would be interesting to monitor this a bit more scientifically in real-time in a variety of environments. Lastly, I think I've seen this here before, or on reddit. I was inspired to do something similar myself - I use a 19" box fan and a purple 3M MERV furnace filter, 20" square. Except the rounded corners they are almost exactly the same size. Some white duct tape around the edges to seal/hold in place. Total cost is about $40 usd for the fan & filter. I would anticipate a similar performance to the leaky MERV's, as mine is filtering more air as a ratio to total air flowed by the fan.
- kcb 5y ago
  You can buy air quality sensor boards that should be pretty easy to integrate with arduino or an rpi. I have one from sparkfun.
- DeRock 5y ago
  Yes, just set up a sensor with home assistant (HA). Incredibly easy. Raspberry pi runs the HA server. The air quality sensor is attached to an ESP32 running ESPHome: https://esphome.io/#air-quality https://esphome.io/#air-quality Almost plug and play. Compiles the FW, flashes it, and the device auto connects to the network and is ready for integration into HA. I used the HM3301 air quality sensor FWIW. You can see my filter here: https://news.ycombinator.com/item?id=28395232 https://news.ycombinator.com/item?id=28395232
- Havoc 5y ago
  >Does anyone know if there is raspberry pi add-on or device that acts as an "air quality" sensor, or similar? Yes. They contain an annoying tiny fan though that gets irritating fast due to high pitch
mynameisash 5y ago
Last year when the west coast was blanketed in smoke, I setup a box fan with a HEPA filter. It probably helped, but we also found that the fan's motor was working really hard. The cord's connection point to the fan started melting (unbeknownst to us), causing the fan to intermittently cut out. I only noticed the problem when, long after the forest fires were gone, we noticed a strong smell of melted plastic coming from the fan itself. All in all, probably a good tradeoff to clean our air during the dangerous smoke levels, but there's the risk of burning out the motor. Be careful out there.
- akozak 5y ago
  You just need to increase the total area of the filter material that the fan is pushing air through. I've seen people shape them into a triangle or box with plastic/tape over seams or openings.
  - fighterpilot 5y ago
    Do we know how the air flow improves in proportion to surface area? I assume it's not quite linear. It would be good to know whether it's worth the effort to do what you're describing.
    - akozak 5y ago
      I'm sure there is an answer to that but this is DIY, so maybe just touch the fan while its been operating for a while, say "damn that sucker's getting hot!" and add more surface area until satisfied you won't burn your house down.
- Isthatablackgsd 5y ago
  That's because box fan are not designed as static pressure fan. They are a airflow fan for the purpose of moving much air as possible with minimal resistance. Static pressure designed as forced pull/push air, it have slightly thick blade and have a different motor for it. The air purifier uses the static pressure fan because of the filter media. Take computer fans for example. There are two common case fan type, the airflow and the static pressure. The airflow fan is for the tower case, the static pressure one is for the radiator (water-cooling kit).
staunch 5y ago
One thing I've found trying to find HEPA filters is that many reviews complain of the filters themselves having some kind of stinky off-gassing. I don't know what the reason for this is, or if it's a problem, but it is worrying. Reviews on most air purifiers seem to have examples of exploding fans and melting components almost burning houses down. It really seems like there's room in the air purification market for a company that is really trustworthy, effective, and affordable. Apple could do it, for example.
- Rebelgecko 5y ago
  I'm pretty happy with my Coway air purifier, and that seems like the consensus based on the reviews I've read. The purifier was pretty affordable, as are the filters (keep in mind that filter costs and electricity consumption can impact cost of ownership much more than the original purchase price!). It isn't Apple, but one of their models does have a striking resemblance to an iPod shuffle: https://m.media-amazon.com/images/I/61716qA3rEL.jpg https://m.media-amazon.com/images/I/61716qA3rEL.jpg
- vkou 5y ago
  > Reviews on most air purifiers seem to have examples of exploding fans and melting components almost burning houses down. I've read the reviews on my air purifier. These sorts of problems were prevalent among the negative reviews. Upon further inspection, the cause of most of these problems was the air purifier being shipped with a shrink-wrapped filter already installed. Unsurprisingly, turning it on without removing the shrink-wrap results a loud and non-working purifier at best - and fire, death, and bedlam at worst.
mholm 5y ago
It would be nice to see this test done with other particulate sizes. I'd wonder how effective this is for smaller particle sizes compared to competitors.
- DeRock 5y ago
  I attached a graph from this morning with raw particulate counts here: https://news.ycombinator.com/item?id=28395232 https://news.ycombinator.com/item?id=28395232
mcc1ane 5y ago
previously - https://news.ycombinator.com/item?id=25442097 https://news.ycombinator.com/item?id=25442097
lxe 5y ago
One of the key features of the expensive purifiers is reduced noise. Box fans are rather loud.
- asdff 5y ago
  I have one that cost me $30 all in and you can't hear it at all on the lowest setting (but still decent airflow when you put your hand down). Honestly the biggest benefit in my home environment is how much it has killed the fungus gnats living in my plants. I checked the filter after running it for a few weeks and it still looked relatively clean, but probably a thousand dead flies were in the bottom of it too. I have taken other measures against these flies with not much success (vinegar traps, sticky paper, watering from the bottom, mosquito bits designed to kill larvae, I tried everything I could find one the internet to no help).
- coding123 5y ago
  On the other hand a loud fan is great to help shut out the neighbors annoying dog barking.
- falcolas 5y ago
  I have box fan filters, and a small room Honeywell filter. On the lowest settings, they're about the same, on the highest, the box fan is definitely a little louder (but moves a lot more air). Turn them down when you're in a room with them - best advice I ever read.
- duffyt 5y ago
  Yeah, I think these DIY filter plans are great and I've built them myself, but I switched to commercial purifiers because of the sound. I don't have measurements but the box fan is substantially louder than either the Blue Air 211 or the Levoit Core 200 that I have.
jedimastert 5y ago
The author later made a more involved DIY filter using a duct fan https://dynomight.net/better-DIY-air-purifier.html https://dynomight.net/better-DIY-air-purifier.html
exhilaration 5y ago
Small aside, but if you're looking for a cheap, decent fan, I recommend this one. Bonus: this Lasko fan is made (or should I say "assembled") in the USA: https://www.homedepot.com/p/Lasko-20-in-Power-Plus-Box-Fan-B20540/300202629 https://www.homedepot.com/p/Lasko-20-in-Power-Plus-Box-Fan-B... My covid project was building a gym in my unfinished basement, and this is the box fan + furnace filter I use to keep the dust levels down.
- ncmncm 5y ago
  Many, perhaps most things with a "Made in USA" sticker are done with prison labor, particularly if made in Louisiana, Mississippi, or Alabama. States have contracts with such for-profit prisons requiring they be kept fully "staffed", railroading as many victims as needed not to pay contract penalties. Police departments are generally agreeable about such policies; everyone they arrest makes them look more important. So, to first order, "Made in USA" implies slave labor.
  - dr_kiszonka 5y ago
    After reading this page, I think Lasko perhaps is not using "slave labour": https://www.lasko.com/social-responsibility/ https://www.lasko.com/social-responsibility/
    - ncmncm 5y ago
      It is always hard to find out whether such declarations are wholly composed by the PR department, with no commitment from management. The page cited does not mention prison labor. The California fire crews do make quite heavy use of prison labor, which seems less problematic than corporate use, except for the safety angle.
      - dr_kiszonka 5y ago
        It does mention it. Just search for "prison labor" :)
Syonyk 5y ago
I built a "four filter fan box" this year when the smoke started rolling in (off MERV 13 filters, I think?), and have also gone about getting the measurement devices to verify that, yes, it really does work! I used a round fan, and a lot of tape to seal things to the fan, though it appears this might not matter as much as I thought. The filter area is sufficient that the fan doesn't really struggle - fan motors burning out from a single filter (as someone else reported) is a well known failure mode, and a good reason not to just strap filters flat to a fan. They rely on the airflow to cool, and it's easy to screw that up. Air quality (in terms of PM2.5 and PM10, the stuff my testers measure) coming out of the fan is very good - far lower than the room, and the filter will start pulling down air particulates in a room fairly quickly. If the room is around 5ug/m^3 PM2.5, the output from the fan is sub-1, and it moves enough air to do quite a few air exchanges per day. It really does make a difference. I'm using a couple Temtop meters to measure air quality, and while they're Chinesium, they do seem to work fairly well. Also, if you ever wondered: Use your stove's vent fans! The air quality difference in PM2.5/PM10 between cooking without those running, vs cooking with them on, is HUGE! I'll see spikes in the kitchen up well past 100ug/m^3 PM2.5 after browning meat, which is worse than it is outside in heavy forest fire smoke! https://www.sevarg.net/2021/08/28/temtop-air-quality-sensors/ https://www.sevarg.net/2021/08/28/temtop-air-quality-sensors...
- TrainedMonkey 5y ago
  Filter's airflow restriction is primarily a function of surface area and filtration medium layers. More surface area results in a better airflow and more filtration medium layers results in a worse airflow. To maximize surface area of a filter, you can get a thicker accordion filter. This will generally provide good airflow and filtration characreristics. Sorry about video links below, I liked them at chart times, so you don't have to watch the video to check out the summary. Here is an example of airflow restriction vs filter. Note that 4 inch Honeywell filter only provides small static pressure rise over no filter option: https://youtu.be/RkjRKIRva58?t=456 https://youtu.be/RkjRKIRva58?t=456 At the same time, same filter is 5th overall in large particle removal: https://youtu.be/RkjRKIRva58?t=661 https://youtu.be/RkjRKIRva58?t=661 Thicker filters cost more, but also last longer. General rule seems to be that when you double the filter thickness, time between replacements doubles as well.
chickenpotpie 5y ago
I keep seeing articles about these and I have to ask: are people actually building them? Sure they work but they’re so ugly. Is it really worth saving $40 for your bedroom to look like a garage?
- pushswap 5y ago
  In my experience, no it's not worth it indoors in a relatively clean environment. I built 3 of these for myself with dust, carbon, & hepa filters, and I used them for over a year. They are louder, use more energy, are less efficient, uglier & cost more money than several store bought alternatives which often include several carbon & hepa filter replacements which is the main expense in building these fans. They work well in the garage though, and if you don't plan on changing the filters frequently they are the cheapest build option.
- Syonyk 5y ago
  I certainly built an ugly filter box this year. What's better? My bedroom having some home built filtration art in it, or the inside of my lungs looking like they've sucked down a lot of forest fire? If you care that deeply about what your room looks like, great. There are plenty of good looking air filters you can pay lots of money for. I'll spend far less to have a far better filter out of the deal.
  - asdff 5y ago
    These articles made sense 5 years ago when there were only a few high priced air filters widely available online, but now the chinese copies have proliferated amazon and other sites and the market is pretty saturated imo. My filter was $30, about the size and look of a larger bluetooth speaker, it is dead silent on the lowest setting, replacement filters another $15 or so, and its good to filter my bedroom pretty heavily in terms of square footage. Can't beat that. At that price we have one of these in a couple different rooms.
    - simias 5y ago
      Ditto. Mine is really quiet too on low settings, which is great if you want to leave it on while you sleep. They also have a carbon layer that's supposed to help with odours and that pollution that can't meaningfully be caught by a simple HEPA filter, although I don't know how effective it is.
DeRock 5y ago
Heres my set up: - 20 inch box fan - 20x20x4 MERV12 filter. The 4-inch pleating is key here, to reduce the air resistance on the box fan. - 1 inch dust pre-filter. This is course, low air resistance, and is for increasing the life of the more expensive MERV12/HEPA filter (so it doesn't get clogged with easy to filter dust). - Both filters are on the intake side of the box fan. This means you don't need a bungee cord because the intake has negative pressure, the filters just "stick". It also means you keep your box fan flowing with only cleaned air. Heres a pic of my setup: https://imgur.com/a/O4I6Udx https://imgur.com/a/O4I6Udx And here's a graph this morning from my air quality sensor, after turning on the fan/filter: https://imgur.com/a/c5GMNcC https://imgur.com/a/c5GMNcC The peak at ~30ug/m^3 PM2.5 is approximately 100 AQI. The fan was on the other side of my 900 sqft house from the sensor.
- chana_masala 5y ago
  Thanks for sharing, that's neat how simple it is. What air quality sensor setup do you have?
  - DeRock 5y ago
    HM3301: https://wiki.seeedstudio.com/Grove-Laser_PM2.5_Sensor-HM3301/ https://wiki.seeedstudio.com/Grove-Laser_PM2.5_Sensor-HM3301... Attached over I2C to an ESP32 running ESPHome: https://esphome.io/#air-quality https://esphome.io/#air-quality Which in turn reports to a raspberry pi running Home Assistant: https://www.home-assistant.io/installation/ https://www.home-assistant.io/installation/ Took about 20 mins to set everything up.
    - jve 5y ago
      > Took about 20 mins to set everything up. Given you have Home Assistant installed and have previously used ESPHome. And attached wires. Otherwise, you may be up to a journey that takes many hours/days. That's the difference experience makes :)
      - DeRock 5y ago
punnerud 5y ago
Posted before: https://news.ycombinator.com/item?id=26106542 https://news.ycombinator.com/item?id=26106542 Warning: The fan motor gets cooled from the airflow. This is drastically altered to something it is not designed for, and can result in fire.
- at_a_remove 5y ago
  So you mount another, smaller fan inside of the box fan to cool the motor ... I wonder how far this can regress.
- ncmncm 5y ago
  So, put the filter on the intake side, and prefer a 4-inch thick filter, if you can find one.
aidenn0 5y ago
Same site, different setup: https://dynomight.net/better-DIY-air-purifier.html https://dynomight.net/better-DIY-air-purifier.html
btbuildem 5y ago
> This was unexpected. I thought the tape would help, but I wouldn’t have been surprised if it hurt instead. Instead, there’s basically no difference at all. I don’t know enough about fluid dynamics to even speculate about what’s happening here, so I won’t try. The fan pumps as much air through the filters as it will, regardless of whether the areas not covered by fans are taped off or not.
everdrive 5y ago
I have one "built" in my house right now. I say "built" as I have simply bungie-tied an air filter to a box fan. I have also hooked it up to one of those analog outlet timers so it's not running the whole day. I'm sure folks on HN could do a much better and more interesting job: - Rasberry Pi Zero runs PM2.5 sensor. - Fan is controlled by Raspberry Pi Zero. - Fan only turns on when PM2.5 levels are over threshold for n seconds. - Fan only turns off when PM2.5 levels are below threshold for n seconds. - Add some collection and graphing of PM2.5 levels over time for fun.
- dfinninger 5y ago
  What PM2.5 Sensor are you using? I'm about to start a build to monitor indoor AQI/CO2.
  - everdrive 5y ago
    So currently I'm not using any. (the list above is what I am NOT doing) Previously, I had sensor from this family hooked up to a Raspberry pi doing measurement only: https://www.amazon.com/AITRIP-Precision-Quality-Detection-Compatible/dp/B08QRJSVW7/ https://www.amazon.com/AITRIP-Precision-Quality-Detection-Co...
    - dfinninger 5y ago
      Oh, ha! So much for my reading comprehension. Thanks for the link. I think that should be compatible with a Pi. I can try it out in a couple weeks when I get some free time to tinker.
  - thamer 5y ago
    I have a PM2.5 sensor hooked to a Raspberry Pi Zero W, I use the PMS5003 ($40 at Adafruit). I have a script running in a screen session that posts its measurements once a minute to a Google Cloud Function which then inserts it into BigQuery, and then a DataStudio dashboard to visualize the data. The cloud cost is $0, it all falls in the free tier. The Raspberry Pi Zero W is just $10. I have a separate station with an SCD-41 CO2 sensor (+ temp/humidity/pressure with an MCP9808) all hooked up to a TinyPico board with Stemma QT that also post metrics to BigQuery in a similar way. And yes I'm planning to combine both into a single device very soon :-)
Zenst 5y ago
Wonder if any PC gaming case manufacturers will ever think to add some HEPA filters to the unit - be good for the PC to have cleaner air going in and that airflow out would be put to good use to cool the PC. I recall seeing a video (BigClive on YT) in which he gets a fan and used a peace of kitchen roll as a filter and found it fairly effective and just dug that out - https://youtu.be/kK7sJq2E0bE https://youtu.be/kK7sJq2E0bE
- mattmaroon 5y ago
  But then you’d have to change the filters and if you didn’t possibly cause airflow problems and then overheating. It would be a nice feature as long as you keep up with it though.
- fighterpilot 5y ago
  Probably cheaper MERV 8 filter would make more sense, since for this application you're mostly concerned about those larger dust particles. Also it seems like this would be a job for the fan manufacturers rather than the PC case manufacturers. The fan just needs a little slot where you can slide in a thin replaceable filter.
- leetcrew 5y ago
  you don't really want something that extreme for case fan intake. you end up "paying" for the cleaner air with higher temps or faster (louder) fans. there are some newer cases with a fine mesh screen over the intake fans. I have these in my fractal meshify 2, and they seem to work pretty well so far.
- Johnny555 5y ago
  I used to have a PC case with a cooling fan filter -- by the time I retired that PC, the dust was nearly an inch thick. I had forgotten that it had a filter, and "out of sight, out of mind". It wasn't even a HEPA filter, just a fine mesh.
kyleblarson 5y ago
I upgraded my heat pump and furnace system last year and spent the extra money to put in a filtration system. We had a two week stretch with AQI of 500+ outside from the fires and never even got a hint of smoke in the house. Great investment.
Someone1234 5y ago
I've made one of these, and they do work extremely well, but they're missing one key thing that I now consider vital: Activated Carbon. A HEPA filter is doing the heavy lifting within any air purifier, it is therefore the most important ingredient. Activated Carbon is well known for removing smells/cents, but it does more than just that: It also neutralizes VOCs and some other gaseous pollutants which can travel through a HEPA filter. This is why I currently have a commercial air filtering solution instead of my box fan + 3M filter, it has suspended Activated Carbon around the HEPA filter fabric. I don't know how I'd achieve that with my box fan solution, and I've not seen anyone else try to address it. Other air purification technologies have safety issues (e.g. ionizers, ozone) or just outright don't work (e.g. UV light inside a channel). HEPA and Activated Carbon have measurable upsides with no downsides aside from their cost. They're the best we have today in spaces occupied by people.
- sparker72678 5y ago
  A DIY solution would be to get activated carbon that's intended for water filtration (fish tanks, etc). Then add a step that moves the air through the media (put it into a mesh box, or heck even just blow the air down onto the carbon that's sitting in a "dish" or some kind. Certainly not as efficient as a commercial all-in-one solution, but it that's not available to you, it can work. You can even make your own "activated charcoal" but it's a pretty messy and time consuming process.
  - malfist 5y ago
    Would activated charcoal for aquarium work thought for gas? We're talking about a significant difference in porosity here. Even going with something like ROX (https://www.bulkreefsupply.com/bulk-premium-rox-0-8-aquarium-carbon.html https://www.bulkreefsupply.com/bulk-premium-rox-0-8-aquarium...) it's going to have much larger pores and much smaller surface area than a carbon coating on a filter
    - sparker72678 5y ago
      Good question. I'm imagining that with a small enough space / large enough quantity of charcoal you'd still get benefits, but it's not like I sat down and tried to do all the math.
martinmunk 5y ago
I had considered something similar, but recently I found that Ikea has started selling air purifiers and sensors for very reasonable money. They even sell an activated carbon filter that you can add. Ikea FÖRNUFTIG: https://www.ikea.com/us/en/p/foernuftig-air-purifier-white-50461961/ https://www.ikea.com/us/en/p/foernuftig-air-purifier-white-5...
- afandian 5y ago
  For years I had planned to make a DIY one, but never got round to it. But a week ago bought one of the IKEA ones, plus a carbon filter, and I've been very happy. The air feels cleaner and somehow just nicer. Lots of reviews say it's underpowered for a normal sized room compared to alternative products. But for me, the alternative is a DIY one that I still wouldn't have got round to making. I'd recommend it.
- leoedin 5y ago
  I've been running one of these for a few months. The filter has slowly turned darker brown - which suggests it's working (not anywhere near forest fires here!). It has a trickle mode which is completely silent and uses about 1W of power. Something you won't get on a DIY fan.
- DoingIsLearning 5y ago
  I read a wired review [0] that says the ikea purifier "blocks 99.95 percent of particles 2.5 microns or larger". Could you confirm the technical spec of the filters you are using? Actual rating? Is it actually HEPA? The ikea website seems to just be pretty vague marketing stuff with no details. [0] https://www.wired.com/story/ikea-fornuftig-air-purifier/ https://www.wired.com/story/ikea-fornuftig-air-purifier/
  - kleyu 5y ago
    You can read more details after you click "Product details". It says that the filter corresponds to EPA12 class, so no HEPA. > The particle filter is tested according to EN 1822-1 and ISO 29463-3 which corresponds to class EPA12.
Hnrobert42 5y ago
This is awesome. I love DIY stuff and love the idea of an affordable clean air solution. For me, the aesthetics, noise, and issues like overworking the fan motor, make the extra $50 for a purpose-built filter worth it.
- ncmncm 5y ago
  Only if it actually works better. But many "purpose-built" units just cost more, but work less well. How do you tell if it will work well, before you spend?
  - Hnrobert42 5y ago
    For the one I purchased, I researched them on Consumer Reports, which independently assessed their efficacy. How do you tell they work less well before you make that assertion?
mrtnmcc 5y ago
I've built a few of these and compared with higher end air purifiers like Alen. The only downside I could find with the DIY box fans was the power consumption is much higher.. about 60W vs 6W for the commercial purifier. That is a $30/yr increased cost in electricity based on CA rates.
- diegocg 5y ago
  Surely you can use less powerful fans?
  - sooheon 5y ago
    It's about power efficiency, not total power draw. Use less powerful fans with an inefficient setup and you're not cleaning your air as well. Engineers put thought into chassis design and motor/fan specs. For this same reason I decided to get a used box from a reputable, long lasting company with top of the line CADR, and I can find plenty of aftermarket HEPA filters that are compatible.
  - mrtnmcc 5y ago
    That was a comparison was roughly for the same air cleaning rate. I think it has to do with the fan architecture. With purpose built purifiers using static pressure fans instead of air fans.
ddlatham 5y ago
Wirecutter performed a similar experiment a few years back. Their results were somewhat different, showing that the DIY version helped significantly, but not nearly as well as the best of the commerical ones they tested: 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... I wonder what accounts for the difference in results.
- ncmncm 5y ago
  Wirecutter pretty consistently produces spurious results, and bad recommendations. Ben Kuhn and Dan Luu have written about this, Cf.
- jly 5y ago
  The wirecutter DIY test used a filter with a rating of FPR 9. Since FPR isn't standardized like MERV or HEPA, it's hard to tell what this means, but likely to be < MERV 13. The OP used a HEPA filter on their testing which is > MERV 16.
amluto 5y ago
Yikes. Putting high pressure drop genuine HEPA filters on a fan like that where most of the air will bypass them defeats the purpose and is a waste of energy and money. Just get a bigger MERV 13 or better filter and cover the whole fan.
- function_seven 5y ago
  > defeats the purpose Based on his testing, it looks like it achieves purpose just fine, and slightly better than the more expensive option. > and is a waste of energy and money His setup has over 5 times the filter area (1,400 cm² at 5 cm thick vs. 300 cm² at 4 cm thick), for $10 more on the filter replacement. Energy use is higher, but it's not clear that it exceeds the savings from cheaper filter replacements. And by having those gaps in the filters, the fan motor is probably being saved from overheating. Plenty of air still gets entrained through the filters, even with some bypass. It's kind of like premature optimization when writing a program. Where you think improvements are needed isn't necessarily where they should be. Test your code, find the hot spots, and improve those. With a filter setup, it might see obvious that a good seal is needed between filter and fan. Turns out, that's not always true.
ryandrake 5y ago
Don’t most home HVAC systems already filter the air blown through them? I know I replace mine every so often. If they do, what does a separate “air purifier” buy you?
- bialpio 5y ago
  Not sure how much you'd gain, but one thing to note here is that I believe not all HVAC systems have an option of running just the blower w/o also heating the air.
  - bluGill 5y ago
    The only furnace I've seen without that option was from the 1950s (probably replaced by now for old age), but I'm sure more exist. It wouldn't be hard from someone who knows power to wire up the fan to run all the time though.
    - bialpio 5y ago
      Furnace is one thing, the wiring in place is another I guess. My house is from the '50s, and the furnace was already replaced, but we have not laid new wiring. I think I need to look into that before the next wildfire season starts...
      - bluGill 5y ago
        If nothing else it should be easy to put a fan switch right on the furnace.
- c_o_n_v_e_x 5y ago
  For central hvac systems, standard systems typically have a filter grille for each return (where the air goes in). You can also add on a higher performance filter on the return side. Something like a Trion Air Bear.
oblib 5y ago
I've been doing this for years using off the shelf filters you can buy at most any hardware store and a 20" box fan. I use the 2" blue masking tape to attach the filters to the fan and seal them so all the air that flows through them is filtered. The blue tape comes off easier than cheap masking tape and without leaving any sticky residue on the fan. I leave the fan running 24/7 and I've bought two of them in the past 15 (or more) years for my home office. I replace the old one earlier this year. It's pretty amazing how much they grab out the air. I live in a pretty rural area and the road to our house is gravel and I'm surrounded by a forest so there's dust and pollen pretty much constantly in the air. And we have cats. Lots of cats. It's also amazing how much cat hair and dander they collect. I can sure tell the difference when they're not on, and when I replace the filters after waiting longer than I probably should.
- drited 5y ago
  How often (if at all) do the filters require washing to keep them going for that long?
  - oblib 5y ago
    Sorry about the slow reply. It depends on how much dust is in the air. I live in an older home on a gravel road surrounded by forest so there's a lot of dust and pollen. It only takes about a week to start seeing some color in the filters. I use the paper disposable filters so washing them isn't an option.
    - drited 5y ago
      Interesting thanks! So I guess you change them weekly then?
nixpulvis 5y ago
Function >= Form, at equality life is good.
jiscariot 5y ago
That looks pretty similar to the Alton Brown way for making beef jerky at home.
019a 5y ago
Absolutely shameless plug: If you'd like a cleaner looking, 3d printed solution for attaching a filter to a pretty standard box fan, in this fashion, we sell some over on Etsy (use the coupon code HACKERNEWS, 20% off) [1] Does it work better than duct tape? Probably not. Duct tape does a great job of creating an airtight seal, whereas with these clamps there is a small gap between the fan and the filter. But, it does make it easier to replace the filter, looks clean, and we've ran tests with an AQI meter; definitely has a positive impact. [1] https://www.etsy.com/listing/980605509/box-fan-filter-holder-set https://www.etsy.com/listing/980605509/box-fan-filter-holder...
wintermutestwin 5y ago
Here in the toxic air of California, I decided to go with an industrial level "DIY" solution: activated charcoal air scrubber with a high CFM fan. Just hopped down to my local indoor grow shop (4 in a 10 mile radius!) and had all manner of choices. Yes, I spent about $150, but I'm quite sure that I am getting much much more than 3x value out of it.
ncmncm 5y ago
When the Caldor fire (now 210 thousand acres, 328 mi^2!) was smoking up Placerville, the downtown Home Depot had, at the main entrance, a big stack of 20x24x1-inch #9 furnace filters, box fans, and rolls of tape, with one all constructed sitting on top, running, black with trapped smoke. Kudos to HD for not jacking up prices. It is best to make them with a four inch thick filter, which HD does not stock, and higher than a #9, likewise, bought online. You can stack a #8 in front of a #17, to make the ~$100 #17 last longer, and stack two box fans (one intake, one exhaust) to get more suction. Maybe stack #8/fan/#17/fan, to keep the fans clean. But the single one-inch #9 filter cleared the smoke from my whole house in no time flat! Don't let "best" stop you from doing "good". An advantage you have from high airflow is that particles that make it through the filter once are likely to get trapped the next time around, or the 20th. Atomizing some water ought to make filters momentarily stickier, until it dries. Fan blades getting dusty can cut airflow in half! (Same on laptop cooling fans; use a vacuum cleaner on your laptop air path.)
- Johnny555 5y ago
  Kudos to HD for not jacking up prices. Not sure all the credit goes to them since if they jacked up the price during the fire, they'd be violating price gouging laws. But it is nice that they put the components for a DIY filter all in once place https://www.oag.ca.gov/consumers/pricegougingduringdisasters https://www.oag.ca.gov/consumers/pricegougingduringdisasters
  - ncmncm 5y ago
    That is news to me, and welcome. By the FAQ, it appears furnace filters must be covered. Box fans might be borderline, but even being seen publicly accused of violating gouging laws would be bad for HD.
    - Johnny555 5y ago
      The statute is broad enough (it includes "emergency supplies", and the CDC has a how-to guide on creating a box-fan filter) that I'm sure they'd be running afoul of the law if they tried to jack up prices for box fans. But yeah, any large retailer would avoid any appearance of price gouging even if they could get away with it legally just to avoid the bad publicity.
newman314 5y ago
I've been keeping an eye on the so-called Corsi-Rosenthal box. I'm probably going to build one or more at some point. There's a bunch of folks that have been optimizing this design on Twitter. The initial results look quite good and properly put together might last an entire school year. Link here: https://www.texairfilters.com/iaq-research-practice-in-action-the-corsi-rosenthal-box-air-cleaner/ https://www.texairfilters.com/iaq-research-practice-in-actio...
- hinkley 5y ago
  You could also go with smaller filters, like 14x20 to reduce the stack size, but the prices don’t scale exactly with surface area, while the pressure does. So I’m not sure how much that really buys you.
  - newman314 5y ago
    IIRC, increased surface area reduces pressure. From what I remember from reading the thread, it appeared to have pretty decent CFM through the use of more and larger filters. EDIT: I found the thread. See https://twitter.com/davidelfstrom/status/1429526273009889286?s=10 https://twitter.com/davidelfstrom/status/1429526273009889286... Thread shows the difference between using 1 and 2 filters. I think he has a separate thread in his Twitter feed measuring a 4/5 filter setup too.
    - aronpye 5y ago
      Increased surface area alone results in higher pressure losses / flow energy losses due to increased frictional losses from the higher surface area.
- minimalist 5y ago
  It is amusing how this person has the temerity to name this configuration (partially) after themselves, while people have been making and documenting these configurations for at least a decade and surely longer.
Havoc 5y ago
Prefer the commercial ones with built in particule sensor because they can ramp up & down dynamically
pacman2 5y ago
No brainer. Basically all commercial purifiers suck (air), the DIY blow it. Big difference!
cf100clunk 5y ago
Some lab tests conducted by a TV show gave the Box Fan With HEPA Filter the highest marks over commercial air purifiers: https://news.ycombinator.com/item?id=26101153 https://news.ycombinator.com/item?id=26101153
- Johnny555 5y ago
  They didn't give the box fan the highest marks, but they found that it outperformed some commercial products, including the $800 Dyson (which has a very bad price-performance ratio)
Johnny555 5y ago
For those that don't want to build their own, I bought one of these about 5 years ago: https://www.whirlpoolairpurifiers.com/product/whirlpool-wp500-whispure-air-purifier-slate-black/ https://www.whirlpoolairpurifiers.com/product/whirlpool-wp50... I measured it last year during bad air quality days, on high, it would take the bedroom from ~150 AQI to less than 10 in about 15 minutes. On low it's quiet enough to be unobtrusive while sleeping. Filters cost around $100 and are rated to last a year (and even after a heavy fire season, I've found that to be true) More expensive than a box fan + filter, but easier to use and a little more attractive. Though this year, I upgraded my furnace with an AprilAire filter box and an MERV13 filter, which has worked surprisingly well, I thought I'd still want to use the standalone HEPA filter in the bedroom, but the furnace filter keeps the entire house below AQI 10 even with outside AQI at 250 (haven't had air worse than that yet this year)
mark_l_watson 5y ago
Good ideas. Several months ago we had bad fires in the mountains of Central Arizona where my wife and I live. I bought a HEPA filter, the complete kit with enclosure, electronic controls, etc. With an extra filter, I spent about $80 and I feel it was money well spent. Making your own for less money is even better.
Causality1 5y ago
Needs a (2020) tag. This article already hit the front page when it was new.
BrickandPorter 5y ago
Product engineer here from a major filter manufacturing company. I've done a ton of tests this summer evaluating DIY box fan filters and I have a couple insights for people building box fan filters this year. 1) Box fans (and other axial fans like your ceiling fan) are terrible at pulling air and an HVAC filter will significantly reduce fan speed due to the added pressure differential across the filter. As a reference, a new MERV 13 filter can reduce fan speed by ~33% when mounted to the intake side, and ~66% if mounted to the front of a box fan. The motor used in your box fan is cooled by the air passing around it, which is why choking off the air flow through your fan can lead to overheating, damaging your fan and creating a fire hazard. If you're going to make a DIY air purifier, mount the filter to the intake side. You'll get much better performance, your fan will stay cleaner, and there's less risk of damage. 2) Some analysis of the test data from the posted article: CADR stands for Clean Air Delivery Rate, measured in cfm. It's basically a measure of how efficiently ambient air in a confined space is purified. A higher CADR means particles are pulled out of the air quicker. For now, just treat this as a relative quality value. Using the PM 2.5 graph for the tiny room experiment in the article, I'd estimate that the DIY filter the author created has a CADR value of 45-50. This is a pretty low value. The room air purifier (RAP) he used is even worse. Then, from the large room graph, I'd estimate the DIY purifier as a CADR of 30-35. This is a rough estimate for two reasons. One, room size is an important variable in this calculation and I don't have an exact value. Two, when I test a purifier, the test chamber starts at a PM 2.5 around 10^5. At the extremely low starting concentrations used in the author's experiments, the percentage of particles that are removed due to natural decay is much more significant. Side note, cigarette smoke is the standard for room air purifier testing. Incense sticks are used less commonly due to their slower particle generation. 3) For wildfire season, I recommend a MERV 13 filter for overall performance, cost effectiveness, and for smoke particle capture (PM 2.5). There is a clear trend of diminishing returns for a DIY box fan filter beyond MERV 12-13 filters, peaking at a CADR value of ~150 for a MERV 13 filter for box fans I've tested at their highest fan speeds. A MERV 13 filter is about 50% better than a MERV 10 in a box fan configuration, while a MERV 14 is actually slightly worse. This will vary based on filter brands, too. Rule of thumb, quality matters. We actually rate our filters a bit lower than their actual performance for a number of reasons. So, my company offers a standing room air purifier with a HEPA filter for about $200. It has a CADR value of 158. A box fan and a MERV 13 filter cost about $40 and has a CADR around 150 - pretty good for a DIY substitution. We even offer cheaper room air purifiers with even lower CADR values. So why buy an expensive room air purifier? First, room air purifiers use a radial fan rather than an axial fan. Axial fans create a low pressure area on the exhaust side, drawing air through the fan. Radial fans draw in lower pressure air near the axle of the fan and push out higher pressure air at an exhaust port at the radius of the fan. Room air purifiers use a radial fan to push air through the high pressure drop HEPA filters they are designed to use. HEPA filters are qualified to remove +99.9% of tiny particles (PM 0.3) in one pass. The thick filtration media requires a high pressure differential to pull air through it. A HEPA filter on an axial box fan is going to kill the motor. If you care about PM 0.3 particles, only a HEPA filter will do the job. Also, longevity. A room air purifier and HEPA filter should run for a year or longer without needing to change filters. An HVAC filter is meant to last 3 months in your home air system under a normal particle load. This lifespan can be much shorter due to poor conditions, such as smoke particles during wildfire season or drywall dust from a renovation project. (Side note - seriously, replace your filter after doing any drywall work. Anything better than a fiberglass filter can clog in just a day or two of dusty drywalling. Also also, fiberglass filters do absolutely nothing, don't buy them.) The room air purifier is designed to run for years with a high pressure differential filter. Your box fan is not. A cheap 20" box fan with an HVAC filter is a good temporary solution, but will not last nearly as long as a room air purifier used daily. Last, noise and style. RAP units are very quiet and blend into your living room space. A large box fan with a strapped on furnace filter makes for an interesting conversation piece, if you could hear your guests. 4) Don't worry about hermetically sealing your 20x20 filter to the box fan. From my experience, a filter with tape sealing every side to the fan is no more efficient than a filter held to the fan at a couple of contact points. Once the fan is on, the intake air will hold the filter close to the fan. Even with a fully sealed filter, remember that axial fans suck at sucking. Your axial box fan will actually draw air around the front corners and into the fan, even without a filter on the back. And with how easy it is to reduce the fan speed of a box fan, you're better off allowing some air through so that the fan runs at a more efficient and safer speed. 5) A filter on a box fan is definitely better than nothing and a good, cheap short term solution. RAP units are great for long term use and capturing all sizes of particulate. I'm not going to try to sell you my brand, but there is one product I advise you avoid. Recently, Lasko has released a box fan with a filter slot as a 2 in 1 product. I've tested this thing with the provided filter and it does not perform nearly as well as a DIY filter you could make. Also, the filter slot does not fit standard 20x20x1 filters. The slot is designed for a slightly smaller, 19x19x1 sized filter, meaning that if you want to buy a replacement filter or a higher quality one than provided, you won't be able to. A normal 20x20 filter (which is actually slightly smaller in W x L nominally) can barely be squeezed into this fan's slot. Just buy a better filter and some tape instead. 6) If you look for DIY box fan filters, you will find examples with 4-5 filters in a cube shape. 5 filters improves filtration efficiency by ~50%, but costs x5 as much. That's $100-$150 in filters that will last just about as long as one filter while only cleaning the air 50% better. Just invest in a room air purifier unit of the same cost instead and enjoy much cleaner air over an entire year.
giardini 5y ago
He mentions cooking: do the recently-introduced air fryers create significant emissions?
- SilverRed 5y ago
  They are basically just a little oven. They don't seem to create anywhere near the smoke level of frying on a pan.
deleted 5y ago
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Wistar 5y ago
For my spouse's classroom, I made a 4-filter-cube and fan filter setup using this video as the basis. https://youtu.be/Y7eL2OAnqc8 https://youtu.be/Y7eL2OAnqc8 I added a fan shroud on the exhaust side of the fan using a piece of cardboard with a circular cutout matching the diameter of the fan blades.
JoshTko 5y ago
What about no filters? Could it be just pushing air around into things in the room is the most effective?
jcims 5y ago
FWIW a 24" wheeled shop fan from Harbor Freight, Tractor Supply, Rural King, etc etc (sometimes HD/Lowes have them) move way more air with more authority than a box fan. They will run $150 but they can be very useful to have around even when you don't need to filter the air. (Note the motors pull more current so you still have to be careful not to overheat them) Some examples: https://www.harborfreight.com/merchandising-promotions/hot-buys/24-inch-high-velocity-shop-fan-93532.html https://www.harborfreight.com/merchandising-promotions/hot-b... https://www.tractorsupply.com/tsc/product/countyline-24-in-direct-drive-tilt-fan-bf24tfcl https://www.tractorsupply.com/tsc/product/countyline-24-in-d... https://www.homedepot.com/p/Commercial-Electric-24-in-Heavy-Duty-2-Speed-Direct-Drive-Tilt-Drum-Fan-BF24TFCE/303061463 https://www.homedepot.com/p/Commercial-Electric-24-in-Heavy-... Axial fans are still better for this application, but these fans are great to have around in summer evenings when the air outside is cooler than it is inside. Put it in one window of the house sucking the air outside, then open windows on the other end of the house and let the cool night air stream in. It's fantastic.
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legohead 5y ago
if you watch any of the woodworker youtube channels, this is basically what they do in their shops. a little more elegant, with wooden enclosures (of course), but the idea is the same -- regular house filter on a fan.
avtolik 5y ago
Does anybody from Europe has a similar DIY setup? I cannot seem to find box fans here. And in my country particular - no rectangular HEPA filters.
JoeAltmaier 5y ago
About taping the fan so all the air goes thru the filter: How does a filter work? Air that goes thru it, gets particles removed. We want to maximize air that goes thru it, and it should be air that has particles in it. To make more air go thru the filter, increase the pressure difference between the dirty side and the clean side. Fan speed is how we control this. And getting clean air away from the fan and dirty air to come close to the fan also seems directly related to fan speed - how fast and how far it ejects clean air. It's all about fan speed, baby. Plugging up parts of the fan unrelated to the filter cross section, can only slow the fan down. I see no reason why it would help in any way.
bawolff 5y ago
> I borrowed a cheap-ish ($100) air quality monitor from a friend. I think it’s made by some company in China and then re-sold by various white-label brands. I can’t figure out who the original manufacturer is. Based on data I’ve seen for the reliability of other air quality monitors, I wouldn’t trust the absolute numbers, but the I think the relative measurements should still be OK. That seems like a pretty big assumption.
stevenhubertron 5y ago
In Colorado I have been using a few different Coway air purifers and they have been able to keep the air quality in my house good, and quickly can recover that good air quality after running my swamp cooler in the morning/evening. Been overall happy with them but perhaps this box fan/merv filter is a good backup to have in case it gets really bad.
travisporter 5y ago
Can anyone provide insight as to why not taping/sealing up the uncovered portion of the fan may improve performance?
stjohnswarts 5y ago
While I'm cool with this for allergy sufferers or "smokey" conditions I think the modern world tries a bit too hard to purify and sanitize everything. Eventually you do have to step outside your home and you can't take your filtered air with you.
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kujaomega 5y ago
It will be good to see the same tests for PM1.0, TVOC and HCHO. But great job
Ashoka_rkt 5y ago
there is a company that's selling DIY air purifiers in India (and probably its neighbouring countries).. https://smartairfilters.com/in/en/product/diy-1-1-air-purifier/ https://smartairfilters.com/in/en/product/diy-1-1-air-purifi... they put the filter in front of the fan, not the air intake aka backside.. they have detailed info on how their purifiers are effective than other costly brands.. probably putting the filter on the front side is more effective?
- snivelus 5y ago
  They've confirmed that they tested it. https://twitter.com/herpiko/status/1388621526757298178 https://twitter.com/herpiko/status/1388621526757298178 I'm using this kind of DIY filter. After of putting the filter in the front of the fan for months, now I modified it into intake mode because of uncomfortable smell occurs whenever I turned the fan on. Some dusts that are already trapped here may have escaped again by sudden push of air.

409 points by ZainRiz 5y ago

It was sub $50 a few weeks ago and comes with a Merv 10 filter. You can always buy the heavy duty Lasko 20 x 20 for sub $30 and stick a MERV 13 Filter behind it when it's on. The suction keeps the filter pinned to the back and you get HEPA with VOC removal. https://www.amazon.com/Lasko-FF305-20-inch-Purifier-Purifying/dp/B089P1Z38J/ref=zg_bs_3303865011_5?_encoding=UTF8&psc=1&refRID=SH1869KWFX1TB5RJQ73P https://www.amazon.com/Lasko-FF305-20-inch-Purifier-Purifyin...

fighterpilot 5y ago
The MERV 13 is neither a HEPA nor VOC filter. It is almost as good as a HEPA filter (it performs the same function albeit it's very slightly less effective), but it doesn't filter out VOCs. For VOCs you need an activated carbon layer.
- hourislate 5y ago
  I do recallusing a 3M filter that removed VOC's but perhaps it was a higher MERV Rating. Here is a substitute. FPR 12 HomeDepot filter. https://www.homedepot.com/p/Honeywell-16-x-25-x-1-FPR-12-Air-Filter-HW12FPR16251/304984285 https://www.homedepot.com/p/Honeywell-16-x-25-x-1-FPR-12-Air... The Honeywell Ultimate Allergen filter is made of a reinforced pleated media designed to capture particles down to 0.3 microns, 300x smaller than the diameter of a human hair. This filter incorporates the power of Arm and Hammer odor reduction technology into the media in the form of performance enhancing dots. Its large surface area improves the efficiency, providing a greater dust-holding capacity and lower energy costs. The filters electrostatic charge helps attract particles that would otherwise pass through. The Honeywell Ultimate Allergen Filter is effective against the following contaminants: dust/lint, airborne dust mite debris, pollen, pet dander, mold spores, bacteria, microscopic allergens, virus carriers, odor, most smoke and smog particles, as well as VOCs.
  - ncmncm 5y ago
    Even a MERV 17 will not clear VOCs. You need a chemical reaction, and MERV describes a purely mechanical property. It is smart to combine an activated-carbon filter with a particulate filter, particularly (heh) if you are operating a 3d printer. Those really foul the air, in ways hard to detect without instruments.

deleted 5y ago

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second--shift 5y ago

Great study & article. Perhaps incense is not the most scientific test, but it's effective enough for this setup. Does anyone know if there is raspberry pi add-on or device that acts as an "air quality" sensor, or similar? It would be interesting to monitor this a bit more scientifically in real-time in a variety of environments. Lastly, I think I've seen this here before, or on reddit. I was inspired to do something similar myself - I use a 19" box fan and a purple 3M MERV furnace filter, 20" square. Except the rounded corners they are almost exactly the same size. Some white duct tape around the edges to seal/hold in place. Total cost is about $40 usd for the fan & filter. I would anticipate a similar performance to the leaky MERV's, as mine is filtering more air as a ratio to total air flowed by the fan.

kcb 5y ago
You can buy air quality sensor boards that should be pretty easy to integrate with arduino or an rpi. I have one from sparkfun.
DeRock 5y ago
Yes, just set up a sensor with home assistant (HA). Incredibly easy. Raspberry pi runs the HA server. The air quality sensor is attached to an ESP32 running ESPHome: https://esphome.io/#air-quality https://esphome.io/#air-quality Almost plug and play. Compiles the FW, flashes it, and the device auto connects to the network and is ready for integration into HA. I used the HM3301 air quality sensor FWIW. You can see my filter here: https://news.ycombinator.com/item?id=28395232 https://news.ycombinator.com/item?id=28395232
Havoc 5y ago
>Does anyone know if there is raspberry pi add-on or device that acts as an "air quality" sensor, or similar? Yes. They contain an annoying tiny fan though that gets irritating fast due to high pitch

mynameisash 5y ago

Last year when the west coast was blanketed in smoke, I setup a box fan with a HEPA filter. It probably helped, but we also found that the fan's motor was working really hard. The cord's connection point to the fan started melting (unbeknownst to us), causing the fan to intermittently cut out. I only noticed the problem when, long after the forest fires were gone, we noticed a strong smell of melted plastic coming from the fan itself. All in all, probably a good tradeoff to clean our air during the dangerous smoke levels, but there's the risk of burning out the motor. Be careful out there.

akozak 5y ago
You just need to increase the total area of the filter material that the fan is pushing air through. I've seen people shape them into a triangle or box with plastic/tape over seams or openings.
- fighterpilot 5y ago
  Do we know how the air flow improves in proportion to surface area? I assume it's not quite linear. It would be good to know whether it's worth the effort to do what you're describing.
  - akozak 5y ago
    I'm sure there is an answer to that but this is DIY, so maybe just touch the fan while its been operating for a while, say "damn that sucker's getting hot!" and add more surface area until satisfied you won't burn your house down.
Isthatablackgsd 5y ago
That's because box fan are not designed as static pressure fan. They are a airflow fan for the purpose of moving much air as possible with minimal resistance. Static pressure designed as forced pull/push air, it have slightly thick blade and have a different motor for it. The air purifier uses the static pressure fan because of the filter media. Take computer fans for example. There are two common case fan type, the airflow and the static pressure. The airflow fan is for the tower case, the static pressure one is for the radiator (water-cooling kit).

staunch 5y ago

One thing I've found trying to find HEPA filters is that many reviews complain of the filters themselves having some kind of stinky off-gassing. I don't know what the reason for this is, or if it's a problem, but it is worrying. Reviews on most air purifiers seem to have examples of exploding fans and melting components almost burning houses down. It really seems like there's room in the air purification market for a company that is really trustworthy, effective, and affordable. Apple could do it, for example.

Rebelgecko 5y ago
I'm pretty happy with my Coway air purifier, and that seems like the consensus based on the reviews I've read. The purifier was pretty affordable, as are the filters (keep in mind that filter costs and electricity consumption can impact cost of ownership much more than the original purchase price!). It isn't Apple, but one of their models does have a striking resemblance to an iPod shuffle: https://m.media-amazon.com/images/I/61716qA3rEL.jpg https://m.media-amazon.com/images/I/61716qA3rEL.jpg
vkou 5y ago
> Reviews on most air purifiers seem to have examples of exploding fans and melting components almost burning houses down. I've read the reviews on my air purifier. These sorts of problems were prevalent among the negative reviews. Upon further inspection, the cause of most of these problems was the air purifier being shipped with a shrink-wrapped filter already installed. Unsurprisingly, turning it on without removing the shrink-wrap results a loud and non-working purifier at best - and fire, death, and bedlam at worst.

mholm 5y ago

It would be nice to see this test done with other particulate sizes. I'd wonder how effective this is for smaller particle sizes compared to competitors.

DeRock 5y ago
I attached a graph from this morning with raw particulate counts here: https://news.ycombinator.com/item?id=28395232 https://news.ycombinator.com/item?id=28395232

mcc1ane 5y ago

previously - https://news.ycombinator.com/item?id=25442097 https://news.ycombinator.com/item?id=25442097

lxe 5y ago

One of the key features of the expensive purifiers is reduced noise. Box fans are rather loud.

asdff 5y ago
I have one that cost me $30 all in and you can't hear it at all on the lowest setting (but still decent airflow when you put your hand down). Honestly the biggest benefit in my home environment is how much it has killed the fungus gnats living in my plants. I checked the filter after running it for a few weeks and it still looked relatively clean, but probably a thousand dead flies were in the bottom of it too. I have taken other measures against these flies with not much success (vinegar traps, sticky paper, watering from the bottom, mosquito bits designed to kill larvae, I tried everything I could find one the internet to no help).
coding123 5y ago
On the other hand a loud fan is great to help shut out the neighbors annoying dog barking.
falcolas 5y ago
I have box fan filters, and a small room Honeywell filter. On the lowest settings, they're about the same, on the highest, the box fan is definitely a little louder (but moves a lot more air). Turn them down when you're in a room with them - best advice I ever read.
duffyt 5y ago
Yeah, I think these DIY filter plans are great and I've built them myself, but I switched to commercial purifiers because of the sound. I don't have measurements but the box fan is substantially louder than either the Blue Air 211 or the Levoit Core 200 that I have.

jedimastert 5y ago

The author later made a more involved DIY filter using a duct fan https://dynomight.net/better-DIY-air-purifier.html https://dynomight.net/better-DIY-air-purifier.html

exhilaration 5y ago

Small aside, but if you're looking for a cheap, decent fan, I recommend this one. Bonus: this Lasko fan is made (or should I say "assembled") in the USA: https://www.homedepot.com/p/Lasko-20-in-Power-Plus-Box-Fan-B20540/300202629 https://www.homedepot.com/p/Lasko-20-in-Power-Plus-Box-Fan-B... My covid project was building a gym in my unfinished basement, and this is the box fan + furnace filter I use to keep the dust levels down.

ncmncm 5y ago
Many, perhaps most things with a "Made in USA" sticker are done with prison labor, particularly if made in Louisiana, Mississippi, or Alabama. States have contracts with such for-profit prisons requiring they be kept fully "staffed", railroading as many victims as needed not to pay contract penalties. Police departments are generally agreeable about such policies; everyone they arrest makes them look more important. So, to first order, "Made in USA" implies slave labor.
- dr_kiszonka 5y ago
  After reading this page, I think Lasko perhaps is not using "slave labour": https://www.lasko.com/social-responsibility/ https://www.lasko.com/social-responsibility/
  - ncmncm 5y ago
    It is always hard to find out whether such declarations are wholly composed by the PR department, with no commitment from management. The page cited does not mention prison labor. The California fire crews do make quite heavy use of prison labor, which seems less problematic than corporate use, except for the safety angle.
    - dr_kiszonka 5y ago
      It does mention it. Just search for "prison labor" :)

Syonyk 5y ago

I built a "four filter fan box" this year when the smoke started rolling in (off MERV 13 filters, I think?), and have also gone about getting the measurement devices to verify that, yes, it really does work! I used a round fan, and a lot of tape to seal things to the fan, though it appears this might not matter as much as I thought. The filter area is sufficient that the fan doesn't really struggle - fan motors burning out from a single filter (as someone else reported) is a well known failure mode, and a good reason not to just strap filters flat to a fan. They rely on the airflow to cool, and it's easy to screw that up. Air quality (in terms of PM2.5 and PM10, the stuff my testers measure) coming out of the fan is very good - far lower than the room, and the filter will start pulling down air particulates in a room fairly quickly. If the room is around 5ug/m^3 PM2.5, the output from the fan is sub-1, and it moves enough air to do quite a few air exchanges per day. It really does make a difference. I'm using a couple Temtop meters to measure air quality, and while they're Chinesium, they do seem to work fairly well. Also, if you ever wondered: Use your stove's vent fans! The air quality difference in PM2.5/PM10 between cooking without those running, vs cooking with them on, is HUGE! I'll see spikes in the kitchen up well past 100ug/m^3 PM2.5 after browning meat, which is worse than it is outside in heavy forest fire smoke! https://www.sevarg.net/2021/08/28/temtop-air-quality-sensors/ https://www.sevarg.net/2021/08/28/temtop-air-quality-sensors...

TrainedMonkey 5y ago
Filter's airflow restriction is primarily a function of surface area and filtration medium layers. More surface area results in a better airflow and more filtration medium layers results in a worse airflow. To maximize surface area of a filter, you can get a thicker accordion filter. This will generally provide good airflow and filtration characreristics. Sorry about video links below, I liked them at chart times, so you don't have to watch the video to check out the summary. Here is an example of airflow restriction vs filter. Note that 4 inch Honeywell filter only provides small static pressure rise over no filter option: https://youtu.be/RkjRKIRva58?t=456 https://youtu.be/RkjRKIRva58?t=456 At the same time, same filter is 5th overall in large particle removal: https://youtu.be/RkjRKIRva58?t=661 https://youtu.be/RkjRKIRva58?t=661 Thicker filters cost more, but also last longer. General rule seems to be that when you double the filter thickness, time between replacements doubles as well.

chickenpotpie 5y ago

I keep seeing articles about these and I have to ask: are people actually building them? Sure they work but they’re so ugly. Is it really worth saving $40 for your bedroom to look like a garage?

pushswap 5y ago
In my experience, no it's not worth it indoors in a relatively clean environment. I built 3 of these for myself with dust, carbon, & hepa filters, and I used them for over a year. They are louder, use more energy, are less efficient, uglier & cost more money than several store bought alternatives which often include several carbon & hepa filter replacements which is the main expense in building these fans. They work well in the garage though, and if you don't plan on changing the filters frequently they are the cheapest build option.
Syonyk 5y ago
I certainly built an ugly filter box this year. What's better? My bedroom having some home built filtration art in it, or the inside of my lungs looking like they've sucked down a lot of forest fire? If you care that deeply about what your room looks like, great. There are plenty of good looking air filters you can pay lots of money for. I'll spend far less to have a far better filter out of the deal.
- asdff 5y ago
  These articles made sense 5 years ago when there were only a few high priced air filters widely available online, but now the chinese copies have proliferated amazon and other sites and the market is pretty saturated imo. My filter was $30, about the size and look of a larger bluetooth speaker, it is dead silent on the lowest setting, replacement filters another $15 or so, and its good to filter my bedroom pretty heavily in terms of square footage. Can't beat that. At that price we have one of these in a couple different rooms.
  - simias 5y ago
    Ditto. Mine is really quiet too on low settings, which is great if you want to leave it on while you sleep. They also have a carbon layer that's supposed to help with odours and that pollution that can't meaningfully be caught by a simple HEPA filter, although I don't know how effective it is.

DeRock 5y ago

Heres my set up: - 20 inch box fan - 20x20x4 MERV12 filter. The 4-inch pleating is key here, to reduce the air resistance on the box fan. - 1 inch dust pre-filter. This is course, low air resistance, and is for increasing the life of the more expensive MERV12/HEPA filter (so it doesn't get clogged with easy to filter dust). - Both filters are on the intake side of the box fan. This means you don't need a bungee cord because the intake has negative pressure, the filters just "stick". It also means you keep your box fan flowing with only cleaned air. Heres a pic of my setup: https://imgur.com/a/O4I6Udx https://imgur.com/a/O4I6Udx And here's a graph this morning from my air quality sensor, after turning on the fan/filter: https://imgur.com/a/c5GMNcC https://imgur.com/a/c5GMNcC The peak at ~30ug/m^3 PM2.5 is approximately 100 AQI. The fan was on the other side of my 900 sqft house from the sensor.

chana_masala 5y ago
Thanks for sharing, that's neat how simple it is. What air quality sensor setup do you have?
- DeRock 5y ago
  HM3301: https://wiki.seeedstudio.com/Grove-Laser_PM2.5_Sensor-HM3301/ https://wiki.seeedstudio.com/Grove-Laser_PM2.5_Sensor-HM3301... Attached over I2C to an ESP32 running ESPHome: https://esphome.io/#air-quality https://esphome.io/#air-quality Which in turn reports to a raspberry pi running Home Assistant: https://www.home-assistant.io/installation/ https://www.home-assistant.io/installation/ Took about 20 mins to set everything up.
  - jve 5y ago
    > Took about 20 mins to set everything up. Given you have Home Assistant installed and have previously used ESPHome. And attached wires. Otherwise, you may be up to a journey that takes many hours/days. That's the difference experience makes :)
    - DeRock 5y ago

punnerud 5y ago

Posted before: https://news.ycombinator.com/item?id=26106542 https://news.ycombinator.com/item?id=26106542 Warning: The fan motor gets cooled from the airflow. This is drastically altered to something it is not designed for, and can result in fire.

at_a_remove 5y ago
So you mount another, smaller fan inside of the box fan to cool the motor ... I wonder how far this can regress.
ncmncm 5y ago
So, put the filter on the intake side, and prefer a 4-inch thick filter, if you can find one.

aidenn0 5y ago

Same site, different setup: https://dynomight.net/better-DIY-air-purifier.html https://dynomight.net/better-DIY-air-purifier.html

btbuildem 5y ago

> This was unexpected. I thought the tape would help, but I wouldn’t have been surprised if it hurt instead. Instead, there’s basically no difference at all. I don’t know enough about fluid dynamics to even speculate about what’s happening here, so I won’t try. The fan pumps as much air through the filters as it will, regardless of whether the areas not covered by fans are taped off or not.

everdrive 5y ago

I have one "built" in my house right now. I say "built" as I have simply bungie-tied an air filter to a box fan. I have also hooked it up to one of those analog outlet timers so it's not running the whole day. I'm sure folks on HN could do a much better and more interesting job: - Rasberry Pi Zero runs PM2.5 sensor. - Fan is controlled by Raspberry Pi Zero. - Fan only turns on when PM2.5 levels are over threshold for n seconds. - Fan only turns off when PM2.5 levels are below threshold for n seconds. - Add some collection and graphing of PM2.5 levels over time for fun.

dfinninger 5y ago
What PM2.5 Sensor are you using? I'm about to start a build to monitor indoor AQI/CO2.
- everdrive 5y ago
  So currently I'm not using any. (the list above is what I am NOT doing) Previously, I had sensor from this family hooked up to a Raspberry pi doing measurement only: https://www.amazon.com/AITRIP-Precision-Quality-Detection-Compatible/dp/B08QRJSVW7/ https://www.amazon.com/AITRIP-Precision-Quality-Detection-Co...
  - dfinninger 5y ago
    Oh, ha! So much for my reading comprehension. Thanks for the link. I think that should be compatible with a Pi. I can try it out in a couple weeks when I get some free time to tinker.
- thamer 5y ago
  I have a PM2.5 sensor hooked to a Raspberry Pi Zero W, I use the PMS5003 ($40 at Adafruit). I have a script running in a screen session that posts its measurements once a minute to a Google Cloud Function which then inserts it into BigQuery, and then a DataStudio dashboard to visualize the data. The cloud cost is $0, it all falls in the free tier. The Raspberry Pi Zero W is just $10. I have a separate station with an SCD-41 CO2 sensor (+ temp/humidity/pressure with an MCP9808) all hooked up to a TinyPico board with Stemma QT that also post metrics to BigQuery in a similar way. And yes I'm planning to combine both into a single device very soon :-)

Zenst 5y ago

Wonder if any PC gaming case manufacturers will ever think to add some HEPA filters to the unit - be good for the PC to have cleaner air going in and that airflow out would be put to good use to cool the PC. I recall seeing a video (BigClive on YT) in which he gets a fan and used a peace of kitchen roll as a filter and found it fairly effective and just dug that out - https://youtu.be/kK7sJq2E0bE https://youtu.be/kK7sJq2E0bE

mattmaroon 5y ago
But then you’d have to change the filters and if you didn’t possibly cause airflow problems and then overheating. It would be a nice feature as long as you keep up with it though.
fighterpilot 5y ago
Probably cheaper MERV 8 filter would make more sense, since for this application you're mostly concerned about those larger dust particles. Also it seems like this would be a job for the fan manufacturers rather than the PC case manufacturers. The fan just needs a little slot where you can slide in a thin replaceable filter.
leetcrew 5y ago
you don't really want something that extreme for case fan intake. you end up "paying" for the cleaner air with higher temps or faster (louder) fans. there are some newer cases with a fine mesh screen over the intake fans. I have these in my fractal meshify 2, and they seem to work pretty well so far.
Johnny555 5y ago
I used to have a PC case with a cooling fan filter -- by the time I retired that PC, the dust was nearly an inch thick. I had forgotten that it had a filter, and "out of sight, out of mind". It wasn't even a HEPA filter, just a fine mesh.

kyleblarson 5y ago

I upgraded my heat pump and furnace system last year and spent the extra money to put in a filtration system. We had a two week stretch with AQI of 500+ outside from the fires and never even got a hint of smoke in the house. Great investment.

Someone1234 5y ago

I've made one of these, and they do work extremely well, but they're missing one key thing that I now consider vital: Activated Carbon. A HEPA filter is doing the heavy lifting within any air purifier, it is therefore the most important ingredient. Activated Carbon is well known for removing smells/cents, but it does more than just that: It also neutralizes VOCs and some other gaseous pollutants which can travel through a HEPA filter. This is why I currently have a commercial air filtering solution instead of my box fan + 3M filter, it has suspended Activated Carbon around the HEPA filter fabric. I don't know how I'd achieve that with my box fan solution, and I've not seen anyone else try to address it. Other air purification technologies have safety issues (e.g. ionizers, ozone) or just outright don't work (e.g. UV light inside a channel). HEPA and Activated Carbon have measurable upsides with no downsides aside from their cost. They're the best we have today in spaces occupied by people.

sparker72678 5y ago
A DIY solution would be to get activated carbon that's intended for water filtration (fish tanks, etc). Then add a step that moves the air through the media (put it into a mesh box, or heck even just blow the air down onto the carbon that's sitting in a "dish" or some kind. Certainly not as efficient as a commercial all-in-one solution, but it that's not available to you, it can work. You can even make your own "activated charcoal" but it's a pretty messy and time consuming process.
- malfist 5y ago
  Would activated charcoal for aquarium work thought for gas? We're talking about a significant difference in porosity here. Even going with something like ROX (https://www.bulkreefsupply.com/bulk-premium-rox-0-8-aquarium-carbon.html https://www.bulkreefsupply.com/bulk-premium-rox-0-8-aquarium...) it's going to have much larger pores and much smaller surface area than a carbon coating on a filter
  - sparker72678 5y ago
    Good question. I'm imagining that with a small enough space / large enough quantity of charcoal you'd still get benefits, but it's not like I sat down and tried to do all the math.

martinmunk 5y ago

I had considered something similar, but recently I found that Ikea has started selling air purifiers and sensors for very reasonable money. They even sell an activated carbon filter that you can add. Ikea FÖRNUFTIG: https://www.ikea.com/us/en/p/foernuftig-air-purifier-white-50461961/ https://www.ikea.com/us/en/p/foernuftig-air-purifier-white-5...

afandian 5y ago
For years I had planned to make a DIY one, but never got round to it. But a week ago bought one of the IKEA ones, plus a carbon filter, and I've been very happy. The air feels cleaner and somehow just nicer. Lots of reviews say it's underpowered for a normal sized room compared to alternative products. But for me, the alternative is a DIY one that I still wouldn't have got round to making. I'd recommend it.
leoedin 5y ago
I've been running one of these for a few months. The filter has slowly turned darker brown - which suggests it's working (not anywhere near forest fires here!). It has a trickle mode which is completely silent and uses about 1W of power. Something you won't get on a DIY fan.
DoingIsLearning 5y ago
I read a wired review [0] that says the ikea purifier "blocks 99.95 percent of particles 2.5 microns or larger". Could you confirm the technical spec of the filters you are using? Actual rating? Is it actually HEPA? The ikea website seems to just be pretty vague marketing stuff with no details. [0] https://www.wired.com/story/ikea-fornuftig-air-purifier/ https://www.wired.com/story/ikea-fornuftig-air-purifier/
- kleyu 5y ago
  You can read more details after you click "Product details". It says that the filter corresponds to EPA12 class, so no HEPA. > The particle filter is tested according to EN 1822-1 and ISO 29463-3 which corresponds to class EPA12.

Hnrobert42 5y ago

This is awesome. I love DIY stuff and love the idea of an affordable clean air solution. For me, the aesthetics, noise, and issues like overworking the fan motor, make the extra $50 for a purpose-built filter worth it.

ncmncm 5y ago
Only if it actually works better. But many "purpose-built" units just cost more, but work less well. How do you tell if it will work well, before you spend?
- Hnrobert42 5y ago
  For the one I purchased, I researched them on Consumer Reports, which independently assessed their efficacy. How do you tell they work less well before you make that assertion?

mrtnmcc 5y ago

I've built a few of these and compared with higher end air purifiers like Alen. The only downside I could find with the DIY box fans was the power consumption is much higher.. about 60W vs 6W for the commercial purifier. That is a $30/yr increased cost in electricity based on CA rates.

diegocg 5y ago
Surely you can use less powerful fans?
- sooheon 5y ago
  It's about power efficiency, not total power draw. Use less powerful fans with an inefficient setup and you're not cleaning your air as well. Engineers put thought into chassis design and motor/fan specs. For this same reason I decided to get a used box from a reputable, long lasting company with top of the line CADR, and I can find plenty of aftermarket HEPA filters that are compatible.
- mrtnmcc 5y ago
  That was a comparison was roughly for the same air cleaning rate. I think it has to do with the fan architecture. With purpose built purifiers using static pressure fans instead of air fans.

ddlatham 5y ago

Wirecutter performed a similar experiment a few years back. Their results were somewhat different, showing that the DIY version helped significantly, but not nearly as well as the best of the commerical ones they tested: 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... I wonder what accounts for the difference in results.

ncmncm 5y ago
Wirecutter pretty consistently produces spurious results, and bad recommendations. Ben Kuhn and Dan Luu have written about this, Cf.
jly 5y ago
The wirecutter DIY test used a filter with a rating of FPR 9. Since FPR isn't standardized like MERV or HEPA, it's hard to tell what this means, but likely to be < MERV 13. The OP used a HEPA filter on their testing which is > MERV 16.

amluto 5y ago

Yikes. Putting high pressure drop genuine HEPA filters on a fan like that where most of the air will bypass them defeats the purpose and is a waste of energy and money. Just get a bigger MERV 13 or better filter and cover the whole fan.

function_seven 5y ago
> defeats the purpose Based on his testing, it looks like it achieves purpose just fine, and slightly better than the more expensive option. > and is a waste of energy and money His setup has over 5 times the filter area (1,400 cm² at 5 cm thick vs. 300 cm² at 4 cm thick), for $10 more on the filter replacement. Energy use is higher, but it's not clear that it exceeds the savings from cheaper filter replacements. And by having those gaps in the filters, the fan motor is probably being saved from overheating. Plenty of air still gets entrained through the filters, even with some bypass. It's kind of like premature optimization when writing a program. Where you think improvements are needed isn't necessarily where they should be. Test your code, find the hot spots, and improve those. With a filter setup, it might see obvious that a good seal is needed between filter and fan. Turns out, that's not always true.

ryandrake 5y ago

Don’t most home HVAC systems already filter the air blown through them? I know I replace mine every so often. If they do, what does a separate “air purifier” buy you?

bialpio 5y ago
Not sure how much you'd gain, but one thing to note here is that I believe not all HVAC systems have an option of running just the blower w/o also heating the air.
- bluGill 5y ago
  The only furnace I've seen without that option was from the 1950s (probably replaced by now for old age), but I'm sure more exist. It wouldn't be hard from someone who knows power to wire up the fan to run all the time though.
  - bialpio 5y ago
    Furnace is one thing, the wiring in place is another I guess. My house is from the '50s, and the furnace was already replaced, but we have not laid new wiring. I think I need to look into that before the next wildfire season starts...
    - bluGill 5y ago
      If nothing else it should be easy to put a fan switch right on the furnace.
c_o_n_v_e_x 5y ago
For central hvac systems, standard systems typically have a filter grille for each return (where the air goes in). You can also add on a higher performance filter on the return side. Something like a Trion Air Bear.

oblib 5y ago

I've been doing this for years using off the shelf filters you can buy at most any hardware store and a 20" box fan. I use the 2" blue masking tape to attach the filters to the fan and seal them so all the air that flows through them is filtered. The blue tape comes off easier than cheap masking tape and without leaving any sticky residue on the fan. I leave the fan running 24/7 and I've bought two of them in the past 15 (or more) years for my home office. I replace the old one earlier this year. It's pretty amazing how much they grab out the air. I live in a pretty rural area and the road to our house is gravel and I'm surrounded by a forest so there's dust and pollen pretty much constantly in the air. And we have cats. Lots of cats. It's also amazing how much cat hair and dander they collect. I can sure tell the difference when they're not on, and when I replace the filters after waiting longer than I probably should.

drited 5y ago
How often (if at all) do the filters require washing to keep them going for that long?
- oblib 5y ago
  Sorry about the slow reply. It depends on how much dust is in the air. I live in an older home on a gravel road surrounded by forest so there's a lot of dust and pollen. It only takes about a week to start seeing some color in the filters. I use the paper disposable filters so washing them isn't an option.
  - drited 5y ago
    Interesting thanks! So I guess you change them weekly then?

nixpulvis 5y ago

Function >= Form, at equality life is good.

jiscariot 5y ago

That looks pretty similar to the Alton Brown way for making beef jerky at home.

019a 5y ago

Absolutely shameless plug: If you'd like a cleaner looking, 3d printed solution for attaching a filter to a pretty standard box fan, in this fashion, we sell some over on Etsy (use the coupon code HACKERNEWS, 20% off) [1] Does it work better than duct tape? Probably not. Duct tape does a great job of creating an airtight seal, whereas with these clamps there is a small gap between the fan and the filter. But, it does make it easier to replace the filter, looks clean, and we've ran tests with an AQI meter; definitely has a positive impact. [1] https://www.etsy.com/listing/980605509/box-fan-filter-holder-set https://www.etsy.com/listing/980605509/box-fan-filter-holder...

wintermutestwin 5y ago

Here in the toxic air of California, I decided to go with an industrial level "DIY" solution: activated charcoal air scrubber with a high CFM fan. Just hopped down to my local indoor grow shop (4 in a 10 mile radius!) and had all manner of choices. Yes, I spent about $150, but I'm quite sure that I am getting much much more than 3x value out of it.

ncmncm 5y ago

When the Caldor fire (now 210 thousand acres, 328 mi^2!) was smoking up Placerville, the downtown Home Depot had, at the main entrance, a big stack of 20x24x1-inch #9 furnace filters, box fans, and rolls of tape, with one all constructed sitting on top, running, black with trapped smoke. Kudos to HD for not jacking up prices. It is best to make them with a four inch thick filter, which HD does not stock, and higher than a #9, likewise, bought online. You can stack a #8 in front of a #17, to make the ~$100 #17 last longer, and stack two box fans (one intake, one exhaust) to get more suction. Maybe stack #8/fan/#17/fan, to keep the fans clean. But the single one-inch #9 filter cleared the smoke from my whole house in no time flat! Don't let "best" stop you from doing "good". An advantage you have from high airflow is that particles that make it through the filter once are likely to get trapped the next time around, or the 20th. Atomizing some water ought to make filters momentarily stickier, until it dries. Fan blades getting dusty can cut airflow in half! (Same on laptop cooling fans; use a vacuum cleaner on your laptop air path.)

Johnny555 5y ago
Kudos to HD for not jacking up prices. Not sure all the credit goes to them since if they jacked up the price during the fire, they'd be violating price gouging laws. But it is nice that they put the components for a DIY filter all in once place https://www.oag.ca.gov/consumers/pricegougingduringdisasters https://www.oag.ca.gov/consumers/pricegougingduringdisasters
- ncmncm 5y ago
  That is news to me, and welcome. By the FAQ, it appears furnace filters must be covered. Box fans might be borderline, but even being seen publicly accused of violating gouging laws would be bad for HD.
  - Johnny555 5y ago
    The statute is broad enough (it includes "emergency supplies", and the CDC has a how-to guide on creating a box-fan filter) that I'm sure they'd be running afoul of the law if they tried to jack up prices for box fans. But yeah, any large retailer would avoid any appearance of price gouging even if they could get away with it legally just to avoid the bad publicity.

newman314 5y ago

I've been keeping an eye on the so-called Corsi-Rosenthal box. I'm probably going to build one or more at some point. There's a bunch of folks that have been optimizing this design on Twitter. The initial results look quite good and properly put together might last an entire school year. Link here: https://www.texairfilters.com/iaq-research-practice-in-action-the-corsi-rosenthal-box-air-cleaner/ https://www.texairfilters.com/iaq-research-practice-in-actio...

hinkley 5y ago
You could also go with smaller filters, like 14x20 to reduce the stack size, but the prices don’t scale exactly with surface area, while the pressure does. So I’m not sure how much that really buys you.
- newman314 5y ago
  IIRC, increased surface area reduces pressure. From what I remember from reading the thread, it appeared to have pretty decent CFM through the use of more and larger filters. EDIT: I found the thread. See https://twitter.com/davidelfstrom/status/1429526273009889286?s=10 https://twitter.com/davidelfstrom/status/1429526273009889286... Thread shows the difference between using 1 and 2 filters. I think he has a separate thread in his Twitter feed measuring a 4/5 filter setup too.
  - aronpye 5y ago
    Increased surface area alone results in higher pressure losses / flow energy losses due to increased frictional losses from the higher surface area.
minimalist 5y ago
It is amusing how this person has the temerity to name this configuration (partially) after themselves, while people have been making and documenting these configurations for at least a decade and surely longer.

Havoc 5y ago

Prefer the commercial ones with built in particule sensor because they can ramp up & down dynamically

pacman2 5y ago

No brainer. Basically all commercial purifiers suck (air), the DIY blow it. Big difference!

cf100clunk 5y ago

Some lab tests conducted by a TV show gave the Box Fan With HEPA Filter the highest marks over commercial air purifiers: https://news.ycombinator.com/item?id=26101153 https://news.ycombinator.com/item?id=26101153

Johnny555 5y ago
They didn't give the box fan the highest marks, but they found that it outperformed some commercial products, including the $800 Dyson (which has a very bad price-performance ratio)

Johnny555 5y ago

For those that don't want to build their own, I bought one of these about 5 years ago: https://www.whirlpoolairpurifiers.com/product/whirlpool-wp500-whispure-air-purifier-slate-black/ https://www.whirlpoolairpurifiers.com/product/whirlpool-wp50... I measured it last year during bad air quality days, on high, it would take the bedroom from ~150 AQI to less than 10 in about 15 minutes. On low it's quiet enough to be unobtrusive while sleeping. Filters cost around $100 and are rated to last a year (and even after a heavy fire season, I've found that to be true) More expensive than a box fan + filter, but easier to use and a little more attractive. Though this year, I upgraded my furnace with an AprilAire filter box and an MERV13 filter, which has worked surprisingly well, I thought I'd still want to use the standalone HEPA filter in the bedroom, but the furnace filter keeps the entire house below AQI 10 even with outside AQI at 250 (haven't had air worse than that yet this year)

mark_l_watson 5y ago

Good ideas. Several months ago we had bad fires in the mountains of Central Arizona where my wife and I live. I bought a HEPA filter, the complete kit with enclosure, electronic controls, etc. With an extra filter, I spent about $80 and I feel it was money well spent. Making your own for less money is even better.

Causality1 5y ago

Needs a (2020) tag. This article already hit the front page when it was new.

BrickandPorter 5y ago

Product engineer here from a major filter manufacturing company. I've done a ton of tests this summer evaluating DIY box fan filters and I have a couple insights for people building box fan filters this year. 1) Box fans (and other axial fans like your ceiling fan) are terrible at pulling air and an HVAC filter will significantly reduce fan speed due to the added pressure differential across the filter. As a reference, a new MERV 13 filter can reduce fan speed by ~33% when mounted to the intake side, and ~66% if mounted to the front of a box fan. The motor used in your box fan is cooled by the air passing around it, which is why choking off the air flow through your fan can lead to overheating, damaging your fan and creating a fire hazard. If you're going to make a DIY air purifier, mount the filter to the intake side. You'll get much better performance, your fan will stay cleaner, and there's less risk of damage. 2) Some analysis of the test data from the posted article: CADR stands for Clean Air Delivery Rate, measured in cfm. It's basically a measure of how efficiently ambient air in a confined space is purified. A higher CADR means particles are pulled out of the air quicker. For now, just treat this as a relative quality value. Using the PM 2.5 graph for the tiny room experiment in the article, I'd estimate that the DIY filter the author created has a CADR value of 45-50. This is a pretty low value. The room air purifier (RAP) he used is even worse. Then, from the large room graph, I'd estimate the DIY purifier as a CADR of 30-35. This is a rough estimate for two reasons. One, room size is an important variable in this calculation and I don't have an exact value. Two, when I test a purifier, the test chamber starts at a PM 2.5 around 10^5. At the extremely low starting concentrations used in the author's experiments, the percentage of particles that are removed due to natural decay is much more significant. Side note, cigarette smoke is the standard for room air purifier testing. Incense sticks are used less commonly due to their slower particle generation. 3) For wildfire season, I recommend a MERV 13 filter for overall performance, cost effectiveness, and for smoke particle capture (PM 2.5). There is a clear trend of diminishing returns for a DIY box fan filter beyond MERV 12-13 filters, peaking at a CADR value of ~150 for a MERV 13 filter for box fans I've tested at their highest fan speeds. A MERV 13 filter is about 50% better than a MERV 10 in a box fan configuration, while a MERV 14 is actually slightly worse. This will vary based on filter brands, too. Rule of thumb, quality matters. We actually rate our filters a bit lower than their actual performance for a number of reasons. So, my company offers a standing room air purifier with a HEPA filter for about $200. It has a CADR value of 158. A box fan and a MERV 13 filter cost about $40 and has a CADR around 150 - pretty good for a DIY substitution. We even offer cheaper room air purifiers with even lower CADR values. So why buy an expensive room air purifier? First, room air purifiers use a radial fan rather than an axial fan. Axial fans create a low pressure area on the exhaust side, drawing air through the fan. Radial fans draw in lower pressure air near the axle of the fan and push out higher pressure air at an exhaust port at the radius of the fan. Room air purifiers use a radial fan to push air through the high pressure drop HEPA filters they are designed to use. HEPA filters are qualified to remove +99.9% of tiny particles (PM 0.3) in one pass. The thick filtration media requires a high pressure differential to pull air through it. A HEPA filter on an axial box fan is going to kill the motor. If you care about PM 0.3 particles, only a HEPA filter will do the job. Also, longevity. A room air purifier and HEPA filter should run for a year or longer without needing to change filters. An HVAC filter is meant to last 3 months in your home air system under a normal particle load. This lifespan can be much shorter due to poor conditions, such as smoke particles during wildfire season or drywall dust from a renovation project. (Side note - seriously, replace your filter after doing any drywall work. Anything better than a fiberglass filter can clog in just a day or two of dusty drywalling. Also also, fiberglass filters do absolutely nothing, don't buy them.) The room air purifier is designed to run for years with a high pressure differential filter. Your box fan is not. A cheap 20" box fan with an HVAC filter is a good temporary solution, but will not last nearly as long as a room air purifier used daily. Last, noise and style. RAP units are very quiet and blend into your living room space. A large box fan with a strapped on furnace filter makes for an interesting conversation piece, if you could hear your guests. 4) Don't worry about hermetically sealing your 20x20 filter to the box fan. From my experience, a filter with tape sealing every side to the fan is no more efficient than a filter held to the fan at a couple of contact points. Once the fan is on, the intake air will hold the filter close to the fan. Even with a fully sealed filter, remember that axial fans suck at sucking. Your axial box fan will actually draw air around the front corners and into the fan, even without a filter on the back. And with how easy it is to reduce the fan speed of a box fan, you're better off allowing some air through so that the fan runs at a more efficient and safer speed. 5) A filter on a box fan is definitely better than nothing and a good, cheap short term solution. RAP units are great for long term use and capturing all sizes of particulate. I'm not going to try to sell you my brand, but there is one product I advise you avoid. Recently, Lasko has released a box fan with a filter slot as a 2 in 1 product. I've tested this thing with the provided filter and it does not perform nearly as well as a DIY filter you could make. Also, the filter slot does not fit standard 20x20x1 filters. The slot is designed for a slightly smaller, 19x19x1 sized filter, meaning that if you want to buy a replacement filter or a higher quality one than provided, you won't be able to. A normal 20x20 filter (which is actually slightly smaller in W x L nominally) can barely be squeezed into this fan's slot. Just buy a better filter and some tape instead. 6) If you look for DIY box fan filters, you will find examples with 4-5 filters in a cube shape. 5 filters improves filtration efficiency by ~50%, but costs x5 as much. That's $100-$150 in filters that will last just about as long as one filter while only cleaning the air 50% better. Just invest in a room air purifier unit of the same cost instead and enjoy much cleaner air over an entire year.

giardini 5y ago

He mentions cooking: do the recently-introduced air fryers create significant emissions?

SilverRed 5y ago
They are basically just a little oven. They don't seem to create anywhere near the smoke level of frying on a pan.

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Wistar 5y ago

For my spouse's classroom, I made a 4-filter-cube and fan filter setup using this video as the basis. https://youtu.be/Y7eL2OAnqc8 https://youtu.be/Y7eL2OAnqc8 I added a fan shroud on the exhaust side of the fan using a piece of cardboard with a circular cutout matching the diameter of the fan blades.

JoshTko 5y ago

What about no filters? Could it be just pushing air around into things in the room is the most effective?

jcims 5y ago

FWIW a 24" wheeled shop fan from Harbor Freight, Tractor Supply, Rural King, etc etc (sometimes HD/Lowes have them) move way more air with more authority than a box fan. They will run $150 but they can be very useful to have around even when you don't need to filter the air. (Note the motors pull more current so you still have to be careful not to overheat them) Some examples: https://www.harborfreight.com/merchandising-promotions/hot-buys/24-inch-high-velocity-shop-fan-93532.html https://www.harborfreight.com/merchandising-promotions/hot-b... https://www.tractorsupply.com/tsc/product/countyline-24-in-direct-drive-tilt-fan-bf24tfcl https://www.tractorsupply.com/tsc/product/countyline-24-in-d... https://www.homedepot.com/p/Commercial-Electric-24-in-Heavy-Duty-2-Speed-Direct-Drive-Tilt-Drum-Fan-BF24TFCE/303061463 https://www.homedepot.com/p/Commercial-Electric-24-in-Heavy-... Axial fans are still better for this application, but these fans are great to have around in summer evenings when the air outside is cooler than it is inside. Put it in one window of the house sucking the air outside, then open windows on the other end of the house and let the cool night air stream in. It's fantastic.

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legohead 5y ago

if you watch any of the woodworker youtube channels, this is basically what they do in their shops. a little more elegant, with wooden enclosures (of course), but the idea is the same -- regular house filter on a fan.

avtolik 5y ago

Does anybody from Europe has a similar DIY setup? I cannot seem to find box fans here. And in my country particular - no rectangular HEPA filters.

JoeAltmaier 5y ago

About taping the fan so all the air goes thru the filter: How does a filter work? Air that goes thru it, gets particles removed. We want to maximize air that goes thru it, and it should be air that has particles in it. To make more air go thru the filter, increase the pressure difference between the dirty side and the clean side. Fan speed is how we control this. And getting clean air away from the fan and dirty air to come close to the fan also seems directly related to fan speed - how fast and how far it ejects clean air. It's all about fan speed, baby. Plugging up parts of the fan unrelated to the filter cross section, can only slow the fan down. I see no reason why it would help in any way.

bawolff 5y ago

> I borrowed a cheap-ish ($100) air quality monitor from a friend. I think it’s made by some company in China and then re-sold by various white-label brands. I can’t figure out who the original manufacturer is. Based on data I’ve seen for the reliability of other air quality monitors, I wouldn’t trust the absolute numbers, but the I think the relative measurements should still be OK. That seems like a pretty big assumption.

stevenhubertron 5y ago

In Colorado I have been using a few different Coway air purifers and they have been able to keep the air quality in my house good, and quickly can recover that good air quality after running my swamp cooler in the morning/evening. Been overall happy with them but perhaps this box fan/merv filter is a good backup to have in case it gets really bad.

travisporter 5y ago

Can anyone provide insight as to why not taping/sealing up the uncovered portion of the fan may improve performance?

stjohnswarts 5y ago

While I'm cool with this for allergy sufferers or "smokey" conditions I think the modern world tries a bit too hard to purify and sanitize everything. Eventually you do have to step outside your home and you can't take your filtered air with you.

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kujaomega 5y ago

It will be good to see the same tests for PM1.0, TVOC and HCHO. But great job

Ashoka_rkt 5y ago

there is a company that's selling DIY air purifiers in India (and probably its neighbouring countries).. https://smartairfilters.com/in/en/product/diy-1-1-air-purifier/ https://smartairfilters.com/in/en/product/diy-1-1-air-purifi... they put the filter in front of the fan, not the air intake aka backside.. they have detailed info on how their purifiers are effective than other costly brands.. probably putting the filter on the front side is more effective?

snivelus 5y ago
They've confirmed that they tested it. https://twitter.com/herpiko/status/1388621526757298178 https://twitter.com/herpiko/status/1388621526757298178 I'm using this kind of DIY filter. After of putting the filter in the front of the fan for months, now I modified it into intake mode because of uncomfortable smell occurs whenever I turned the fan on. Some dusts that are already trapped here may have escaped again by sudden push of air.