A good way to build many air filters on the cheap
TLDR: ↓
With access to a 3D printer and some lack of regard for aesthetics, you can build a Levoit 300 (a popular air filter) clone for roughly 25% of the price.
And against the cheapest possible competition, it is roughly price per volume of air competitive/better but much quieter + modular + not soft vendor locked to their filters.
Why not just buy n many X filters?
If there is one thing I associate the LW community with, that would be the undying love for Eliezer Yudkowsky and his role as the rightful caliph[1] Lumenators and Air Filters.
Ever since my allergy diagnosis, I’ve thought of various schemes to cover my house with the latter. The obvious decision is to acquire as many as IKEA/Levoit/CheapInc. filters as fast and as cheaply per CFM (cubic feet per minute) as possible. Then proceed to plant them around the place and call it a done deal.
Sadly, a quick proof of concept somewhat shattered my dream. After listening to how loud some of these fans are in person, I realized the advertised CFM all correspond to the highest fan setting—and they are very noisy (effectively in the 50~60 db range). And I didn’t want to spend hundreds on what are essentially glorified pressure optimized fans on a box.
Also, I am not the biggest fan of having IoT devices sprinkled in my living quarters—too many of these filters have smart features I don’t want. If I didn’t write the firmware, I certainly don’t want anything with more compute and IO than a Raspberry PI pico controlling my fans and light bulbs.
Somewhere, somehow, someone must have had this problem and wrote something about it?
There is an obvious solution
Between “A box taped together with filter as sides and a box fan on top” and “custom designed fan enclosure to win some obscure competition by 0,3% margin”, there is a great solution proposed by the BigClive—a fan mounted on a cylindrical filter.
BigClive’s solution is elegant. By combining pressure optimized off-the shelf parts (computer fans) with the bare minimal of essentials (the filter itself as the filter and structure, and a PSU), you can just get a air filter without any of the bells and whistles. And, to free yourself from filter shape vendor lock, if you have access to a 3D printer, you can just print the right sized cone to mount your filter on.
Now, BigClive’s solution is not perfect, at least in this specific implementation. The fan he used is not a good fit for the purpose—likely not optimized for high static pressure (even the ones optimized would only get you max 80% of the advertised air flow figures). And the filter itself is quite top heavy and could be pulled off from a simple brush with the cord. But the solution is good.
The shopping list
For illustrative purposes, here is the pricing in USD at Amazon for the essential parts + some negligible cost in 3d printed plastics. In the perfect scenario, $100 (assuming some tax) gets you 5 mini air filters that each does 55 CFM nominally at around 25 db.
But we can do better. By adopting the mantra of “Bigger is Better”, we can instead use the slightly more expensive P14 fans that does 75 CFM at the expense of an additional $10. Now we have $110 for 5 filters at nominal 75 CFM at 25db. Assume 60% of the air flow of the advertised figure under static pressure, we have...[2]
For our filter: $0.50 per CFM at “very quiet”.
Against the old market leader, Levoit’s 300/300s series:
Note, the 135 CFM figure is at max volume (at ~60w). If we compare it to the 24 db setting, assuming the cubic root relation between power and flow holds and some extrapolation between the power consumptions at various settings[3], it is likely to be around 30~50CFM at the lowest setting. Levoit 300 is priced at $100 for the US market before tax and delivery. However, I am sure I am being too generous here because the general anectodal evidence suggest that, even at the lowest setting, the fan is annoying and easily audible, which suggest it to be more 30db ~ 40db range than the claimed 24db.
At generous reading for the Levoit 300: $2 per CFM at “very quiet”.
How is it working out?
It is working out just fine! Here is the first prototype that I built with a Noctua fan and some other things I had lying around—currently connected to my computer so it would turn on as I turn it on.
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Obvious Scott Alexander callback here. It is too good to not repeat at least once in a while.
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See comment thread below with Zac.
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https://www.techgearlab.com/reviews/health-fitness/air-purifier/levoit-core-300 - See
When set to medium and used for two hours, the Core 300 used .05 kilowatts.vs an different claim here: https://www.airpurifierfirst.com/review/levoit-core-300/
I’d run the numbers for higher-throughput, lower-filtration filters—see eg cleanairkits writeup—but this looks great!
Thanks! I have built these before I ran into cleanairkits and the school of thought that “lower efficiency with higher throughput is better”—I think per dollar, their CADR is likely quite a bit better! Looking at their Exhalaron line up—we have what is essentially two of my style of filter glued and tension-ed down in a portable package.
And a similar HEPA filter is used here as well. With two fans that are each nominally figure of 75 CFM. 92cfm / 2 / 75cfm = 61% - instead of the 80% figure I handwaved! (the new numbers should be up soon that is based on a more economical P14 setup instead of the P12.)
upvoted, i think this article would be better with comparison to the recommendations in thomas kwa’s shortform about air filters
I think Thomas’s “Instead of using HEPA to ‘one-shot’ (original design intention) the air filtration task, the ‘few-shot’ approach with much higher through put with a MERV 13ish lvl of efficency is generally better” is mostly correct. I see that Dynomight’s IKEA filter investigations have also made a similar conclusion (although it is more in the case of HEPA vs MORE HEPA).
However, I didn’t want to 3D print/jerryrig an enclosure to fit in the recommended filters, and where I am, I couldn’t source a nice self-supporting (non-HEPA) filter that I can easily plop a fan onto. But if I had some more spare time, the cleanairkits folks that Zac mentioned built basically a “quieter rectangular box with fans on the small sides, and non-HEPA filters on the big sides”—and that’s what I want eventually.
In a hypothetical, bad future where we have to do VaccinateCA 2.0 against e.g. bird flu, I personally wonder if “aggressively help people source air filters” would be a pre-vaccine-distribution-time step we would consider. (Not canon! Might be very wrong! Just idle musing)