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.

1. ^{^}

   Obvious Scott Alexander callback here. It is too good to not repeat at least once in a while.

2. ^{^}

   See comment thread below with Zac.

3. ^{^}

   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/