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Comment by BasilPH | Hacker News Reader

I've built dynomight's DIY filter[^1], and I love it. No DRM issues either.

[^1]: https://dynomight.net/better-DIY-air-purifier.html

Very cool. Anyone seen a mod that adds activated carbon? A lot of DIY filters are very cost-effective, but the lack of activated carbon is a big downside compared to commercial solutions.

There's really only two air cleaning technologies that are worthwhile right now:

- HEPA filters (remove PM 2.5 particulate and above).

- Activated carbon (neutralizes harmful gases).

The only other thing that may be worthwhile is a pre-filter, but that is mostly a cost saving measure rather than actually improving air filtration.

Unfortunately commercial filters are full of gimmicks that don't work or may even be harmful due to Ozone generation (e.g. Ionizers, UV lights, et al.).

You could buy an activated charcoal filter built to attach to the same inline fan booster from most garden stores that carry indoor growing equipment. They're popular with people growing cannabis indoors to remove the smell from their grow area.

Something like this:

https://www.amazon.com/dp/B07GSKFBL8/

My Philips air purifier has separate HEPA and carbon filters that can be replaced independently from each other. And also a pre-filter mesh that collects large dust particles and can be vacuumed out.

Perhaps a stupid question. Why go to the trouble of bolting together four filters, instead of attaching the duct fan to a cylindrical HEPA filter? Then you just need two plates: one to adapt the duct inlet and one to seal the base of the filter.

I think, as with many DIY projects, one must look to the pot growing community. They solved the problem of "how do I not stink out my apartment" a long time ago - duct fans and HEPA/carbon filters are the mainstay of grow rooms.

Link? I'm interested.

However, I suspect availability plays into it. Almost anyone can pick up a couple of rectangular HEPA filters and a box fan from the local hardware or big box store.

I've never seen a cylidrical HEPA filter. So, I probably need to order that from an industrial supplier.

And, aren't duct fans kind of noisy? Do they make quiet ones?

In addition, this fan/filter seems even easier: https://smartairfilters.com/en/blog/cannon/

The duct fan + filter combo is how lots of cylindrical consumer units work (e.g. Levoit, Philips, etc). They just use crappy fans which don't really pull any air through, or they're very expensive for what they are. In terms of availability, I think the current popularity helped, otherwise you could also look at the auto market, lots of cylindrical filters there and they'd be fine for pulling out stuff like pollen, though you might want a carbon filter as well.

I have no idea what's required for HEPA certification, but lots claim to be:

https://www.amazon.com/LEVOIT-Core-Replacement-High-Efficien...

It has an outer dust mesh, the HEPA filter and a carbon insert. They're all the same though, I think you could use any manufacturer's filter. Longevity isn't really an issue if a supplier goes bust, just re-make the adapter ring. Pretty much all you need and you don't need to worry about sealing the corners.

According to OP's link, they use a duct fan booster which is a lot quieter than a box fan (at least the claim is it can go down to 16 dB).

Though a question is does a can-shape system work better? Or would you get the same effect if you used a single flat filter with the same "unrolled" area? I have no idea, and there's so much uncertainty involved with DIY testing.

Hm, I feel like cylindrical HEPA filters are common for shop vacs and widely available? Searching "HEPA filter shop vac" on home depot gets what looks like plenty; or are these not what would work for that kind of design, maybe we're talking about different things? They are "cylindrical" by being pleated.

Cost and availability. Cylinder filters tend to be way more expensive ($30-50 per, I've seen true HEPA cylinder ones "for allergy sufferers" aka medical space, >$130) than rectangular (a few bucks). Rectangular also comes in a wider variety of form factors than cylinders, with more different filter styles (eg different thickness, with carbon, high flow, true HEPA/MERV >13).

I go back to that page often. I might finally buy the parts tonight and actually make it instead of just thinking about it :)

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