Those fans will not have sufficient static pressure to be useful. These makeshift purifiers may be great in a pinch (wildfire season and stores are all out), but far from the best.

▲ schiffern 10 hours ago | parent | next [-]

This is indeed the "conventional wisdom," but if you do the math it turns out this is incorrect.

The trick is to use high-airflow MERV 13 filters (ideally 3M 1900, which has high pleat count and low resistance) instead of high-resistance HEPA filters, so axial PC fans can generate enough pressure.

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bradknowles 10 hours ago | parent [-]

At that point, you’re not doing any actual air filtering. Just use them as fans instead.

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gpm 10 hours ago | parent [-]

Merv-13 means it filters at least 75% of the particles passing through it.

For a standalone air filter which is pushing the filtered air into the same room its drawing from, we can assume reasonably good mixing of the intake and exhaust air. Thus to be equivalent to a HEPA-filter it only needs 1/3rd more airflow... Which really isn't much.

This doesn't follow for a a fancier setup where you're taking dirty air and blowing it into a cleanroom or something... but that's not how we're typically using household air filters.

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CoffeeOnWrite 7 hours ago | parent [-]

Is it safe to assume all particles are equally as likely to be filtered, or are there particles that HEPA will filter that a MERV-13 will never filter no matter the number of cycles?

	▲	schiffern 6 hours ago \| parent [-]
		75% refers to the Most Penetrating Particle Size (MPPS), so they intentionally measure at the worst-case size so you don't have that problem. All other sizes are filtered at a higher percent.

▲ jerlam 10 hours ago | parent | prev | next [-]

The linked PC-fan purifier has been tested and shown to purify air faster than the conventional alternatives, and are not makeshift, so I'm not sure what you are referring to.

▲ kube-system 10 hours ago | parent | prev [-]

From the specs it looks like they are probably closer to higher-static-pressure server fans, the CADR is a few hundred CFM and it pulls 30+ watts. And as with many of these designs, you can compensate for low pressure performance by increasing filter surface area... the greater the surface area the lower the pressure drop. Of course, the problem with this is you are just spending more money on filters, which is the expensive part.

▲ jerlam 8 hours ago | parent | next [-]

The filter cost for the AirFanta 3 doesn't seem particularly high? In fact it seems cheaper than competitors.

All the purifier companies realized they make more money selling overpriced filters, so $60+ for the OEM filters is common nowadays. Despite it just being a standard HEPA filter cut to a specific size.

Airfanta 3 Pro filters (four) - $57

Coway Mighty Filter - $60

Coway Mighty2 Filter - $70-$80

I'd be concerned about the availability of filters after a few years. Some of these companies have been changing their models and filter type every year to add some nebulous feature. Others just go out of business.

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kube-system 7 hours ago | parent [-]

Yeah, the retail prices of filters is all over the place because of the popularity of razor/blade model of pricing. I'm not talking about that, I'm talking about the BOM itself -- this device is using 4x the filter media of comparable filters, which would typically have one filter that size.

HEPA media itself is a commodity and aftermarket filters (and the BOM cost to the OEM) tend to level out at whatever rate is minimally profitable per area of filter media.

The upfront cost of this device is high for what you get, because you're mostly buying filter media. The device itself is not much of a polished product. I'm sure it works fine but I wouldn't want one in my living room.

	▲	jefftk 5 hours ago \| parent [-]
		I have three in my living room, and while they're not fashion they blend in fine: https://photos.app.goo.gl/wvRc4hEYqQep4zgd9 I leave them on setting 2 (unless someone is sick or we're having a lot of people over), and we get ~425 CFM for about $40/y in power (~5W each).

▲ schiffern 10 hours ago | parent | prev [-]

  >Of course, the problem with this is you are just spending more money on filters, which is the expensive part.

For up-front cost, this is true.

For cost over time, twice the filters means twice the replacement interval, so the additional filter cost cancels out.

▲ kube-system 7 hours ago | parent [-]

But since you bought mostly media, you didn't get a housing that hides the media, and you get to look at any collected dirt for that longer interval as well...

I'm sure it works but like any engineering decision it has pros and cons.

▲ schiffern 7 hours ago | parent | next [-]

  >you get to look at any collected dirt

If that's your biggest worry in life, seems like a high-class problem! Many people are satisfied 'just' protecting their heart and lung health.

However, if you really don't want to look at it, you can always flip most DIY designs inside-out so the fans blow in and dust collects on the inside instead of the outside! This doesn't clean the room quite as efficiently, but if the appearance of dust is a major concern that should be a fix.

	▲	kube-system 6 hours ago \| parent [-]
		Yeah it’s functionally fine but I would opt to buy a more polished product for the above reasons. Probably would be great for a shop or similar, which is where I’ve done DIY purifiers myself. My personal favorite for cutting down on dust during home projects is a 20x20 furnace filter slapped on the intake side of a box fan. It requires zero extra parts or assembly. Flipping the fans isn’t great because then you have to clean the fans, this is why draw-through is the standard configuration. Also many designs like this rely on negative pressure to seal properly or even stay together. You might just have the lid take off like a helicopter.

▲ jefftk 5 hours ago | parent | prev [-]

It just looks grey, but the housing is also grey, so I don't think it generally reads to people as dirty.