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Comment by kn100 | Hacker News Reader

This is a really cool project but frankly you can do it a lot cheaper. Find the cheapest box fan you can find, and find the cheapest HEPA filter you can, and strap the HEPA filter to the front. To improve air purification even futher, slap an activated carbon filter on the front too, and your air purifier now filters VoCs! For cheap Activated Carbon Filters, look into the kind used for Aquarium filters. They'll work just as well for air as far as I know.

In China there is a whole company founded on doing just this, although they've since pivoted into selling specific air purification hardware at a significant discount. See: https://smartairfilters.com/cn/en/product/diy-1-1-air-purifi...

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If you want this to actually work, you take a box fan, and 2 filters. You make a triangle and then some cardboard on the top and bottom to fill the gap. This increases surface area and reduces the stress on the fan due to pressure drop. Slapping a filter directly on to the box fan is going to kill the fan very quickly. https://i.imgur.com/SX1RloH.jpg

I ran a few cheap box fans with filters directly over them for several years and none of them failed, I have heard this claim a lot but it seems overstated. The triangle setup will significantly increase the airflow and thus the effectiveness of filtration though.

I guess it depends on the quality of filter. If you're rocking a high merv filter for smoke the pressure drop can be significant.

I've done this too for years and no issues.

Clarification: triangle goes on the in or out side of the fan?

Doesn't really matter as long as the filters are in the correct orientation. However it would be beneficial if the air is clean before it goes through the fan.

Put it on the intake. It will keep the fan clean.

Right. And in general, duct tape and popsicle sticks take care if that housing.

Or ditch the cardboard, and use 5 square HVAC filters to make a true "box fan".

https://imgur.com/gallery/jtECP9C

The HEPA filters are the expensive component. Adding more filters doesn't improve net effectiveness.

Cardboard is the vastly more cost-effective option. And taping a HEPA filter directly to the back of the fan effective and proven.

Actually it does. You'll get higher CFM through the system and it will take 5x longer to saturate the filters with dust. The cost per month will actually be similar, granted you'll have a higher startup cost with 5 filters instead of 1.

The time-to-saturate is a wash when you factor in per-filter rates. You're achieving a 4x longer life per filter, but multiplying that by the 4x filters you are using.

The net filtration rate per filter will fall as that's a function of flow rate * filtration size.

Yes, the four- or five-filter version will probably have a slightly better overall filtration rate ... but that's really not the principle constraint here. A single-filter design will drop particle density by ~90% in about 30 minutes. That's sufficient. It's doubtful the final particle density measurement will be much reduced (though if someone's got the data I'd be interested in seeing it), or that this reduction is clinically significant from a health perspective. Remember that you're likely going outside the filtered area, so achieving perfection inside but spending a substantial fraction of your time in unfiltered or far-more-poorly-filtered air, isn't much of a win.

Meantime, you're tying up 3-4 filters that you don't really need, and which others could make effective use of, for a very low marginal imrovement in your own experience.

Where the filters themselves are in low supply -- typical for a region where a wildfire has errupted, and particularly true where filters aren't a high-volume item as with Australia presently -- designs which economise on filter use are a net social benefit.

I don't know why, but I love the fact that someone is selling filters strapped to fans on a reasonably glossy and well-made page.

It does kind of look like a joke doesn't it? A demo/how-to masquerading as a shop page.

Here's a video from the University of Michigan showing just how effective these cheap DIY air filters can be: https://www.youtube.com/watch?v=kH5APw_SLUU

This may or may not work, cheap box fans aren't designed to push/pull air through obstructions (ie: low static pressure) and HEPA filters are giant obstructions. So instead of the air going through the filter it'll go through gaps, around the edges of the blades, etc.

I used a cheap box fan for some DIY ventilation and the loss in airflow is massive after filter and ducting.

couldn't you easily test this by sticking your hand (wet it first) in the middle section of the filter and see if you feel any air coming through

You could, but I think the goal would be to make sure the design will work before going through the effort of making it. If you already have all the parts though definitely worth a try.

I have a box fan on low 24x7 with a 12 on the input and a 7 on the output. That's not a HEPA rated filter set, but it's fantastic. Allergy season just went away (while in my room) when I started using it a few years ago. I also tried a 3 on the output, and that worked as well, but the flow was noticeably lower. The fan is cheap, $20. I change the filters every few months, and it always looks like I waited too long.

I used a cheap box fan and a furnace filter held simply by pressure differential.

Flow drops a lot, but it still works. Filter was dirty after some time. It was hilariously undersized for my purpose (serious belt-sander action leveling sub-floors), but air does move.

I have smartairfilter's Cannon purifier. As all those standalone purifiers it works only with windows completely closed.

I went for ventilation systems that take air from outside, filter it and then bring it inside the room. Namely Xiaomi MJXFJ-300-G1: https://www.xiaomitoday.com/xiaomi-mijia-air-purifier-mjxfj-... I've installed one in every bedroom.

That seems quite expensive for what it is. For that price I'd expect it to have an efficient heat exchanger.

Can confirm: did this when the wildfires hit SF a couple years ago, and it really worked!!

My "measurement" was my lungs when working from home: coughing before, no coughing after. Filters turned black.

Would something like this this work sufficiently well to capture airborne fur/dander from a tabby cat? Or will there be a vastly different outcome from an expensive one?

Yes! We have two box fans with HEPA filters in our bedroom to combat the cat fur/dander. It's made a world of difference. You can see it working as the filter captures everything. Have to change them monthly... but so worth it. #YMMV #YouCatsMayVary

Do you strap them to the front or the rear of the fan? Doesn't having them on the front (i.e. where the air blows out) mean that the fan will get dirty since it's sucking the air in?

You may wish to use a cheap thin washable pre-filter either way. It will catch a lot of the hair before it hits the fine filtration, which you can then just wash off the pre-filter, making it easier and cheaper to maintain.

Do you have a link to one of the cheap thin washable pre-filters that I can look at as a reference for what to buy?

Get some window screen from your local home improvement big box store.

know a way to get them to US reasonably? cost is like 10x when shipping to US :(

In the US, grab a basic 20" box fan and toss on a 20x20x1 furnace filter like this one:

https://www.homedepot.com/p/Honeywell-20-in-x-20-in-x-1-in-A...

For extra flow, set up a triangle with the fan on one side and 2 of these filters on the intake end as the other two sides, and duct tape some cardboard to seal the top and bottom.

I came here to post the very same link :D

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