Comment by nucleardog | Hacker News Reader

Comment by nucleardog | Hacker News Reader

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The “more throughput for your buck” seems to hold even as you move up beyond “filter taped on a fan”.

The air filter I built out ran me about $200 without the filters. (Lumber, inline duct fan, foam tape for sealing, window screen, some other accoutrements. The bulk of the cost was the fan.)

The filters are two 3M MPR1500/MERV12 5” deep furnace filters that ran $75. They’re prefiltered with some activated carbon sheet and some window screen (to avoid the filters clogging with pet hair). This is a similar filter to what runs in my furnace and they generally last over a year in there—the 5” filters have a lot of surface area.

So all in let’s round up and call it $300. Looking at Amazon and ignoring the questionably rated all-caps-manufacturer style products, looks like they would get me an air purifier rated for 600 square feet at two air changes per hour. Similar deal at Home Depot.

At two changes per hour mine’s good for about 3000 square feet. Which means in the space it’s in it can do more like 10 air changes per hour, or one every 6 minutes.

In reality that’s a lot of moving air and makes quite a lot of noise. Day-to-day I’m running it at around 20% and sitting at two changes per hour. But when we’re cooking something smoky or during wildfire season, it’s one knob to turn up a bit and the air cleans up in here almost immediately.

I run an AirQ which measures PM1/2.5/10, among other things. Without the filter on they all hover around 20ug/m3. When we’re frying food it can easily hit 300-400ug/m3 in here and unless we open up a bunch of windows it takes a long time to come down.

With the filter on it’s 0. Not 0.something, just a flat 0. If I turn it off and get it up to 300-400 in here and then turn it back on to about 60%, we’re back below 100 within 15 minutes, and down to something that rounds off to zero about a half hour after that.

Which is all a long ramble to not really say anything except that it was a fun project.

This sounds amazing. Do you have a reference to the materials and possibly plans? Sounds like a great design to open source.

I believe he's referring to a Corsi Rosenthal filter:

https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box

It's somewhere on my infinitely expanding todo list to actually document it properly somewhere (though likely on my personal site where literally nobody will see it). Overall though it's pretty conceptually simple and straightforward to build if you've got some basic tools and are moderately handy.

I pulled a few photos of the build off my phone and threw them online. I should preface them with "I'm a software engineer, not a carpenter.". You can probably get a good idea of the whole thing from these: https://files.bogon.cloud/d/9d8d2f9b9e8f454ba305

Conceptually it's just a wooden tube the size of a 16x25 furnace filter. The middle section houses an inline duct fan which pulls air from inside the box and exhausts out a hole in the top. That draws air in through the giant holes at the end of the tube and blows it out the top. The filters slot in on either end, so any air drawn in is pulled through the filters. The whole box is built out of half a sheet of OSB and a couple lengths of 2x2 and couple small pieces of 2x4.

I used some neoprene rubber tape (the particular one I bought was sold for wrapping pipes to insulate them) where I attached the outer shell to the frame as well as under the attachment points for the fan to help seal the "inner box" and keep vibration down. Used way more screws than I needed just to try and make sure everything was as air tight as possible.

The box is sized such that the 2x2 frame matches the _outer_ size of the filter which leaves the hole a bit (3" on either dimension) smaller than the typical filter in order to accommodate most of the typical variations in filter sizes. (A 25x16 filter is... apparently never 25x16. That's actually how I ended up with these 3M filters--bought them for my furnace then found out it takes some weird even more non-standard size.) The frame the filter sits against also has some neoprene tape stapled to it to help create a seal when the filter's sitting against it.

The filter isn't actually held in place by anything but friction and the fact that I don't really move this thing around much. I built another 2x2 frame to sit outside the filter to help hold it in place as well as to have somewhere to staple some window screen so the filters didn't immediately plug with pet hair. (Last couple images show the amount of hair and dust that it catches after running it for a bit.) Since that's not fixed in place by anything you could actually put any filter _up to_ 5" deep in there--anything smaller and the frame can just slide further in. (Though I'm not sure something like a 1" filter would support the air volume the fan can pull. In a pinch you could at least put _something_ in and just run it at a lower speed though.)

The hole the fan sits in just has another strip of neoprene tape run around the inside to seal it to the top board so I don't have any major leaks there. I did end up stapling another piece of window screen over the hole just to make sure my kid or cats didn't throw any toys in there and wreck the fan.

The activated charcoal filter is just a 16x48" sheet I bought off of Amazon and cut in half and set against the filters. The frame + screen holds it against the filter well enough.

You probably can't see in any photos, but my carpentry skills are not _that_ good, so I did wind up with a few small gaps between the outer shell boards where the filters sit that weren't sealed off by anything since there's no 2x2/etc inside that part. Ended up just filling them with a glue gun. Once I was done I used a kleenex to check for any major leaks--held it around all the joins, edges of the filter frame, etc, and watched whether it got sucked towards it. I'm sure _some_ air is getting in somewhere, but definitely the vast majority of the air is going through the filters. Even with the duct fan barely above a whisper, it's pulling enough air that a balled up kleenex will stick anywhere on either filter.

I... I guess if anyone has any questions or feedback feel free! I kinda just cobbled this together one weekend when I got annoyed at the wildfire smoke, so I'm sure there's probably some room for improvement.

Thank you for sharing this. Your carpentry skills being “not that good” is a feature - most of ours aren’t either.

Do you have any photos of your filter system? I am curious about the form factor that uses two 5” filters, rather than the four filter rigs I have seen.

Posted a few in reply to a sibling comment, they're here: https://files.bogon.cloud/d/9d8d2f9b9e8f454ba305

IMG007 is the mostly-complete build and probably answers your question most directly.

Basically it's just a wooden tube with a fan inside in the middle that draws air from inside the tube and exhausts it out the top. I stick a furnace filter in each end of the tube so the air that's getting sucked in is pulled through the filters.

Perfect. Thank you.

I did a similar thing. Built a plywood box, used cheap box fans, MERV12 4", with a replaceable activated filter. Didn't think to use the screen door covering for pet hair. That's a good fix. The plywood box contains... the cat litter box, one for each litter box. And then I have a few other air filters throughout the home of similar, box=>box fan=>MERV12 filter.

Oh man... we've got a couple cats as well and I hadn't even thought of that. We've currently got a couple litter boxes inside the TV stand so even just cutting another couple holes in the back and adding a 120mm fan with some cut-to-size MERV13 + activated charcoal or something would probably go a long way to keeping the dust and smell down.

Thanks for the ideas!

If you have a spare bathroom, and have a duct fan, with some 4" ducting, and a couple of MERV filters to prevent dust going up the duct, guess where you can duct the litter box odour too? Look up.

But yes, a fan that can pull some semi-decent airflow, I run the box fans on lowest setting possible, I could probably run 12v 120mm or 200mm computer fans and get the same effect, and pull that air through a charcoal filter and box filter, it cuts out all dust, allergens and honestly, just about all the odour. The cats have to drop an absolute stinker to overcome the filters. If you hit Amazon, you will find a lot of euphemistic phrases on pages for ducted fans, activated filters, and so forth, for "filtering for your tomato plants that you grow indoors." That's the kind of equipment you can research. The odour from smelly, indoor "tomato plants" are not even in the same league for what my cats produce on a daily basis.

Edit: Wait, I'm replying to original author who built the ducted fan version. You already know about the euphemistic language on Amazon as you have no doubt DYOR (done-your-own-research). Apologies.

Edit part-deux: The activated filters I picked up come in a big sheet, and then I cut them down to size to fit the back of the MERV. I simply tape them on with gaffer tape and they hold until I am ready to change them out, a single sheet of filter lasts well over a year before it loses odour control. I experimented with layers of activated filter, but, anecdotally, it didn't seem to make much difference. I think the thick MERV filter slows down the air flow enough that the odour causing particles linger around the activated filter for a longer period of time, so they have more chance to bond.

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