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The most efficient and inexpensive air purifier I have found consists of a cheap box fan with a furnace filter taped to the back of it. You can even get a fancy

The most efficient and inexpensive air purifier I have found consists of a cheap box fan with a furnace filter taped to the back of it. You can even get a fancy furnace filter and still come out way ahead financially. The other nice aspect of this approach is that you get much more air throughput for a fraction of the cost of a dedicated air purifier.

dillydogg 3y ago
I remember reading about the Corsi-Rosenthal boxes when the pandemic was at it's peak. https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box
francisofascii 3y ago
I wonder if a box fan will cause too much airflow, which would cause all the allergens to be spewed all over the room. You might need to run this system when not using the room.
- iambateman 3y ago
  I don’t think so. The box fan can only pull air through the filter, so it should pull everything out of the air which it can in a few minutes. This kind of fan can do multiple air changes per hour, depending on how many sqft it’s supporting.
nucleardog 3y ago
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.
- andrei_says_ 3y ago
  This sounds amazing. Do you have a reference to the materials and possibly plans? Sounds like a great design to open source.
  - haneefmubarak 3y ago
    I believe he's referring to a Corsi Rosenthal filter: https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box
  - nucleardog 3y ago
    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 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.
pxx 3y ago
This is a fairly good concept but it is not efficient, especially if you're using 1" filters. The operating cost is high because the fan uses a lot of electricity compared to better designed units. See this analysis: https://onlinelibrary.wiley.com/doi/10.1111/ina.13163 https://onlinelibrary.wiley.com/doi/10.1111/ina.13163 Even with fairly reasonable electricity numbers you should look into building a more complicated box. And if your time is of import the dedicated purifiers start winning again after you amortize the startup cost (also they typically are better engineered for noise concerns).
- Ajedi32 3y ago
  Thanks, that's exactly the sort of information I've been looking for! Though I disagree with the methodology used to calculate ongoing (operating) costs, as it seems that doesn't include the cost of replacement filters, only the cost of electricity. Many dedicated air filters have replacements that cost hundreds of dollars, so that seems like a huge oversight when comparing to standard MERV 13 furnace filters that I can pick up for $10 a pop. (The electricity price they used in their calculations is also nearly double what I pay, but that's an understandable deviation given that electricity prices can vary wildly by region.)
rajnathani 3y ago
Well, the interesting thing about the post is that it is specifically about VOCs. Airflow based air purification through Carbon filters and HEPA/MERV filters barely do anything to reduce VOCs (keep in mind that VOCs themselves are of many subtypes).
Ajedi32 3y ago
I wish there were more commercial air filters that accepted standard furnace filters. They're super inexpensive compared to the proprietary filters designed for specific air purifiers, have tons of surface area (allowing for good airflow), and there are plenty available with MERV 13 ratings. There's an argument to be made that MERV 13 isn't a good substitute for the higher rated carbon/HEPA filters that most dedicated air filters are designed to use, but I've read some things that make me question that assumption. Specifically, it sounds like, at least to a point, lower efficiency filters can be compensated for by increasing airflow, resulting in air passing through the filter multiple times?[1] I never got a satisfying answer to that question. [1]: https://dynomight.net/ikea-purifier/ https://dynomight.net/ikea-purifier/
walleeee 3y ago
I am partial to the "Cuboid" as described here https://dynomight.net/better-DIY-air-purifier.html https://dynomight.net/better-DIY-air-purifier.html. Quiet, easy, effective, and get all the foam you need from the duct fan packaging.

dillydogg 3y ago

I remember reading about the Corsi-Rosenthal boxes when the pandemic was at it's peak. https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box

francisofascii 3y ago

I wonder if a box fan will cause too much airflow, which would cause all the allergens to be spewed all over the room. You might need to run this system when not using the room.

iambateman 3y ago
I don’t think so. The box fan can only pull air through the filter, so it should pull everything out of the air which it can in a few minutes. This kind of fan can do multiple air changes per hour, depending on how many sqft it’s supporting.

nucleardog 3y ago

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.

andrei_says_ 3y ago
This sounds amazing. Do you have a reference to the materials and possibly plans? Sounds like a great design to open source.
- haneefmubarak 3y ago
  I believe he's referring to a Corsi Rosenthal filter: https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box
- nucleardog 3y ago
  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 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.

pxx 3y ago

This is a fairly good concept but it is not efficient, especially if you're using 1" filters. The operating cost is high because the fan uses a lot of electricity compared to better designed units. See this analysis: https://onlinelibrary.wiley.com/doi/10.1111/ina.13163 https://onlinelibrary.wiley.com/doi/10.1111/ina.13163 Even with fairly reasonable electricity numbers you should look into building a more complicated box. And if your time is of import the dedicated purifiers start winning again after you amortize the startup cost (also they typically are better engineered for noise concerns).

Ajedi32 3y ago
Thanks, that's exactly the sort of information I've been looking for! Though I disagree with the methodology used to calculate ongoing (operating) costs, as it seems that doesn't include the cost of replacement filters, only the cost of electricity. Many dedicated air filters have replacements that cost hundreds of dollars, so that seems like a huge oversight when comparing to standard MERV 13 furnace filters that I can pick up for $10 a pop. (The electricity price they used in their calculations is also nearly double what I pay, but that's an understandable deviation given that electricity prices can vary wildly by region.)

rajnathani 3y ago

Well, the interesting thing about the post is that it is specifically about VOCs. Airflow based air purification through Carbon filters and HEPA/MERV filters barely do anything to reduce VOCs (keep in mind that VOCs themselves are of many subtypes).

Ajedi32 3y ago

I wish there were more commercial air filters that accepted standard furnace filters. They're super inexpensive compared to the proprietary filters designed for specific air purifiers, have tons of surface area (allowing for good airflow), and there are plenty available with MERV 13 ratings. There's an argument to be made that MERV 13 isn't a good substitute for the higher rated carbon/HEPA filters that most dedicated air filters are designed to use, but I've read some things that make me question that assumption. Specifically, it sounds like, at least to a point, lower efficiency filters can be compensated for by increasing airflow, resulting in air passing through the filter multiple times?[1] I never got a satisfying answer to that question. [1]: https://dynomight.net/ikea-purifier/ https://dynomight.net/ikea-purifier/

walleeee 3y ago

I am partial to the "Cuboid" as described here https://dynomight.net/better-DIY-air-purifier.html https://dynomight.net/better-DIY-air-purifier.html. Quiet, easy, effective, and get all the foam you need from the duct fan packaging.