Air purifiers are highly suboptimal and could be >2.5x better.
Some things I learned while researching air purifiers for my house, to reduce COVID risk during jam nights.
An air purifier is simply a fan blowing through a filter, delivering a certain CFM (airflow in cubic feet per minute). The higher the filter resistance and lower the filter area, the more pressure your fan needs to be designed for, and the more noise it produces.
HEPA filters are inferior to MERV 13-14 filters except for a few applications like cleanrooms. The technical advantage of HEPA filters is filtering out 99.97% of particles of any size, but this doesn’tmatter when MERV 13-14 filters can filter 77-88% of infectious aerosol particles at much higher airflow. The correct metric is CADR (clean air delivery rate), equal to airflow * efficiency. [1, 2]
Commercial air purifiers use HEPA filters for marketing reasons and to sell proprietary filters. But an even larger flaw is that they have very small filter areas for no apparent reason. Therefore they are forced to use very high pressure fans, dramatically increasing noise.
Originally people devised the Corsi-Rosenthal Box to maximize CADR. They’re cheap but rather loud and ugly, but later designs have fixed this.
(85% confidence) Wirecutter recommendations (Coway AP-1512HH) have been beat by ~2.5x in CADR/$, CADR/noise, CADR/floor area, and CADR/watt at a given noise level, just by having higher filter area; the better purifiers are about 2.5x better at their jobs. [3]
At noise levels acceptable for a living room (~40 dB, Wirecutter’s top pick on medium), CleanAirKits and Nukit sell purifier kits that use PC fans to push air through commercial MERV filters, getting 2.5x CADR at the same noise level, footprint, and energy usage [4]. These are basically handmade but still achieve cost parity with used Coways, 2.5x CADR/$ against new Coways, and use cheaper filters.
At higher noise levels (Wirecutter’s top pick on high), there are kits and DIY options meant for garages and workshops that beat Wirecutter in cost too.
However, there exist even better designs that no one is making.
Someone on Twitter made a wall-mounted prototype with PC fans that blocks fan noise, reducing noise by another few dB and reducing the space requirement to near zero. If this were mass-produced flat-pack furniture (and had a few more fans), it would likely deliver ~300 CFM CADR (2.7x Wirecutter’s top pick on medium, enough to meet ASHRAE 241 standards for infection control for 6 people in a residential common area or 9 in an office), be really cheap, and generally be unobtrusive enough in noise, space, and aesthetics to be run 24⁄7.
A seller on Taobao makes PC fan kits for much less than cleanairkits (reddit discussion). One model is sold on Amazon for a big markup, but it’s not the best model, takes 4-7 weeks to ship, is often out of stock, and don’t ship to CA where I live. If their taller (higher area) model shipped to CA I would get it over the cleanairkits one.
V-bank filters should have ~3x higher filter area for a given footprint, further increasing CADR by maybe 1.7x.
If I’m right, the fact that these are not mass-produced is a major civilizational failing.
[1] For large rooms, another concern is getting air to circulate properly.
[2] One might worry that the 20% of particles that pass through MERV filters will be more likely to pass through again, which would put a ceiling on the achievable purification. But in practice, you can get to the air quality of a low-grade cleanroom with enough MERV 13 filtration, even if the filters are a few months old. Also, MERV filters get a slight efficiency boost from the slower airflow of a PC fan CR box.
[3] Most commercially available air purifiers have even worse CADR/$ or noise than the Wirecutter picks.
[4] The Wirecutter top pick was tested at 110 CFM on medium; the CleanAirKits Luggable XL was tested at 323 CFM at around the same noise level (not sure of exact values as measurements differ, but the Luggable is likely quieter) and footprint with slightly higher power usage.
But an even larger flaw is that they have very small filter areas for no apparent reason.
Is reducing cost of manufacturing filters ‘no apparent reason’?
It seems like literally the most important reason… the profit margin of selling replacement filters would be heavily reduced, assuming pricing remains the same.
I don’t think that a small HEPA filter is necessarily more expensive to produce than a larger MERV filter. I think they are using other rationale to make their decision about filter types.
Their perception of public desirability/marketability is likely the biggest factor in their decision here. Components of their expectation here likely include:
Expecting consumers to want a “highest possible quality” product, measured using a dumb-but-popular metric.
Expecting consumers to prioritize buying a sleek-looking smaller-footprint unit over a larger unit. Also, cost of shipping smaller units is lower, which improves the profit margin.
Wanting to be able to sell replacements for their uniquely designed filter shape/size, rather than making their filter maximally compatible with commonly available furnace filters cheaply purchaseable from hardware stores.
Isn’t a major point of purifiers to get rid of pollutants, including tiny particles, that gradually but cumulatively damage respiration over long-term exposure?
Yes, and all of this should apply equally to PM2.5, though on small (<0.3 micron) particles MERV filter efficiency may be lower (depending perhaps on what technology they use?). Even smaller particles are easier to capture due to diffusion so the efficiency of a MERV 13 filter is probably over 50% for every particle size.
A brief warning for those making their own purifier: five years ago, Hacker News ran a story, “Build a do-it-yourself air purifier for about $25,” to which someone replied,
One data point: my father made a similar filter and was running it constantly. One night the fan inexplicably caught on fire, burned down the living room and almost burned down the house.
Luckily, that’s probably not an issue for PC fan based purifiers. Box fans in CR boxes are running way out of spec with increased load and lower airflow both increasing temperatures, whereas PC fans run under basically the same conditions they’re designed for.
Any interest in a longform post about air purifiers? There’s a lot of information I couldn’t fit in this post, and there have been developments in the last few months. Reply if you want me to cover a specific topic.
I’ve noticed that every air purifier I used fails to reduce PM2.5 by much on highly polluted days or cities (for instance, the Aurea grouphouse in Berlin has a Dyson air purifier, but when I ran it to the max, it still barely reduced the Berlin PM2.5 from its value of 15-20 ug/m^3, even at medium distances from Berlin). I live in Boston where PM2.5 levels are usually low enough, and I still don’t notice differences in PM [I use sqair’s] but I run it all the time anyways because it still captures enough dust over the day
Sounds like you use bad air purifiers, or too few, or run them on too low of a setting. I live in a wildfire prone area, and always keep a close eye on the PM2.5 reports for outside air, as well as my indoor air monitor. My air filters do a great job of keeping the air pollution down inside, and doing something like opening a door gives a noticeable brief spike in the PM2.5.
Good results require: fresh filters, somewhat more than the recommended number of air filters per unit of area, running the air filters on max speed (low speeds tend to be disproportionately less effective, giving unintuitively low performance).
Yes, one of the bloggers I follow compared them to the PC fan boxes. They look very expensive, though the CADR/size and noise are fine.
My guess is Dyson’s design is particularly bad. No way to get lots of filter area when most of the purifier is a huge bladeless fan. No idea about the other one, maybe you have air leaking in or an indoor source of PM.
Thanks, I didn’t realize that this PC fan idea had made air purifiers so much better since I bought my Coway, so this post made me buy one of the Luggable kits. I’ll share this info with others.
Air purifiers are highly suboptimal and could be >2.5x better.
Some things I learned while researching air purifiers for my house, to reduce COVID risk during jam nights.
An air purifier is simply a fan blowing through a filter, delivering a certain CFM (airflow in cubic feet per minute). The higher the filter resistance and lower the filter area, the more pressure your fan needs to be designed for, and the more noise it produces.
HEPA filters are inferior to MERV 13-14 filters except for a few applications like cleanrooms. The technical advantage of HEPA filters is filtering out 99.97% of particles of any size, but this doesn’t matter when MERV 13-14 filters can filter 77-88% of infectious aerosol particles at much higher airflow. The correct metric is CADR (clean air delivery rate), equal to airflow * efficiency. [1, 2]
Commercial air purifiers use HEPA filters for marketing reasons and to sell proprietary filters. But an even larger flaw is that they have very small filter areas for no apparent reason. Therefore they are forced to use very high pressure fans, dramatically increasing noise.
Originally people devised the Corsi-Rosenthal Box to maximize CADR. They’re cheap but rather loud and ugly, but later designs have fixed this.
(85% confidence) Wirecutter recommendations (Coway AP-1512HH) have been beat by ~2.5x in CADR/$, CADR/noise, CADR/floor area, and CADR/watt at a given noise level, just by having higher filter area; the better purifiers are about 2.5x better at their jobs. [3]
At noise levels acceptable for a living room (~40 dB, Wirecutter’s top pick on medium), CleanAirKits and Nukit sell purifier kits that use PC fans to push air through commercial MERV filters, getting 2.5x CADR at the same noise level, footprint, and energy usage [4]. These are basically handmade but still achieve cost parity with used Coways, 2.5x CADR/$ against new Coways, and use cheaper filters.
At higher noise levels (Wirecutter’s top pick on high), there are kits and DIY options meant for garages and workshops that beat Wirecutter in cost too.
However, there exist even better designs that no one is making.
jefftk devised a ceiling fan air purifier which is extremely quiet.
Someone on Twitter made a wall-mounted prototype with PC fans that blocks fan noise, reducing noise by another few dB and reducing the space requirement to near zero. If this were mass-produced flat-pack furniture (and had a few more fans), it would likely deliver ~300 CFM CADR (2.7x Wirecutter’s top pick on medium, enough to meet ASHRAE 241 standards for infection control for 6 people in a residential common area or 9 in an office), be really cheap, and generally be unobtrusive enough in noise, space, and aesthetics to be run 24⁄7.
A seller on Taobao makes PC fan kits for much less than cleanairkits (reddit discussion). One model is sold on Amazon for a big markup, but it’s not the best model, takes 4-7 weeks to ship, is often out of stock, and don’t ship to CA where I live. If their taller (higher area) model shipped to CA I would get it over the cleanairkits one.
V-bank filters should have ~3x higher filter area for a given footprint, further increasing CADR by maybe 1.7x.
If I’m right, the fact that these are not mass-produced is a major civilizational failing.
[1] For large rooms, another concern is getting air to circulate properly.
[2] One might worry that the 20% of particles that pass through MERV filters will be more likely to pass through again, which would put a ceiling on the achievable purification. But in practice, you can get to the air quality of a low-grade cleanroom with enough MERV 13 filtration, even if the filters are a few months old. Also, MERV filters get a slight efficiency boost from the slower airflow of a PC fan CR box.
[3] Most commercially available air purifiers have even worse CADR/$ or noise than the Wirecutter picks.
[4] The Wirecutter top pick was tested at 110 CFM on medium; the CleanAirKits Luggable XL was tested at 323 CFM at around the same noise level (not sure of exact values as measurements differ, but the Luggable is likely quieter) and footprint with slightly higher power usage.
Is reducing cost of manufacturing filters ‘no apparent reason’?
It seems like literally the most important reason… the profit margin of selling replacement filters would be heavily reduced, assuming pricing remains the same.
I don’t think that a small HEPA filter is necessarily more expensive to produce than a larger MERV filter. I think they are using other rationale to make their decision about filter types. Their perception of public desirability/marketability is likely the biggest factor in their decision here. Components of their expectation here likely include:
Expecting consumers to want a “highest possible quality” product, measured using a dumb-but-popular metric.
Expecting consumers to prioritize buying a sleek-looking smaller-footprint unit over a larger unit. Also, cost of shipping smaller units is lower, which improves the profit margin.
Wanting to be able to sell replacements for their uniquely designed filter shape/size, rather than making their filter maximally compatible with commonly available furnace filters cheaply purchaseable from hardware stores.
Isn’t a major point of purifiers to get rid of pollutants, including tiny particles, that gradually but cumulatively damage respiration over long-term exposure?
Yes, and all of this should apply equally to PM2.5, though on small (<0.3 micron) particles MERV filter efficiency may be lower (depending perhaps on what technology they use?). Even smaller particles are easier to capture due to diffusion so the efficiency of a MERV 13 filter is probably over 50% for every particle size.
A brief warning for those making their own purifier: five years ago, Hacker News ran a story, “Build a do-it-yourself air purifier for about $25,” to which someone replied,
Luckily, that’s probably not an issue for PC fan based purifiers. Box fans in CR boxes are running way out of spec with increased load and lower airflow both increasing temperatures, whereas PC fans run under basically the same conditions they’re designed for.
Any interest in a longform post about air purifiers? There’s a lot of information I couldn’t fit in this post, and there have been developments in the last few months. Reply if you want me to cover a specific topic.
Have you seen smartairfilters.com?
I’ve noticed that every air purifier I used fails to reduce PM2.5 by much on highly polluted days or cities (for instance, the Aurea grouphouse in Berlin has a Dyson air purifier, but when I ran it to the max, it still barely reduced the Berlin PM2.5 from its value of 15-20 ug/m^3, even at medium distances from Berlin). I live in Boston where PM2.5 levels are usually low enough, and I still don’t notice differences in PM [I use sqair’s] but I run it all the time anyways because it still captures enough dust over the day
Sounds like you use bad air purifiers, or too few, or run them on too low of a setting. I live in a wildfire prone area, and always keep a close eye on the PM2.5 reports for outside air, as well as my indoor air monitor. My air filters do a great job of keeping the air pollution down inside, and doing something like opening a door gives a noticeable brief spike in the PM2.5.
Good results require: fresh filters, somewhat more than the recommended number of air filters per unit of area, running the air filters on max speed (low speeds tend to be disproportionately less effective, giving unintuitively low performance).
Yes, one of the bloggers I follow compared them to the PC fan boxes. They look very expensive, though the CADR/size and noise are fine.
My guess is Dyson’s design is particularly bad. No way to get lots of filter area when most of the purifier is a huge bladeless fan. No idea about the other one, maybe you have air leaking in or an indoor source of PM.
Thanks, I didn’t realize that this PC fan idea had made air purifiers so much better since I bought my Coway, so this post made me buy one of the Luggable kits. I’ll share this info with others.