[HN Gopher] I Made a Quieter Air Purifier
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I Made a Quieter Air Purifier
Author : crescit_eundo
Score : 60 points
Date : 2025-11-24 21:31 UTC (6 days ago)
(HTM) web link (chillphysicsenjoyer.substack.com)
(TXT) w3m dump (chillphysicsenjoyer.substack.com)
| runariot wrote:
| Why do computer fans blow air inside?
| brtv wrote:
| I think this done to create a slight overpressure which helps
| keep the dust out.
| pixl97 wrote:
| Your question isn't very clear... Is it why do they use
| computer fans? (My guess is that is what they had). Or is it
| why do you blow instead of suck?
| chr-s wrote:
| You want to suck air through the filters rather than blow into
| them. For one, this keeps the fans cleaner but also it's more
| efficient, reducing turbulence inside the box.
|
| https://old.reddit.com/r/crboxes/ is a good resource if you're
| looking to make your own. There's been a general shift from
| large box fans to PC fans because their performance/noise ratio
| is better.
| calvinmorrison wrote:
| Corsi-Rosenthal... they taped some filters to a fan, something
| the poor folk have been doing forever and academia acts like they
| invented fire. It's caveman technology with a fancy name.
|
| embarassing
| pkd wrote:
| It didn't really come out of academia in that way. It was an
| innovation borne out of necessity during Covid. I don't think
| I've seen anybody using a box design for it. If you have a
| source, I'll be happy to be corrected.
|
| Regardless technology is often named after people who made it
| popular, especially when original inventors are unknown or too
| diffuse.
| tguvot wrote:
| not that much of innovation.
|
| i build one like this back in 2018 during california fires.
| used it mostly in garage for filtering air when i work on
| "things". decommissioned it last week.
|
| i guess i am not the only one who came up with this idea
| prior to covid as this is pretty obvious
| tehjoker wrote:
| i did think the 4-sided design using the cardboard box as a
| base was very efficient. one of the contributions here was
| Corsi and Rosenthal did this in a lab and reported
| particulate counts, flow rates, and energy usage. So there
| was some kind of intellectual contribution to broaden
| adoption by giving it more of an imprimatur of quality
|
| other refinements were using a shroud to prevent back flow
| tguvot wrote:
| back in 2018, during fires, when air filters were
| unobtanium, on bayarea subreddits (and probably here)
| were floating a lot of posts of DIY filter made from box
| fan + hvac filters. plenty of people also posted
| particulate count reduction charts.
|
| you don't need a lab to figure out that replacing 1"
| filter with 4" filter and even better arranging multiple
| filters in a box will improve flow rates and filtration
| (especially for box fans that not really designed for
| static pressure)
|
| corsi&rosental work, imho, is equivalent to lab work
| reporting that you can move more water through 10" pipe
| than through 1" pipe.
| tehjoker wrote:
| I can see that argument, but I think what they
| contributed was that there's something approaching a
| cost-optimal design. they reported that the 4-sided
| design with box fan had higher CADR than many if not all
| commercially available filters though it often used more
| power and was louder, so the filtration per dollar was
| very good
| tguvot wrote:
| p100 masks will have optimal filtration on HEPA level per
| dollar.
| tehjoker wrote:
| can't argue with that
| tguvot wrote:
| I think we should wait for Corsi and Rosenthal lab work
| on this topic
| tehjoker wrote:
| a difference is that those masks were designed and
| certified at that level, which aides adoption
| tguvot wrote:
| labs don't certify that mask is more effective per dollar
| compared to usual air filters.
|
| hence extra research by Corsi and Rosenthal is needed.
| After they will do the measurements they can be declared
| as inventors of p100 masks, that will be known as
| Rosenthal-Corsi masks
| turtlebits wrote:
| They're the ones who did the actual testing/measurements. I
| had a box fan attached to a single filter (for wildfire
| smoke) but was unaware of how inefficient it was.
| scosman wrote:
| I had the same insight for in-home air movement. Purpose built
| inter-room fans from broan/etc are 3x louder and several times
| more expensive than computer fans at the same CFM. I've been very
| happy with them.
|
| https://scosman.net/blog/using_in_wall_computer_fans_for_hom...
| lozenge wrote:
| There are continuous extraction fans, they're called dMEV
| systems. Eg https://www.epicair.co.uk/products/vent-axia-lo-
| carbon-reviv...
|
| There's also systems which can move air both ways through the
| whole house - https://www.ventilation-
| alnor.co.uk/index/support/alnor-know...
|
| Your DIY solution looks a lot cheaper though.
| ortusdux wrote:
| Vevor makes drop-in floor/wall register fans in the $35-40
| price range. The 'proper' solution is inline booster fans in
| each leg of the ducts, but retrofitting can be expensive
| depending on access.
| tguvot wrote:
| if you have house (with attic), jump ducts are nicer solution
| erwincoumans wrote:
| I wonder if they used silent computer fans? I love the quiet
| Noctua fans, and replace all noisy fans with them: fans in pc,
| wifi router, playstation 5, mister fpga, robots, jetson orin etc.
| tyre wrote:
| My favorite DIY air purifier setup is buying this Conway, myself:
|
| https://www.amazon.com/dp/B01728NLRG
|
| Quiet, efficient, works super well
| notatoad wrote:
| yeah, i get why people were doing the DIY air purifier thing
| during covid when air purifiers were hard to come by, and the
| box fan version at least has cost-effectiveness going for it.
|
| but 5x arctic cooling PC fans is ~$100. the commercial versions
| are easily available, more effective, no more expensive, and
| don't look like a box of furnace filters taped together.
| blargey wrote:
| Huh? 5x Arctic P12 is $24 on Amazon right now, no sale going
| on. And the whole Corsi-Rosenthal trend started specifically
| because consumer air purifiers were tested to be both noisier
| and less effective at their job.
|
| But I'd honestly pay a premium for a commercial air purifier
| that just has a bunch of 120mm/140mm fan mounts instead of
| their "maybe tolerable at Very Low" integrated box-fan
| equivalent.
|
| In general, all I've learned from online reviews of "quiet"
| appliances is that different people have _very_ different
| definitions / criteria for "quiet".
| amluto wrote:
| If you're willing to buy something off the shelf, get one of
| the IKEA filters. They're more efficient, quieter, and use less
| expensive media.
| xnx wrote:
| Are those 200mm fans?
| jsheard wrote:
| They're from Arctic Cooling, so 140mm at most. They don't make
| anything bigger than that.
| ruralfam wrote:
| Appreciate any thoughts you all have re: this post. For years I
| have been using Noctua NF-P14 fans to circulate air in house to
| distribute heat in the winter from our wood stove. E.g. cut holes
| in the walls, and circulate remote rooms using the fans. Has
| worked great, and the Noctuas have been rock solid.
|
| Recently a daughter moved into a really nice apartment close to a
| major university/freeway where she will live for the number of
| years it takes to get a Phd. I got concerned about tire dust. So
| I am about to start building a really nice air DIY air filter
| using eight Noctua NF-P14s (about 1000 cfm). XMas present.
|
| I really wanted to use merv-13, but got quite worried about air
| flow restrictions, plus cost to replace (assume monthly). Instead
| I went with two 12x24 Carter reusable electrostatic merv-8
| filters. I use Carter filters on my house blower, and really like
| them (just washed them... scary how much junk is in household
| air). Also, I got the 12x24 direct from Carter for a very low
| price as they were returns. Note: This is NOT a low cost project,
| but I just got scared re: merv-13 so went with what I know.
|
| Anyway, the final product will NOT be like this guy's DIY. I will
| use my somewhat decent woodworking skills to fashion a good
| looking standing "lamp like" appliance that should look good in
| most living rooms. I am thinking of going with knotless cedar as
| I really like working with cedar, and there are some mills here
| in NW WA where one can go to get such wood (not a HomeDepot
| specialty).
|
| My question is whether an electrostatic merv-8 filter would do
| well with tire dust. I am not looking to create "clean room"
| conditions in the apartment. Just get rid of some of the bad
| stuff. I am very weak re: understanding filters, mervs, etc.
| APPRECIATE any insights. Thx, RF
| tim333 wrote:
| I recently bought one of these. It's pretty quiet (on the low
| setting) - you quite likely wouldn't hear it near a freeway
| https://www.amazon.co.uk/gp/aw/d/B071SLZRRV
|
| vid of the noise levels https://youtu.be/wOc0TM1ErYA?t=195
| bsilvereagle wrote:
| Check out the plot in the header, and find the particulate size
| you care the most about: https://www.frdmtoplay.com/nagivating-
| air-purification/
| ruralfam wrote:
| Thanks. However note that the site dropped electrostatic
| filters to simplify things. My understanding is that for non-
| static-affected particles merv-13 would obviously out perform
| merv-8 for smaller particles. However the promise of
| electrostatics is that the materials in the filters create a
| e-stat field that makes them more efficient re: particles
| like dust. Certainly the two electrostatic merv-8 filters on
| my hvac blower capture A LOT of dust (fine particles). Since
| you clean them in a bathtub by filling the tub and washing
| the filters thru them, I can attest that there is A LOT of
| really fine particulates being captured.
|
| The lead line for this article pretty much reflects the
| reason for my post: "The air purifier marketplace is an apt
| metaphor for how a particle must feel while being trapped in
| a filter - at every turn there's a new acronym or regulatory
| agency or purifier type."
| amluto wrote:
| Are you talking about an electrostatic precipitator (metal
| plates with a power supply) or an electrostatic filter
| (fibers with a surface charge)? Electrostatic precipitators
| are neat, but there are basically no standards for them and
| they're not cheap to operate. For charged fibers, I see no
| a priori reason to expect amazing performance or to expect
| them to remain charged after a bath. If they met MERV 13
| standards, they would say so.
|
| In any case, if you see lots of gunk, that's not the hard-
| to-filter stuff. I can say, as the proud owner of a
| monstrous HEPA filter with a dirt cheap noting-special MERV
| 8 pre filter and an utterly boring metal louver before
| that, all continuously collecting outsize air, the louver
| gets a bit gunky, the MERV 8 filter turns black after a
| while, and the HEPA filter is indistinguishable from brand
| new. This whole system replaced an older "ISO ePM1" (yes,
| the manufacturer conveniently forgot the number after that,
| but it's MUCH higher spec than MERV 8), and the indoor air
| quality as measured by a little particle sensor suggested
| that the ePM1 filter missed about 50% of the outdoor PM2.5,
| whereas the new system produces air that measures zero
| across all particle sizes. And that ePM1 filter did a fine
| job of turning black :)
|
| Get a particle counter and test your system!
|
| P.S. the HEPA system uses less power and will cost less to
| operate over time because it is HUGE but has the same flow
| rate.
| amluto wrote:
| To make a nice air purifier, you want to deliver clean air at
| some respectable rate, where "clean air" is a notional amount
| of completely pure air, and you want this to work for all
| particle sizes. If you move 1000cfm (that's a whole lot BTW)
| through a filter that removes 60% of the worst-case particles,
| that's 600cfm of clean air.
|
| At some point I found a nice chart, IIRC from the EPA, showing
| the efficiency plotted vs particle sizes for a variety of
| filters. IIRC the filters generally split into two categories:
| those with decent efficiency all the way down to zero microns
| and those with very poor efficiency at small sizes. IIRC the
| split was around MERV 12. Obviously your filter is not the
| filter in the chart.
|
| So I would go with MERV 13 or even a bit higher. Also, keep in
| mind that pressure drop is related to the velocity of air
| through the filter, so a physically larger filter will have
| lower pressure drop at the same flow rate. But the need to
| replace a filter is related to collected gunk per unit area, so
| doubling your filter area will cost twice as much but last
| twice as long and will use less power and run quieter.
|
| Also, electrostatic filters can lose their charge from exposure
| to various contaminants.
|
| edit: it was the chart here, also mentioned down thread.
|
| https://www.frdmtoplay.com/nagivating-air-purification/
|
| Portable Air Cleaners, Furnace, and HVAC Filters. 3ed. EPA
| 402-F-09-002
|
| And I remembered a bit wrong. Even MERV 10 will pick up the
| smallest particles, but MERV 8 may miss some. But for good
| performance at the most penetrating size, you want MERV 12-ish.
| For a single-pass filter (filtering outdoor air as it enters),
| you want much higher - MERV 16 or even HEPA or near-HEPA, if
| you want acceptable performance against potentially nasty
| outdoor conditions due to wildfire or nasty human particle
| sources.
| dpark wrote:
| If these 5 little fans do the job as well as a full size box fan,
| I have to wonder:
|
| 1. Are box fans just really terrible? I would expect the amount
| of airflow from a box fan to absolutely demolish these little
| case fans.
|
| 2. Does airflow not actually matter that much? Assuming the box
| fan really does move far more air, that would imply that air
| filtration is somehow not driven by air flow. Or else the testing
| methodology is flawed.
| skhameneh wrote:
| At first glance, it appears to have lower negative pressure and a
| much higher filter surface area than most air purifiers.
|
| AFAIK PC fans are designed to move air at low pressure while
| quality air purifiers are designed to handle more pressure.
|
| Other than that, it's a neat writeup. Would like to see a follow-
| up with static pressure considerations.
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(page generated 2025-11-30 23:01 UTC)