I made a quieter air purifier | Hacker News Reader

I made a quieter air purifier

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131 pointscrescit_eundo65 comments

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...

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.

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.

Houses with rads don’t have ducts or registers. There’s a whole other class of solutions for these houses.

if you have house (with attic), jump ducts are nicer solution

The general rule about computer fans is that the bigger they are the the quieter they are (i.e. 140mm > 120mm etc.) It's a wierd market gap that large "commercial" air moving fans are so loud.

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.

Box fans are a fire hazard when used continuously to pull air through filtration media. They are not designed for that.

Computer cooling fans also might not have been designed to handle that much "load", but in the case of the box fan, we have an actual report of someone who almost burned down the house:

https://news.ycombinator.com/item?id=21382994

One report? You can find one report of anything. It might even be true.

Yeah, it's just as likely they had it plugged in improperly or the wires were fraying or something. Might as well say it was spontaneous combustion.

I had a dishwasher catch fire because the installers didn't properly connect the power cable. That doesn't mean dishwashers are inherently a fire hazard.

The 5 little fans are on high while the box fan is on low. The box fan is capable of much higher airflow, but the objection is noise.

The PC case fans have been specifically designed to be as quiet as possible. The Lasko box fan ... well, these are cheap fans. They're designed to be manufactured at the lowest price possible while still meeting the low quality bar of "It's acceptable considering I got the cheapest option that WalMart carries". But in general yes, box fans are terrible.

I’m surprised that those little fans can push more air than a box fan while still being quieter. But I guess each of those 5 little fans cost as much as the box fan so maybe my surprise is unwarranted.

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

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

Check out the plot in the header, and find the particulate size you care the most about: https://www.frdmtoplay.com/nagivating-air-purification/

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."

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.

Thanks so much. The merv-8 to HEPA discontinuity is a great insight. RF.

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.

A deep merv-13 with a lot of pleats can have a very reasonable pressure drop - you just have to shop a little more carefully.

I would stick with merv-13 because you'll get solid performance across a lot of things you might want to remove, from viruses to general pm2.5 and things like volatilized cooking oil. Clean air is awesome and tire dust isn't the only thing that's annoying.

I agree completely re: mev-13 == optimal solution. But the word "pragmatic" hits me hard. Merv-13 when new/clean start out with pretty restrictive flow. They catch a lot of particles so restriction increases rapidly. At some point the CFM loss means the filter is much less optimal. All the studies I read used new filters, smoke-filled rooms, a day's treatment. It is obviously impractical and very, very expensive to replace a merv-13 filter every few days. There are no reusable merv-13 filters that I could find. If there is a study about merv-13 effectiveness over 30 days vs. merv-8 I would love to see it. I would love to use merv-13, but just cannot get my head around how it is a practical, affordable solution across years and years of use, and let's say a month between filter renewal. Let me know if you have good insights as I am pretty worn out researching this. Thx, RF

I've been down this rabbit hole for a while now and sadly can't seem to find the article about the filtrete vs others, but there's been some people who tested the 'load' on these furnace filters and the Filtrete filters far exceeded everyone else in terms of airflow as the filters loaded up.

Re: Filter costs - stock up when costco has them on sale, which seems like every few months. They've got Filtrete 2500 (merv 14) for 3 filters for ~$35 if I remember correctly. I use them in my CR boxes and those I built for family (which I give them with a 3 pack of new filters and also instruct them to refill during costco sales)

I built one for a local stray cat rescue where it literally sits in the middle of the living space for 10+ cats, it's 4 months now since they started using it and the filters look quite dirty but the air flow is surprisingly still very good. (4x Noctua NF-F12 iPPC 3000 fans and 4x 16"x25"x1" Filtrete 2500 filters)

Here's some info re: which merv levels work best with various fan combinations (looks like if you're going to go with higher merv you'll need the static pressure to be able to continue to pull through them with any reasonable airflow once they're getting loaded up with stuff) - https://www.cleanairkits.com/blogs/news/what-happens-to-cadr...

I ran a corsi-rosenthal box 24/7 for a year during covid with 4 merv-13 filters and the airflow stayed pretty good. Depends of course - we don't have pets and were running the HVAC filter full time also so it was a pretty clean environment. I would bet that the lack of a pre-filter would kill things fast if you had pet hair or lots of dust. But I suspect 6 months is totally reasonable from a "provides effective filtration" perspective.

Remember that as the filter starts to get dirty, its filtration effectiveness actually increases, though the airflow rate drops. CADR will drop but less than just watching airflow would predict.

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.

With a multi-fan set-up, would it help to vary the individual fan RPMs, to spread the noise over a broader range of frequencies? Would a 9-fan grid at slightly lower RPMs improve this further?

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.

I use a bank of eight Noctuas to vent from the rooom with our wood stove to the main hall. Then use more units with two sets of Noctuas ganged to move from the hall to various remote rooms. Been using them for more years than I can remember. Silent, never failed yet. It has been great vs. using house fans (my initial approach). BTW: Our local thrift store has a huge amount of various DC power supplies from donations. I sift thru them to find the right volts/amp combo needed for the ganged units, and generally spend about a dollar per supply. HTH, RF.

Oh, boy. During the great California fire of 2020, I went down a very deep rabbit hole looking to air purification. I studied what is out there. I compared their characteristics with regard to filter media quality, air cycles, PM2.5 performance, VOC's, energy consumption, and so on, but my goal was to have performance and near silence. I learned about static pressure and how important it is to choose the right blower for the job (impeller and blower vs fan; and yes, a blower creates vastly more static pressure than these fans). I looked into what quantities of charcoal actually deliver meaningful VOC reduction and how often they must be replaced (you need lbs of it; that single filter on the front of most purification units does nothing but act as a pre-filter). I really should have written all my research and discovery down but if you are serious about it, I came to the following conclusions:

1. A furniture company that decides to integrate a purification system into their offerings can hit it big. Imagine a big, heavy dresser in your bedroom that sucks in air from the bottom and pushes up to the top or a TV stand that where different compartments are filter media. The mass of these reduces noise and vibration and allow for larger, slower spinning blowers and larger surface area of filter media (less static pressure, longer replacement intervals, easily hold pounds of activated charcoal).

2. One needs to pump in oxygen from the outside and exchange the indoor air with the outside air. Circulating and cleaning the air in the home, especially with modern doors and windows, will become unpleasant in a short while, especially during wildfire season. The positive pressure this creates also helps pollutants stay out, if done correctly.

I would absolutely go the diy route using large filters, activated carbon, housed in a wooden box with a blower and implement something like AC Infinity's in-line filtration systems to pull outside air in through a window, with a large carbon filter on one end. Or if a homeowner, set up a more permanent solution. https://acinfinity.com/inline-fan-systems/

I've bought a few of these, and love them: https://www.cleanairkits.com/

This is something that's annoyed me for ages. Why are extractor fans so loud and horrible sounding when PC fans are almost silent? I'm surprised someone like Noctua doesn't go into house fans. But maybe the market isn't there as they're more difficult to fit and people seem to keep them for decades. Or perhaps they just wouldn't work as well outside of that nice cosy PC case.

Good PC case fans are incredibly quiet, move a lot of air and last forever. That's the nice thing about having some many companies competing over optimizing them for so long.

I have a similar setup, just premade from CleanAirKits.

Allergies are gone, house has far less dust and cat hair floating around.

Can't do anything for wood smoke though, that would need carbon filters - which run out quite quickly.

My favorite DIY air purifier setup is buying this Conway, myself:

https://www.amazon.com/dp/B01728NLRG

Quiet, efficient, works super well

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.

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".

conway filter posted above, you can't really hear it from 10ft when it runs on low speed (it has sensor and he autoadjusts ).

i also have iqair which according to reviews is quiet at low speed. in my experience this quiet sounds like airplane (i got it replaced once. apparently it's just the way it is).

I think you're missing his point entirely. The problem with these retail purifiers is that you either get quiet or effective. You don't get both.

Sure, you can't hear it from 10ft away - but how much air is it moving at that setting?

I have various configurations of these PC fan setups and the Arctic P14 Pro that you can get 5 for $32 on amazon are honestly wildly effective and designed for applications with some static pressure (radiators and such).

So we're back to; effective or quiet, you're only going to get both with the PC fans, for now.

I have the one I linked and it is both. Set to auto, it is quiet most of the time and only ramps up when I’m cooking or the air quality is bad.

there is also added benefit of not taking half of the room. i think conway footprint is 1/6th of The Cube if not less.

Rabbit Air (i think they closely related to conway) you can hang on the wall as art piece.

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.

I have two. I agree it's quiet on the lowest speed but it's loud on the medium and high speeds so mine are setup to run 24/7 on the lowest speed regardless of the air quality. The air quality in my house is good and they run nearly always at the low speed anyways. Buy them while they are on sale.

Why do computer fans blow air inside?

I think this done to create a slight overpressure which helps keep the dust out.

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?

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.

I think it's because there are no grills on the outside. If the fans were sucking air out of the box, dust would build up on the outside, and bumping it would dislodge dust back into the environment.

With the fans blowing in, all the dust is on the inside of the box (and on the fans).

The box fan version also blows air into the box

Are those 200mm fans?

They're from Arctic Cooling, so 140mm at most. They don't make anything bigger than that.

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

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.

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

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

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.

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

p100 masks will have optimal filtration on HEPA level per dollar.

can't argue with that

I think we should wait for Corsi and Rosenthal lab work on this topic

a difference is that those masks were designed and certified at that level, which aides adoption

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

The p100 masks are already certified so you can shop based on cost. Filters + fans are not certified so you have to know something more to understand what you are getting

not the point/example i was making. nm

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.

just to follow your logic. lets say I invent car. Corsi and Rosenthal measure it to discover how more efficient it is compared to horse, and from that point it's known as their invention

It's known as their technique (not invention) because it got popularized via of word of mouth. Same as the Fosbury flop or Larsen truss.

If you create some novel car design that gets discussed enough, maybe it will get popularized and get named after you. ie Bangle butt, for the E65 BMW 7 series.

You should tell us your name and publish the result so we can credit you for the work. Would love to call it whatever your name is instead.

i find it shameful to take credit for obvious things that other people probably did before me.

i refused to sign my name on a bunch of patents at work because they were too obvious (they were still granted USPTO)

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