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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.

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

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.

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

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.

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."

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.

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.

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".

Why do computer fans blow air inside?

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.

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?

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

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

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.

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