Comment by ruralfam | Hacker News Reader

Comment by ruralfam | Hacker News Reader

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.

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.

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.

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

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