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Comment by wintermutestwin | Hacker News Reader

If you want to get an indoor air filter to clean up the toxic air that enters your home, I recommend an industrial solution. Go down to your local indoor growing shop and check out the carbon filters that the pot growers use to scrub the weed smell.

Get a filter like this: https://indoorgrowing.com/collections/vortex/products/profil...

And pair it with an inline fan to match like this: https://indoorgrowing.com/collections/vortex/products/vortex...

That gets you 400CFM of air cleaning power for ~$500. (you can get this stuff for much cheaper used as the margin on indoor pot growing has tanked)

Compare to a "designed for the home" air purifier like this: https://oransi.com/products/mod-hepa-air-purifier

Compared to my proposed industrial solution, the air purifier costs $100 more and is about 1/4 the CFM. The only pluses are that it looks pretty and is roughly 2/3 the DBA when running.

View on HN

If anyone's interested in a cheaper and less powerful version I've done tests on a DIY purifier that's based on a similar design: https://dynomight.net/better-DIY-air-purifier.html It costs around $100 to build and delivers around 104 CFM (2.95 m^3/min) on high and around 71 CFM (2.02 m^3/min) on medium (where it's pretty quiet). Interestingly, this is nearly an equal "deal" to your solution: it's around 1/5 the price and around 1/4 as powerful.

If people are put off of installing an air purifier because of they price, I think they should try not to make perfect the enemy of good: Due to the way half-lives work, you get the biggest benefit in terms of cleaner air from the "first" CFM of power that you install.

That dude (you?) compares his contraption to the same three filters taped randomly on a box fan. That's a little silly. I'm surprised it filters anything at all, as the air would simply flow between and around the filters. During Oregon's 2020 Labor Day fires many of us just taped standard 20"x20"x1" home filters to a standard box fan. You can tape all the way around it ensuring that all air passes through the filter, and the filters have pretty good ratings (we used MERV 11 and 13 filters). We found that they worked very well. Here's a picture of one of ours (we had three running in different places): https://photos.app.goo.gl/XkEKBdt5m8nN8NU7A

I don't have any numbers, but I strongly suspect for purposes of wildfire smoke they work as well as what that article is proposing.

It is me! I'll note that I've also done experiments comparing the effects of taping and not taping around the filters, which you can see here:

https://dynomight.net/2020/12/15/some-real-data-on-a-DIY-box...

It's awesome that you've done all this work. I have to say, though, that this could just as well be evidence that your experiment is not designed terribly well. One would never build a home furnace with a filter that has gaps all the way around it.

Certainly you wouldn't, but keep in mind the blower in a home furnace is designed to push air through a filter. This is not the case with box fans, where (at least with the filters I used) the fan throughput vastly decreases when you add the tape. I'm pretty confident in my experiments, but I wouldn't claim this as a general result: It's just a sort of coincidence that the two factors (pushing more air overall and funneling a larger fraction of that air through the filters) happen to almost exactly cancel out. If you have a more powerful fan or you have filter media that creates less of a pressure barrier, then I'm pretty sure you will see a benefit from taping. But I was just worried about the box fan motor so I decided to remove them.

The purpose of these filters is entirely different. A furnace filter needs to prevent large particles from passing it, as such particles can deposit on the coil. Its utility is essentially -(particles missed). A room air purifier exists to remove particles. Its utility is (particles removed). Blowing additional air around that bypass the filter in an air purifier is harmless, whereas it can destroy your furnace or especially your A/C.

This is why the IKEA air purifier is a better design than a HEPA purifier.

Also people most certainly do build furnaces with gaps around the filter.

This is all wrong. Our furnace filters (and all furnace filters) filter particles specifically to clean the air. And with a MERV 13 or higher rating, they even filter Pm 2.5.

And, no, I’ve never seen a single furnace filter with intentional gaps around the filter.

In addition to using the wrong type of filter, "roughly 2/3 the DBA" is not a terribly helpful statement because decibels are a log-scale unit. A 60dB sound will carry about 100x the energy of a 40dB sound. It's not super problematic at these levels, but if it were going from 60 to 80 then there's a real hearing damage concern.

Actually, your ears are pretty close to logarithmic in perception so a difference of 20-40dB is a lot less than you might expect. Below a certain level ~40dBA it's hard to discern, because other noise sources dominate, but in a perfect anechoic chamber or alone in a cave you can hear significantly lower holding your breath. I've usually heard about sound Pressure levels at a specific distance (eg 1m) where 20dB is a 10x increase (not 10dB) while for sound POWER levels you're correct 10dB is a 10x increase and power is closer to our logarithmic perception.

Human hearing easily covers (and can distinguish loudness) over 5-6 orders of magnitude with ~1-3dB resolution.

   0dB is the limit of a young ear
   10dBA is normal breathing at 1m
   20dBA is a soft whisper at 10ft
   30dBA is a soft whisper (1m)
   40dBA is a very quiet library (no distance)
   50dBA is a coffee percolator (at 1m)
   60dBA is normal conversation 1m
   70dBA is a TV or a toilet flush 1m
   80dBA is an alarm clock 1m
   90dBA is a blender or shouting 1m
   100dBA is a circular saw or motorcycle 1m
   110dBA is a loud concert (no distance) or leaf blower (1m)
   120dBA is an ambulance siren or jack-hammer 1m

The health danger cited by the article is PM 2.5 particles. From a quick skim, your proposed solution does nothing to help with fine particles.

You are correct and the grandparent comment (relative to mine) is misguided -- at least as far as wildfire smoke is concerned.

Although wildfire smoke probably contains other pollutants, fine particulates are by far the most important, and the filter recommended by grandparent does not even claim to remove particulates: "effectively remove 99.5% of volatile organic compounds", it claims.

I have owned both kinds of filters -- the ones that remove volatile organic compounds (including activated-carbon ones as well as other types) and the ones that remove particulates (all of which happened to be HEPAs although I acknowledge that there are probably other types effective at removing wildfire smoke) -- since the 1980s. Also, I used my HEPAs to protect my lungs from the severe episodes of wildfire smoke in the Bay Area in Nov 2018 and Sep 2020.

I am (because of chronic illness) much more sensitive to volatile organic compounds than most people and have never been tempted to use my volatile-organic-compound filter against wildfire smoke, so not only is the filter advocated by grandparent insufficient for protecting lungs against wildfire smoke, it is IMO unnecessary.

>does not even claim to remove particulates: "effectively remove 99.5% of volatile organic compounds"

These filters are sold to pot growers who are specifically trying to remove the pot smell (VOCs) from the air so that's the focus of their claims.

Maybe I am making an incorrect assumption, but aren't VOCs smaller particles than the "bad stuff" from wildfire smoke?

VOCs are individual molecules (in gaseous form) which of course are much smaller than wildfire-smoke particles and other forms particulate air pollution.

The point is that activated-carbon filters do little to remove wildfire smoke from the air.

Thanks for pointing out the issue. I'll make covers out of Merv 13 Filter Material. Too bad I can't measure CFM to test for the reduction of flow...

> The only pluses are that it looks pretty and is roughly 2/3 the DBA when running.

Well, the HEPA will remove a lot more particles than just a carbon air filter.

I don't think a carbon filter is what you want for wildfire smoke. A carbon filter will remove some chemicals in the air, but for particulates, which is mostly what smoke is, you need a particle filter, for example a HEPA filter. It doesn't need to be HEPA though -- a furnace filter will remove a lot of the particulates.

Wildfire smoke is both particulates and strong odors from VOCs. A MERV13 particulate filter (HEPA is likely overkill) reduces PM2.5 and a carbon filter reduces the odor carrying VOCs.

I recently switched from a carbon filter with a canfan to a medifyair MA-40. I think the medifyair is far superior to the straight carbon filter. It also is significantly quitter than my old set up and still pushed ~220 CPM. They are a little pricey but well worth it in my opinion.

An industrial air scrubber with a 3-stage filter at 500 CFM is now about $500 new on Amazon. They were a lot more expensive during the height of the pandemic. They're great, also critical if you are cooking with natural gas.

No referral link: https://www.amazon.com/s?k=air+scrubber

Nice find!

The reviews for one of them says it's very loud and meant for constructions sites. Maybe one of them is quieter? (post a link if you find it!)

These are loud, about as loud as a good stove hood. They are quieter than the hydroponic scrubbers recommended above. This isn't really the kind of device you have on all the time. I use it with an AQI monitor during high-particulate events like cooking, smog, wildfires, etc., to clean my apartment's air. The AQI outside where I live has routinely been around 100-150 PM 2.5, while a few runs for 5-15 minutes every time I leave or enter keeps it around 5-15.

It is hard but possible to have a conversation when it is on full speed. Mine has a knob you can adjust the fan speed, which obviously trades off with CFM, but makes it easier to have a conversation.

But they work really fast at max speed. I cook a lot in a hot oil wok on a gas stove. My "hood" is a fan built into my microwave above the oven that just recirculates air in the room, and I have just one small window in my kitchen. If I forget to turn my scrubber on while cooking, it only takes about 10-15 minutes for my PM2.5 sensor at the other end of my kitchen to go from 500+ to about 10.

I have an Oransi air filter, they are extremely well made and are priced accordingly. I have two so during work(wfh) both me and my Wife can be working in clean air envs(in our offices) and at night we put them into our bedrooms so we are breathing clean air overnight. Fun fact they also work well to get strong cooking smells out of the house/kitchen. Cannot recommend them enough.

getting rid of strong kitchen smells and cleaning my air, i'm sold.

This is a rather different thing. The carbon filter removes VOCs and maybe some ozone and NOx — it’s for gasses. A normal air purifier is for small particles. You can certainly do both, and you can surely add a MERV 13 or better filter to the “industrial” solution, but as you describe them, they address different problems.

Does anyone know of an air purifier that does not use proprietary filters?

It would be wonderful to pair some of the features of dedicated air purifiers (low noise, unobtrusive design, etc.) with the availability of standard 3M MERV HVAC filters stocked at Lowe's/Home Depot.

That would be so great. I was struggling to find this recently when shopping for an air purifier!

box fan

We used to have one of those. They're so so loud and it's a bad kind of noise that wears on you. We've since moved to a admittedly more expensive iq air air filter that provides up tp 320 cfm and is nearly silent. Totally worth it.

Carbon filters the smell, but not particles. You need HEPA filter for that.

This isn't HEPA, but far superior option to your typical 1" HVAC filter. Higher MERV, less restrictive, lasts multiples longer:

Honeywell Home FC40R1011

The home unit claims it goes up to 350 CFM

That's just the fan speed rating - not the filter's rating.

It's specs say it covers 1,312 sq ft (2 air changes per hour) Assuming 10foot ceilings, that comes out to 109CFM

Is there any effective method for air purification that doesn't involve disposable supplies?

No, sadly. Maybe trucking in liquified clean air and boiling it off into your house to create a positive pressure.

I wonder how feasible it would be to construct an elastic air filter. When it's full you could take it outside, stretch it out, and blow all the crap out with a leaf blower.

Maybe some day! But current materials science doesn’t have anything like that. It’s not usually about physical hole size, but rather electrostatic attraction (in the case of HEPA filters) or adsorption (in the case of activated carbon). There’s no “hole” to open to let things drop out. It’s a surface that things are somewhat bonded to.

Maybe you could tote the machine outside, stretch it, and then electronically reverse the charge to make it easy to blow off contaminants.

Maybe bubbling the air through water or a mist/air counterflow like scrubbers in coal plants? Throw in some UVC light source to keep germs from growing in the water. Are there UVC LEDs yet?

There also are electrostatic filters but they produce ozone.

There are, and they're finally getting cheap!

https://www.ebay.com/itm/114268507196

$4-5 USD per unit really isn't bad (plus power supply and resistors or controllers) compared to what they were a few years back.

(saw these on extractions & ire, the ones he got must be very durable since he way way way overpowered them)

Joan you have to do something with the dirty water ;P.

The carbon filters like the one I linked could have been designed so that the activated charcoal could be replaced. Unfortunately, The companies who make these filters choose profits over environmental sustainability.

Its like printers

You can look into the electrostatic cleaners with a washable filter

Do those actually work, though?

For noise, you use insulated flexable duct snaked back and forth in several s-curves.

Completely agree. Terrabloom is another good option with noise dampened products.

How loud is it?

The fans used to push or pull air through a carbon filter are typically pretty loud. Hard not to be when you are moving so much air. You can get quitter ones but they are expensive. If you are going the carbon filter/canfan route its most likely going to be loud. You can create a box around the fan with something like MDF to reduce the noise.

~68dba compared to the Oransi at 49dba (assuming max fan speed). The sound is only from the movement of air and is directly correlative to how much CFM you are moving.

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