Better Air Purifiers

Like you, I'm also somewhat surprised there hasn't been more innovation here. I'm also disappointed by the current market for HEPA filters, and the widespread perception of HEPA supremacy. To some extent, part of the reason why we see low innovation may be connected to the problem of low HVAC competence, which I've written about previously

In my view there's a point somewhere around a corsi box and possibly before where we're probably better off embracing the HVAC trade and all of the tools and materials this encompasses. This probably means replacing tape with tin snips, and cardboard with sheet metal. These tools and materials exist for the express purpose of reducing time and expense, while delivering effective, aesthetically pleasing results. This should be done in a way that advances diy projects, but opens up an entirely new world of fans, ducts and filters of myriad stock sizes, each as separate components that can be placed together in a variety of configurations and installed virtually anywhere in a building.

As an example, vevor has a 6", 400 cfm fan for ~$50. This is compact enough to be installed near the ceiling of a closet, which would greatly reduce motor noise in drawing air from/supplying to the adjacent room. Adding a sidewall filter grill and filter will run another $115, and we'd probably need some ductwork and a supply register. But at this point we probably have a permanent, quiet, discreet system for delivering >350 cfm cadr for <$250 in materials.

Benevolent_Rain @ 2025-05-15T08:06 (+5)

Just woke up to what you mention about optimization in HVAC installation, here in Sweden they mount ductwork components with something akin to TDC/TDF - it is literally easier than Lego! It is so fast I was blown away working with it. And no special tools required. Awesome podcast you did btw on Pigeon Hour!

JesseSmith @ 2025-05-15T16:33 (+6)

Very similar to systems in the US! My sense is that many people have an ugh field wrt embracing basic trade knowledge, which is a significant impediment to even intermediate level projects. Glad you enjoyed the podcast!

Thomas Kwa @ 2025-05-18T21:47 (+2)

A pair of CR boxes can also get 350 CFM CADR at the same noise level for less materials cost than either this or the ceiling fan, and also have much less installation cost. E.g. two of this CleanAirKits model on half speed would probably cost <$250 if it were mass-produced. This is the setup in my group house living room and it works great! DIY CR boxes can get to $250/350 CFM right now.

The key is having enough filter area to make the static pressure and thus power and noise minimal-- the scaling works out such that every doubling of filter area at a given CADR decreases noise by 4.5 dB, assuming noise is proportional to power and pressure goes as (face velocity)^1.5, which are common rules of thumb. I'd guess that the pair of CR boxes has 5x more filter area, so an 11dB advantage for the closet sound isolation to make up. MERV filters also get slightly higher efficiency when the face velocity is slower.

I have used inline fans for other purposes and even the air passing through a 6" duct generates some noise and adds static pressure. With a CR box you're doing the minimal work necessary to filter air.

Standard HVAC parts do have many advantages though. The aesthetics are unmatched and all parts are likely to be available, and they're very durable.

JesseSmith @ 2025-05-18T23:06 (+3)

Are you limiting your noise estimates to strictly airflow/static pressure? The research I'm aware of suggests Corsi boxes perform at >40db and up to 60db on high speed (~350 cfm). https://housefresh.com/corsi-rosenthal-box-review . During the pandemic this was widely discussed as a major impediment to their adoption. 

Upsizing terminations substantially reduces room-facing air velocity. Assuming 400 cfm, the velocity at the termination I link will be roughly 175 fpm. This is extremely low, and I'd expect to achieve 20-25 db. Most residential return registers are undersized and thus outside ACCA's duct design standard of 500 fpm. It's also less than standards widely used for quiet spaces (200-300 fpm). You could continue pursuing improvements here via upsizing further if you're so inclined, or using clever placement to put the filtration system away from the room activities or behind (offset) cabinetry or furnishings. 

Thomas Kwa @ 2025-05-18T23:33 (+2)

That's a box fan CR box; the better design (and the one linked) uses PC fans which are better optimized for noise. I don't have much first-hand experience with this, but physics suggests that noise from the fan will be proportional to power usage, which is pressure * airflow, if efficiency is constant, and this is roughly consistent with various tests I've found online.

Both further upsizing and better sound isolation would be great. What's the best way to reduce duct noise in practice? Is an 8" flexible duct quieter than a 6" rigid duct or will most of the noise improvement come from oversizing the termination, removing tight bends or installing some kind of silencer device? I might suggest this to a relative.

JesseSmith @ 2025-05-19T00:13 (+3)

Ah, sorry, the CR reference threw me off. That link seems to suggest those are in the 40 db range. 

Duct design uses equivalent length as a metric across duct types and fittings with 1 foot of straight metal pipe as a '1'. The problem with flex is that each foot has an equivalent length of 1.5, which adds a moderate amount to system length (although to keep this in perspective many near air handler transitions are >100'). For something like this I'd probably do a short run of flex near the motor (both sides) to dampen motor/vibration noise and transition to metal beyond. Avoid cornering with flex! In general, remote fans, eliminating line of sight on the motor and oversizing ducts/terminations will be shockingly quiet to most room occupants. If you want to get super clever, you can start doing offset openings in walls by cutting grilles high on one side and low on the other, but it can be difficult to pull sufficient air while keeping velocity low (400 cfm = 6 stud bays).

Thomas Kwa @ 2025-05-19T18:28 (+2)

You can get down to 25 dB by running two at half speed. Fan noise is proportional to RPM^5, so 50% speed will mean -15dB noise. The fans just need enough static pressure to maintain close to 50% airflow at 50% speed.

JesseSmith @ 2025-05-20T13:37 (+3)

Do you have tested numbers? Going from high to low on 4 filter CR boxes shows a reduction in airflow in the range of 30%-40% and db reduction around ~13%.

JesseSmith @ 2025-05-20T13:38 (+1)

Thomas Kwa @ 2025-05-21T20:00 (+15)

(edited to fix numbers, I forgot 2 boxes means +3dB)

dB is logarithmic so a proportional reduction in sound energy will mean subtracting an absolute number of dB, not a percentage reduction in dB.

HouseFresh tested the AirFanta 3Pro https://housefresh.com/airfanta-3pro-review/ at different voltage levels and found:

• 12.6 V: 56.3 dBA, 14 minutes
• 6.54 V: 43.3 dBA, 28 minutes

So basically you subtract 13 dB when halving the CADR. I now realize that if you have two boxes, the sound energy will double (+3dB) and so you'll actually only get -10 dB from running two at half speed. So a more accurate statement for the Airfanta would be that for -15dB noise at the same CADR, you need something like 2.8 purifiers running at 36% speed. It's still definitely possible to markedly lower noise by adding more filter area.

Your box fan CR box data tell a similar story. If logarithmic scaling is accurate, the sound reduction for halving CADR would be ln(1/2)/ln(165/239)*(8 dB) = 15 dB, or 12 dB for maintaining CADR with double the units. It just doesn't have a speed low enough to get these low noise levels (and due to the box fan's low static pressure you might need to add more filters per fan at low speeds).

Airfanta's absolute noise levels are high for a CR box type design but this is a device that retails for 298 CNY = $41 USD in China, runs at high speed, and uses near-HEPA (95%) rather than MERV filters so is to be expected.

JesseSmith @ 2025-05-22T19:58 (+1)

Bummer. Operating at 25 db would have been really impressive for a room filter but it looks like it's not even close. Extrapolating from David Elfstrom's airflow estimates it looks like ~1/2 cfm would put it >40 db, which isn't great. So if we want to create a satisfactory occupant experience we probably shouldn't put down the tin snips just yet.

Jeff Kaufman 🔸 @ 2025-05-19T13:18 (+2)

That's a box fan CR box

A filter with PC fans isn't a "CR box" -- a Corsi-Rosenthal box is specifically a design based on box fans. For example, Wikipedia, US Davis, Clean Air Crew, and the Corsi-Rosenthal Foundation mention only box fans.

Thomas Kwa @ 2025-05-19T18:25 (+5)

Usage varies-- the top five posts on /r/crboxes all use PC fans. Other guides do too,  and CleanAirKits and Nukit both describe themselves as PC fan CR boxes.

Jeff Kaufman 🔸 @ 2025-05-20T18:06 (+2)

Interesting! I hadn't realized people had started using the term this way!

As a non-engineer not working in this space, I can't say much but I've had a lot of similar thoughts (ceiling mounted, outer casing, non-HEPA). I do have my question about your design (and GPT o4-mini-high is pretty skeptical), wondering whether the fan really draws all the air through the filters. This design also doesn't work well in smaller rooms? I personally like "fandeliers" or ceiling-mounted CR boxes that include lighting and actually look decent.

I agree that there seems to be a lot of improvement up for grabs, and I would love to see a company start innovating here and - most importantly - scale up production. 

The problem, however, seems like it would be demand: air purifiers aren't in high demand, and consumers don't notice the difference between high and low CADR (they do notice noise). A company could bet on creating a large, cheap stockpile to sell off at high prices during an epidemic, but I imagine it'd be hard to get investors on board for that. This space probably needs market-shaping, such as advance market commitments or indoor air quality regulations (e.g. mandated PM2.5 levels) creating demand.

Great article, makes a lot of sense. It might make sense to launch towards commercial sectors first - there they are more interested in increasing air quality at low cost and low noise. Just realized that some hair salons have purifiers - I think there are lots of small hair particles floating around that I think you don't want to inhale.

On replacing filters: There is actually this study from the national labs that indicate filters hardly need replacement at all. I think it is even worse than razorblades that grow dull - filters last forever! It is a shame the torrent of marketing now has the public believing the filters need frequent replacement - that will be a hard myth to kill for a startup.

Jeff Kaufman 🔸 @ 2025-05-15T10:11 (+6)

There is actually this study from the national labs that indicate filters hardly need replacement at all. I think it is even worse than razorblades that grow dull - filters last forever!

That study is looking at nuclear facilities, but I'm not sure it generalizes to environments with more particulates in the air. In a dusty enough environment your filter will surely get clogged up to the point where you're not able to move much air through it!

Benevolent_Rain @ 2025-05-15T10:49 (+2)

Yeah perhaps. But on my purifiers there is at least one screen sorting out a lot of debris. And then I vacuum the filters every now and then. I guess one might notice if the airflow significantly reduces. I am running a test on mine to see how long they last. You have prompted me to get a new filter and later on just test how much air flow is reduced with the old filters - thanks for the inspiration. But I think one can safely run the home air filter for much longer than the 6-12 months suggested by manufacturers.

JesseSmith @ 2025-05-15T16:40 (+3)

Increasing pressure drop across a filter is probably the best proxy for whether a filter needs replacement. In the field, a new, high MERV filter will often have a measured pressure drop between .15 and .3 inches of water column. An increase of ~.5iwc is probably sufficient to indicate replacement. Where high merv filters are used, it's not uncommon to have very fine particulate be the primary source of clogging which won't be visible so visible dirt isn't entirely reliable. It's not uncommon for sophisticated control systems to automate this. It's also fairly accessible as a diy tech.

Benevolent_Rain @ 2025-05-15T17:39 (+2)

Do you know why, apart from increased energy usage and perhaps fan wear and tear, one needs to replace filters as pressure increases? My understanding is that filter efficiency does not decrease but maybe I missed something. And I agree measuring the pressure drop is best, but just thinking for some DIY, at-home quick fix without any special tools, it might just make sense to check if the airflow is more or less the same.

Jeff Kaufman 🔸 @ 2025-05-15T18:30 (+4)

Efficiency in terms of fraction of removed particles wouldn't decrease, but because CFM will decrease efficiency in terms of CADR will too.

Thomas Kwa @ 2025-05-17T19:02 (+10)

Efficiency can decrease too, especially when there are lots of very small particles like smoke. See this reddit thread: https://www.reddit.com/r/crboxes/comments/1fznar2/comment/lr2j404/.

My understanding is the small particles can basically cover the surface area of the fibers and block their electric field. Here's an image from one of the linked studies showing filters that are (a) clean, (b) after one test, and (c) having absorbed 2 grams / m^2 of smoke and having its efficiency drop from 92% to 33%.

JesseSmith @ 2025-05-15T18:42 (+3)

Fan energy is a mixed bag depending on motor type. Constant airflow fans have a fairly flat pressure:CFM curve and will increase energy but PSC motors will see much lower airflow and reduced energy as a consequence.

In heat/AC systems airflow reductions outside a fairly narrow range will cause shutdown/failure due to high heat/freezing coils. In filter only systems you'd probably see low airflow and premature motor burnout. In standard motor assemblies going beyond .2 iwc increase would widely be considered quite aggressive (I have a fairly high risk tolerance for crazy stuff in buildings and probably wouldn't do this)

Benevolent_Rain @ 2025-05-17T05:04 (+2)

I guess just a last question from me: Does it seem necessary to you to replace filters every 6-12 months as manufacturers suggest? I have run filters for >2 years and cannot really discern any increased load on the fan motors, just listening for changes in motor sound. Nor can I sense any significant reduction in air flow, just feeling the air flow with my hand. I am asking as I am telling people not to replace the filters as often as manufacturers suggests - it is expensive and people might think that their purifiers do not work well after a filter is 6-12 months old.

JesseSmith @ 2025-05-18T22:38 (+7)

My experience is mainly in central HVAC systems and I mostly agree that filters could have a much long service life. However, I've seen significantly shorter life via fine particulate via the use of tap water in humidifiers. Ime this will clog a MERV 13 filter in a few weeks. If you can afford a manometer (~$100) capable of reading increments of IWC ~.1, testing pressure is quite simple and many more people would benefit from adding this type of measurement to their skillet! I'm happy to provide further guidance!

Benevolent_Rain @ 2025-05-19T05:57 (+3)

Yeah if $100 can save you even just 3 filter replacements, that sounds like a good investment. Maybe I should do this myself. For now, I will just hope motor sound + my intuition/tacit knowledge of air purifier air flow is enough for me to realize when a filter definitely needs replacing. Thanks Jesse!

Love the design here! 
One doubt I have is that in India, where I live, the air quality is worst during winter - a time of year that you wouldn't want a ceiling fan on. Would be interested if you have any thoughts on an alternative to use in winter.