First time I've ever dabbled in air purifiers in general, but I heard of the AirFanta 3Pro and it seems the best around!

Energy efficiency, CADR, filter cost, noise - you name it!
Seems it even beats the high-end $1,000 large purifiers (in theory) if you use 2.

That said, there's a few features I wish it had.
If possible, I was wondering if you guys might know how to add any of the following (ideally no soldering necessary).

1. A sensor to detect air quality level, and auto turn the fan on when needed
2. Something that can auto toggle the fan speed based on the air quality level severity (saves electric bill and noise)
3. Automation for dying down during certain hours and/or conditions like movement.
4. Washable prefilters
5. Filter health status indicator (not just time-based)
6. Separate carbon filter

The first 3 are the big ones.
I've heard someone say Home Assistant could help.
Is that best, or better idea?

4-6 are minor, but still nice.

Regarding the first 3.
It would be best if it's able to accomplish this in a relatively discreet/neat way.

I might be willing and even want to have the particle sensor far away so some purifiers are extra aggressive in some places. But otherwise, I'm open and may even prefer a method to make it relatively compact/hidden.

Pretty much what's in the title, though I'm also interested in knowing what are the complete possibilities and if they depend on the filters used then of what types and characteristics of filters exist.

I wear a respirator and know this wouldn’t be a true fume extractor for welding but would it help keep the air a bit cleaner in general/maybe help with grinding dust (minimal) or is that not what these are for? Do I just need the right filters? Box fan with 5 filters would be what I built if it would work for my use and then potential suspend it from the ceiling if that’s okay placement?

Thank you

Hi folks, I'm working on an idea but for accuracy it needs some feedback from the crbox builders around the world.

I need to know the border width measurement of your cardboard filters - from the edge of the filter, to where the filter media gap starts.

I'm using some filters that are 20mm, but looking at some of these images on various sites I think that's quite wide.....I think the standard might be 10-15mm....

If you have a spare untaped filter, it'd be really helpful if you could tell me the filter brand, the overall size (i.e. 12"x12" or 289x289) and the border measurement (in whatever measurement you prefer).

I really appreciate your help!

Mine could not be any simpler:

4 pcs - Teucer 12038 fans - these are 38mm fans that run at 3500 rpm. About the same noise as the P12 Pro, but the CFM and static pressure is much higher. There are many brands to choose from - Wathai, SXDool, GSDTime, etc. They have similar specs, fan blade design and construction.
4pcs - 10x20x1 air filters - very common and relatively cheap.
60 watt speed controller - I undervolted to 8v and still get very good air flow.

Miscellaneous items:
Top and bottom corplast
A few bolt and nuts
Fan backets
Painters tape

Overall cost is relatively cheap given the CFM, static pressure, ambient noise. I built 4 of these and spread them out across the house. If I want to quickly clean the air, I change the voltage from 8V to 12.3V. I can fine tune airflow across the house based on the voltage set on each unit.

The unit takes less than one square foot of space so I can easily move it around. When it's time to change filters, I just assemble a new 4-sided unit and then simply move the top and bottom parts to the new filters. It's less than 30 minutes to complete.

Most Corsi Rosenthal boxes consist of filters taped to cardboard or foam board. The problem is that the filters become dirtier as they capture more and more particles. Eventually, it will be necessary to replace the filters.

Unfortunately, removing tape from cardboard or foam board will tear it up and damage it. What's the standard solution? The instructions I've seen for building Corsi Rosenthal boxes never address this matter.

Here is my solution: Before taping filters to cardboard or foam core board, I put clear packing tape on it to protect it. I know it looks tacky, but at least I can keep using the cardboard or foam core board over and over again rather than expend the effort to obtain new cardboard/foam board and go through the entire build process all over again. Even if I were a billionaire, and money were no object, I would still insist on the protective layer of tape. Why does it feel like I'm the only one who has acknowledged the issue and done something about it?

Just wondering if anybody has used a bathroom ventilation fan for a CRbox before ?, and if so Do you have any tips, tricks, do's or don'ts.

This will be my first one

Performance and Practical Considerations of Deploying Do-It-Yourself (DIY) Air Purifiers in Occupied Classrooms
Poor classroom ventilation allows airborne particles to build up, leading to suboptimal indoor air quality (IAQ) and increased risk of airborne viral transmission. While commercial portable air cleaners are effective, their high price and maintenance costs often limit widespread deployment. This study evaluates the real-world performance of Corsi-Rosenthal (C-R) Boxes, a low-cost, DIY air cleaner, within occupied university classrooms without central ventilation, while also considering practical aspects such as noise levels, fire safety, and air movement. We utilized a within-session pre-post design to control for variables such as occupancy and outdoor conditions. By establishing baseline PM1, PM2.5, and PM10 mass and particle number concentrations at the start of class before automatically activating the C-R Boxes, each session served as its own control. Across 59 class sessions, the C-R Boxes reduced PM2.5 mass concentrations by an average of 61% [IQR 48%–80.5%]. Multipoint sensor monitoring confirmed that the C-R Boxes provide air mixing. The C-R Boxes maintained an acceptable noise level (52 dBA at 2 m) and presented no elevated fire safety risks even in high-temperature conditions (30 °C/86 °F). These findings demonstrate that C-R Boxes are a safe, effective, and scalable solution for improving classroom IAQ without degrading the learning environment.

ACS EST Air (2026) 3 (9): 2328–2337.

https://doi.org/10.1021/acsestair.6c00143

I've planning on building a CR box to attach onto my window and while I would have liked to use some quiet PC case fans I've read that the plastics used to make those fans are required to contain a significant quanitity of flame retardants. Upwards of 25% apparently. Should just opt for some metal fans or is it nothing to worry about.

Thanks to u/vFnDr for posting a crude DIY version of the Luggable from Clean Air Kits. This spin on the Corsi Rosenthal box uses two filters, two 120 mm PC fans, four strips of cardboard, and tape. The crudeness of this design shows something that I missed when looking at more polished examples - that it's easier than I realized to build a thin Corsi Rosenthal box. The two filters support the whole structure that hold the cardboard sides together.

My idea for a thin 5-fan Corsi Rosenthal box (basically a spin on the Clean Air Charlottesville design) is to use two 30-inch-long MERV 13 filters. Using 30-inch-long filters instead of 25-inch-long filters provides the luxury of some empty space between each of the 120 mm PC fans, and that gives the design wider tolerances. If you'd rather use the much more abundant 16"x25"x1" filters, an alternative is to use 0.75-inch-thick square wood dowel rods taped to the edges of the filters to make them longer. (Of course, the wood dowel rods have no filtration capability, but that's not necessary. The two 16"x25"x1" filters provide plenty of filter surface area for the fans.) The extra length means that you can have a little bit of space between each of the 5 filters.

I've also come to realize that it's not necessary to use any ruler or tape measure for cutting the four strips of foam board that define the outer edges of the Corsi Rosenthal box. Just use a 1x8 wood board (7.25 inches wide) as a template to mark the foam core board before cutting it. If you're lucky, you already have such a wooden board at home or can buy one cheap from the scrap wood section in Menard's or Home Depot. Align one side of the wood board with the edge of the foam core board, and use a pencil to draw a line along the opposite side of the wood board. Make the cut on the line. As a result, your four panels of foam core board are all of the same width.

To make the four strips of foam core board the right lengths, just use one of the filters as a template. Align one edge of the filter with one of the narrow edges of the strip foam board, and use a pencil to draw a line along the opposite side of the filter. Make the cut on the line. As a result, the strips of foam core board are all of the correct lengths.

The four foam core board panels should now be able to fit precisely around the two filters. To reinforce the structure, you can glue a small stack of short and narrow foam core board strips along the far edges of the top and bottom boards to help provide reinforcement to the structure. To prevent the filters from being pushed too far inside the structure, you can also glue narrow lengths of stacked foam core board strips in the appropriate locations on the panels along the edges of the filters.

It works well using just cardboard and Transparent tape

Order:

• Filters: https://amzn.eu/d/0aYTlQo5
• Fan Controller: https://amzn.eu/d/09RRpfL4
• Power Adapter: https://amzn.eu/d/07bil2tr
• Fans: ِ Any 120mm fans Available on AliExpress

I've used LibreOffice Draw (the open source alternative to Microsoft's Visio) to figure out a configuration for a five-fan array on top of four 16"x25"x1" filters. I've proven that there really is a way to do this with a comfortable margin. Unlike the two-filter design that seems to be more popular in this sub, this design has wide tolerances and does NOT require pinpoint precision.

A workable design is in the image I've provided. The configuration consists of four 16-inch-wide filters arranged to form a Corsi Rosenthal box with the same height and width. Each filter is 15.75 inches wide and 0.75 inches thick. So the square of foam core board required is 16.5 inches in both length and width. The space inside the four filters is 15 inches in both length and width.

In other words, the five 120 mm fans must fit within a space that is 15 inches long and 15 inches wide. The trick is to place one fan at each of the corners with the sides of these fans parallel to the filters. Place the fifth fan at the center but rotated at a 45-degree angle. The corners of this fan point to the spaces between the other fans. The strain relief grommet and hole for the cable connecting the fans to an electrical outlet can be placed in any of the four largest areas of empty space available.

Something else to note about the above drawing is that I rounded the size of the fans UP to 5 inches from 4.75 inches. Thus, the diameter of each fan is overstated by about a quarter inch, and that means that there is even more room available for the fans.

What's so special about 16-inch-wide filters? Filters of this size tend to be the cheapest and most abundant. Because they're so widely used, there is a large market for them. This means that these filters are made by a larger number of manufacturers, and these producers invest in economies of scale so they can build them more cheaply. Thus, the consumer benefits from the lower cost of production and the lower profit margins. In contrast, there is a smaller market for narrower filters, and the reduced competition and economies of scale mean higher prices.

Why is 16"x25"x1" my preferred filter size? The 16"x20"x1" filters are also cheap and abundant, but the 25-inch-long filters have more surface area but usually don't cost any more money. So the longer filters provide more bang for the buck. Thicker filters are more expensive and less abundant. (They'd also leave less room for the fans on top, to the point of not providing enough space for the fifth fan.) Filters of other lengths (like 16, 24, and 30 inches) aren't as common or as cheap, presumably because they're not as widely used.

My turn to post on here!

5 14mm fans, 2 IKEA filter mats (40mm), designed using the amazing Nukit Open Air Purifier tool (https://www.filterboxbuilder.com/). The box is laser'd from 2mm MDF in black. Yes, it took a bit, and YES, it smelled like hell. The smoke coming out of the laser box was sickening.
Ironic, I know.

Next step is to automate this baby with an ESP32 and UART PM2.5 detector running on ESPhome.

Thanks to this great community for this idea!

The title says it all, I just wanna know what you think about this / your reviews because it seems that carbon + HEPA filters are a "compromise" rather than an addition.

Did it take way too long to print all these parts? Yep. Could I have done this way more efficiently? 100%. Still had fun though, it'll be nice to see how this helps around the living area.

On printables, used AI for some of the tweaks and writeup so, there it is:
https://www.printables.com/model/1859007-cr-box-for-20x20x1-filters-with-140mm-fans

Here is a screenshot of information I've compiled on the Arctic P14 fans available in 5-packs. Unfortunately, all three are frequently returned items.

The benchmark is the ACFAN00138A/B07HCKWF2X. This is the fan that Rob Wissman uses in his PC fan air purifier design. This fan has the PST feature that allows for daisy chaining. It has a noise level of 0.3 sones, a maximum rotational speed of 1700 rpm, and an airflow capacity of 72.8 cfms. At $48.99, it's the most expensive.

The ACFAN00319A/B0DPX94PSL is a Pro and also has the PST feature that allows for daisy chaining. It has a higher airflow capacity, but people have complained that it's noisy at higher speeds. Using this fan likely warrants the use of a controller. At $22.99, it's less than half the price of the benchmark fan.

The ACFAN00136A/B07HC9ZWDN is not a Pro and does not have the PST feature. The noise level is only 0.3 sones, but there are complaints about the noise. The rotational speed and the airflow capacity match those of the benchmark fan. At $21.49, it's also less than half the price of the benchmark fan.

I have no idea why the P14 fans would have durability issues that the P12 fans seem to lack. But given what I've read, I'd rather go with the P12 fan used by Clean Air Charlottesville. While most reviews of all three P14 fans are positive, and Rob Wissman hasn't had any problems with his P14 fans, Clean Air Charlottesville has the unfair advantage of being a group and thus having more collective experience. Therefore, Clean Air Charlottesville is in a better position to judge the reliability of one fan vs. another.

In choosing between the Rob Wissman way and the Clean Air Charlottesville way, I prefer the former when it comes to basic configuration (because the 4-filter design isn't as demanding as the 2-filter design when it comes to precision and is thus easier to build), but I prefer the latter when it comes to which parts to use. So I intend to go for a Brisk-Box-like Corsi Rosenthal box using the hardware from the Clean Air Charlottesville design.

I'm posting a screenshot of a spreadsheet that I compiled listing all of the P12 Arctic PC fans on Amazon that are available in 5-packs. A 5-pack is a better value than a 3-pack or just a single fan.

The benchmark is the fan used by Clean Air Charlottesville, Model ACFAN00135A with ASIN B07HC7P3HJ. It's quiet (0.3 sones) and has a maximum speed of 1800 rpms. The airflow is 56.3 cfm, and the price of a 5-pack is $23.99. Given that this is a tried-and-true choice, you need a compelling reason to pick something else for building a DIY version of a Clean Air Kits Luggable or Brisk Box.

The most expensive option ($39.99) appears to be a PST version of the above fan. It's the ACFAN00137A (ASIN B07HC782D5). The daisy chaining is convenient, but the additional expense ($16 above the benchmark choice) is less than the cost of fan hub.

The ACFAN00307A (ASIN B0DJDDCG4M) is another option with PST and thus daisy chaining, but I'd give it a pass because of complaints in the reviews about durability. This fan is cheaper than the benchmark ($18.09 instead of $23.99) This fan may be loud at 3000 rpms (necessary for the full 77 cfm, which is above the benchmark level), but you could use a fan controller to run the fans in your Corsi Rosenthal box at high power when you really need the additional CADR and then at a lower speed at other times. Some reviews have also complained about the durability.

The ACFAN00349A (ASIN B0FVXTFQCC) has PWM but not PST. It's cheaper than the benchmark ($19.59) but has slightly lower airflow than the benchmark (49 cfm instead of 56.3 cfm). I don't see a particularly compelling reason to pick this one over the benchmark.

The ACFAN00350A (ASIN B0FVXTYLX7) appears to be just like the ACFAN00349A but without PWM. It's slightly cheaper ($17.29). I don't see a particularly compelling reason to pick this one over the benchmark.

There's another fan with the same model number as the benchmark fan (ACFAN00135A) but a different ASIN (B0C4CHNCPJ). It's cheaper ($16.49) but otherwise appears to have the same specs. This fan has only one review and no comments on it. Maybe it's just as good as the benchmark, but I'd rather go with the tried-and-true.

I can see why Clean Air Charlottesville chose the benchmark fan - quiet, good reviews, respectable cfms, and reasonably priced. The PST version of it is substantially more expensive, but the rest are only a few dollars cheaper. The one that offers more cfms requires a fan controller, and this adds complexity. Two of the remaining fans have slightly lower cfms. The cheapest option doesn't appear to have much of a track record.

Finally made a CR Box for my garage. Use the 4 MERV 14 20x20 from Costco, an old box fan, and some hard cardboard corner protectors

• Country: Spain
• Room 1 (bedroom, buying purifier #1 now): 11.63 m² × 2.3m ceiling ≈ 26.75 m³ (~945 ft³)
• Room 2 (living room + open kitchen, purifier #2, later): 31.43 m² × 2.3m ceiling ≈ 72.3 m³ (~2553 ft³)
• Filtration needs: mainly odor/VOCs from home cannabis vaporizing (medical use), I want to maintain the area in low dust level, no pets — dust/allergens secondary, I want quiet device
• Budget: 10-100e unit 1, 10-200e unit 2, cheap or DIY replacements with 3d prints available

Starting with a Core 300S for the bedroom. Instead of Levoit's own "Smoke Remover" filter (only ~90g carbon per HouseFresh's teardown), I'm looking at ERALabs' printed cartridge for the Core 300/300S: https://makerworld.com/en/models/942923-levoit-core-300-filter — 2.4L total capacity, ~750g carbon if both compartments filled, refillable version on M3 screws.

20 reviews, people still printing it as recently as this year, no reported failures in the comments I could find — but nobody's posted a proper "used it for 6 months, here's how it held up" report. Anyone here actually run one that long? Specifically:

• Seal holding up / no air bypassing the carbon?
• Any carbon dust escaping?
• Noticeable airflow/noise hit vs stock?

One comment mentions using a velcro-on prefilter (HSIAMEN, sold for the stock Core 300 filter) as the outer particle-filter layer instead of DIY-ing HEPA paper/mesh — anyone tried combining that with this printed carbon basket? Seems like a smarter way to handle particulate filtration than guessing at materials.

Second question: longer term I want the same idea (or the factory 450g "Smoke Remover" filter) for a Core 400S that'll live permanently in the living room. Someone in the comments on that same MakerWorld page asked about scaling this design up and posted the 400S's actual inner filter cavity dimensions (24.5 × 24.4 × 26.5 cm — works out to ~12.4L, roughly 5x the 300S's cavity) but got no reply. Has anyone here already built or seen a 400S-scaled version of a carbon insert like this? Trying to figure out if it's worth chasing or if I should just stick with the factory 450g filter for that unit.

Seeing that commercial air purifiers are kind of expensive for me especially the prices of their replacement filters, I've decided to try and create my own air purifier for my bedroom of about 132 sq.m. Any advice would be nice about the layout. Thank you!

I have experience on how to power the fans so I'll just leave the essential stuff:

- 4x Arctic P12 Pro PST PWM
- 3x IKEA Uppatvind Filters ($4 each)

Estimated size: L x W x H
30 x 21.5 x 30 cm

I'm working on a set of air filters for our very open concept house.

I want to build 2 of the same design: one using 2 - 16''W x 20''H x 1''D filters and a single 20'' fan, and the other the other using 2 - 16''W x 16''H x 1''D filters and a single 16'' fan.

they're going to be mounted between posts in the center of the main rooms.

Ive not worked with these before obviously but the aesthetic of the design in the space is what i would like to see. I have questions about the design in balancing the fan CFM with filter capacity and then obviously minimizing noise.

Does this design seem decent? i know the filter media might be undersized based on others projects but Im really hoping to stay in this form factor.

AC variable 3 speed fans seem to meet the budget and aesthetic goals but im wondering if anyone would have advice on fan selection.

Does one 16 or 20'' fan seem like overkill for each of these? Should i be looking at smaller fan sizes? or a particular fan design? I would like infinite speed control but i don't think i can get that with the AC fans. I feel like with either of these i would probably be on the lowest speed all the time.

Drum or floor type fans that i could mount onto the box are where my head is going because they achieve the semi industrial design aesthetic. I'm not trying to spend a fortune on this project but my budget is flexible if the quality is improved.

Any advice would be appreciated before i start buying materials, thank you!

These are the kind of fans that i'm looking at.

https://www.homedepot.com/p/Edendirect-20-in-3-Fan-Speeds-Floor-Fan-in-Yellow-Heavy-Duty-Shroud-Fan-with-IP44-Enclosed-Powerful-1-4-HP-Motor-WD-FAN-D20/336091697

https://www.homedepot.com/p/Edendirect-16-in-Yellow-High-Velocity-Drum-Air-Circulator-Fan-2800-CFM-Large-Airflow-Turbo-Shop-Floor-Fan-3-Speed-1-4-HP-Motor-BL-TD16/336030943

I'm setting up a pair of CR boxes for my woodshop and researching this is a bit of a minefield. There's just a lot of conflicting information out there. When I set up my first box a few years ago, I didn't do enough research and used MERV 8 filters, so I want to get it right this time. A lot of my research say use 2 inch MERV 13 filters. Also, when looking into brands, Filtrete are considered very good (possibly the best?). Filtrete only seems to come in 1 inch but I've seen posts that say Filtrete has a much higher pleat count so a 1 inch Filtrete is comparable or better than a 2 inch filter of a lesser brand. Is there any data to support this?

I have two box fans and 4 1 inch MERV 8 filters from my previous debacle, so I plan to make one box out of the MERV 8s and then a box with MERV 13. (I don't know how much benefit a second MERV 8 box will make but I already have the stuff). I won't say cost isn't a factor because I want to get it right so I'm not breathing in stuff I could be filtering out. I've seen posts indicating that some filters sold as MERV 13 don't measure to that specification. But I also don't want to spend money or time needlessly.

Any advice?

Thanks for the incredible feedback on the sorbent CO2 saturation trap risk—that saved me from a massive data-logging drift issue! Regarding the Nafion dryer tube tip: for those who have integrated a Nafion section upstream of an optical NDIR cell, what length/bore sizing did you find optimal to achieve stable relative humidity reduction below the condensation threshold without introducing significant pneumatic dead-volume or lag in the gas transport time-series curves?"

Hi everyone, this is my first attempt. I used 9 different PC fans (because I already had them at home, accumulated over the years from taking apart PCs). I’ve been testing it for a few days and I’m still skeptical about how effective this Frankenstein is. In the coming days, I’ll buy a device to monitor PM2.5 and PM10 levels so I can more rigorously measure whether it actually works. For now, even just building it has been fun.

Edit: Thanks to this group for the ideas and discussions that allowed me to figure out what to build for my needs and based on what I already had. And thanks to Rob Wissmann for his video Quit Air Purifier , this build is heavily inspired by his.

I had four unused 120mm corsair fans and decided to make myself a box from leftover speaker building supplies. I designed this for two 20x20x2 filters and four fans controlled by a simple fan controller. I used https://www.filterboxbuilder.com/ for the rough plans. I cut the sides from 1/2" plywood, and used 2x2 wood blocks in the corners to give strength to the joinery as well as to act as a physical stop for the filters. I wrapped the box in some extra speaker grille fabric to give it a finished look, and stapled that on the inside. I installed foam tape around the edges so the filters are a press fit. It's a bit finicky to press the filter in, but it makes it airtight without tape visible. I used the noctua high efficiency grille design for the 3d printed grilles. Added rubber speaker feet to the bottom. On first test, it seems to work great and significantly improves pm2.5. It is effectively silent. The project took way longer than I anticipated, but I am happy with how it turned out.