The Corsi-Rosenthal box[0] design using a standard box fan and furnace filters has been tested and verified to be very effective (and cheap):<p>[0] <a href="https://cleanaircrew.org/box-fan-filters/" rel="nofollow">https://cleanaircrew.org/box-fan-filters/</a>
The intention of the OP/post might be good, but the comparison (<a href="https://dynomight.net/img/cuboid_purifier/boxfan.jpg" rel="nofollow">https://dynomight.net/img/cuboid_purifier/boxfan.jpg</a>) makes no sense whatsoever.<p>They have just thrown 3 filters on top of the box fan with lots of gaps on the side. Air will <i>mostly</i> follow the path of lease resistance and not go through the filters. You need a filter that is the same size, 20x20, as the box fan and then the filter(s) will need to be sealed, via duct tape or so, around the fan for it to be effective.
What this and the box fans seem to miss is that prop fans have very little ability to generate static pressure. You will move more volume over the static pressure loss of the filters if you use a centrifugal type of fan. A little looking on Amazon shows those at about 3x the price of the prop boosters for the same rated cfm.<p>If you add even a little bit of duct to extend approximately 3 duct diameters on the outlet it will make the fan a lot more efficient by allowing the airflow to stabilize and lower buffeting noise at the outlet. There athere are some of the centrifugal fans that include a noise suppressor for more money that's basically a short duct that's double walled and perforated on the inside.
The quickest way to build one, IMO, is using a hot glue gun. It takes quite a bit of hot glue to get it nicely sealed (maybe 10 sticks or so), but it's a LOT faster than using tape & it's more forgiving - you just "caulk" the seams liberally. The non-standard, longer length glue sticks make it especially quick. I feel like my hot-glued results have generally also been a bit faster/easier to pull partially apart, too, for when it's time to do filter replacements.<p>I personally prefer to use two filters on each side, one as a "pre-filter" (cheaper carbon/charcoal, MERV ~10ish), with better (higher MERV, ~13 if possible) rated filters underneath. You can then replace the prefilters more often, but the filters underneath won't require replacement nearly as much.<p>I wrote about building them with hot glue on my blog[0] -- shameless plug! :)<p>0 - <a href="https://dantonio.info/projects/10-minute-corsirosenthal-box/" rel="nofollow">https://dantonio.info/projects/10-minute-corsirosenthal-box/</a>
Note that the OP uses ASHRAE's definition of CADR (nominally airflow rate * removal efficiency) and not AHAM's more complicated definition which accounts for contamination rate.<p>Some details about AHAM's CADR calculation are here: <a href="https://frdmtoplay.com/nagivating-air-purification/" rel="nofollow">https://frdmtoplay.com/nagivating-air-purification/</a>
I saw a pretty good comparison video the other day of different DIY options:
<a href="https://youtu.be/KobogQMf_Xc" rel="nofollow">https://youtu.be/KobogQMf_Xc</a><p>"What is the best DIY Air Filter?"<p>EDIT: I built one for my workshop but in my house with all the windows/doors closed it seems that I can get by with a much smaller filter, there's not that much smoke/particles that keep getting in IMO.
There’s a simpler construction of this, using tape and the fan or filter cardboard packaging, here: <a href="https://cleanaircrew.org/box-fan-filters/" rel="nofollow">https://cleanaircrew.org/box-fan-filters/</a>
<i>- Two bungee cords (free).</i><p><i>- Tape (free).</i><p><i>- Two pieces of packaging foam (free).</i><p>Not that those are expensive, but I'm not sure why the link says they are free.
Related question:<p>As filters age, they change colour. Mine start white and turn dark grey when old.<p>However, I noticed that even the outlet side got darker. That made me wonder if a 'blocked' filter was actually allowing more small particulates through (for example perhaps because all the surface area that a bit of PM2.5 could electrolytically stick to has already been stuck to).<p>Does this sound plausible? Anyone got test results for small particulates of different materials and how the filter performs as it ages?
I couldn't afford this without having my own shop space. The parts are cheap, but after setting up something the size of a mini fridge that's covered in bungee cords in my little apartment, the marriage counseling would be costly.
The Cuboid might be better, but because he used two completely different fans, this is not a valid comparison. The Cuboid uses a ducted fan, where the tall duct will increase the flow rate by promoting expansion in the exhaust. The other fan is just a non ducted (or very poorly ducted) box fan. It looks like he also only does one trial for his measurements. <i>Most of the efficiency CADR/power probably comes from using a better fan</i>, which is the whole purpose of adding a long duct.<p>The extra effectiveness might also come from the Cuboid fan being elevated off the floor. With the box fan standing on the floor and pointing horizontally, it has an additional solid boundary restricting intake air. The vertical pointing fan in the Cuboid does not have this restriction.<p>A proper test would use the same fan in both designs. The non-Cuboid fan would be oriented upwards and mounted on stilts. Multiple trials should be done with the results averaged. He omitted a chart showing both particles counts vs. time of both approaches, which would be the most important figure to include.
This is almost exactly the design used by BlueAir purifiers <a href="https://www.blueair.com/us/blue-family.html" rel="nofollow">https://www.blueair.com/us/blue-family.html</a><p>Can recommend.
The article didn't mention the static pressure of the fan. Typically, for a "suck" type fan, you want higher static pressure and there are fans which are designed expressly for this.
> If you take the top pick from the Wirecutter and read user reviews carefully, you’ll see that roughly one person a week reporting that their unit exploded.<p>I mean, if you read colander reviews on Amazon, you'll find people who somehow lost a hand to it.
<p><pre><code> The design should also fix the worst parts of using a box fan:
1. Make less noise.
2. Use less electricity.
</code></pre>
Point #1 is not the worst part of a box fan for many people, myself included. I have a lot of fond memories from growing up where a box fan was running at night due to the outside heat and lack of central AC. It made sleeping a much easier and more pleasant experience.
(2021)<p>I'm currently dreaming up a ceiling fan replacement that has a much lower profile and also cleans the air with a standard furnace filter.
On the topic of air quality, can anyone recommend a "fine dust meter" (or really just "regular dust meter")? I would prefer totally non-wireless (USB preferred, handheld okay).<p>For CO2, the ZGm053U has been a great device. But now I would like to measure all the dust I see in a dark room with a bright flashlight.
I'm speaking out of ignorance and not criticism, why not just go with a commercial product at that price point?<p>I can imagine many theoretical reasons like sourcing and availability but are there things beyond actual conjecture here
I only use <a href="https://www.sharpconsumer.com/home-appliances/hg40el/" rel="nofollow">https://www.sharpconsumer.com/home-appliances/hg40el/</a><p>It runs silently at lowest and humidifies the air too.<p>It uses one container of water per day.<p>I filter the tap water so it doesn't clog up the filter.<p>I disable the ionisation feature which makes particles stick to the walls and furniture.
Working in this field and living in Northern Thailand which has similar high levels of air pollution from wildfires every year, I have spend many years making my house safe from air pollution.<p>Box fans with HEPA filters are a good immediate and emergency solution but have their drawbacks, mainly:<p>1) The fan and filter fights against particles coming in from outside, through small cracks under the doors, windows etc. So the higher the outdoor air pollution, the more the fan needs to fight against it. At extremely high pollution levels they will get into problems maintaining healthy air indoors.<p>2) Sealing the gaps and making your house/room more air tight, increases CO2 levels significantly which leads to drowsyness and decreased mental performance. So you always work with a trade-off.<p>3) Often you need to run the fans on highest level to be effective which creates quite a high level of noise.<p>4) Ideally you want something demand controlled that switches automatically on/off or regulates its speed based on the pollution levels.<p>5) The HEPA filters filter primarily PM2.5 but the wildfires also produce hazardous gases like NOx, VOCs which you would need carbon filters to remove.<p>With my company AirGradient (we offer open source air quality monitors [1]), we monitor thousands of indoor spaces and the only technology that we found consistently working -no matter of the level of air pollution- are positive pressure systems.<p>A positive pressure fresh air system takes air from outside, filters it through HEPA/Carbon filters and pumps it into the room. Thereby pressurizing the room and thus preventing dirty air from entering the room.<p>With this system you just need to achieve a slightly higher pressure then outdoors and then you can consistently achieve zero PM levels inside -even when hazardous outdoors. As a result the system, often runs at low fan speeds and does not create much noise.<p>Additionally, due to the fact that it uses fresh air, the CO2 levels remain low and you can often add carbon filters to the system.<p>I made a comparison test of this type of system [2] and a detailed description how it works [3].<p>[1] <a href="https://www.airgradient.com/open-airgradient/kits/" rel="nofollow">https://www.airgradient.com/open-airgradient/kits/</a><p>[2] <a href="https://www.airgradient.com/open-airgradient/blog/air-purifier-vs-positive-pressure-fresh-air-system-an-unfair-battle/" rel="nofollow">https://www.airgradient.com/open-airgradient/blog/air-purifi...</a><p>[3] <a href="https://www.airgradient.com/open-airgradient/blog/positive-pressure-systems-for-clean-air/" rel="nofollow">https://www.airgradient.com/open-airgradient/blog/positive-p...</a>
You can also make this with a box fan, construct a box with 4 filters of roughly the same size as the fan, the bottom of the box can be cardboard. Did this back in 2019 and it works great, though I ended up switching to a Coway a few years later because of the noise.
Why does it work better?<p>I see that in the test he compares four filters being used completely in the cube setup to three being used incompletely on a box fan. But that doesn't seem like it can be the entire explanation -- particularly given the reduced airflow of the duct fan.
The haphazard assembly of his box fan doesn't look ideal. If you're going to do it that way at least get a 20x20 size furnace filter and use some duct tape to seal it.