Time to build: about 45 mins (not including soldering the power supply)
Materials:
(4) $5 Arctic P12 fans
(2) $10 10x20x1 1200D filtrete MERV-11 filters
(2) $7 20x20x1 1500 MERV-12 filtrete filters (pack of 12 off ebay)
duct tape
zip ties
cardboard
12V power supply (free because I soldered a random old adapter, but normally $10 plus $3 or so for a splitter. Or get the fans with built-in splitters)
Total: approx $55, next one will cost about $70.
Roughly 225cfm and extremely quiet. #CRBox
@Andres4NY if folks want a kit version of this thing, I have been very happy with my Clean Air Kit C-R box. https://www.cleanairkits.com. One thing to note, if you want MERV 13 (best for catching viral particles) you’ll need to upgrade to the Filtrete 1900.
Here's the 4 PC fan #CRBox next to a traditional 20" box fan CR Box. I'm betting I could fit 8 PC fans with those same filters. Cutting the shroud for the box fan is the biggest hassle of a traditional CR box, I really appreciated not having to worry about that for the PC fan version.
lol, I went downstairs and was like, "damnit, someone left the cr box unplugged again" (this happens on occasion when people decide they need some quiet from the loud box fan). But no, it was running, it's just that quiet.
I added another 4 Arctic P12 fans to this and it fits perfectly. No need for a piece of cardboard, and I didn't need to modify the filters at all. Two 20x20x1 filters, and two 10x20x1 filters. Should be roughly 450cfm, and it feels like a lot more air moving compared to the (noisier and more expensive) 6 fan AC Infinity box next to it. This is only about $40 worth of PC fans.
@enobacon@Andres4NY I was thinking the same thing. Even high-end iPPC Noctua fans see their flow drop in half when put in front of a radiator or a simple mesh filter; and they have insane static pressure designed for this kind of application. The P12 have 2.2mm H2O of static pressure vs 7.8 for the Noctua. I expect this CR box isn't going to do anything.
@Andres4NY@kawazoe I don't doubt that it's doing something, just not 450cfm. Interesting that they're getting the same particle-count results as the box fan, I thought bigger blades were more efficient (all other things being equal, which the AC motor isn't.) But the box fan probably suffers similar loss of flow due to pressure since it's also axial.
@enobacon@kawazoe In another post Rosenthal guesstimates that a 6-fan design, spec'd at 320cfm, is likely 220-270cfm for the cr box. Which feels right to me; the pressure of the air blowing out of that same 6-fan design (ac infinity 12cm fans) feels on par with my coway ap-1215, while my 8-fan designs feels like it's pushing more air. Meanwhile the box fan w/ shroud is pushing twice as hard.
@Andres4NY@enobacon The amount of fans don't change their SP, only the CFM. This is because more fans means more area to push the air through. From what I've seen (I've only looked into this quickly) MERV filters can have resistance between 1mm and 70mmH2O depending on their grade. I expect 13s to land around 4 or 8, as the pressure impact is exponential. This means a single Noctua iPPC fan might be more effective than 8 of the P12, just because it can overcome the filters resistance.
@Andres4NY 😂 sorry about that. Seriously though it would be cool if you can measure the cfm now vs after n months, as the resistance will increase when the filters fill up. Is an anemometer the way to do that?
@enobacon Unfortunately I don't have a good way to measure cfm (and of course I care about CADR most of all). My typical test is a piece of paper; does it stick to the filters? Can I make it float above the fans? Not quantitative in the least, but gives me a ballpark of whether air is a) moving around the room, and b) through the filters. Significant differences between filters & fans with just that simple test.
@enobacon I'm still ignoring that other dumbass who was like, "it CAN'T work because I've built PCs before and therefore I know everything about static pressure and air filters without ever actually trying it!"
But it's been 3 months of 24/7 usage. You can see the filters are dirtier (the cats sometimes rub against this one). AQI was around 40 today, and we had the windows open briefly, but a pair of these keeps AQI very low.
@enobacon@pleaseclap Qualitatively, the PC fans win out in terms of size and noise over box fan, and while the CADR is clearly lower, it's not THAT much lower.
That same location used to have a box fan CR box. When my wife spent time on her laptop in the dining room or cooking in the kitchen, she would sometimes turn off the box fan because it was bothering her. The 2 PC fan CR boxes that replaced it have not been complained about or shut off.
@enobacon@pleaseclap If you have ideas for quantitative measurements that don't involve buying hundreds of dollars worth of specialized equipment, I'm willing to try stuff.
@Andres4NY@pleaseclap without an anemometer, IDK. Maybe you could measure the force on a parachute of known area, using a hanging scale or small weights (and screening so they don't fall into the fan.) The flow at the edges is complicated as it disperses, maybe there are tables to estimate that based on geometry and measured velocity while confined within an output duct.
@Andres4NY@enobacon "It spends more time on because we can tolerate it on" is a great observation
You've probably already tinkered with this but I would think the filters used would have a significant impact on outcomes, too: you might see tradeoffs like more CFM with lower air quality
@enobacon wow... welp, if someone's reading this, we now have a proof that this guy doesn't care about science and is in for the dogma. I wouldn't trust anything they'd say if they can't even refute a simple "SP isn't CFM" argument with data or adjust their control when someone points out it is probably flawed...
@kawazoe your statement that it won't do anything is kinda discrediting if it does something, I'm curious as to an actual cfm per watt but was hoping to convince someone else to measure it... so be nice
@brainwane Sure, I'm planning to build another one soon. More fans arriving tomorrow. Or did you mean building one for you? I'm happy to give links for materials.
@Andres4NY I was thinking I could join you and learn more about the nuts and bolts of building one, and then I would feel more comfortable buying parts and building a few on my own
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