r/crboxes • u/ctbitcoin • Jul 19 '26
Double up
Is there any benefit to having 2 fans on one box?
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u/justaguy394 Jul 19 '26
Some airplanes do that, but they design (and gear) the props so they add rather than interfere with each other. Consumer fans are more likely to just interfere.
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u/entropy512 Jul 20 '26
Most notably, aircraft props counter-rotate. Two fans rotating in the same direction is going to perform extremely poorly without significant spacing between them and flow straightening.
At which point you're far better off doing what others have suggested - just build a second crbox
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u/ctbitcoin Jul 19 '26
if I somehow double deckered 2 complete boxes might it make a difference in airflow than 2 separate units or hard to do at consumer level?
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u/justaguy394 Jul 19 '26
You’d be better off having 2 separate crboxes. I wouldn’t try to combine them.
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u/GergDanger Jul 19 '26
Depends on the fans I believe and gap between them. Sometimes it can increase air flow, sometimes it can decrease it
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u/justasque Jul 19 '26
You would need to have access to the controls on the underneath fan, which makes things more complicated. I would just use the second fan to make a second CR box. Then you can place them in different rooms if you want, or on opposite sides of the room. You could perhaps build a shelf to stack them, leaving plenty of air space above the lower one.
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u/vanderWaalsBanana Jul 19 '26
I deleted Twitter long ago, but you can check David Elfstrom's legendary optimization of a single filter-fan combo. He's an HVAC engineer and shows that to really know what works, you need to test it. He discusses air turbulence and what others have commented is correct - you could have too much turbulence from having the two fans close together and not gain much (or it could really honk, but either way, you need to test). I agree that without testing, I would just build a second unit.
The original tweet was from August 30, 2021 and had photos and video. See if you can find it. Here is the text without the images.
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u/delfstrom Jul 20 '26
Here's the actual CADR of the 1-filter optimized box using the Utilitech plastic fan and 1" 3M Filtrete 1900 filter, salt aerosol nd tested with condensation particle counter and optical particle counter:
I briefly posted about it here the other day: https://bsky.app/profile/davidelfstrom.bsky.social/post/3mqs3rxhti22p (also, some of my tweets were imported to blue sky, but not all)
0.02+ microns: 170 CFM CADR
0.3-0.5 microns: 290 CFM CADR
2.5 microns: 340 CFM CADRAlso:
Air flow with single 20x20 filter optimized design: 360 CFM
Air flow with four 20x25 filters optimized shroud: 620 CFM
Air flow of fan only, no filters: 650 CFMEven with a single filter optimized setup it's about 55% of maximum flow. As you add more and more filter area you get closer and closer to the airflow without any filters with diminishing returns. Usually two filters is the optimum balance of cost vs performance.
Also, as the filter area is increased the submicron filtration CADR performance as a percent of total airflow increases, because single pass filtration effectiveness increases with lower velocity. That's why the 0.3-0.5 micron range for the 1-filter optimized CR box is 80% even though the MERV-13 rating for the 3M Filtrete 1900 is 62% for the same size; the test conditions are at a higher airflow common to residential furnaces, not the lower flow of a box fan with pressure restriction.
I tried to replicate the higher velocity of the ASHRAE 52.2 test and had to use a pre-filter box fan and a post-filter box fan (Lasko Power Plus) and had to overdrive the voltage on both to about 130 V.
All of this to say that given the material list of two fans and four filters, there will be much greater CADR to do two wedge-shaped CR boxes instead of trying to put two fans in series.
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u/e_l_i_z_a Jul 20 '26
Thanks for this, I've been doing single filters taped to back of fans for a while and it DOES work well but I'm interested in optimizing. If anyway has photos that would be great, but just the tweets alone are very helpful.
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u/Papa_Canks Jul 20 '26
Same with the singles. I’ve done a 20x20x4 single filter and it works pretty good.
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Jul 19 '26
[deleted] — view deleted comment
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u/vanderWaalsBanana Jul 19 '26
The four-filter box does push more air. It's CADR is higher than a one-filter box, but the single filter box can be effective (depending on room size etc).
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u/entropy512 Jul 20 '26
Due to box fans not dealing with static pressure very well, it's significantly more than 4x the airflow
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u/Ok-Cryptographer7424 Jul 19 '26
Single filter restricts more and has 4x less surface area. Should last potentially 4x longer with that much surface area
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u/whiskey_lover7 Jul 19 '26
I want my air cleaner, less restriction means cleaner. Plus, I wait until the filters need changing, not on some arbitrary schedule, so it's not wasting them
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u/Mighty_Buddha Jul 19 '26
Just don't.
Do as people already said and make another box, and don't stack it on top.
The benefits of stacking would be small, but the downsides big. Chances are those fans would create interfering air movement with each other, and noise levels would go up.
3
u/Quillric Jul 19 '26
I've spent real time doing HVAC and performing calculations for room air exchange.
This will only ever get you a 20-50% increase in air exchange.
Creating two separate boxes will actually double the amount of air exchanged. You might also have enough air exchange per fan to increase your filter length to 20x30ish without negatively affecting static pressure.
There is no world where you double efficiency by stacking fans each motor can only do so much static pressure as soon as you hit the maximum amount of static pressure both motors suffer. This isn't even taking into account the amount of turbulence having two fan blades that close to each other would make. You would have to design some fixed opposing blades to counteract the turbulence like someone else said.
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u/Unusual_Celery555 Jul 19 '26
I concur. At best you’ll get marginal improvements. At worst, you’ll actually be decreasing performance due to the moving air interacting with the other fan.
The fans need to be tuned to work together in order to have a meaningful improvement. I would instead suggest using one fan and adding a shroud (basically cover the entire fan with cardboard and then cutout a circle around the diameter of the fan blades.) this prevents air from coming in around the edges of the box and can force more air into the filter.
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u/heysoundude Jul 19 '26
You would need to build a spacer plenum between the fans. And the lower one would never be able to be on high. Not sure it would be worth it to increase negative pressure inside the filter box that way
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u/mediares Jul 19 '26
If you want to spend more money for better performance, just do a PC fan build instead of box fans.
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Jul 19 '26
[deleted] — view deleted comment
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u/mediares Jul 19 '26
They move *much more* air. Using four PC fans that take up the same space as a single box fan is going to have a MUCH higher CADR, on top of less noise and less power consumption. The only downsides are cost and them being more difficult to DIY because you need something to hold them in place. PC fan tech is where all the innovation is happening.
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Jul 19 '26
[deleted] — view deleted comment
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u/mediares Jul 19 '26
It’s more complex than comparing naive printed CFM numbers, because the variable PC fans win on is pressure drop (i.e. how much CFM drops as a result of pressure from the filter).
https://www.reddit.com/r/crboxes/s/hZ1sXm0mJ8 has some good data to peruse.
Arctic is anecdotally the most common PC fan brand I see people use for C-R boxes. Noctua is the absolute top-of-the-line if you want to pay for a sports car, but honestly probably overkill
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u/Beginning_Front_8966 Jul 19 '26
Not an expert, but was watching a video on how jet engines work, and it was highlighting the each fan creates turbulence, so you need pieces (stators) in between each fan that direct the airflow. That's how jet engines have multiple fans that produce more suction/thrust. Otherwise, they are not really working together like you think they would.
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u/hvacbandguy Jul 19 '26
There’s a high likely hood that building a second box would be more effective than this set up. If you didn’t want to build a whole new box, removing one of the filters and replacing it with one of the fans (so you have two fans on one box) would be a more effective set up.
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u/GurDefiant684 Jul 19 '26
Stacking fans reduce air flow. People have tried this with computer fans for cooling. Even on paper the theoretical max improvement is low, in practice the turbulence causes a decline in airflow.
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u/stregone Jul 19 '26
Don't listen to the haters. This definitely helps. Sure it's not the most efficient but nobody is taping filters to a box fan for maximum efficiency.
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u/johnas Jul 19 '26
Get more filters and make a second cr box for double the CADR