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Comment by breput | Hacker News Reader

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):

[0] https://cleanaircrew.org/box-fan-filters/

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Indeed, linked in that article is this which I built yesterday and works very very well.

https://www.texairfilters.com/a-variation-on-the-box-fan-wit...

-EDIT- I should also mention that you don't need 5 filters for this (MERV 13 is expensive), you can use 4 and a piece of cardboard for the 6th side that is touching the ground.

Yeah mine has 4 filters with a box fan on top and cardboard that the box fan came in as the base. All taped together with duct tape. Brain dead simple and cheap. Used some more duct tape to fill in the "corners" of the box fan. Mine blows positive pressure into the box, I just saw some designs that suck negative pressure out of the box, I don't know if that matters, but I've got a PM2.5 meter and mine worked fine last year during the smoke days in Seattle.

I think it matters because with negative pressure you are causing the box to close and cover the gaps, whereas when you blow you are pushing on the box causing the gaps to expand.

I would think that you want to keep dust out of your fan motor/bearings etc. so you want the fan to be sucking out of the filter, not the other way around.

Those fans normally would operate without any filters on either side, so the dust won’t really matter.

... they'll still function, but if you've ever had one you'll know that dust does matter. They're a pain to clean and the various crap in your air builds up on every edge. Definitely want it sucking filtered if that's that's an option.

I see this all the time, why use multiple 1" thick filters to build a box, when you could just use 1x 4" filter. They are pleated, and the surface area should be equivalent. I documented my setup here before: https://news.ycombinator.com/item?id=28395232

Having a greater surface area to filter the air through will decrease the static pressure and be nicer on the fan motor. If OP was putting the filters in series, then yes, it should have no effect. But they are putting the filter in parallel.

My point was that 4x 1" filters assembled into a box is the same surface area as 1x 4" filter, but the latter is much easier to assemble.

Agreed, but it might cause your fan to burn out much quicker, or be less productive if you're using a box fan, since the static pressure differential will be approximately 16x between the two.

Why? If they have the same surface area, I would think they would have the same pressure differential?

Imagine calculating all possible routes an air molecule could take to escape after it is blown in. In the box case, the air molecule has 4 potential escape routes instead of 1 in the direct stacked filter case. And each escape route has it traversing 1/4 the amount of filter material.

Respectfully, I think you don't understand how air filters work. Air does not pass through 4x the filter material in a 4 inch filter. Instead, the 4 inch filter is pleated, folded like an accordion, so that each air molecule passes through the exact same thickness of filter as a 1 inch filter of the same MERV rating, but ~3x more surface area allows for ~3x more air to pass through. This is the only reason for making a thicker filter - to allow greater air flow. A box fan is generally operating below the capacity of the 4 inch filter, which is designed to allow for high cfm whole-house airflow.

???

4 x 1" filters, each with a surface area of 10 square megamicrons, provide a total of 40 square megamicrons when assembled into a box.

A single 1 x 4" filter strapped to a fan gives a total of 10 square megamicrons.

How are those the same surface area?

Ah! I get it, because of the pleating. I guess it depends on whether the greater incoming airflow interferes with itself once it gets squeezed between pleats.

Can confirm this works great. I wish there were more options for higher quality 20x20 box fans though. I’ve had Lasko fans that got loud after a few months, and I assume that’s a bearing going bad.

There are commercial fans but they’re louder and more expensive.

If there’s a belt from the motor to the fan you’re in for a good time.

I have the same, without the 1" pre-filter in your case. The air flow is excellent, though it does get a little loud.

Exactly what I do, but with a 5", 20x25" MERV 16 filter. I run two of these on box fans when it's smokey in the summer, and it works very well. Box fans do get loud though.

> why use multiple 1" thick filters to build a box, when you could just use 1x 4" filter.

Probably uncertainty over how effective fans are when immediately obstructed, and maybe concerns about particles possibly getting deflected backwards if a filter messes with a smooth, laminar flow.

This is, if we think of two kinds of airflow:

1. [laminar flow](https://en.wikipedia.org/wiki/Laminar_flow ), which is like a smoothly flowing river;

2. [turbulent flow](https://en.wikipedia.org/wiki/Turbulence ), which is like erratic flow inside a mixer;

then it seems like a box-fan might effect a laminar-flow further away from the blades (under normal circumstances), but could it be more turbulent near the blades?

The boxed-in design seems to try to dodge assumptions about how air flows near the blades, reducing uncertainty about how turbulent-flows might screw with it.

I just built one in preparation for a home remodel. I want to try to clean out any excess dust while we’re living through a remodel.

Or even all the excess dust kicked up from a digging out old things we didn’t know he still had…

How does this stack up in terms of noise versus the one in the OP?

I have a few box fans filters and one cuboid fan, it's not actually the cuboid though, I used a circular hepa filter with the inline duct booster fan.

I leave the fan duct fan running on low (right at the midpoint of the speed dial so technically medium I guess) 24/7 in my kitchen out of eye site because it's so quiet.

So, in my experience, much quieter, quiet enough that people don't notice it running all the time. I have the dial in reach though so I can crank it all the way if something burns on the stove, clears the air out shockingly fast.

I got some sound proof panels and crappily bungee corded them around the fan and it didn't have a noticeable effect. I think if I put more effort into the sound baffles it would reduce the sound, but the whole setup without baffles is quiet enough I haven't bothered.

It's constructed using a regular 3 speed box fan, so it isn't going to be significantly louder than that and possibly quieter depending on how it is shrouded and insulated.

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