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Comment by DeRock | Hacker News Reader

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

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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.

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