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Comment by dynm | Hacker News Reader

> Just from inspection, that should increase the total volume of air which is cleaned by a factor of 2.

Are you aware of any data that supports this? It could be true for all I know but (contrary to my expectations as well) the experiments I've done don't really seem to support this. See the "taping" experiment here: https://dynomight.net/2020/12/15/some-real-data-on-a-DIY-box...

You might be referring to non-HEPA furnace air filters which (by design) have much better airflow, but typically don't capture the smallest particles, which have the largest health impact.

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Just run highly rated MERV filters. I use a bit of a bulky MERV 14, but as long as you can cycle the total air volume of the space you're cleaning quickly, anything with 20+% uptake on small particles will work over time.

The experiments and data on that site (I assume it's yours?) don't give me nearly the same conclusions you do - Almost all decent filter setups will hit the same particulate equilibrium asymptote with the only real difference being the duration it takes to get there - a negligible plus or minus half an hour. Speed is not aim. It's the maintenance of the steady-state that is.

If the filter stays on, the air stays clean (barring total capacity issues - you can't filter a huge house with a single box fan).

So the only real roadblocks are human factors that would 1) make people skip making a filtering setup and 2) make people want to turn off the filter. This is where the real UX comes into play.

> Almost all decent filter setups will hit the same particulate equilibrium asymptote with the only real difference being the duration it takes to get there

I'm not doubting this, but do you have any evidence? According to the specs, by definition a MERV 14 should leave at most 25% of all particles size 0.3 microns or larger in one pass, while a HEPA filter can leave at most 0.03%. It's plausible but not totally obvious to me to me that this means that a MERV 14 will leave at most a fraction 0.25^k after k passes (though it's plausible). If so, that would suggest you need around 6 passes through a MERV filter to get equivalent filtration to 1 pass through a HEPA filter. (Airflow might be 6 times as high.)

Box fan with a 20x20x1 filter makes the fan work much harder.

20x20x2 or 20x20x4 filters have much larger surface area, which is better.

Creating a cube with four 20x20x2 is the best in terms of energy use and air output.

And then you add a fan shroud for even more efficiency.

https://www.texairfilters.com/how-to-improve-the-efficiency-...

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