[rampant_ai]

Most box fans will struggle with a single filter right up against them. The blade design on a typical box fan is designed for velocity, not pressure. Also, I don't know the physics as to why, but you want to be pulling air through the filter rather than pushing. It's why car radiators have the fan on the engine side.

The best setup is to use multiple filters arranged in a box shape, since more filter surface area = less restriction on air flow.

There's some great examples here:

https://cleanaircrew.org/box-fan-filters/

[themaninthedark]

When you are pushing air, you are building a high pressure zone. When you are pulling, you make a low pressure zone.

As the pressure in front of the fan builds up, it starts to push back against the blades of the fan until it reaches an equilibrium.

[Tade0]

> Also, I don't know the physics as to why, but you want to be pulling air through the filter rather than pushing

I guess pulling seals the gap between the fan and the filter and pushing only widens it.

[channel_t]

The primitive "filter taped to box fan" model worked fine for me in Portland during the massive fires of 2020. Maybe not the greatest thing for the fan, but my indoor AQI sensor would go from reading like 250+ to under 100 pretty quickly. And of course, the filters basically turned black in a short period of time.