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Comment by mynameisash | Hacker News Reader

Last year when the west coast was blanketed in smoke, I setup a box fan with a HEPA filter. It probably helped, but we also found that the fan's motor was working really hard. The cord's connection point to the fan started melting (unbeknownst to us), causing the fan to intermittently cut out. I only noticed the problem when, long after the forest fires were gone, we noticed a strong smell of melted plastic coming from the fan itself.

All in all, probably a good tradeoff to clean our air during the dangerous smoke levels, but there's the risk of burning out the motor.

Be careful out there.

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That's because box fan are not designed as static pressure fan. They are a airflow fan for the purpose of moving much air as possible with minimal resistance. Static pressure designed as forced pull/push air, it have slightly thick blade and have a different motor for it. The air purifier uses the static pressure fan because of the filter media.

Take computer fans for example. There are two common case fan type, the airflow and the static pressure. The airflow fan is for the tower case, the static pressure one is for the radiator (water-cooling kit).

Technically, since you're sending the air through the filter multiple times, you don't need a HEPA filter. HEPA filters are optimized for single-pass airflows.

And there are thicker filters which give you more pleats, and thus less air impedance, in addition to the "triangle" method suggested by a sibling comment.

You just need to increase the total area of the filter material that the fan is pushing air through. I've seen people shape them into a triangle or box with plastic/tape over seams or openings.

Do we know how the air flow improves in proportion to surface area? I assume it's not quite linear. It would be good to know whether it's worth the effort to do what you're describing.

I'm sure there is an answer to that but this is DIY, so maybe just touch the fan while its been operating for a while, say "damn that sucker's getting hot!" and add more surface area until satisfied you won't burn your house down.

If you notice your fan motor or cord getting hot you can reduce the pressure differential (and therefore torque/load on the motor) by allowing some airflow to get around the filter- i.e. push the filter a bit to the side or add a tiny gap between the filter and the fan by sticking in a little block of wood.

This will come at a cost of filtration effectiveness (less airflow going through the filter) but will save your fan in the long run. Make sure not to let your gap or hole get too big or your filter will stop flowing enough air to work.

And of course, air will circulate through the fan multiple times over time so it doesn't have to catch everything in one go.

I'm glad you caught that before it caught fire.

The ~$25 box fan I got for a DIY air filter was simply failing to push much air through the 3M filter, but I didn't think to check whether the cord/plug or motor was getting hot in addition.

Or burning your house down!

Try putting the filter on the suction side of the fan. This will put less load than normal on the fan and there should still be enough airflow to cool the motor.

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