A simple device for indoor air management

Note: I wrote this in much more of a hurry than I normally write things, because I have noticed that I get way too meticulous, so it takes forever and I eventually give up. I hope it is still readable/​useful!

Like many of us living in the Bay Area and other parts of the Western US, I have been dealing with smoke, and like most people who live and work indoors, I sometimes deal with stuffy, high-CO2 air. Smoke is bad for health, mood, productivity, sleep, and comfort. CO2 is bad for all the same things except perhaps health. Even people who live in places without seasonal wildfires might benefit from being able to filter out pollution or allergens without closing off their house and building up CO2.

The standard way to handle smoke is to buy air purifiers that pull air from inside the room, run it through a HEPA filter to remove particulates, and push it back out into the room. These work well for keeping particulate levels low when the air is not too smoky. However, if the air is very smoky, the only way they can keep up is if only a small amount of air is entering the house from outside. This means either closing the windows or buying more air purifiers. The trouble with closing all the windows is that CO2 created by people in the house will build up, and the trouble with buying more air purifiers is that it is expensive and impractical for high levels of particulates.

The solution is to filter air as it enters the house. An air filter that is pulling in smoky air from outside removes more particulates per minute for the same throughput than one that is filtering less particulate-filled air. An additional benefit of this is that it serves the purpose of actively cycling air into the house, rather than allowing it to passively happen through an open window. I spent some time searching for filters that work this way, and I was unable to find one. I also looked a bit for an in-window air conditioner that included a HEPA filter, but did not find one. So I built one myself. It was actually quite easy and inexpensive.

The filter consists of a cardboard box, a HEPA filter, an in-line duct fan, a short length of flexible ducting, and a cardboard flap. The box has a large rectangular hole cut into one side and a 6“ circular hole cut into another side. The fan is attached to the 6” hole using duct tape (I used “all-weather” duct tape, since some of it would be exposed to heat, UV, and dust, though I do not know if it actually makes any difference), the HEPA filter is taped over the rectangular hole, and the duct is attached to the fan. The box is placed in the window with the filter facing outward and the fan pointing to the side. The duct is bent so that it blows filtered air into the room (or into my face, as is sometimes the case when I am working at my desk). The flap covers the rest of the window to limit the amount of unfiltered air coming into the apartment. All the parts combined cost less than $100. It may not be hard to get it below $50, depending on how much throughput you want and what kind of filters you can find.

The air purifier installed in my apartment

I actually built this a couple weeks ago, but I did not get a chance to measure its performance until my air quality monitor arrived in the mail and there was enough smoke outside to actually test it. I did test it, and it does seem to mostly work.

CO2 and particulates inside and outside my house, while changing various parametersCO2 and particulates inside and outside my house, while changing various parameters

This data does not entirely make sense to me. in particular, the whole thing seems to be performing substantially worse toward the end while I’m out of the house, which is also significantly worse than it was for most of today.

It generally works better for removing particulates and worse for cycling out CO2 than I’d thought and hoped that it might. The fan seems to be moving less air than it is rated for, but I do not think it is designed for operating with much of a pressure difference. I will probably order a bit beefier fan and rebuild the whole thing with a more durable box.

All in all, I’m happy with it. The parts were cheap and it only took about 30 min to assemble. I’m happy to answer questions if anyone else is interested in building one.