Around Christmas 2021 I put together a big air purifier on top of our bookcase. Somehow I didn't blog about it or take a picture, but here's it in the background of a birthday:

The basic idea was that I didn't have a good place to put a filter cube, commercial air purifiers were scarce, and a different configuration of filters seemed like it could work well. It's a box fan with eleven filters: five along the side, five along the top, and one at the end. It glows because I built a regular light bulb into it, though without any means of replacing the bulb when it died (which it eventually did).

The design relies on a good seal with the wall, but when I initially set it up I was worried about peeling the paint off so I used masking tape there. Which eventually pulled away:

Which means it's probably been doing about nothing for a while. I considered fixing it, but there were other issues with the design: the box fan was pretty loud on "high" and the fan didn't point a sensible direction. And at this point commercial purifiers are much more widely available. So I ordered three AP-1518R purifiers, which are very similar to (and filter-compatible with!) the AP-1512HH we already had, and replaced the big purifier:

I've laid the purifiers on their backs, so the fresh air circulates towards people's heads and I can reach the controls from the ground. I'm not sure whether this is a good idea—the internal squirrel cage fan would have been engineered expecting loads from a different direction, and this might not be good for it? Any guesses?

I previously tested one of these against a filter cube and estimated a CADR of 219 CFM on high, 90% of what that filter cube gave on high. When the big purifier was new and sealed properly it was probably in this range or a bit worse: having nearly 3x the filters as a standard filter cube should have given less restriction on the airflow, but there's probably highly diminishing returns and the position of the outflow was bad. If I run the three new purifiers on medium they're far quieter than the big purifier while giving more clean air than it did before it started leaking. And I have the option to run them on high if the room is very crowded, where they're about the same volume as the big purifier while delivering about 3x the clean air.