Is there a reason you'd use HEPA instead of having a bigger MERV filter and relying on multiple passes to get the air clean? I thought MERV filters were a better fit if you're recirculating air using a small blower and HEPA does a better job if you can support the pressure drop and absolutely need the particles to be filtered on the first pass.
You can indeed take this route [0] and even DIY something with a box fan. But I think the jury is a bit out on this. Quote from that link below. You also need to move that much more air to get the multiple passes required and that can get pretty noisy.
> They claim that around 90% of particles sized 0.3 microns are larger [for a MERV] are eliminated in a single pass. That’s good, but not totally reassuring. The question is, does it remove 99% in two passes? If 90% of the particles in the ambient air were large and the filter only catches large particles, then additional passes would never get rid of the most dangerous small particles. This is why I trust HEPA filters a bit more: since they remove almost all particles in one pass, I’m confident they should remove almost all particles eventually.
Also, there is the idea that HEPA filters act like a sieve. They don't. This article from the same guy is a great read [1]. So basically, multiple passes for the win.
> Air filters easily catch both large and small particles. It’s the intermediate regime where things are hard. The size where the filter performs worst is called the Most Penetrating Particle Size (MPPS). Typically this is around 0.15 microns.
