Thread by @Michael59503746

I was randomly thinking about viral defenses, and wanted to talk for a minute about filtration (e.g., HEPA, CR-Box, Clean Air Kit, etc.).

I see a LOT of folks asking, “What’s a good HEPA to buy?”

That’s the wrong initial question to ask. 🧵>>

Your FIRST question should be, “What clean air delivery rate (CADR) do I need to achieve at least 6 air changes per hour (ACH) in >each< space that needs filtration?”

You can have the nicest, coolest HEPA ever made, but it won’t do what you need if it’s not scaled to the space.

There are online calculators like this one to sort this out.

Enter the room’s area (length x width), height, and 6 for ACH, and it will show you the needed CADR (in cubic feet/min or CFM).

omnicalculator.com/construction/a…

So, for example, if you have a 15 x 15 room (225 sq ft) that’s 8 feet high, 6 ACH would require a total CADR of 180 CFM; 12 ACH (WAY better!) would be 360 CFM.

So you’d need one or more units whose total output = 180 (to achieve 6 ACH) or 360 (12 ACH).

Granted, if you have central HVAC and a MERV13 filter, that can also contribute to your overall ACH. But for the sake of this exercise, we’re just looking at what the in-room filters need to provide.

The “one or more” units is a key point: you don’t need one massive filter for a big room - you can use two or more smaller ones (which may well be cheaper!) whose combined CADR hits your requirement. They’ll also filter the air more evenly across the space.

Do this for each room/space that needs its air cleaned.

THEN it’s time to go filter shopping, whether it’s for a HEPA, CR-Box (don’t forget the newer 12V fan designs, which are much quieter!), Clean Air Kits, etc.

But as you shop, check to make sure which fan setting (typically high/med/low) provides which CADR. Product blurbs often just provide the CADR for the high setting, so MAKE SURE you take that into account, especially since high = louder.

Finally, do NOT trust the “filters X area” claims unless the info specifies the CADR and ACH. I’ve seen a lot of these where it winds up being something like 1 or 2 ACH for the claimed space, which obviously is no bueno. /end/