@voxofgod@jorts.horse Less effective than what? I mentioned that the Corsi-Rosenthal Box generally performs _better_ than common HEPA purifiers from Amazon, WalMart, etc. For HVAC systems, it depends on the type. Assuming a ducted, forced-air system, the best plan is to stick a Filtrete 1900, 2200, or 2500 filter on each return intake. If you have a ductless, window, or portable AC unit, you're better off using a separate device - like this CR Box - as opposed to messing with the airflow of the AC.