Awareness of how important air quality is to human health has grown in our industry, as well as in society at large. The COVID-19 virus invigorated sales of household air cleaners, and the abbreviation IAQ became as ubiquitous in conversations as NFL, ROI, and, for builders, CYA. Innovations like the Comparetto Cube (five filters and a box fan) and the Corsi-Rosenthal box (just four filters) were offset in the marketplace by opportunists putting a computer fan behind a filter media in a sleek housing and offering it for sale online. While some of those devices did offer some benefit, many of the air cleaners dropped off on front porches by masked Amazon drivers were boxes of “snake oil” with little hope of providing improved health outcomes for their owners, as referenced in articles by Allison Bailes and others trying to save consumers from a bad purchase.

In terms of function, air cleaners are simple machines and typically have just two key elements: a fan and a filter. Like a miniature forced-air furnace without any heating (or cooling), they simply pull air into a housing and push it back out through a filter to capture some level of particulate—nothing more. The level of filtration varies by unit but like any range hood or bath fan, these small units can filter the air in a localized area only. The limitation on the volume they clean or “scrub” is a function of their size (filter surface area with fan strength) and location (the air they can access).

The typical interior of a home allows for communication of air from one room to the next through hallways and under and around closed doors, and from one floor to another. But despite these pathways, most existing homes have relatively little air circulation without the use of central air handlers and distribution ductwork. As a result, these tiny, localized air scrubbers have limited reach, and unlike with a Corsi-Rosenthal Box, which has a larger filter surface area and decent airflow, you need multiple units to handle multiple rooms.

In theory, you could set the fan in an existing, centrally ducted air handler to run continuously to circulate air to these individual filtered locations. But in recent years, we’ve shifted away from recommending continuous operation of the air handler, as this can lead to high humidity, especially in tighter, better insulated homes, which call for less cooling and therefore less dehumidification of the constantly circulated air. Also, hoping that the air circulated to these spaces will efficiently find its way to these air scrubbers for cleaning and redistribution throughout the house is too optimistic. Eventually, the air should in theory find the filter, but the volume of air flowing through the filter devices is fixed, and many units don’t separate their supply and return by more than a few inches, so they may scrub the same air over and over without cleaning the dirtier air that can’t get in the mix.

Rather than rely on small, room-by-room, plug-in units, we favor installing a whole-house HEPA filtration system, specifically the Fantech Hero HS300 appliance. Fantech boasts “3 stage filtration,” citing a prefilter with carbon capturing 90% of particles between 3 and 10 microns and a HEPA filter to capture 99.97% of particles .3 microns and larger. This unit has two speed settings—220 and 300 cfm—and though it operates independently from the air handler, it uses the existing ductwork system to access the entire volume of the household air for constant filtration. It’s an insulated box with an inlet at the top, a prefilter with carbon, then the fan, the HEPA filter, and outlet at the bottom. For us, the insulation is primarily for sound control since the unit isn’t heating or cooling the air, though it also would allow for installation in an unconditioned space.

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