The Filter You Install Isn’t the Filter You Replace

Researchers at Purdue University have been tracking how HVAC filters perform after installation, not just when new, and found their efficiency shifts a lot over time. MERV ratings (a standard scale for how well a filter traps particles) are measured on fresh filters, but the study ran filters for over two years in real outdoor air and found some got better with age while others got worse. A basic MERV 8 filter actually improved to a MERV 14-level performance after a year, as trapped dust helped it catch more particles. A MERV 13 "electret" filter, which relies partly on an electric charge to grab particles, lost that charge over time and dropped to roughly MERV 10 performance after about six months. A MERV 14 filter held steady for nearly two years until physical damage caused it to fail.

The research also found that standard MERV ratings don't account for ultrafine particles, the very smallest pollutants, which made up most of the outdoor particles entering the test system. A separate Purdue study found that swapping a MERV 14 filter for a HEPA filter cut these ultrafine particles indoors by 94%.

There's a tradeoff to consider too: as filters load up with debris, they get harder for the fan to push air through, which can raise energy use or reduce airflow in the house. And bringing in more outdoor air, often assumed to improve air quality, can backfire during events like wildfires, since it let in more ozone in the tests even as it lowered indoor CO2.