![Smaller, secondary air filter behind main filter in air return of residential HVAC system. : r/whatisthisthing Smaller, secondary air filter behind main filter in air return of residential HVAC system. : r/whatisthisthing](https://i.redd.it/m72qauh4lt181.jpg)
Smaller, secondary air filter behind main filter in air return of residential HVAC system. : r/whatisthisthing
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Amazon.com: AC Infinity Inline Filter Box 6" with High Efficacy Filter, Whole House HVAC Filtration Kit, Fits 6-Inch Inline Duct Fans for Fresh Air Intake and Ventilation : Tools & Home Improvement
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