For most homes, MERV 8–11 is the sweet spot: real filtration without choking airflow. Change it every 1–3 months — monthly with pets or allergies, and set a phone reminder because nobody remembers otherwise.
The mistake that costs people furnaces: jamming in the highest-MERV filter the store sells. Ultra-dense filters on systems not designed for them restrict airflow, overheat the furnace and can crack heat exchangers over time.
A dirty filter is the cheapest problem in HVAC and the cause of a shocking share of service calls — weak airflow, short-cycling, frozen A/C coils. Sixty seconds a month prevents all of it.
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