The size comes first
Filters are labelled with a nominal size, such as 16 by 25 by 1 inches. Take the size off the frame of the filter you have now, or measure the slot. A filter that is too small lets air bypass it entirely.
What MERV means
MERV stands for minimum efficiency reporting value, a scale from 1 to 16 set by a standard from ASHRAE. A higher number means the filter traps smaller particles, such as fine dust and some smoke.
A denser filter also resists airflow more. Many home systems are fine with mid-range ratings, but check your furnace or air handler manual, or ask a technician, before you move up the scale.
Where HEPA fits
HEPA filters trap very fine particles, but most central systems are not built to push air through one. They are common in portable room purifiers, where a fan is designed around the filter.
How often to change it
Follow the interval printed on the filter, and check it monthly. Pets, renovation dust, nearby wildfire smoke and constant running all fill a filter faster.
General information only. For wiring, refrigerant or gas work, use a licensed professional.
