· by Michael Blair, Nashville's Air Duct Cleaning
Filter ratings measure how small a particle is captured. Capturing more requires either more resistance or more surface area, and choosing the first without the second causes problems.
What the rating means
MERV rates capture efficiency across particle sizes. Higher numbers capture smaller particles. A basic filter protects the equipment; a mid-range one captures pollen and dust; higher ones capture finer particulates.
Rating is not the same as airflow resistance, but on a one-inch filter the two rise together.
The restriction problem
A high-rated one-inch filter has the same face area as a basic one and much finer media, so resistance rises. That reduces airflow, which reduces capacity, raises energy use and can freeze the coil.
The answer is more surface area — a four or five inch media cabinet delivers a high rating at low resistance.
- Mid-range one-inch filters suit most systems
- Four or five inch media gives high rating at low resistance
- Check the filter seals in the track, not just its rating
- Change one-inch filters every one to three months here
- Media filters typically last six to twelve months
Sealing matters more than rating
Air takes the easiest path. A filter that does not seal in its track lets air bypass it entirely, and a high-rated filter with a gap performs worse than a basic one that fits.
It is the most common filtration failure and it costs nothing to check.
Intervals in Middle Tennessee
One-inch pleated filters want changing every one to three months, at the short end during pollen season and with pets. Media filters typically run six to twelve months.
A loaded filter restricts airflow and stops capturing, so intervals matter as much as the initial choice.