You may have heard that high-efficiency filters are bad for your system. You may also have heard that they are the best thing you can do for your air. Both can be true. It depends on the filter and the system it is in.
What MERV means
MERV is a rating from 1 to 16 for how well a filter captures particles. A basic fiberglass filter might be MERV 4 and catches mostly large dust. A MERV 13 filter captures a large share of the fine particles that matter most for health, including smoke and many allergens.
Where it goes wrong
Better filtration means denser filter media, and denser media resists airflow more. When a MERV 13 filter is squeezed into a 1-inch slot that was designed for a basic filter, the blower has to work much harder to pull air through it.
We see the effect as high static pressure. Airflow drops, the coil runs too cold, comfort suffers, and in some cases the coil can even freeze.
How to get both
- Use more filter area. A 4 to 5 inch media cabinet has many times the surface area of a 1-inch filter, so it captures more with far less resistance.
- Make sure there is enough return air. An undersized return is the most common reason a good filter becomes a problem.
- Measure it. We check the pressure drop across the filter so we know it is helping, not hurting.
- Change it on time. A loaded filter restricts airflow more every week it stays in.