The Carter MERV 8 Permanent Electrostatic Air Filter is a non-replaceable filter. The 20″x25″x1 version we tested contained no pleats, as other washable filters. Our favorite feature of this filter is that it can be custom-sized to fit any 1-inch filter slot/grille. While this type of filter does not require replacement, its filtration performance is among the worst we have ever tested.

The Filtest permeability score for this model came at 5.3 (higher means better), which makes its pressure-delta almost three times higher than some of the best performers. A higher pressure delta (which represents the resistance to airflow of a filter) would put more strain on your furnace (or AC) blower fan, which would lead to higher energy consumption and would potentially shorten the life of your equipment (compared to filters with a better permeability score).

Most importantly, this model’s filtration performance at the sub-micron level is nonexistent (0.3% @ 0.3-1µm) and poor at best at the 1-3µm level (28.3%). 46% at the 3-10µm level is not great too. It is worth mentioning that when we tested it, its MERV rating came out at 6 (vs. the advertised 8).