Maintenance
Which Air Filter to Buy for the System You Have
Buy the size printed on the filter that is already in the system. The frame says something like 16x25x1. The last number is the thickness. A 16x25x1 and a 16x25x4 are not interchangeable unless the rack was built for both, and most racks were not.
MERV
MERV is the rating on the frame. For most central systems, MERV 8 is the right everyday filter. MERV 11 is reasonable if someone in the house has allergies and the system is not already struggling for air. MERV 13 in a one-inch slot is a common way to starve an older blower.
The air has to get through the media. If a new filter makes the supply air weaker, or the indoor coil ices up, the rating is too high for that system. Go back down.
Thickness
A one-inch filter is a screen you replace often. A four- or five-inch media filter, in a cabinet made for it, holds more dust and can stay in for three to six months. Do not fold a thick filter into a thin slot. Do not run the system with no filter while you wait on the right size. The coil will collect what the filter would have caught, which is how many no-cool calls start.
The ones to skip
The cheap spun-fiberglass pad stops almost nothing. A basic pleated MERV 8 costs a little more and is the one to keep in a closet. Washable filters are usually a coarse mesh. They only stay harmless if they are fully dry before they go back in. A damp one, in a humid climate, grows mildew.
Brand matters less than size, MERV, and whether the spare is in the house when the old one fails the light test. Buy a six-pack of the size you measured.
If the only filter slot means pulling a furnace door you would rather not pull, ask a technician whether that system can take a filter at the return grille instead. Some systems are built for one location. Adding a second filter in line with the first is how airflow dies.