How often to change your AC filter in North Texas, and which one to buy

Three things load a filter faster in Collin County than the box suggests. The cooling season is long, so the blower runs more hours. The air outside carries oak pollen in spring, ragweed in fall, mountain cedar in winter and blackland dust whenever it has not rained. And a lot of our homes have ducts in a 140-degree attic with small gaps that pull attic dust into the return. A one-inch pleated filter that is rated for 90 days is usually gray at 30 here.

MERV is the filtration rating. MERV 8 catches dust and pollen. MERV 11 adds most pet dander and finer particles and is a good default for the area. MERV 13 catches smoke and more allergens but restricts airflow enough that one-inch versions should only go in systems designed for them. A four-inch media cabinet at the air handler is the better way to get MERV 13 if allergies matter in your house: more surface area, less resistance, and one change a year.

Skip the thin fiberglass filters unless a technician told you the system needs them; they protect the equipment and nothing else. Skip the washable electrostatic ones, which restrict airflow as they load and rarely get washed.

• Most homes: one-inch pleated MERV 8 to 11, changed monthly in summer
• Allergies or pets: four-inch media MERV 11 to 13 at the air handler, changed every six months
• Never: a filter so dense you cannot easily blow through it

Turn the system off at the thermostat first so the blower does not pull dust past the open slot.

1. 1Find it. Either a return grille in a hallway ceiling or wall (open the latches) or a slot at the air handler in the attic or closet.
2. 2Note the size printed on the old frame, like 16x25x1, and the arrow showing airflow direction.
3. 3Slide the new one in with the arrow pointing toward the unit and away from the room.
4. 4Write the date on the frame edge in marker. Set a reminder on your phone for the next one.
5. 5Turn the system back on.

A loaded filter drops airflow across the indoor coil. In cooling season that lets the coil get too cold and freeze; in heating season it lets the furnace overheat and shut down on its limit switch. Both are the most common no-cool and no-heat calls a technician runs, and both start with a filter nobody changed. The blower motor also works harder against the restriction, and a motor replacement runs several hundred dollars.