The short answer: check it every month, and change a standard 1-inch filter at least every three months. Thicker filters last longer, and the washable filters in mini-split and window units need cleaning far more often.
The longer answer depends on the filter and on your home. Pets, dust, allergies, and how many hours a day the system runs all shorten the interval.
By filter type
| Filter | Typical interval | Notes |
|---|---|---|
| 1-inch fiberglass | About every 30 days | The thin, see-through kind. Cheap, and it loads up fast. |
| 1-inch pleated | Every 60 to 90 days | The most common home filter. Check monthly and change sooner if it looks gray. |
| 4 to 5-inch media filter | Every 6 to 12 months | More surface area, so it lasts much longer. |
| Washable mini-split or window AC filter | Clean every 2 to 4 weeks in heavy use | Rinse, let it dry fully, and put it back. Check your unit's manual. |
What shortens the interval
- Pets. Hair and dander load a filter quickly.
- Allergies or asthma in the house. A fresher filter keeps more out of the air.
- Dusty surroundings, like a desert climate, a dirt road, or renovation work nearby.
- A system that runs most of the day, as in a long hot summer.
- More people at home, more of the time.
How to tell it's due
Hold it up to a light. If you can't see light through it, or the pleats are packed with gray dust, change it. Other signs are weaker airflow from the vents, more dust settling around the house, and a system that runs longer than usual to reach the set temperature.
Getting the right replacement
The size is printed on the cardboard frame, for example 16 x 25 x 1. Write it down somewhere you'll find it, because standing in the store trying to remember it is how the job gets put off another month.
The arrow on the frame shows airflow direction. It points toward the furnace or air handler, away from the return grille. For the rating, MERV 8 to 13 suits most homes. A higher MERV filters finer particles but restricts airflow more, so check what your system is designed for before going higher.
Common questions
How often should I change my AC filter?
Check it monthly. Change a 1-inch pleated filter every 60 to 90 days, a thin fiberglass filter about every 30 days, and a 4 to 5-inch media filter every 6 to 12 months. Change it sooner with pets, dust or allergies.
What happens if I don't change the filter?
Airflow drops, so the system runs longer and uses more energy to do the same job. Dust builds up on the coil and in the ducts, and in bad cases the system can freeze up or overheat and shut down.
Which way does the filter go in?
Follow the arrow on the frame. It points in the direction the air flows, toward the furnace or air handler.